Catarina Casais

RAR1 and its interacting partner SGT1 play a central role in plant disease resistance triggered by a number of resistance (R) proteins. We identified cytosolic heat shock protein 90 (HSP90), a molecular chaperone, as another RAR1 interacting protein by yeast two-hybrid screening. RAR1 interacts with the N-terminal half of HSP90 that contains the ATPase domain. HSP90 also specifically interacts with SGT1 that contains a tetratricopeptide repeat motif and a domain with similarity to the cochaperone p23. In Arabidopsis, the HSP90 inhibitor geldanamycin reduces the hypersensitive response and abolishes resistance triggered by the R protein RPS2 against Pseudomonas syringae pv. tomato DC3000 (avrRpt2). One of four Arabidopsis cytosolic HSP90 isoforms, AtHSP90.1 is required for full RPS2 resistance and is rapidly induced upon pathogen challenge. We propose that RAR1 and SGT1 function closely with HSP90 in chaperoning roles that are essential for disease resistance.

Lack of the barley (Hordeum vulgare) seven-transmembrane domain MLO protein confers resistance against the fungal pathogen Blumeria graminis f. sp. hordei (Bgh). To broaden the basis for MLO structure/function studies, we sequenced additional mlo resistance alleles, two of which confer only partial resistance. Wild-type MLO dampens the cell wall-restricted hydrogen peroxide burst at points of attempted fungal penetration of the epidermal cell wall, and in subtending mesophyll cells, it suppresses a second oxidative burst and cell death. Although the Bgh-induced cell death in mlo plants is spatially and temporally separated from resistance, we show that the two processes are linked. Uninoculated mutant mlo plants exhibit spontaneous mesophyll cell death that appears to be part of accelerated leaf senescence. Mlo transcript abundance increases in response to Bgh, rice (Oryza sativa) blast, wounding, paraquat treatment, a wheat powdery mildew-derived carbohydrate elicitor, and during leaf senescence. This suggests a broad involvement of Mlo in cell death protection and in responses to biotic and abiotic stresses.

Innate immunity signaling pathways in both animals and plants are regulated by mitogen-activated protein kinase (MAPK) cascades. An Arabidopsis MAPK cascade (MEKK1, MKK4/MKK5, and MPK3/MPK6) has been proposed to function downstream of the flagellin receptor FLS2 based on biochemical assays using transient overexpression of candidate components. To genetically test this model, we characterized two mekk1 mutants. We show here that MEKK1 is not required for flagellin-triggered activation of MPK3 and MPK6. Instead, MEKK1 is essential for activation of MPK4, a MAPK that negatively regulates systemic acquired resistance. We also showed that MEKK1 negatively regulates temperature-sensitive and tissue-specific cell death and H2O2 accumulation that are partly dependent on both RAR1, a key component in resistance protein function, and SID2, an isochorismate synthase required for salicylic acid production upon pathogen infection. Innate immunity signaling pathways in both animals and plants are regulated by mitogen-activated protein kinase (MAPK) cascades. An Arabidopsis MAPK cascade (MEKK1, MKK4/MKK5, and MPK3/MPK6) has been proposed to function downstream of the flagellin receptor FLS2 based on biochemical assays using transient overexpression of candidate components. To genetically test this model, we characterized two mekk1 mutants. We show here that MEKK1 is not required for flagellin-triggered activation of MPK3 and MPK6. Instead, MEKK1 is essential for activation of MPK4, a MAPK that negatively regulates systemic acquired resistance. We also showed that MEKK1 negatively regulates temperature-sensitive and tissue-specific cell death and H2O2 accumulation that are partly dependent on both RAR1, a key component in resistance protein function, and SID2, an isochorismate synthase required for salicylic acid production upon pathogen infection. The recognition of pathogen-derived