BackgroundChlorosis of leaf tissue normally observed during pathogen infection may result from the degradation of chloroplasts. There is a growing evidence to suggest that the chloroplast plays a significant role during pathogen infection. Although most degradation of the organelles and cellular structures in plants is mediated by autophagy, its role in chloroplast catabolism during pathogen infection is largely unknown.ResultsIn this study, we investigated the function of autophagy in chloroplast degradation during avirulent Pst DC3000 (AvrRps4) infection. We examined the expression of defensive marker genes and suppression of bacterial growth using the electrolyte leakage assay in normal light (N) and low light (L) growing environments of wild-type and atg5-1 plants during pathogen treatment. Stroma-targeted GFP proteins (CT-GFP) were observed with LysoTracker Red (LTR) staining of autophagosome-like structures in the vacuole. The results showed that Arabidopsis expressed a significant number of small GFP-labeled bodies when infected with avirulent Pst DC3000 (AvrRps4). While barely detectable, there were small GFP-labeled bodies in plants with the CT-GFP expressing atg5-1 mutation. The results showed that chloroplast degradation depends on autophagy and this may play an important role in inhibiting pathogen growth.ConclusionAutophagy plays a role in chloroplast degradation in Arabidopsis during avirulent Pst DC3000 (AvrRps4) infection. Autophagy dependent chloroplast degradation may be the primary source of reactive oxygen species (ROS) as well as the pathogen-response signaling molecules that induce the defense response.

Autophagy has been implicated as a cellular protein degradation process that is used to recycle cytoplasmic components under biotic and abiotic stresses and so restrict programmed cell death (PCD). In this study, we report a novel regulatory mechanism by which NADPH oxidase respiratory burst oxidase homolog D (RBOHD) regulated pathogeninduced autophagy and hypersensitive (HR) cell death. We found that the Pseudomonas syringae pv tomato bacteria DC3000 expressing avrRps4 (Pst-avrRps4) induction of RBOHD-dependent reactive oxygen species (ROS) production promoted the onset of autophagy, whereas a pretreatment with an NADPH oxidase RBOHD inhibitor reversed this trend. The inhibitor significantly blocked pathogen-induced autophagosome formation and ROS increase. Moreover, we also show that in the wild-type and atrbohF mutant, Pst-avrRps4-induced cell death was limited, whereas in the case of the atrbohD mutant, the infection triggered a spreading-type necrosis. Our results demonstrate that the RBOHDdependent ROS accumulation stimulated autophagosome formation and limited HR cell death.

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