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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Translocation of phospholipase A2α to apoplasts is modulated by developmental stages and bacterial infection in Arabidopsis</title><author><name sortKey="Jung, Jihye" sort="Jung, Jihye" uniqKey="Jung J" first="Jihye" last="Jung">Jihye Jung</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><institution> Division of Biosystems and Bioengineering, University of Science and Technology,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kumar, Krishna" sort="Kumar, Krishna" uniqKey="Kumar K" first="Krishna" last="Kumar">Krishna Kumar</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff3"><institution> Department of Biological Sciences, Chungnam National University,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Lee, Hyoung Yool" sort="Lee, Hyoung Yool" uniqKey="Lee H" first="Hyoung Yool" last="Lee">Hyoung Yool Lee</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Park, Youn Il" sort="Park, Youn Il" uniqKey="Park Y" first="Youn-Il" last="Park">Youn-Il Park</name><affiliation><nlm:aff id="aff3"><institution> Department of Biological Sciences, Chungnam National University,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Cho, Hyung Taeg" sort="Cho, Hyung Taeg" uniqKey="Cho H" first="Hyung-Taeg" last="Cho">Hyung-Taeg Cho</name><affiliation><nlm:aff id="aff4"><institution> School of Biological Sciences and Genomics and Breeding Institute, Seoul National University,</institution><country>Seoul, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Ryu, Stephen Beungtae" sort="Ryu, Stephen Beungtae" uniqKey="Ryu S" first="Stephen Beungtae" last="Ryu">Stephen Beungtae Ryu</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><institution> Division of Biosystems and Bioengineering, University of Science and Technology,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">22719742</idno><idno type="pmc">3376726</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3376726</idno><idno type="RBID">PMC:3376726</idno><idno type="doi">10.3389/fpls.2012.00126</idno><date when="2012">2012</date><idno type="wicri:Area/Pmc/Corpus">000F47</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Translocation of phospholipase A2α to apoplasts is modulated by developmental stages and bacterial infection in Arabidopsis</title><author><name sortKey="Jung, Jihye" sort="Jung, Jihye" uniqKey="Jung J" first="Jihye" last="Jung">Jihye Jung</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><institution> Division of Biosystems and Bioengineering, University of Science and Technology,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kumar, Krishna" sort="Kumar, Krishna" uniqKey="Kumar K" first="Krishna" last="Kumar">Krishna Kumar</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff3"><institution> Department of Biological Sciences, Chungnam National University,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Lee, Hyoung Yool" sort="Lee, Hyoung Yool" uniqKey="Lee H" first="Hyoung Yool" last="Lee">Hyoung Yool Lee</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Park, Youn Il" sort="Park, Youn Il" uniqKey="Park Y" first="Youn-Il" last="Park">Youn-Il Park</name><affiliation><nlm:aff id="aff3"><institution> Department of Biological Sciences, Chungnam National University,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Cho, Hyung Taeg" sort="Cho, Hyung Taeg" uniqKey="Cho H" first="Hyung-Taeg" last="Cho">Hyung-Taeg Cho</name><affiliation><nlm:aff id="aff4"><institution> School of Biological Sciences and Genomics and Breeding Institute, Seoul National University,</institution><country>Seoul, Korea</country></nlm:aff></affiliation></author><author><name sortKey="Ryu, Stephen Beungtae" sort="Ryu, Stephen Beungtae" uniqKey="Ryu S" first="Stephen Beungtae" last="Ryu">Stephen Beungtae Ryu</name><affiliation><nlm:aff id="aff1"><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><institution> Division of Biosystems and Bioengineering, University of Science and Technology,</institution><country>Daejeon, Korea</country></nlm:aff></affiliation></author></analytic><series><title level="j">Frontiers in Plant Science</title><idno type="eISSN">1664-462X</idno><imprint><date when="2012">2012</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>Phospholipase A<sub>2</sub> (PLA<sub>2</sub>) hydrolyzes phospholipids at the <italic>sn</italic>-2 position to yield lysophospholipids and free fatty acids. Of the four paralogs expressed in <italic>Arabidopsis</italic>, the cellular functions of PLA<sub>2</sub>α <italic>in planta</italic> are poorly understood. The present study shows that PLA<sub>2</sub>α possesses unique characteristics in terms of spatiotemporal subcellular localization, as compared with the other paralogs that remain in the ER and/or Golgi apparatus during secretory processes. Only PLA<sub>2</sub>α is secreted out to extracellular spaces, and its secretion to apoplasts is modulated according to the developmental stages of plant tissues. Observation of PLA<sub>2</sub>α-RFP transgenic plants suggests that PLA<sub>2</sub>α localizes mostly at the Golgi bodies in actively growing leaf tissues, but is gradually translocated to apoplasts as the leaves become mature. When <italic>Pseudomonas syringae</italic> pv.