Abstract
The plant hormone abscisic acid (ABA) is involved in an array of plant processes, including the regulation of gene expression during adaptive responses to various environmental cues. Apart from its well-established role in abiotic stress adaptation, emerging evidence indicates that ABA is also prominently involved in the regulation and integration of pathogen defense responses. Here, we demonstrate that exogenously administered ABA enhances basal resistance of rice (Oryza sativa) against the brown spot-causing ascomycete Cochliobolus miyabeanus. Microscopic analysis of early infection events in control and ABA-treated plants revealed that this ABA-inducible resistance (ABA-IR) is based on restriction of fungal progression in the mesophyll. We also show that ABA-IR does not rely on boosted expression of salicylic acid-, jasmonic acid -, or callose-dependent resistance mechanisms but, instead, requires a functional Galpha-protein. In addition, several lines of evidence are presented suggesting that ABA steers its positive effect on brown spot resistance through antagonistic cross talk with the ethylene (ET) response pathway. Exogenous ethephon application enhances susceptibility, whereas genetic disruption of ET signaling renders plants less vulnerable to C. miyabeanus attack, thereby inducing a level of resistance similar to that observed on ABA-treated wild-type plants. Moreover, ABA treatment alleviates C. miyabeanus-induced activation of the ET reporter gene EBP89, while derepression of pathogen-triggered EBP89 transcription via RNA interference-mediated knockdown of OsMPK5, an ABA-primed mitogen-activated protein kinase gene, compromises ABA-IR. Collectively, these data favor a model whereby exogenous ABA enhances resistance against C. miyabeanus at least in part by suppressing pathogen-induced ET action in an OsMPK5-dependent manner.
Highlights
The plant hormone abscisic acid (ABA) is involved in an array of plant processes, including the regulation of gene expression during adaptive responses to various environmental cues
Six rice cultivars, including four indica and two japonica lines, were screened with two C. miyabeanus strains, both of which were isolated from diseased rice in field plots at the International Rice Research Institute in the Philippines (Supplemental Table S1)
Consistent with previous reports (Ahn et al, 2005), treatment with 0.1 mM jasmonic acid (JA) yielded no significant protection against C. miyabeanus (Fig. 1A), even though this concentration is high enough to induce JA-responsive JIOsPR10 transcription (Jwa et al, 2001)
Summary
The plant hormone abscisic acid (ABA) is involved in an array of plant processes, including the regulation of gene expression during adaptive responses to various environmental cues. The primary mode of interaction between the SA and JA/ET signaling pathways appears to be mutual antagonism with corresponding trade-offs between biotroph resistance, on the one hand, and resistance to necrotrophic pathogens and insect herbivores, on the other hand (Bostock, 2005; Stout et al, 2006; Spoel et al, 2007) This is likely an oversimplified model, as synergistic actions of SA and JA/ET have been reported as well (Van Wees et al, 2000; Mur et al, 2006; Adie et al, 2007; Truman et al, 2007). Fine-tune regulation of the antagonism and cooperation between JA and ET depends on the balance of activation by both hormones of ERF1 and MYC2, two opposing transcription factors that differentially regulate divergent branches of the JA signaling pathway involved in the response to necrotrophic pathogen attack and wounding, respectively (Berrocal-Lobo et al, 2002; Lorenzo et al, 2003, 2004)
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