molecules, so called pathogen-associated molecular patterns (PAMPs), 4The abbreviations used are: PAMP, pathogen-associated molecular pattern; MAPK, mitogen-activated protein kinase; MAPKK, MAPK kinase; MAPKKK, MAPKK kinase; DAB, 3,3′-diaminobenzidine; WT, wild type; SA, salicylic acid; R, resistance; MS, Murashige and Skoog; RT, reverse transcription; BisTris, 2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol. triggers innate immunity responses in both animals and plants (1Nurnberger T. Brunner F. Kemmerling B. Piater L. Immunol. Rev. 2004; 198: 249-266Crossref PubMed Scopus (915) Google Scholar). In animals, PAMP recognition is often mediated by receptors such as Toll and Toll-like receptors containing leucine-rich repeats (2Hayashi F. Smith K.D. Ozinsky A. Hawn T.R. Yi E.C. Goodlett D.R. Eng J.K. Akira S. Underhill D.M. Aderem A. Nature. 2001; 410: 1099-1103Crossref PubMed Scopus (2837) Google Scholar). In plants, PAMPs are also recognized by leucine-rich repeat-containing receptor kinases. For example, Arabidopsis FLS2 is a receptor kinase required for innate immune responses triggered by a bacterial flagellin (3Gomez-Gomez L. Boller T. Trends Plant Sci. 2002; 7: 251-256Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar). Similar to animal pathogens, many plant pathogens introduce effector proteins that are able to suppress PAMP-triggered immune responses (4Chisholm S.T. Coaker G. Day B. Staskawicz B.J. Cell. 2006; 124: 803-814Abstract Full Text Full Text PDF PubMed Scopus (2119) Google Scholar). Plants, on the other hand, recognize such effectors, directly or indirectly by resistance (R) proteins and trigger rapid defense responses, including localized cell death (4Chisholm S.T. Coaker G. Day B. Staskawicz B.J. Cell. 2006; 124: 803-814Abstract Full Text Full Text PDF PubMed Scopus (2119) Google Scholar). The signal transduction following pathogen recognition by R proteins and receptor kinases in plants are thought to be mediated by MAPK cascades, since a number of MAPK components are known to be activated in these processes (5Asai T. Tena G. Plotnikova J. Willmann M.R. Chiu W.L. Gomez-Gomez L. Boller T. Ausubel F.M. Sheen J. Nature. 2002; 415: 977-983Crossref PubMed Scopus (2041) Google Scholar, 6Pedley K.F. Martin G.B. Curr. Opin. Plant Biol. 2005; 8: 541-547Crossref PubMed Scopus (234) Google Scholar). However, the specificity or requirement of individual MAPK signaling components in disease resistance is poorly defined, since many of these components also are activated by various environmental cues (7Nakagami H. Pitzschke A. Hirt H. Trends Plant Sci. 2005; 10: 339-346Abstract Full Text Full Text PDF PubMed Scopus (544) Google Scholar). Recently, a transient reverse genetic approach based on virus-induced gene silencing provided genetic evidence for requirement of some MAPK cascade components in disease resistance. For example, tobacco MEK2, a MAPK kinase (MAPKK), and its substrate MAPKs, SIPK and WIPK, were shown to be essential for full N-mediated resistance against Tobacco mosaic virus (8Jin H.L. Liu Y.D. Yang K.Y. Kim C.Y. Baker B. Zhang S.Q. Plant J. 2003; 33: 719-731Crossref PubMed Scopus (167) Google Scholar). The likely upstream activator of MEK2 is MAPKKKα, which is essential for Pto-mediated resistance in tomato and Nicotiana benthamiana (9del Pedley K.F. Pozo O. Martin G.B. EMBO J. 2004; 23: 3072-3082Crossref PubMed Scopus (274) Google Scholar). Consistently, silencing MEK2, SIPK, or WIPK resulted in reduction of localized cell death associated with Pto resistance (9del Pedley K.F. Pozo O. Martin G.B. EMBO J. 2004; 23: 3072-3082Crossref PubMed Scopus (274) Google Scholar). In addition, another MAPK cascade, NPK1-MEK1-NTF6, is also essential for N-mediated resistance and Pto-dependent cell death (9del Pedley K.F. Pozo O. Martin G.B. EMBO J. 2004; 23: 3072-3082Crossref PubMed Scopus (274) Google Scholar, 11Liu Y.L. Schiff M. Dinesh-Kumar S.P. Plant J. 2004; 38: 800-809Crossref PubMed Scopus (216) Google Scholar). These data suggest that multiple MAPK cascades operate in order to establish disease resistance. Using a transient gain of function approach in Arabidopsis protoplast, a complete MAPK cascade consisting of MEKK1, MKK4/MKK5 (MEK2 orthologs), and MPK3/MPK6 (WIPK/SIPK orthologs, respectively) was proposed as a downstream signaling process of the flagellin receptor FLS2 (5Asai T. Tena G. Plotnikova J. Willmann M.R. Chiu W.L. Gomez-Gomez L. Boller T. Ausubel F.M. Sheen J. Nature. 