Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity.

It is an apparent conundrum how plants evolved effector-triggered immunity (ETI), involving programmed cell death (PCD), as a major defence mechanism against biotrophic pathogens, because ETI-associated PCD could leave them vulnerable to necrotrophic pathogens that thrive on dead host cells. Interestingly, during ETI, the normally antagonistic defence hormones, salicylic acid (SA) and jasmonic acid (JA) associated with defence against biotrophs and necrotrophs respectively, both accumulate to high levels. In this study, we made the surprising finding that JA is a positive regulator of RPS2-mediated ETI. Early induction of JA-responsive genes and de novo JA synthesis following SA accumulation is activated through the SA receptors NPR3 and NPR4, instead of the JA receptor COI1. We provide evidence that NPR3 and NPR4 may mediate this effect by promoting degradation of the JA transcriptional repressor JAZs. This unique interplay between SA and JA offers a possible explanation of how plants can mount defence against a biotrophic pathogen without becoming vulnerable to necrotrophic pathogens

ZmXTH1, a new xyloglucan endotransglucosylase/hydrolase in maize, affects cell wall structure and composition in Arabidopsis thaliana

15 pages, 9 figures.-- PMID: 18316315[PubMed].-- Supporting information available at: http://jxb.oxfordjournals.org/content/59/4/875/suppl/DC1Xyloglucan endotransglucosylase/hydrolases (XTHs; EC 2.4.1.207 and/or EC 3.2.1.151) are enzymes involved in the modification of cell wall structure by cleaving and, often, also re-joining xyloglucan molecules in primary plant cell walls. Using a pool of antibodies raised against an enriched cell wall protein fraction, a new XTH cDNA in maize, ZmXTH1, has been isolated from a cDNA expression library obtained from the elongation zone of the maize root. The predicted protein has a putative N-terminal signal peptide and possesses the typical domains of this enzyme family, such as a catalytic domain that is homologous to that of Bacillus macerans beta-glucanase, a putative N-glycosylation motif, and four cysteine residues in the central and C terminal regions of the ZmXTH1 protein. Phylogenetic analysis of ZmXTH1 reveals that it belongs to subgroup 4, so far only reported from Poaceae monocot species. ZmXTH1 has been expressed in Pichia pastoris (a methylotrophic yeast) and the recombinant enzyme showed xyloglucan endotransglucosylase but not xyloglucan endohydrolase activity, representing the first enzyme belonging to subgroup 4 characterized in maize so far. Expression data indicate that ZmXTH1 is expressed in elongating tissues, modulated by culture conditions, and induced by gibberellins. Transient expression assays in onion cells reveal that ZmXTH1 is directed to the cell wall, although weakly bound. Finally, Arabidopsis thaliana plants expressing ZmXTH1 show slightly increased xyloglucan endohydrolase activity and alterations in the cell wall structure and composition.This work was funded by the Spanish ‘Ministerio de Ciencia y Tecnología’ (BIO2001-1140). VG was financed by a pre-doctoral grant from the ‘Generalitat de Catalunya’ (2003-FI00090). In addition, VG was funded by two grants from the Generalitat de Catalunya (2005-BE00104 and 2006-BE00668) for her work performed at Professor Fry's and Dr Ruel's laboratories. SF was financed by a post-doctoral grant from the ‘Generalitat de Catalunya’ (2003PIV-A-00033) and by an I3P contract from the ‘Consejo Superior de Investigaciones Científicas’. DC-R was financed by the Spanish ‘Ministerio de Educacion y Ciencia’ (‘Ramon y Cajal’ Program). This work was carried out within the framework of the ‘Xarxa de Referència en Biotecnologia’ from the ‘Generalitat de Catalunya’. SCF was funded by the UK Biotechnology and Biological Sciences Research Council. We are indebted to Dr Castresana (IBMB-CSIC) for his advice on the phylogenetic analyses, Dr Capellades for her technical support and the sequencing, and the greenhouse teams of IBMB-CSIC.Peer reviewe

Down-regulation of the maize and Arabidopsis thaliana caffeic acid O-methyl-transferase genes by two new maize R2R3-MYB transcription factors