~<italic>tomato</italic> DC3000 carrying the avirulent factor <italic>avrRpm1</italic> infects the apoplasts of host plants, PLA<sub>2</sub>α rapidly translocates to the apoplasts where bacteria attempt to become established. <italic>PLA</italic><sub>2</sub>α promoter::GUS assays show that PLA<sub>2</sub>α gene expression is controlled in a developmental stage- and tissue-specific manner. It would be interesting to investigate if PLA<sub>2</sub>α functions in plant defense responses at apoplasts where secreted PLA<sub>2</sub>α confronts with invading pathogens.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Bechtold, N" uniqKey="Bechtold N">N. Bechtold</name></author><author><name sortKey="Pelletier, G" uniqKey="Pelletier G">G. Pelletier</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Brown, W J" uniqKey="Brown W">W. J. 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Front Plant Sci</journal-id><journal-id journal-id-type="iso-abbrev">Front Plant Sci</journal-id><journal-id journal-id-type="publisher-id">Front. Plant Sci.</journal-id><journal-title-group><journal-title>Frontiers in Plant Science</journal-title></journal-title-group><issn pub-type="epub">1664-462X</issn><publisher><publisher-name>Frontiers Research Foundation</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">22719742</article-id><article-id pub-id-type="pmc">3376726</article-id><article-id pub-id-type="doi">10.3389/fpls.2012.00126</article-id><article-categories><subj-group subj-group-type="heading"><subject>Plant Science</subject><subj-group><subject>Original Research Article</subject></subj-group></subj-group></article-categories><title-group><article-title>Translocation of phospholipase A2α to apoplasts is modulated by developmental stages and bacterial infection in Arabidopsis</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Jung</surname><given-names>Jihye</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref><xref ref-type="aff" rid="aff2"><sup>2</sup></xref></contrib><contrib contrib-type="author"><name><surname>Kumar</surname><given-names>Krishna</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref><xref ref-type="aff" rid="aff3"><sup>3</sup></xref></contrib><contrib contrib-type="author"><name><surname>Lee</surname><given-names>Hyoung Yool</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author"><name><surname>Park</surname><given-names>Youn-Il</given-names></name><xref ref-type="aff" rid="aff3"><sup>3</sup></xref></contrib><contrib contrib-type="author"><name><surname>Cho</surname><given-names>Hyung-Taeg</given-names></name><xref ref-type="aff" rid="aff4"><sup>4</sup></xref></contrib><contrib contrib-type="author"><name><surname>Ryu</surname><given-names>Stephen Beungtae</given-names></name><xref ref-type="aff" rid="aff1"><sup>1</sup></xref><xref ref-type="aff" rid="aff2"><sup>2</sup></xref><xref ref-type="author-notes" rid="fn001"><sup>*</sup></xref></contrib></contrib-group><aff id="aff1"><sup>1</sup><institution> Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB),</institution><country>Daejeon, Korea</country></aff><aff id="aff2"><sup>2</sup><institution> Division of Biosystems and Bioengineering, University of Science and Technology,</institution><country>Daejeon, Korea</country></aff><aff id="aff3"><sup>3</sup><institution> Department of Biological Sciences, Chungnam National University,</institution><country>Daejeon, Korea</country></aff><aff id="aff4"><sup>4</sup><institution> School of Biological Sciences and Genomics and Breeding Institute, Seoul National University,</institution><country>Seoul, Korea</country></aff><author-notes><fn fn-type="edited-by"><p>Edited by: <italic>Xuemin Wang, University of Missouri-St Louis and Donald Danforth Plant Science Center, USA</italic></p></fn><fn fn-type="edited-by"><p>Reviewed by: <italic>Stephan Pollmann, Universidad Politécnica de Madrid, SpainIngo Heilmann, Martin-Luther-University Halle-Wittenberg, Germany Ying Gu, Pennsylvania State University, USA</italic></p></fn><corresp id="fn001">*Correspondence: <italic>Stephen Beungtae Ryu, Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), 125 Gwahak-ro, Yuseong-gu, Daejeon 306-809, Korea. e-mail:<email>sbryu@kribb.re.kr</email></italic></corresp><fn fn-type="other" id="fn002"><p>This article was submitted to Frontiers in Plant Physiology, a specialty of Frontiers in Plant Science.