Genetic dissection of salicylic acid-mediated defense signaling networks in Arabidopsis.

Properly coordinated defense signaling networks are critical for the fitness of plants. One hub of the defense networks is centered on salicylic acid (SA), which plays a key role in activating disease resistance in plants. However, while a number of genes are known to affect SA-mediated defense, relatively little is known about how these gene interact genetically with each other. Here we exploited the unique defense-sensitized Arabidopsis mutant accelerated cell death (acd) 6-1 to dissect functional relationships among key components in the SA hub. We show that while enhanced disease susceptibility (eds) 1-2 and phytoalexin deficient (pad) 4-1 suppressed acd6-1-conferred small size, cell death, and defense phenotypes, a combination of these two mutations did not incur additive suppression. This suggests that EDS1 and PAD4 act in the same signaling pathway. To further evaluate genetic interactions among SA regulators, we constructed 10 pairwise crosses in the acd6-1 background among mutants defective in: SA INDUCTION-DEFICIENT 2 for SA biosynthesis; AGD2-LIKE DEFENSE 1, EDS5, and PAD4 for SA accumulation; and NONEXPRESSOR OF PR GENES 1 for SA signaling. Systematic analysis of the triple mutants based on their suppression of acd6-1-conferred phenotypes revealed complex and interactive genetic relationships among the tested SA genes. Our results suggest a more comprehensive view of the gene networks governing SA function and provide a framework for further interrogation of the important roles of SA and possibly other signaling molecules in regulating plant disease resistance.

Genetic analysis of acd6-1 reveals complex defense networks and leads to identification of novel defense genes in Arabidopsis.

Pathogen infection leads to the activation of defense signaling networks in plants. To study these networks and the relationships between their components, we introduced various defense mutations into acd6-1, a constitutive gain-of-function Arabidopsis mutant that is highly disease resistant. acd6-1 plants show spontaneous cell death, reduced stature, and accumulate high levels of camalexin (an anti-fungal compound) and salicylic acid (SA; a signaling molecule). Disruption of several defense genes revealed that in acd6-1, SA levels/signaling were positively correlated with the degree of disease resistance and defense gene expression. Salicylic acid also modulates the severity of cell death. However, accumulation of camalexin in acd6-1 is largely unaffected by reducing the level of SA. In addition, acd6-1 shows ethylene- and jasmonic acid-mediated signaling that is antagonized and therefore masked by the presence of SA. Mutant analysis revealed a new relationship between the signaling components NPR1 and PAD4 and also indicated that multiple defense pathways were required for phenotypes conferred by acd6-1. In addition, our data confirmed that the size of acd6-1 was inversely correlated with SA levels/signaling. We exploited this unique feature of acd6-1 to identify two genes disrupted in acd6-1 suppressor (sup) mutants: one encodes a known SA biosynthetic component (SID2) and the other encodes an uncharacterized putative metalloprotease (At5g20660). Taken together, acd6-1 is a powerful tool not only for dissecting defense regulatory networks but also for discovering novel defense genes.