The maize (Zea mays L.) caffeic acid O-methyl-transferase (COMT) is a key enzyme in the biosynthesis of lignin. In this work we have characterized the involvement of COMT in the lignification process through the study of the molecular mechanisms involved in its regulation. The examination of the maize COMT gene promoter revealed a putative ACIII box, typically recognized by R2R3-MYB transcription factors. We used the sequence of known R2R3-MYB factors to isolate five maize R2R3-MYB factors (ZmMYB2, ZmMYB8, ZmMYB31, ZmMYB39, and ZmMYB42) and study their possible roles as regulators of the maize COMT gene. The factors ZmMYB8, ZmMY31, and ZmMYB42 belong to the subgroup 4 of the R2R3-MYB family along with other factors associated with lignin biosynthesis repression. In addition, the induction pattern of ZmMYB31 and ZmMYB42 gene expression on wounding is that expected for repressors of the maize COMT gene. Arabidopsis thaliana plants over-expressing ZmMYB31 and ZmMYB42 down-regulate both the A. thaliana and the maize COMT genes. Furthermore, the over-expression of ZmMYB31 and ZmMYB42 also affect the expression of other genes of the lignin pathway and produces a decrease in lignin content of the transgenic plants.This work was funded by the Spanish “Ministerio de Ciencia y Tecnología” (BIO2001-1140). S-F was financed by a post-doctoral grant from the Autonomous Government of Catalonia (2003PIV-A-00033). F.M-S was financed by the European Project INCO-II (ICA4-CT-2000-30017) and by the “Consorci CSIC-IRTA” laboratory. T-M was financed by the European Project MapMaize (PL 962312). D-C.R was financed by the Spanish “Ministerio de Ciencia y Tecnología” (“Ramón y Cajal” Program). This work was carried out within the framework of the “Centre de Referència de Biotecnologia” (CERBA) from the Autonomous Government of Catalonia.Peer reviewe

Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity.

It is an apparent conundrum how plants evolved effector-triggered immunity (ETI), involving programmed cell death (PCD), as a major defence mechanism against biotrophic pathogens, because ETI-associated PCD could leave them vulnerable to necrotrophic pathogens that thrive on dead host cells. Interestingly, during ETI, the normally antagonistic defence hormones, salicylic acid (SA) and jasmonic acid (JA) associated with defence against biotrophs and necrotrophs respectively, both accumulate to high levels. In this study, we made the surprising finding that JA is a positive regulator of RPS2-mediated ETI. Early induction of JA-responsive genes and de novo JA synthesis following SA accumulation is activated through the SA receptors NPR3 and NPR4, instead of the JA receptor COI1. We provide evidence that NPR3 and NPR4 may mediate this effect by promoting degradation of the JA transcriptional repressor JAZs. This unique interplay between SA and JA offers a possible explanation of how plants can mount defence against a biotrophic pathogen without becoming vulnerable to necrotrophic pathogens

The maize ZmMYB42 represses the phenylpropanoid pathway and affects the cell wall structure, composition and degradability in Arabidopsis thaliana

The involvement of the maize ZmMYB42 R2R3-MYB factor in the phenylpropanoid pathway and cell wall structure and composition was investigated by overexpression in Arabidopsis thaliana. ZmMYB42 down-regulates several genes of the lignin pathway and this effect reduces the lignin content in all lignified tissues. In addition, ZmMYB42 plants generate a lignin polymer with a decreased S to G ratio through the enrichment in H and G subunits and depletion in S subunits. This transcription factor also regulates other genes involved in the synthesis of sinapate esters and flavonoids. Furthermore, ZmMYB42 affects the cell wall structure and degradability, and its polysaccharide composition. Together, these results suggest that ZmMYB42 may be part of the regulatory network controlling the phenylpropanoid biosynthetic pathway.This work received financial support from the CONSOLIDER-INGENIO programme (CSD2007-00036) from the Spanish Ministerio de Ciencia e Innovación. F.M. Sonbol was financed by the European Project INCO-II (ICA4-CT-2000-30017), by the CRAG laboratory, and by the “Instituto de Cooperación con el Mundo Árabe, Agencia Española de Cooperación Internacional”. S. Fornale was financed by a post-doctoral grant from the “Generalitat de Catalunya” (2003PIV-A-00033) and by an I3P contract from the “Consejo Superior de Investigaciones Científicas”. D. Caparrós-Ruiz was financed by the Spanish “Ministerio de Educación y Ciencia” (“Ramón y Cajal” Program). This work was carried out within the framework of the “Xarxa de Referència de Biotecnologia” (XarBa) from the Autonomous Government of Catalonia.Peer reviewe