</p></fn></author-notes><pub-date pub-type="epub"><day>18</day><month>6</month><year>2012</year></pub-date><pub-date pub-type="collection"><year>2012</year></pub-date><volume>3</volume><elocation-id>126</elocation-id><history><date date-type="received"><day>22</day><month>12</month><year>2011</year></date><date date-type="accepted"><day>25</day><month>5</month><year>2012</year></date></history><permissions><copyright-statement>Copyright © Jung, Kumar, Lee, Park, Cho and Ryu.</copyright-statement><copyright-year>2012</copyright-year><license license-type="open-access" xlink:href="http://www.frontiersin.org/licenseagreement"><license-p> This is an open-access article distributed under the terms of the <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by-nc/3.0/">Creative Commons Attribution Non Commercial License</ext-link>, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.</license-p></license></permissions><abstract><p>Phospholipase A<sub>2</sub> (PLA<sub>2</sub>) hydrolyzes phospholipids at the <italic>sn</italic>-2 position to yield lysophospholipids and free fatty acids. Of the four paralogs expressed in <italic>Arabidopsis</italic>, the cellular functions of PLA<sub>2</sub>α <italic>in planta</italic> are poorly understood. The present study shows that PLA<sub>2</sub>α possesses unique characteristics in terms of spatiotemporal subcellular localization, as compared with the other paralogs that remain in the ER and/or Golgi apparatus during secretory processes. Only PLA<sub>2</sub>α is secreted out to extracellular spaces, and its secretion to apoplasts is modulated according to the developmental stages of plant tissues. Observation of PLA<sub>2</sub>α-RFP transgenic plants suggests that PLA<sub>2</sub>α localizes mostly at the Golgi bodies in actively growing leaf tissues, but is gradually translocated to apoplasts as the leaves become mature. When <italic>Pseudomonas syringae</italic> pv.~<italic>tomato</italic> DC3000 carrying the avirulent factor <italic>avrRpm1</italic> infects the apoplasts of host plants, PLA<sub>2</sub>α rapidly translocates to the apoplasts where bacteria attempt to become established. <italic>PLA</italic><sub>2</sub>α promoter::GUS assays show that PLA<sub>2</sub>α gene expression is controlled in a developmental stage- and tissue-specific manner. It would be interesting to investigate if PLA<sub>2</sub>α functions in plant defense responses at apoplasts where secreted PLA<sub>2</sub>α confronts with invading pathogens.</p></abstract><kwd-group><kwd>phospholipase A<sub>2</sub></kwd><kwd>translocation</kwd><kwd>apoplast</kwd><kwd>bacterial infection</kwd><kwd>subcellular localization</kwd></kwd-group><counts><fig-count count="5"></fig-count><table-count count="0"></table-count><equation-count count="0"></equation-count><ref-count count="24"></ref-count><page-count count="7"></page-count><word-count count="0"></word-count></counts></article-meta></front><body><sec><title>INTRODUCTION</title><p>Phospholipase A<sub>2</sub> (PLA<sub>2</sub>) is widespread throughout nature and stereospecifically catalyzes the hydrolysis of phospholipids at <italic>sn</italic>-2 to produce lysophospholipids and free fatty acids, which are important mediators or precursors in signal transduction pathways in animal cells (<xref ref-type="bibr" rid="B18">Schaloske and Dennis, 2006</xref>; <xref ref-type="bibr" rid="B3">Burke and Dennis, 2009</xref>). There is evidence that plant PLA<sub>2</sub>s are also involved in diverse biological and physiological processes such as senescence, wound healing, elicitor and stress responses, defense against pathogens, and the induction of secondary metabolite accumulation (<xref ref-type="bibr" rid="B23">Wang, 2001</xref>,<xref ref-type="bibr" rid="B24">2004</xref>; <xref ref-type="bibr" rid="B16">Ryu, 2004</xref>; <xref ref-type="bibr" rid="B19">Scherer et al., 2007</xref>; <xref ref-type="bibr" rid="B20">Seo et al., 2008</xref>; <xref ref-type="bibr" rid="B8">Kirik and Mudgett, 2009</xref>; <xref ref-type="bibr" rid="B14">Mansfeld, 2009</xref>; <xref ref-type="bibr" rid="B5">Froidure et al., 2010</xref>; <xref ref-type="bibr" rid="B13">Liao and Burns, 2010</xref>).</p><p>There are four PLA<sub>2</sub> paralogs in <italic>Arabidopsis</italic>: <italic>PLA<sub>2</sub></italic>α, <italic>PLA<sub>2</sub></italic>β, <italic>PLA<sub>2</sub></italic>γ, and <italic>PLA<sub>2</sub></italic>δ. The paralogs <italic>PLA<sub>2</sub></italic>γ and <italic>PLA<sub>2</sub></italic>δ are expressed solely in pollen, localized in the endoplasmic reticulum (ER)/Golgi bodies and ER, respectively, and mediate pollen germination and tube growth (<xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>). <italic>PLA<sub>2</sub></italic>β is localized in the ER and expressed in different tissues such as young seedlings, elongating flower stems, and pollen, and it mediates cell elongation, shoot gravitropism, stomatal opening, and pollen development (<xref ref-type="bibr" rid="B10">Lee et al., 2003</xref>; <xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>). Although <italic>PLA<sub>2</sub></italic>α appears to be ubiquitous in diverse organs (<xref ref-type="bibr" rid="B17">Ryu et al., 2005</xref>; <xref ref-type="bibr" rid="B15">Mansfeld and Ulbrich-Hofmann, 2007</xref>; <xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>), its temporal and spatial expression dynamics in different tissues and its subcellular translocation during different developmental stages are unknown.