2002; 415: 977-983Crossref PubMed Scopus (2041) Google Scholar). In these experiments, transiently expressing a MEKK1 derivative lacking the N-terminal regulatory domain (MEKK1-ΔN) resulted in activation of MKK5. Similarly, a constitutive active form of either MKK4 (MKK4a) or MKK5 (MKK5a) activated MPK3 and MPK6. Flg22 activates MPK3 and MPK6 in a FLS2-dependent manner. Furthermore, transient expression of MEKK1-ΔN, MKK4a, or MKK5a led to enhanced resistance against Pseudomonas syringae pv. tomato (Pst), supporting the model for the complete MAPK cascade in defense responses (5Asai T. Tena G. Plotnikova J. Willmann M.R. Chiu W.L. Gomez-Gomez L. Boller T. Ausubel F.M. Sheen J. Nature. 2002; 415: 977-983Crossref PubMed Scopus (2041) Google Scholar). However, whether full-length MEKK1 directly interacts with and activates MKK4 and MKK5 remains to be shown. Instead, MEKK1 was shown to directly interact with MKK1 and MKK2, but not with MKK4, in yeast two-hybrid assays (12Mizoguchi T. Ichimura K. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. FEBS Lett. 1998; 437: 56-60Crossref PubMed Scopus (103) Google Scholar, 13Ichimura K. Mizoguchi T. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. Biochem. Biophys. Res. Commun. 1998; 253: 532-543Crossref PubMed Scopus (155) Google Scholar). Interestingly, the N-terminal regulatory domain of MEKK1 also interacts with MPK4, but not MPK3, indicating that MEKK1 may act as a scaffold to establish specific phosphorylation relays for the MEKK1-MKK1/MKK2-MPK4 cascade (12Mizoguchi T. Ichimura K. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. FEBS Lett. 1998; 437: 56-60Crossref PubMed Scopus (103) Google Scholar, 13Ichimura K. Mizoguchi T. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. Biochem. Biophys. Res. Commun. 1998; 253: 532-543Crossref PubMed Scopus (155) Google Scholar). Consistent with these observations, flg22 treatment activates MKK1 to phosphorylate and activate MPK4 (14Teige M. Scheikl E. Eulgem T. Doczi F. Ichimura K. Shinozaki K. Dangl J.L. Hirt H. Mol. Cell. 2004; 15: 141-152Abstract Full Text Full Text PDF PubMed Scopus (733) Google Scholar, 15Droillard M.J. Boudsocq M. Barbier-Brygoo H. Lauriere C. FEBS Lett. 2004; 574: 42-48Crossref PubMed Scopus (130) Google Scholar). Because of these potential discrepancies in the specificity of signaling pathways during defense response, we tested the existing hypotheses by isolating two mutants of MEKK1 to examine the genetic requirement of this protein for activation of the various MAPKs downstream of FLS2. We show here that MEKK1 is required for activation of MPK4, but not MPK3 and MPK6, in response to flg22. Similar to the mpk4 mutants, the mekk1 plants exhibit constitutive expression of certain defense genes, such as PR1, PR2, and PR5. These data suggest that MEKK1 and MPK4 are the components of a MAPK cascade regulating plant defense responses. Furthermore, loss of MEKK1 results in temperature-dependent H2O2 accumulation and cell death in a tissue-specific manner. Interestingly, the H2O2 accumulation and cell death is partly RAR1- and SID2-dependent, indicating a potential link between MEKK1 and R protein-dependent defense signaling pathways. Plant Materials and Growth Conditions—The Arabidopsis mekk1-1 (SALK_052557) and mekk1-2 (Gabi-Kat 813G10) mutants (Col background) were generated and in the of the Kim P. J. P. C. C. A. E. H. L. H. M. E. M. C. P. L. M. T. E. A. W.L. 2003; PubMed Scopus Google Scholar, B. K. B. Plant Mol. Biol. 2003; PubMed Scopus Google Scholar). were by of using and The mekk1 mutants used in this were Arabidopsis mutants P. P. A. K. Dangl J.L. Plant Cell. 2002; PubMed Scopus Google M. P. A. Eulgem T. E. F. C. Dangl J.L. Plant Cell. 2002; PubMed Scopus Google and J. G. Ausubel F.M. Nature. 