A key role for ALD1 in activation of local and systemic defenses in Arabidopsis

Abstract

SummaryThe Arabidopsis thaliana agd2-like defense response protein1 (ald1) mutant was previously found to be hypersusceptible to the virulent bacterial pathogen Pseudomonas syringae and had reduced accumulation of the defense signal salicylic acid (SA). ALD1 was shown to possess aminotransferase activity in vitro, suggesting it generates an amino acid-derived defense signal. We now find ALD1 to be a key defense component that acts in multiple contexts and partially requires the PHYTOALEXIN DEFICIENT4 (PAD4) defense regulatory gene for its expression in response to infection. ald1 plants have increased susceptibility to avirulent P. syringae strains, are unable to activate systemic acquired resistance and are compromised for resistance to the oomycete pathogen Peronospora parasitica in mutants with constitutively active defenses. ALD1 and PAD4 can act additively to control SA, PATHOGENESIS RELATED GENE1 (PR1) transcript and camalexin (an antimicrobial metabolite) accumulation as well as disease resistance. Finally, ALD1 and PAD4 can mutually affect each other's expression in a constitutive defense mutant, suggesting that these two genes can act in a signal amplification loop.

Two redundant receptor-like cytoplasmic kinases function downstream of pattern recognition receptors to regulate activation of SA biosynthesis in Arabidopsis

“…Chromatin immunoprecipitation-sequencing analysis identified many regulators of plant immunity such as FMO1 and ALD1 as target genes of SARD1 and CBP60g. FMO1 has been shown to function in SAindependent pathogen resistance (Bartsch et al, 2006), whereas ALD1 plays a critical role in biosynthesis of pipecolic acid and plant defense responses (Song et al, 2004;Návarová et al, 2012). Consistent with reduced expression of SARD1 and CBP60g in pcrk1 pcrk2, induction of FMO1 and ALD1 by bacterial infection is also compromised in pcrk1 pcrk2 plants.…”

Genetic Dissection of Salicylic Acid-Mediated Defense Signaling Networks in Arabidopsis

“…Plants were grown in growth chambers with light intensity at 200 mmol m 22 s 21 , 60% humidity, and 22°. The single mutants (acd6-1, ald1-1, npr1-1, pad4-1, eds5-1, and sid2-1), the double mutants (acd6-1ald1-1, acd6-1eds5-1, acd6-1npr1-1, acd6-1pad4-1, and acd6-1sid2-1), and the triple mutants (acd6-1ald1-1pad4-1 and acd6-1npr1-1pad4-1) were previously described (Song et al 2004b;Lu et al 2009). eds1-2, which was introgressed to the Col-0 background, was kindly provided by Jane Parker at Max Planck Institute for Plant Breeding Research.…”

“…The cell death and dwarf phenotypes are sensitized to the change of defense levels in acd6-1. We have taken advantage of this unique feature of acd6-1 in a genetic analysis to understand functional relationships between several SA regulators and in a suppressor screen to identify novel defense genes (Song et al 2004b;Lu et al 2009; Wang et al 2011a,b).In this article, we exploited acd6-1 in a systematic dissection of functional relationships among components in the SA hub. We showed that while eds1-2 and pad4-1 individually suppressed acd6-1-conferred phenotypes, a combination of these two mutations did not incur additional suppression.…”

“…The cell death and dwarf phenotypes are sensitized to the change of defense levels in acd6-1. We have taken advantage of this unique feature of acd6-1 in a genetic analysis to understand functional relationships between several SA regulators and in a suppressor screen to identify novel defense genes (Song et al 2004b;Lu et al 2009; Wang et al 2011a,b).…”

“…However, when a mutation in one defense gene already leads to enhanced disease susceptibility, additional susceptible phenotypes caused by mutations in two or more defense genes can be difficult to detect on the basis of standard disease assays. Therefore, more sensitive methods should be developed to assess the functional relationships between certain components on defense networks.A small Arabidopsis mutant acd6-1 is being used to develop one such method and has already revealed new insights into the SA signaling networks (Song et al 2004b;Lu et al 2009). ACD6 is a type II SA regulator that was shown to be a major determinant of fitness in Arabidopsis (Lu et al 2003;Todesco et al 2010).…”