</p><p>In this study, we report that PLA<sub>2</sub>α in <italic>Arabidopsis</italic> moves from the ER/Golgi apparatus to the apoplasts as the leaves become mature, and that <italic>PLA<sub>2</sub></italic>α gene expression is controlled in both a developmental stage- and organ-dependent manner. Several lines of evidence suggest that secretory proteins or proteins enhancing secretory pathways play important roles in plant defense responses (<xref ref-type="bibr" rid="B22">Wang et al., 2005</xref>; <xref ref-type="bibr" rid="B9">Kwon et al., 2008</xref>; <xref ref-type="bibr" rid="B21">Sup Yun et al., 2008</xref>). Thus, we examined if the secretion of PLA<sub>2</sub>α to apoplasts is modulated by pathogen infection. Interestingly, translocation of PLA<sub>2</sub>α to apoplasts was rapidly enhanced in response to the inoculation of <italic>Pseudomonas syringae</italic> pv. <italic>tomato</italic> DC3000 carrying <italic>avrRpm1</italic> (<italic>Pst-avrRpm1</italic>). These observations suggest that PLA<sub>2</sub>α proteins secreted into apoplasts in response to bacterial infection may play a role in host defense responses.</p></sec><sec sec-type="materials|methods" id="s1"><title>MATERIALS AND METHODS</title><sec><title>PLANT MATERIALS AND REAGENTS</title><p><italic>Arabidopsis thaliana</italic> (Col-0) plants were grown in soil pots at 22°C, 60% relative humidity, with a 16-h photoperiod and a photon flux density of 110 μmol m<sup>-2</sup> s<sup>-1</sup>.</p></sec><sec><title>GUS STAINING</title><p>For histochemical localization studies, a <italic>PLA<sub>2</sub></italic>α-promoter::GUS (<italic>ProPLA<sub>2</sub></italic>α::GUS) construct was cloned by incorporating the <italic>PLA<sub>2</sub></italic>α sequence upstream of the ATG start codon (from -1175 bp to +3 bp) into the <italic>Hind</italic>III and <italic>Bam</italic>HI sites of the pBI101 vector. The resulting plasmids were inserted into <italic>Agrobacterium tumefaciens</italic> strain EHA105, which was transformed into <italic>Arabidopsis</italic> using the floral dip method as described previously (<xref ref-type="bibr" rid="B1">Bech1998told and Pelletier, 1998</xref>). Histochemical GUS assays to show tissue-specific <italic>PLA<sub>2</sub></italic>α expression at different developmental stages were performed as previously described (<xref ref-type="bibr" rid="B6">Jefferson et al., 1987</xref>). Tissues from <italic>ProPLA<sub>2</sub></italic>α::GUS-transformed plants were immersed in GUS solution [1 mM X-gluc, 100 mM sodium phosphate buffer (pH 7.0), 0.5 mM K<sub>3</sub>Fe(CN)<sub>6</sub>, 0.5 mM K<sub>4</sub>Fe(CN)<sub>6</sub>, 10 mM EDTA, and 0.1% (v/v) Triton X-100] and incubated for 12 h at 37°C due to its weak staining. After GUS staining, 100% ethanol was used to remove the chlorophyll.</p></sec><sec><title>SUBCELLULAR LOCALIZATION OF PLA<sub>2</sub>α-RFP IN <italic>ARABIDOPSIS</italic></title><p>To investigate the dynamics of <italic>PLA<sub>2</sub></italic>α subcellular localization, transgenic <italic>Arabidopsis</italic> plants carrying <italic>Pro35S::PLA<sub>2</sub></italic>α-RFP were generated (<xref ref-type="bibr" rid="B12">Lee et al., 2010</xref>). Leaf tissues from the <italic>PLA<sub>2</sub></italic>α-RFP transgenic <italic>Arabidopsis</italic> plants were viewed at different developmental stages using a laser scanning confocal microscope (Meta system, Zeiss) after incubation in water or 1 N KNO<sub>3</sub> for 5 min to trigger plasmolysis. RFP-fluorescence was excited at 543 nm and the emitted fluorescence was collected with a band-pass filter at 560–615 nm.</p></sec><sec><title>CO-LOCALIZATION ASSAY OF PLA<sub>2</sub>α-RFP AND ST-GFP</title><p>To investigate whether PLA<sub>2</sub>α-RFP proteins are co-localized with a Golgi marker ST-GFP, transgenic <italic>Arabidopsis</italic> plants expressing both PLA<sub>2</sub>α-RFP and ST-GFP were generated (<xref ref-type="bibr" rid="B12">Lee et al., 2010</xref>). Close-to-mature leaves of 3-week-old transgenic plants were observed with a laser scanning confocal microscope (Meta system, Zeiss). RFP and GFP fluorescence was detected using at 543/560–615 nm and 488/505–530 nm excitation/emission filter sets, respectively.