2001; PubMed Scopus Google were provided by J. M. and M. For the were on Murashige and with and in the for with and were A. H. Plant Cell. PubMed Scopus Google Scholar, H. Zhang Y.D. Plant J. Scopus Google Scholar). following were used for MEKK1 mekk1-1 mekk1-2 mekk1-2 and M.J. P. K. B.J. 2002; PubMed Scopus Google provided by P. P. A. K. Dangl J.L. EMBO J. 2003; PubMed Scopus Google provided by M. J. G. Ausubel F.M. Nature. 2001; PubMed Scopus Google Scholar). and of were in and proteins were with a containing as K. Mizoguchi T. T. Shinozaki K. Plant J. PubMed Google Scholar). of proteins were by the and and were as K. Mizoguchi T. T. Shinozaki K. Plant J. PubMed Google Scholar, C. A. K. A. K. P. 2002; PubMed Scopus Google Scholar). were with of S.Q. H. Plant Cell. 1998; 10: Google kinase assays were in of the containing of and protein as a substrate for The was by the of on a the protein was by for MPK3 were using a the was against to the N-terminal of MPK4 to and were K. Mizoguchi T. T. Shinozaki K. Plant J. PubMed Google Scholar). and were in and was using to the was with with and in using were with of of to the was used for to the The following were used for MPK3 and MPK4 were Plant Cell. 2004; PubMed Scopus Google Scholar). The mekk1 and of MEKK1 in defense responses, we two of MEKK1 in the which we as mekk1-1 (SALK_052557) and mekk1-2 (Gabi-Kat 813G10) in with the of H. Hirt of The in mekk1-1 was in the the and mekk1-2 was in the of mekk1-1 and mekk1-2 mutants were wild plants on However, as as the we in mekk1 mutants The were and to the The was also mekk1-1 and mekk1-2 and on The plants also showed in We mekk1-1 and mekk1-2 as plants, which were The of and in the was a and for mekk1-1 and These data that the were by the loss of MEKK1 MEKK1 for MPK3 and MPK6 The complete MAPK cascade proposed by (5Asai T. Tena G. Plotnikova J. Willmann M.R. Chiu W.L. Gomez-Gomez L. Boller T. Ausubel F.M. Sheen J. Nature. 2002; 415: 977-983Crossref PubMed Scopus (2041) Google that MEKK1 has a regulatory in the FLS2 signaling to the phosphorylation of MPK3 and MPK6 activation of MKK4 and MKK5. To test this model we and mekk1 with flg22 and an kinase by using specific for MPK3 and MPK6. Consistent with MPK3 and MPK6 were activated by the flg22 treatment in plants In we that MPK3 and MPK6 were activated by flg22 In the activation in mekk1 were in on these we that MEKK1 is for MPK3 and MPK6 activation by flg22. MPK4 by flg22 was shown to be activated upon flg22 treatment (14Teige M. Scheikl E. Eulgem T. Doczi F. Ichimura K. Shinozaki K. Dangl J.L. Hirt H. Mol. Cell. 2004; 15: 141-152Abstract Full Text Full Text PDF PubMed Scopus (733) Google Scholar, 15Droillard M.J. Boudsocq M. Barbier-Brygoo H. Lauriere C. FEBS Lett. 2004; 574: 42-48Crossref PubMed Scopus (130) Google Scholar). To MEKK1 is in MPK4 activation by we a kinase for Flg22 activated MPK4 in but in mekk1 plants protein kinase of MPK3, MPK4, and MPK6 were in we protein of these MAPKs in and mekk1 by We that MPK3 and MPK4 to in MPK6 were in the To examine protein of MPK3 and MPK4 in mekk1 are to accumulation of we We of MPK3 and MPK4 in mekk1 is that of MPK3 in mekk1 is to the protein in the These data also show that loss of MPK4 in mekk1 in response to flg22 is not to MPK4 protein These data that MEKK1 is required for activation of MPK4 triggered by flg22. of MEKK1 in and with are to cell death mutants that constitutive defense responses E. Rev. Mol. Biol. 2004; PubMed Scopus Google Scholar). we that the of mekk1 mutants may be to cell To test this we mekk1 plants with a for K. L. F. C. P. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). We in the of in both mekk1-1 and plants showed the between the were this of in the were in the mekk1 mutants we also to on these we that MEKK1 negatively regulates tissue-specific cell H2O2 in mekk1 death by either of of cell death or activation of R proteins upon pathogen often with accumulation of H2O2 K. P. Plant Mol. Biol. PubMed Scopus Google Scholar, 2004; PubMed Scopus Google Scholar). To H2O2 production in mekk1 mutants, we with DAB, which in an H2O2 H. Zhang Y.D. Plant J. Scopus Google Scholar). In plants, and were and data not In many in the were with in both mekk1-1 and mekk1-2 mutants with the of mekk1 mutants. In addition, we that many were also in mekk1 mutants In this the was the the in the cell The in the not with the since we not These results suggest that H2O2 accumulation is associated with cell death in the but not in the in mekk1 that constitutive defense responses and cell death often which be by T. M. H. S. K. Shinozaki K. Plant J. 2005; PubMed Scopus Google Scholar). We in the in the of mekk1 mutants with was in the of plants In to and was in the and in the in we tested mekk1 mutants expression of in the of a pathogen or We accumulation of PR1, PR2, and for salicylic acid and and pathways B.J. Trends Full Text Full Text PDF PubMed Scopus Google Scholar). We that expression of PR1, PR2, and was in mekk1 mutants, that of was We were not able to disease resistance in these mutants of and these data suggest that loss of MEKK1 function to constitutive activation of the responses. of cell death and H2O2 accumulation in mekk1 mutants were in the we the MEKK1 expression with this containing the MEKK1 was to the gene and We that the was in in and and expression of in was also in not but in of an expression of MEKK1 in the We also expression in showed expression MEKK1 expression is tissue-specific and with the cell death and H2O2 accumulation in of mekk1 of R protein signaling and constitutive defense responses in the cell death mutants are often by T. M. H. S. K. Shinozaki K. Plant J. 2005; PubMed Scopus Google Scholar, S. S. J. A. C. F. S. K. P. Plant Cell. 2004; PubMed Scopus Google Scholar, S. Dinesh-Kumar S.P. C. Baker B. Cell. Full Text PDF Scopus Google Scholar). To test whether these the mekk1 we assays by a were on for and to in or mekk1 plants were with and the plants the Similarly, mekk1 plants on for were the mekk1 plants were not suppress the since mekk1 plants in the and we cell death and H2O2 accumulation in mekk1 plants the with plants the number of and was and the in mekk1 were with the of cell death and H2O2 These data also that the temperature-dependent process is upstream of cell death and H2O2 To the and be to MAPK in mekk1 mutants, we MAPK assays using and mekk1 plants for Flg22 activated MPK3, MPK4, and MPK6 in plants, indicating that the signaling pathways were this In MPK4 was in mekk1 mutants. reduction of MPK4 activation by flg22 in mekk1 mutants is not to and MPK3 and MPK6 were in mekk1 mutants, supporting that MEKK1 is for MPK3 and MPK6 activation by flg22. Interestingly, MPK3 and MPK6 were not this of MPK3 and MPK6 was to and in mekk1 the is likely to be to by the of mekk1 in or The temperature-dependent cell death in mekk1 plants that signaling pathways may be activated in the To test this we mekk1 or mutants that exhibit loss of disease resistance against of and P. P. A. K. Dangl J.L. Plant Cell. 2002; PubMed Scopus Google Scholar, M. P. A. Eulgem T. E. F. C. Dangl J.L. Plant Cell. 2002; PubMed Scopus Google Scholar). was or of the in or mutants with mekk1 not Interestingly, the was in the plants were mekk1 To examine cell death and H2O2 accumulation patterns were also in mutants, we with and We that or in the in mekk1 plants The number of or in both and was in but in and The in of mekk1 mutants was also by the by was not to the R cell H2O2 and are often dependent on K. P. Plant Mol. Biol. PubMed Scopus Google Scholar, S. K. Sci. S. A. 2004; PubMed Scopus Google Scholar). To test the cell H2O2 and in mekk1 is also by mekk1 was plants, which are not able to upon pathogen J. G. Ausubel F.M. Nature. 