BAH1/NLA, a RING-type ubiquitin E3 ligase, regulates the accumulation of salicylic acid and immune responses to Pseudomonas syringae DC3000

Salicylic acid (SA) is a primary factor responsible for exerting diverse immune responses in plants and is synthesized in response to attack by a wide range of pathogens. The Arabidopsis (Arabidopsis thaliana) sid2 mutant is defective in a SA biosynthetic pathway involving ISOCHORISMATE SYNTHASE1 (ICS1) and consequently contains reduced levels of SA. However, the sid2 mutant as well as ICS-suppressed tobacco (Nicotiana benthamiana) still accumulate a small but significant level of SA. These observations along with previous studies suggest that SA might also be synthesized by another pathway involving benzoic acid (BA). Here we isolated a benzoic acid hypersensitive1-Dominant (bah1-D) mutant that excessively accumulated SA after application of BA from activation-tagged lines. This mutant also accumulated higher levels of SA after inoculation with Pseudomonas syringae pv tomato DC3000. Analysis of the bah1-D sid2 double mutant suggested that the bah1-D mutation caused both ICS1-dependent and -independent accumulation. In addition, the bah1-D mutant showed SA-dependent localized cell death in response to P. syringae pv tomato DC3000. The T-DNA insertional mutation that caused the bah1-D phenotypes resulted in the suppression of expression of the NLA gene, which encodes a RING-type ubiquitin E3 ligase. These results suggest that BAH1/NLA plays crucial roles in the ubiquitination-mediated regulation of immune responses, including BA- and pathogen-induced SA accumulation, and control of cell death.