Chloroplast Redox Status Modulates Genome-Wide Plant Responses during the Non-host Interaction of Tobacco with the Hemibiotrophic Bacterium Xanthomonas campestris pv. vesicatoria

Abstract

Non-host resistance is the most ample and durable form of plant resistance against pathogen infection. It includes induction of defense-associated genes, massive metabolic reprogramming, and in many instances, a form of localized cell death (LCD) at the site of infection, purportedly designed to limit the spread of biotrophic and hemibiotrophic microorganisms. Reactive oxygen species (ROS) have been proposed to act as signals for LCD orchestration. They are produced in various cellular compartments including chloroplasts, mitochondria and apoplast. We have previously reported that down-regulation of ROS build-up in chloroplasts by expression of a plastid-targeted flavodoxin (Fld) suppressed LCD in tobacco leaves inoculated with the non-host bacterium Xanthomonas campestris pv. vesicatoria (Xcv), while other defensive responses were unaffected, suggesting that chloroplast ROS and/or redox status play a major role in the progress of LCD. To better understand these effects, we compare here the transcriptomic alterations caused by Xcv inoculation on leaves of Fld-expressing tobacco plants and their wild-type siblings. About 29% of leaf-expressed genes were affected by Xcv and/or Fld. Surprisingly, 5.8% of them (1,111 genes) were regulated by Fld in the absence of infection, presumably representing pathways responsive to chloroplast ROS production and/or redox status during normal growth conditions. While the majority (∼75%) of pathogen-responsive genes were not affected by Fld, many Xcv responses were exacerbated, attenuated, or regulated in opposite direction by expression of this protein. Particularly interesting was a group of 384 genes displaying Xcv responses that were already triggered by Fld in the absence of infection, suggesting that the transgenic plants had a larger and more diversified suite of constitutive defenses against the attacking microorganism compared to the wild type. Fld modulated many genes involved in pathogenesis, signal transduction, transcriptional regulation and hormone-based pathways. Remarkable interactions with proteasomal protein degradation were observed. The results provide the first genome-wide, comprehensive picture illustrating the relevance of chloroplast redox status in biotic stress responses.

Transcriptional and Metabolic Profiling of Potato Plants Expressing a Plastid-Targeted Electron Shuttle Reveal Modulation of Genes Associated to Drought Tolerance by Chloroplast Redox Poise

“…Noteworthy, ethylene metabolism and signaling were consistently down-regulated in Stpfld252 plants, including 29 genes encoding transcription factors of the apetala2/ethylene-responsive family ( Figure 3 , Table S1 ). These observations also agree with earlier reports on Fld-expressing tobacco plants [ 31 ].…”

“…Among genes induced in Stpfld252 plants there was a remarkable over-representation of those associated to protein degradation by the proteasome ( Figure 3 , Table S1 ). A similar induction pattern has been observed in Fld-expressing tobacco plants [ 31 ], indicating that plastid-located Fld exerts a key regulatory role in the accumulation of proteasomal components in different species, presumably through modulation of chloroplast redox chemistry.…”

“…Moreover, the introduction of Fld in plants via genetic engineering increased tolerance to multiple stresses [ 15 , 22 , 40 ] and complemented Fd-deficient plants [ 41 ]. While this protective effect is clearly associated to Fd substitution, Fld affects plant development [ 22 , 42 ], and transcriptional profiles ( Figure 3 , [ 31 ]) in the absence of stress, when Fd levels are not down-regulated. The functional basis for these effects remain yet to be elucidated, but they depend on the relative proportions of the two electron carriers [ 40 ], suggesting that they interact with common partners in the redox network of the chloroplast, and that the magnitude of the Fld impact might vary between species accumulating different Fd amounts.…”

Reactive oxygen species generated in chloroplasts contribute to tobacco leaf infection by the necrotrophic fungus Botrytis cinerea

“…NADPH-consuming reactions involved in different chloroplastic metabolic pathways, including the Calvin cycle, are early targets of inhibition by different stresses. Along with the decrease in Fd expression that occurs under stress conditions (Tognetti et al, 2006;Zurbriggen et al, 2009;Pierella Karlusich et al, 2017), this leads to the accumulation of NADPH, limitation of physiological acceptors (oxidized Fd and NADP + ) and, as a consequence, the over-reduction of intermediate components of the photosynthetic electron transfer chain (PETC) in the thylakoid membrane. Under these conditions, electron transfer to alternative acceptors such as oxygen leads to the generation of ROS and, in turn, oxidative stress and the further inhibition of photosynthesis, initiating a self-propagating process of ROS production and cell damage (Tognetti et al, 2006).…”

Chloroplast‐targeted antioxidant protein protects against necrotrophic fungal attack

“…They are also interested in determining what signals exit plastids in response to changes in redox status and which are suppressed by Fld, as well as the underlying mechanisms associated with tissue death and virulence. They will use genetic and pharmacological approaches as well as genome-wide transcriptional, proteomic and metabolic analyses, analogous to those used in studies of hemibiotrophic pathogens (Zurbriggen et al, 2009;Pierella et al, 2017).…”