2001; PubMed Scopus Google Scholar, C. Plant Cell. Google Scholar). We that the mekk1 cell and H2O2 were not in not However, to these mekk1 the was not as as in These data that the associated with cell death and H2O2 accumulation and are mediated by and Arabidopsis MEKK1 was proposed to a in plant immunity overexpression of domain activates of defense responses, including MPK6, and in a transient (5Asai T. Tena G. Plotnikova J. Willmann M.R. Chiu W.L. Gomez-Gomez L. Boller T. Ausubel F.M. Sheen J. Nature. 2002; 415: 977-983Crossref PubMed Scopus (2041) Google Scholar). However, genetic of mekk1 mutants here that in to the MEKK1 is for activation of of MPK3 and MPK6. are to this may be other to MEKK1 in of MPK3 and MPK6 the of overexpression using the N-terminal regulatory domain is this domain may be in the specificity of with downstream MAPK components Curr. Opin. Plant Biol. 2003; PubMed Scopus Google Scholar). For example, animal MEKK1 its N-terminal regulatory domain phosphorylate but is to signal downstream J. A. C. Sci. S. A. PubMed Scopus Google Scholar). Similarly, the N-terminal domain of Arabidopsis MEKK1 the MPK4 K. Mizoguchi T. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. Biochem. Biophys. Res. Commun. 1998; 253: 532-543Crossref PubMed Scopus (155) Google may act as a scaffold to the phosphorylation overexpression of MAPK cascade components may also in activation of but downstream signaling components. In MPK3 and MPK6 be activated by flg22 MEKK1, indicating that be other in activation of MPK3 and MPK6. We showed that MEKK1 is essential for activation of the of MEKK1 interacts directly with MPK4, but not MPK3 or MPK6 in yeast K. Mizoguchi T. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. Biochem. Biophys. Res. Commun. 1998; 253: 532-543Crossref PubMed Scopus (155) Google K. and K. the model is that MEKK1 and MPK4 are components of a MAPK signaling MPK4 negatively regulates expression of defense M. P. H. E. B. M. M.J. O. J. Cell. Full Text Full Text PDF PubMed Scopus Google MEKK1, an upstream component of MPK4, may also be as a of such defense gene to PR1, PR2, and are in expression in and but expression in is also by both MEKK1 and MPK4, with the that MEKK1 MPK4 M. P. H. E. B. M. M.J. O. J. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). The MAPKK for the flg22 signal MEKK1 to MPK4 is MKK1 interacts with both MEKK1 and MPK4 in yeast and is activated by flg22 or H2O2 in (12Mizoguchi T. Ichimura K. Irie K. Morris P. Giraudat J. Matsumoto K. Shinozaki K. FEBS Lett. 1998; 437: 56-60Crossref PubMed Scopus (103) Google Scholar, M. Scheikl E. Eulgem T. Doczi F. Ichimura K. Shinozaki K. Dangl J.L. Hirt H. Mol. Cell. 2004; 15: 141-152Abstract Full Text Full Text PDF PubMed Scopus (733) Google Scholar). Furthermore, MKK1 is able to phosphorylate MPK4 but not MPK3 or MPK6 upon flg22 and H2O2 treatment (14Teige M. Scheikl E. Eulgem T. Doczi F. Ichimura K. Shinozaki K. Dangl J.L. Hirt H. Mol. Cell. 2004; 15: 141-152Abstract Full Text Full Text PDF PubMed Scopus (733) Google Scholar). However, of mekk1 are not to of For the mekk1 plants are in mpk4 are but not M. P. H. E. B. M. M.J. O. J. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). MEKK1 also may function other MAPKs that to the MAPK as MPK4 K. Shinozaki K. Tena G. Sheen J. A. M. Zhang S.Q. Hirt H. C. E. Morris J. Trends Plant Sci. 2002; 7: Full Text Full Text PDF PubMed Scopus Google Scholar). Similarly, another may activate the MPK4 since activation of MPK4 in mekk1 was and data and data of showed that such as trigger activation of both and and for expression of defense genes, such as PR1, PR2, and the M.J. Boudsocq M. Barbier-Brygoo H. Lauriere C. FEBS Lett. 2004; 574: 42-48Crossref PubMed Scopus (130) Google Scholar). of both of is that defense gene is by a of and the of MEKK1 not activation of the MPK4 but also to of that MEKK1 may an in and of MAPK cascades. MPK3 are in Because MPK3 is by T. Irie K. T. K. Matsumoto K. Shinozaki K. Sci. S. A. PubMed Scopus Google is that enhanced MPK3 expression is to of cell death in However, MPK6 protein MPK6 is upon flg22 treatment in that of also We show here that MEKK1 is in the and as a in cell death and H2O2 that the cell death and H2O2 accumulation are since the in The of the signal that triggers tissue-specific cell death and H2O2 accumulation remains also candidate for such a signal is a known cell death in plants A. C. Cell. Full Text PDF PubMed Scopus Google Scholar). H2O2 is a and signaling that the A. C. Cell. