High air humidity dampens salicylic acid pathway and plant resistance via targeting of NPR1

Abstract

The occurrence of plant disease is determined by interactions among host, pathogen and climate conditions. Air humidity has long been recognized to profoundly influence diseases in the phyllosphere and high air humidity (e.g., after rain falls) is known as a prerequisite for numerous disease outbreaks in the field1-3. However, the molecular basis of how high humidity interferes with plant resistance mechanisms to favor disease remained elusive. Here we show that high humidity is associated with an "immune-compromised" status of plants, revealed by lower expression of defense genes during bacterial infection of Arabidopsis plants. Examination of humiditys effect on individual immune pathways showed that the accumulation and signaling of salicylic acid (SA), an essential hormone conferring plant resistance against infectious microbes4,5, are significantly inhibited under high humidity. Surprisingly, NPR1 protein, an SA receptor and central transcriptional co-activator of SA-responsive genes6-9, accumulated to a significantly higher level in the nucleus under high humidity. Further investigation indicated a decreased binding affinity of NPR1 protein to the target gene promoter, suggestive of an "inactive" nature of NPR1, under high humidity and an impaired ubiquitination and degradation of NPR1 protein, likely due to down-regulation of Cullin 3-mediated cellular ubiquitination pathway and 26S proteasome pathway under high humidity. Our study uncovers disruption of NPR1 protein turnover as a major mechanism, by which high humidity dampens plant immune strength against pathogens, and provides new insights into the long-observed air humidity influence on diseases in nature.