NO2 Enhances Pathogen Resistance

Nitrogen dioxide (NO₂) forms in plants under stress conditions, but little is known about its 
physiological functions. Using a variety of techniques, Mayer et al. (10.1104/pp.18.00704) have examined the effects of fumigating Arabidopsis with 10 ppm NO₂ for 1 h, a treatment that does not cause 
visible leaf symptoms or ion leakage. Although visual symptoms were absent, the expression of genes related to pathogen resistance was induced. Fumigated plants developed basal disease resistance, or 
pattern-triggered immunity (PTI), against the necrotrophic fungus Botrytis cinerea and the hemibiotrophic bacterium Pseudomonas syringae. The authors also demonstrate that both the salicylic acid (SA) and jasmonic acid (JA) signaling pathways are required for full expression of NO₂-induced 
resistance against B. cinerea. An early peak of SA accumulation immediately after NO₂ exposure was followed by transient accumulation of oxophytodienoic acid. 
NO₂ also induced the simultaneous expression of genes involved in the biosynthesis and catabolism of jasmonates.  JA and JA-isoleucine did not accumulate but rather their catabolites did. NO₂-treated plants also emitted the volatile 
monoterpene α-pinene and the sesquiterpene longifolene, which might 
function in signaling or defense. NO₂-triggered B. cinerea 
resistance was dependent on enhanced early callose deposition and specific defense-related genes. In 
summary, exogenous NO₂ triggers basal pathogen resistance, pointing to a possible role for 
 endogenous NO₂ in defense signaling. Additionally, the results of the study point to the possible involvement of 
jasmonate catabolism and volatiles in pathogen immunity.