</p></sec><sec><title>BACTERIAL INOCULATION OF PLANTS</title><p><italic>Pseudomonas syringae</italic> pv. <italic>tomato</italic> DC3000 carrying <italic>avrRpm1</italic> (<italic>Pst-avrRpm1</italic>) were obtained from Y. J. Kim (Korea University, Seoul, Korea). Plants were inoculated by spreading a bacterial suspension (1 × 10<sup>8</sup> CFU ml<sup>-1</sup> in 0.015% v/v Silwet L-77 and 10 mM MgCl<sub>2</sub>) onto the adaxial leaf surfaces of <italic>Arabidopsis</italic> carrying <italic>Pro35S::PLA<sub>2</sub></italic>α-RFP. Plants designated as NT were given no treatment, whereas mock plants were treated with 0.015% v/v Silwet L-77 and 10 mM MgCl<sub>2</sub>. All the data presented in this study were obtained from at least three independent replicates.</p></sec></sec><sec><title>RESULTS</title><sec><title>HISTOCHEMICAL ANALYSIS OF GUS ACTIVITY OF THE <italic>PLA<sub>2</sub>α</italic> PROMOTER</title><p>Although RT-PCR analysis shows that <italic>PLA<sub>2</sub></italic>α transcripts are present in different parts of <italic>Arabidopsis</italic> tissues (<xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>), there is little information regarding PLA<sub>2</sub>α gene expression at different developmental stages. To elucidate the cell type-specific expression patterns of the <italic>PLA<sub>2</sub></italic>α gene, transgenic <italic>Arabidopsis</italic> lines were generated that expressed the <italic>beta-glucuronidase</italic> (<italic>GUS</italic>) reporter gene under the control of the <italic>PLA<sub>2</sub></italic>α promoter (<bold>Figure <xref ref-type="fig" rid="F1">1</xref></bold>). GUS activity was detected in the cotyledons, the shoot apex, the hypocotyl, and the vascular tissues of 7-day-old germinated seedlings (<bold>Figure <xref ref-type="fig" rid="F1">1A</xref></bold>). Strong GUS activity was detected in the shoot apex in 14-day-old seedlings and 3-week-old plants, and was preferentially expressed in young leaves rather than old leaves (<bold>Figures <xref ref-type="fig" rid="F1">1A</xref>–<xref ref-type="fig" rid="F1">C</xref></bold>). No GUS activity was detected in the roots at this stage. In 5-week-old plants, GUS expression was found in the cauline leaves, sepals, styles, and pedicel of reproductive tissues (<bold>Figure <xref ref-type="fig" rid="F1">1D</xref></bold>). The apical end of the pedicel is particularly dark-stained, apparently due to its thickened cell tissues based on a comparison with the GUS staining of the control transgenic plants harboring the <italic>35S</italic> promoter fused with the <italic>GUS</italic> gene (<bold>Figure <xref ref-type="fig" rid="F1">1I</xref></bold>). In plants transformed with <italic>ProPLA<sub>2</sub></italic>α::GUS, GUS expression was also detected in the developing siliques (<bold>Figures <xref ref-type="fig" rid="F1">1E</xref>–<xref ref-type="fig" rid="F1">H</xref></bold>) and in the main roots of flowering plants (<bold>Figure <xref ref-type="fig" rid="F1">1J</xref></bold>). Taken together, these data indicate that <italic>PLA<sub>2</sub></italic>α gene expression is controlled in a unique developmental stage- and tissue-specific manner.</p><fig id="F1" position="float"><label>FIGURE 1</label><caption><p><bold>Spatial and temporal expression of <italic>PLA<sub>2</sub></italic>α.</bold> Spatiotemporal expression patterns of the <italic>PLA<sub>2</sub></italic>α gene in transgenic <italic>Arabidopsis</italic> plants harboring the <italic>PLA<sub>2</sub></italic>α promoter fused with the <italic>GUS</italic> gene. Promoter activity was visualized by histochemical GUS staining. <bold>(A)</bold> Seven-day-old plant. <bold>(B)</bold> Fourteen-day-old plant. <bold>(C)</bold> Three-week-old plant. <bold>(D)</bold> Flower cluster, cauline leaf, and stem of a 5-week-old plant. <bold>(E–H)</bold> Carpels and developing siliques of a 5-week-old plant. <bold>(I)</bold> Pedicel of the control transgenic plants harboring the <italic>35S</italic> promoter fused with the <italic>GUS</italic> gene. <bold>(J)</bold> Root of a 6-week-old plant. Bars = 2 mm.</p></caption><graphic xlink:href="fpls-03-00126-g001"></graphic></fig></sec><sec><title>SUBCELLULAR LOCALIZATION OF PLA<sub>2</sub>α</title><p><xref ref-type="bibr" rid="B12">Lee et al. (2010)</xref> reported that fluorescence signals for PLA<sub>2</sub>α-fusion proteins were observed at the Golgi apparatus of root hair cells. However, <xref ref-type="bibr" rid="B5">Froidure et al. (2010)</xref> showed time-dependent localization of PLA<sub>2</sub>α using a transient expression system incorporating <italic>N. tabacum</italic>. The YFP reporter fused with PLA<sub>2</sub>α was detected in cytoplasmic vesicles around the nucleus 36 h after agroinfiltration to tobacco leaves, and was detected at the extracellular spaces outside the cells at a later time point (48 h after agroinfiltration). To resolve these inconsistencies, we investigated in more detail the subcellular localization of PLA<sub>2</sub>α by analyzing the fluorescence of fusion proteins in transgenic plants carrying <italic>Pro35S::PLA<sub>2</sub></italic>α-RFP. The leaves of 4-week-old <italic>PLA<sub>2</sub></italic>α-RFP transgenic