Figure 1.

Impact of the ssi2 Mutant on Defense Signaling in Arabidopsis.

SA and JA are important signaling molecules in plant defense. JA signaling is required for the pathogen-activated expression of PDF1.2 and resistance to the necrotrophic pathogen B. cinerea. SA signaling is required for the pathogen-activated expression of the PR genes and resistance to Psm. The EDS5 and SID2 genes are required for SA synthesis; the loss-of-function eds5 and sid2 mutations block SA synthesis. SA signaling is activated via both NPR1-dependent and -independent mechanisms. In addition to SA, an unknown pathogen-activated factor is required for signaling through the NPR1-independent pathway. The loss of the SSI2-encoded stearoyl-ACP desaturase activity in the ssi2 mutant has pleiotropic effects on plant defense responses. The ssi2 mutant allele promotes (+) the spontaneous development of lesions containing dead cells, the accumulation of increased SA levels, and the constitutive expression of NPR1-dependent and -independent defense mechanisms, which confer high-level expression of PR genes and enhanced resistance to Psm. By contrast, the ssi2 mutant interferes with (−) the ability of JA/MeJA to activate PDF1.2 expression and exhibits enhanced susceptibility to B. cinerea. 18:1 application restores the JA-inducible expression of PDF1.2 in the ssi2 mutant plant (Kachroo et al., 2001), suggesting a role for an 18:1-derived factor, which is limiting in the ssi2 mutant plant, in promoting JA signaling.