Regulation of defense gene expression by Arabidopsis SRFR1

Reduced growth and viability is a common phenotype of plants with constitutively activated pathogen defenses. One branch of the plant innate immunity system, effector-triggered immunity, is especially potent and requires tight control to enable normal plant development. While some facets of this control that directly regulate resistance protein abundance or activity have been documented, general control of effector-triggered signaling sensitivity is poorly understood. We recently identified SUPPRESSOR OF rps4-RLD 1 (SRFR1), a novel negative regulator of avrRps4-triggered immunity. Mutations in SRFR1 were previously shown not to induce constitutive high expression of the defense gene PR1, and to be fully susceptible to the virulent Pseudomonas syringae pv. tomato strain DC3000. SRFR1 encodes a tetratricopeptide repeat-containing protein with weak similarity to transcriptional repressors in other organisms. By transient expression in Nicotiana benthamiana, SRFR1 was localized to the nucleus. Here we investigate more carefully whether expression of defense genes is misregulated in srfr1 mutant plants. Consistent with the hypothesized function of SRFR1 as a negative transcriptional regulator, we find that mRNA levels of several defense genes are upregulated in srfr1 mutants.