seedlings were viewed using a laser scanning confocal microscope. The results showed that the subcellular localization of PLA<sub>2</sub>α was dependent on the developmental stages of leaf tissue. PLA<sub>2</sub>α-RFP fusion proteins were present primarily at the Golgi apparatus in pre-mature young leaves (<bold>Figure <xref ref-type="fig" rid="F2">2A</xref></bold>), whereas in mature leaves, they were detected primarily in the apoplasts (<bold>Figure <xref ref-type="fig" rid="F2">2B</xref></bold>). Even after cell plasmolysis was induced by treatment with 1 N KNO<sub>3</sub> for 5 min, the PLA<sub>2</sub>α-RFP signal remained in the extracellular spaces or diffused into the gap between the cell wall and the plasma membrane that is induced by plasmolysis (<bold>Figure <xref ref-type="fig" rid="F2">2C</xref></bold>). These results indicate that PLA<sub>2</sub>α is indeed localized in the apoplasts of mature leaves.</p><fig id="F2" position="float"><label>FIGURE 2</label><caption><p><bold> Subcellular localization of PLA<sub>2</sub>α at different leaf ages in PLA<sub><bold>2</bold></sub>α-RFP transgenic <italic>Arabidopsis</italic> plants.</bold><bold>(A,B)</bold> Epidermal cells in pre-mature leaf tissues <bold>(A)</bold> and mature leaf tissues <bold>(B)</bold> from <italic>Pro35SPLA<sub>2</sub></italic>α-RFP transgenic <italic>Arabidopsis</italic> plants (4-week-old) were viewed using confocal microscopy. <bold>(C)</bold> Mature leaf tissues were incubated in 1 N KNO<sub>3</sub> to induce plasmolysis before being viewed with confocal microscopy. The slightly decreased clarity of images in <bold>(C)</bold> appears to result from the diffusion of PLA<sub>2</sub>α-RFP into the expanded apoplasts due to plasmolysis. White arrows in <bold>(C)</bold> indicate the plasmolyzed plasma membrane, whereas black arrows indicate extracellular spaces. Fluorescent (top), bright field (middle), and merged images (bottom) are presented. Bars = 20 μm.</p></caption><graphic xlink:href="fpls-03-00126-g002"></graphic></fig></sec><sec><title>CO-LOCALIZATION OF PLA<sub>2</sub>α WITH A GOLGI MARKER</title><p>As secretion of proteins to apoplasts is known to occur through ER and Golgi bodies, PLA<sub>2</sub>α-RFP signals were mostly detected at the Golgi bodies in pre-mature young leaves (<bold>Figure <xref ref-type="fig" rid="F2">2A</xref></bold>). However, the fluorescent spots become gradually bigger as the leaves become mature, leading us to suspect that they may be other cellular organelles. To investigate if the big PLA<sub>2</sub>α fluorescent spots are real Golgi bodies, we performed co-localization assay of PLA<sub>2</sub>α with a Golgi body marker, sialyltransferase (ST). Close-to-mature leaves of transgenic <italic>Arabidopsis</italic> plants expressing both PLA<sub>2</sub>α-RFP and ST-GFP were observed with a laser scanning confocal microscope. As shown in <bold>Figure <xref ref-type="fig" rid="F3">3</xref></bold>, big spots of PLA<sub>2</sub>α-RFP signals are mostly overlapped with the spots of a Golgi body marker (ST-GFP), confirming that PLA<sub>2</sub>α is localized in Golgi apparatus before secretion to the apoplasts. We found that the apparent big size spots result from aggregation of several Golgi bodies and strong brightness of RFP-fluorescence. Aggregation of Golgi bodies appears to be gradually enhanced as the leaves become mature.</p><fig id="F3" position="float"><label>FIGURE 3</label><caption><p><bold> Co-localization of PLA<sub><bold>2</bold></sub>α-RFP with ST-GFP, a Golgi body marker</bold>. Close-to-mature leaves of transgenic <italic>Arabidopsis</italic> plants expressing both PLA<sub>2</sub>α-RFP and ST-GFP were observed with a laser scanning confocal microscope. PLA<sub>2</sub>α-RFP signals are mostly overlapped with ST-GFP, a Golgi body marker. ST-GFP <bold>(A)</bold>, PLA<sub>2</sub>α-RFP <bold>(B)</bold>, bright field <bold>(C)</bold>, and merged image <bold>(D)</bold> are presented. Bar = 20 μm.</p></caption><graphic xlink:href="fpls-03-00126-g003"></graphic></fig></sec><sec><title>PLA<sub>2</sub>α TRANSLOCATES TO APOPLASTS IN RESPONSE TO THE INOCULATION OF AVIRULENT BACTERIA</title><p>As leaves become mature, PLA<sub>2</sub>α is secreted into the apoplast, where it generates its lipid products, lysophospholipids and free fatty acids. The lipid products have been suggested to function as bio-active molecules that mediate a variety of cellular processes. Apoplasts are an important site for the interaction of plant cell defense mechanisms with invading bacteria, which attempt to become established in the apoplasts. If PLA<sub>2</sub>α positively participates in defense responses to pathogen attack, we hypothesized that its translocation to the apoplasts would be enhanced when pathogens are inoculated. As speculated, the translocation of PLA<sub>2</sub>α to the apoplasts was enhanced at 3 h post-inoculation of avirulent bacteria, <italic>Pst-avrRpm1</italic>, in young leaves (<bold>Figure <xref ref-type="fig" rid="F4">4C</xref></bold>) and in close-to-mature leaves (<bold>Figure <xref ref-type="fig" rid="F5">5C</xref></bold>), as compared to non-treated controls (NT) and 0.015% Silwet/10 mM MgCl<sub>2</sub>-treated mocks (<bold>Figures <xref ref-type="fig" rid="F4">4A</xref>,<xref ref-type="fig" rid="F4">B</xref> and <xref ref-type="fig" rid="F5">5A</xref>,<xref ref-type="fig" rid="F5">B</xref></bold>).