Circadian clock-dependent and-independent response of Phaseolus vulgaris to Pseudomonas syringae

As sessile organisms, plants have to fine tune their immune responses in anticipation and in response to the exogenous attacks. One of the early steps in antimicrobial defense is the production of ROS. ROS are produced by plasma membrane anchored oxidases, characterized as respiratory burst oxidase homologs (RBOHs) and per-oxidases, with the most extensively studied in bean, the French bean peroxidase 1 (FBP1). We have investigated the effect of water, the elicitor flagellin 22 (flg22) and a virulent Pseudomonas syringae pathovar phaseolicola bacterial strain on transcriptional changes induced in the nine Phaseolus vulgaris oxidases (PvRBOHA-PvRBOHI) and FBP1 in a time-dependent manner during 24 h following treatments in the morning or in the evening. Moreover, the time-dependent oxidative burst generation and the in-planta bacterial growth were studied. Our results show that the microbe-triggered response outreaches the elicitor-triggered response in intensity and duration. More importantly, infection reinforces the circadian rhythm in the expression of FBP1 and abolishes the rhythm of the two oxidases PvRBOHC and PvRBOHD with, however, significantly enhanced levels and specific characteristics in each case. Among the other members of the PvRBOH gene family, PvRBOHB is unique regarding its rapid response and pattern of expression. In addition, the expression of the clock genes PvLHY and PvTOC1 is attenuated with all treatments. Our results reveal that post-invasively, the oxidative defense of P. vulgaris de-pends on transcriptional reprogramming of specific PvRBOH genes that develops non-rhythmically as an emer-gency mechanism, while peroxidase FBP1 is part of the rhythmical pre-invasive and post-invasive defense with emphasis during the morning.

Automated summary

As sessile organisms, plants need to adjust their immune responses to external attacks. This study investigated the effect of water, flagellin 22 (flg22), and a pathogenic bacterial strain on the transcriptional changes and oxidative burst generation in bean plants. Results showed that microbe-triggered response had a greater and longer-lasting impact compared to elicitor-triggered response. Infection also affected the expression of clock genes and specific oxidases, indicating a circadian rhythm in plant defense.