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). Similar to a MEKK1 in MEKK1 is and activated upon H2O2 treatment H. S. Hirt H. J. Biol. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). and H. In the of MEKK1, cell such as in the may to H2O2 and in H2O2 accumulation and cell We also expression of MEKK1 in but not cell death and H2O2 accumulation were in mekk1 expression of MEKK1 and the defense are also The and in mekk1 are by and in and indicating that MEKK1 is not essential for plant or Instead, this may a potential link between and defense signaling many of which are temperature-sensitive and T. M. H. S. K. Shinozaki K. Plant J. 2005; PubMed Scopus Google Scholar, S. S. J. A. C. F. S. K. P. Plant Cell. 2004; PubMed Scopus Google Scholar, S. Dinesh-Kumar S.P. C. Baker B. Cell. Full Text PDF Scopus Google Scholar, C. Plant Cell. Google Scholar, K. P. Trends Plant Sci. 2003; 8: Full Text Full Text PDF PubMed Scopus Google Scholar). interacts with and and to of R proteins P. P. A. K. Dangl J.L. Plant Cell. 2002; PubMed Scopus Google Scholar, P. P. A. K. Dangl J.L. 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In this MEKK1 is by a R which is activated by of on the expression such an R be in the but so in The of such a in may not be to cell death but for the cell death was in the but not in in mekk1 mutants. is with the model proposed for R protein in defense S. S. J. A. C. F. S. K. P. Plant Cell. 2004; PubMed Scopus Google Scholar, C. S. J. A. L. A. C. J. K. EMBO J. 2006; PubMed Scopus Google Scholar). To this in mekk1 was also in the of RAR1, which is required for R protein number of MAPKKK, MAPKK, and MAPK are in the Arabidopsis but of been genetically complete MAPK cascades were proposed but were by the genetic on the and expression of MAPK cascade components is also genetic and expression as as in protein of the individual MAPK components are to signaling pathways in defense responses. We A. and K. for for of the and T. for of the We also the for the Arabidopsis mutants and J. M. and M. for Arabidopsis mutants.

The polymorphic barley (Hordeum vulgare) Mla locus harbors allelic race-specific resistance (R) genes to the powdery mildew fungus Blumeria graminis f sp hordei. The highly sequence-related MLA proteins contain an N-terminal coiled-coil structure, a central nucleotide binding (NB) site, a Leu-rich repeat (LRR) region, and a C-terminal non-LRR region. Using transgenic barley lines expressing epitope-tagged MLA1 and MLA6 derivatives driven by native regulatory sequences, we show a reversible and salt concentration-dependent distribution of the intracellular MLA proteins in soluble and membrane-associated pools. A posttranscriptional process directs fourfold greater accumulation of MLA1 over MLA6. Unexpectedly, in rar1 mutant plants that are compromised for MLA6 but not MLA1 resistance, the steady state level of both MLA isoforms is reduced. Furthermore, differential steady state levels of MLA1/MLA6 hybrid proteins correlate with their requirement for RAR1; the RAR1-independent hybrid protein accumulates to higher levels and the RAR1-dependent one to lower levels. Interestingly, yeast two-hybrid studies reveal that the LRR domains of RAR1-independent but not RAR1-dependent MLA isoforms interact with SGT1, a RAR1 interacting protein required for the function of many NB-LRR type R proteins. Our findings implicate the existence of a conserved mechanism to reach minimal NB-LRR R protein thresholds that are needed to trigger effective resistance responses.