</p><fig id="F4" position="float"><label>FIGURE 4</label><caption><p><bold>Translocation of PLA<sub>2</sub>α to apoplasts was enhanced by the inoculation of bacteria, <italic>Pst-avrRpm1</italic>, in pre-mature young leaves.</bold><bold>(A–C)</bold> Images showing increased fluorescence intensity and vesicle sizes followed by the translocation of PLA<sub>2</sub>α to apoplasts at 3 h post-inoculation of <italic>Pst-avrRpm1 </italic><bold>(C)</bold> compared to the no-treatment control <bold>(A)</bold> and 0.015% Silwet/10 mM MgCl<sub>2</sub>-treated mock <bold>(B)</bold> in pre-mature young leaves where PLA<sub>2</sub>α is normally localized primarily in Golgi bodies. Fluorescent (top), bright field (middle), and merged images (bottom) are presented. Bars = 20 μm.</p></caption><graphic xlink:href="fpls-03-00126-g004"></graphic></fig><fig id="F5" position="float"><label>FIGURE 5</label><caption><p><bold>Enhanced translocation of PLA<sub>2</sub>α to apoplasts in response to the inoculation of <italic>Pst-avrRpm1</italic> in close-to-mature leaves.</bold><bold>(A–C)</bold> Significant enhancement of PLA<sub>2</sub>α translocation to apoplasts was observed at 3 h post-inoculation of <italic>Pst-avrRpm1</italic><bold>(C)</bold> compared to the no-treatment control <bold>(A)</bold> and 0.015% Silwet/10 mM MgCl<sub>2</sub>-treated mock <bold>(B)</bold> in close-to-mature leaves where PLA<sub>2</sub>α is already detected at a low level in the apoplasts. Fluorescent (top), bright field (middle), and merged images (bottom) are presented. Bars = 20 μm.</p></caption><graphic xlink:href="fpls-03-00126-g005"></graphic></fig></sec></sec><sec><title>DISCUSSION</title><p><italic>PLA<sub>2</sub></italic>α is expressed in a tissue- and developmental stage-specific manner in <italic>Arabidopsis</italic> plant tissues. Relatively strong activities of the <italic>PLA<sub>2</sub></italic>α promoter were observed in actively growing seedlings and young leaves. Expression decreased slightly as leaves became mature. This expression pattern of <italic>PLA<sub>2</sub></italic>α is different from that of <italic>PLA<sub>2</sub></italic>β, which is expressed at a very low level in the mature leaves (<xref ref-type="bibr" rid="B10">Lee et al., 2003</xref>). This pattern of expression is also observed in the cauline leaves of the inflorescence stems, which display strong expression of <italic>PLA<sub>2</sub></italic>α but low expression of <italic>PLA<sub>2</sub></italic>β. At the young seedling and reproductive organ developmental stages, both <italic>PLA<sub>2</sub></italic> paralogs display similar expression patterns; strong expression in actively growing tissues and reproductive organs such as sepals, pedicels, and styles of open flowers, but low expression in petals, stigmas, and ovaries. Expression of both <italic>PLA<sub>2</sub></italic> paralogs was detected in developing siliques but not in maturing seeds. However, <italic>PLA<sub>2</sub></italic>α was not expressed in pollen tissues, in contrast to the strong expression of <italic>PLA<sub>2</sub></italic>β. In the root, <italic>PLA<sub>2</sub></italic>α was expressed at the late stages of growth, whereas <italic>PLA<sub>2</sub></italic>β was expressed in roots from seedling stages (<xref ref-type="bibr" rid="B10">Lee et al., 2003</xref>). These results suggest that PLA<sub>2</sub>α and PLA<sub>2</sub>β may play a role in plant growth and development in harmony with holding their own cellular roles at the different cellular localization and expressing tissues.</p><p><italic>Arabidopsis PLA<sub>2</sub></italic> genes encode proteins with N-terminal signal peptides, which are predicted to be secreted via ER and Golgi bodies to apoplasts and/or vacuoles. PLA<sub>2</sub>β has a KTEL sequence at its C-terminus, which is similar to the canonical ER-retention signal KDEL, and was shown to be localized in the ER (<xref ref-type="bibr" rid="B20">Seo et al., 2008</xref>). The <italic>PLA<sub>2</sub></italic>γ and <italic>PLA<sub>2</sub></italic>δ isoforms, which are solely expressed in pollen, are localized in the ER and/or Golgi (<xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>). <italic>PLA<sub>2</sub></italic>β, <italic>PLA<sub>2</sub></italic>γ, and <italic>PLA<sub>2</sub></italic>δ, which share high sequence homologies with each other, are expressed during pollen growth and development and play critical roles during pollen germination and tube growth (<xref ref-type="bibr" rid="B7">Kim et al., 2011</xref>). In contrast to <italic>PLA<sub>2</sub></italic>γ and <italic>PLA<sub>2</sub></italic>δ, <italic>PLA<sub>2</sub></italic>β is expressed in tissues such as actively growing leaves and elongating stems, and regulates shoot cell elongation and stem gravitropism, likely as a downstream component of auxin signaling (<xref ref-type="bibr" rid="B10">Lee et al., 2003</xref>). In addition, <italic>PLA<sub>2</sub></italic>β is expressed in guard cells in response to light and modulates light-induced stomatal opening (<xref ref-type="bibr" rid="B20">Seo et al., 2008</xref>).