Abstract
We showed previously that nitrogen (N) limitation decreases Arabidopsis resistance to Erwinia amylovora (Ea). We show that decreased resistance to bacteria in low N is correlated with lower apoplastic reactive oxygen species (ROS) accumulation and lower jasmonic acid (JA) pathway expression. Consistently, pretreatment with methyl jasmonate (Me‐JA) increased the resistance of plants grown under low N. In parallel, we show that in planta titres of a nonvirulent type III secretion system (T3SS)‐deficient Ea mutant were lower than those of wildtype Ea in low N, as expected, but surprisingly not in high N. This lack of difference in high N was consistent with the low expression of the T3SS‐encoding hrp virulence genes by wildtype Ea in plants grown in high N compared to plants grown in low N. This suggests that expressing its virulence factors in planta could be a major limiting factor for Ea in the nonhost Arabidopsis. To test this hypothesis, we preincubated Ea in an inducing medium that triggers expression of hrp genes in vitro, prior to inoculation. This preincubation strongly enhanced Ea titres in planta, independently of the plant N status, and was correlated to a significant repression of JA‐dependent genes. Finally, we identify two clusters of metabolites associated with resistance or with susceptibility to Ea. Altogether, our data showed that high susceptibility of Arabidopsis to Ea, under low N or following preincubation in hrp‐inducing medium, is correlated with high expression of the Ea hrp genes in planta and low expression of the JA signalling pathway, and is correlated with the accumulation of specific metabolites.
Highlights
Plants are usually exposed simultaneously to multiple stresses and many reports have shown that one stress can affect the plant's response to another stress
We showed previously that the susceptibility of nonhost Arabidopsis to the necrotroph Erwinia amylovora (Ea) is reduced in high N, with lower titres of Ea than in low N (Fagard et al, 2014)
The underlying mechanisms are not known, it is thought that increased susceptibility to fire blight under high N fertilizer conditions in Malus is due to the increased presence of growing tissue (Fallahi & Mohan, 2000)
Summary
Plants are usually exposed simultaneously to multiple stresses and many reports have shown that one stress can affect the plant's response to another stress. We observed strong brown staining in Ea- inoculated leaves of plants grown in both high and low N conditions (Figure 3a).
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