</p><p>PLA<sub>2</sub>α localizes primarily at Golgi bodies in actively growing young leaves but translocates to apoplasts as the leaves become mature. In root tissues, PLA<sub>2</sub>α localizes at Golgi bodies (<xref ref-type="bibr" rid="B12">Lee et al., 2010</xref>). Localization of PLA<sub>2</sub>α in Golgi bodies was confirmed by co-localization assay with a Golgi body marker. Studies in animal cells indicate that lysophospholipids, which are generated by PLA<sub>2</sub>, modulate retrograde trafficking and the cisternal structure of the Golgi complex by modifying membrane tubule formation (<xref ref-type="bibr" rid="B4">de Figueiredo et al., 1998</xref>; <xref ref-type="bibr" rid="B2">Brown et al., 2003</xref>). As in animal cells, PLA<sub>2</sub> at Golgi bodies in plant cells may play an important role in the intracellular trafficking of proteins. Membrane fusion, formation, and intracellular trafficking at the Golgi bodies are prominent processes during active growth stages of plant tissues. Consistent with this hypothesis, Golgi-localized PLA<sub>2</sub>α in root hairs appeared to act in the trafficking of PIN proteins (<xref ref-type="bibr" rid="B12">Lee et al., 2010</xref>). In actively growing young leaf tissues, PLA<sub>2</sub>α localized at Golgi bodies may facilitate growth and development by mediating vesicular trafficking.</p><p>Once leaf tissues are mature, PLA<sub>2</sub>α translocates to the apoplasts by way of Golgi bodies. Among the four<italic> Arabidopsis</italic> PLA<sub>2</sub> paralogs, only PLA<sub>2</sub>α translocates to the apoplasts of mature leaves. The reason for PLA<sub>2</sub>α movement from ER and Golgi bodies to apoplasts in the mature leaves is unknown. It could be speculated that as leaves are mature the demand for PLA<sub>2</sub>α activity diminishes in ER and Golgi bodies since the demand for vesicular trafficking for active growth decreases. If so, PLA<sub>2</sub>α may rather translocalize to the apoplasts, probably, in order to fulfill some other mission in the apoplasts of the mature leaves. Translocation of PLA<sub>2</sub>α is supported by its unique enzyme characteristics. The optimal pH range for PLA<sub>2</sub>α activity is quite broad compared with other PLA<sub>2</sub> paralogs, from pH 6 to 11 (<xref ref-type="bibr" rid="B11">Lee et al., 2005</xref>), so that PLA<sub>2</sub>α may fully be functional not only in the ER and Golgi bodies but also in the apoplasts. In contrast, PLA<sub>2</sub>β, which is localized in the ER, has a narrow range of optima from pH 6 to 7, whereas PLA<sub>2</sub>γ and PLA<sub>2</sub>δ, which are localized in the ER/Golgi bodies and ER of pollens, respectively, have optima at pH 7–9 and pH 8–9, respectively (<xref ref-type="bibr" rid="B11">Lee et al., 2005</xref>).</p><p>Structurally, apoplasts are formed by a continuation of the cell walls of adjacent cells and the associated extracellular spaces. The apoplast is important for a plant's interaction with the environment and microbes. The apoplast is also a site for cell-to-cell communication. Therefore, we examined the dynamics of PLA<sub>2</sub>α secretion to apoplasts in response to bacterial attack. When <italic>Pst-avrRpm1</italic> infects host plants, the pathogens cannot enter into the cytoplasm but remain in the apoplasts and attempt to become established. Our results indicate that PLA<sub>2</sub>α rapidly translocates to apoplasts in response to pathogen invasion. These data prompt us to speculate that PLA<sub>2</sub>α may play a certain role in the apoplasts, where host cells confront invading pathogens. Thus, it would be of interest to investigate the role of PLA<sub>2</sub>α in a variety of cellular processes, including host defense responses to pathogen attacks.</p></sec><sec><title>Conflict of Interest Statement</title><p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p></sec></body><back><ack><p>We thank H. J. Kim for helping us to write the manuscript. This work was supported by a 2nd Bio-Green grant (PJ009029) from Rural Development Administration, by a grant (IPET 109106-3); by a grant from the KRIBB Initiative Program; and by a Joint Research Support Project of University of Science and Technology. 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