Proteostatic Regulation of MEP and Shikimate Pathways by Redox-Activated Photosynthesis Signaling in Plants Exposed to Small Fungal Volatiles

Abstract

Microorganisms produce volatile compounds (VCs) with molecular masses of less than 300 Da that promote plant growth and photosynthesis. Recently, we have shown that small VCs of less than 45 Da other than CO2 are major determinants of plant responses to fungal volatile emissions. However, the regulatory mechanisms involved in the plants’ responses to small microbial VCs remain unclear. In Arabidopsis thaliana plants exposed to small fungal VCs, growth promotion is accompanied by reduction of the thiol redox of Calvin-Benson cycle (CBC) enzymes and changes in the levels of shikimate and 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway-related compounds. We hypothesized that plants’ responses to small microbial VCs involve post-translational modulation of enzymes of the MEP and shikimate pathways via mechanisms involving redox-activated photosynthesis signaling. To test this hypothesis, we compared the responses of wild-type (WT) plants and a cfbp1 mutant defective in a redox-regulated isoform of the CBC enzyme fructose-1,6-bisphosphatase to small VCs emitted by the fungal phytopathogen Alternaria alternata. Fungal VC-promoted growth and photosynthesis, as well as metabolic and proteomic changes, were substantially weaker in cfbp1 plants than in WT plants. In WT plants, but not in cfbp1 plants, small fungal VCs reduced the levels of both transcripts and proteins of the stromal Clp protease system and enhanced those of plastidial chaperonins and co-chaperonins. Consistently, small fungal VCs promoted the accumulation of putative Clp protease clients including MEP and shikimate pathway enzymes. clpr1-2 and clpc1 mutants with disrupted plastidial protein homeostasis responded weakly to small fungal VCs, strongly indicating that plant responses to microbial volatile emissions require a finely regulated plastidial protein quality control system. Our findings provide strong evidence that plant responses to fungal VCs involve chloroplast-to-nucleus retrograde signaling of redox-activated photosynthesis leading to proteostatic regulation of the MEP and shikimate pathways.

Glucose-6-P/phosphate translocator2 mediates the phosphoglucose-isomerase1-independent response to microbial volatiles

“…Although transcript abundance on its own cannot be used to infer changes in the proteome and fluxes in central metabolism ( Nakaminami et al, 2014 ; Schwender et al, 2014 ), this indicated that enhanced incorporation of cytosolic G6P into chloroplasts caused by increased GPT2 expression in leaves could be involved in the plant’s response to small VCs. However, our differential proteomic analyses did not detect any statistically significant accumulation of GPT2 protein in leaves promoted by small VCs ( Supplemental Tables S1 and S2 ; Sánchez-López et al, 2016b ; Ameztoy et al, 2021 ). These analyses also did not detect statistically significant higher levels of GPT2 protein in WT leaves than in gpt2-1 and pgi1-2gpt2-1 leaves ( Supplemental Tables S3, S6, and S7 ).…”

“…Predicted locations of these proteins using the SUBA4 Arabidopsis protein subcellular localization database ( Hooper et al, 2017 ) included almost all cellular compartments, but the locations associated with the greatest number of proteins were the cytosol and plastid ( Supplemental Table S1 and Supplemental Figure S3B ). Nearly 70% of these proteins were identified as differentially expressed by small VCs in a previous differential proteomic study using a Col-O background ( Ameztoy et al, 2021 ) ( Supplemental Table S1 ). No statistically significant changes in the levels of GPT2 protein were observed upon small fungal VC treatment ( Supplemental Table S2 ).…”

“…Recently, using a “box-in-box” in vitro co-cultivation system in which plants were grown in the vicinity of microbial cultures covered with charcoal filters, we showed that VCs with a molecular mass less than ca. 45 Da (hereinafter designated as “small VCs”) are important determinants of plant responses to microbial volatile emissions ( Ameztoy et al, 2019 , 2021 ; García-Gómez et al, 2019 , 2020 ; Gámez-Arcas et al, 2022 ). Regulation of these responses is primarily nontranscriptional and involves global changes in the proteome ( Ameztoy et al, 2021 ) and thiol redox proteome, particularly in photosynthesis- and starch biosynthesis-related proteins ( Li et al, 2011 ; Ameztoy et al, 2019 ).…”

“…45 Da (hereinafter designated as “small VCs”) are important determinants of plant responses to microbial volatile emissions ( Ameztoy et al, 2019 , 2021 ; García-Gómez et al, 2019 , 2020 ; Gámez-Arcas et al, 2022 ). Regulation of these responses is primarily nontranscriptional and involves global changes in the proteome ( Ameztoy et al, 2021 ) and thiol redox proteome, particularly in photosynthesis- and starch biosynthesis-related proteins ( Li et al, 2011 ; Ameztoy et al, 2019 ). Responses to small VCs also involve CK-mediated mechanisms wherein long-distance communication between roots and the aerial part of the plant play important roles ( García-Gómez et al, 2019 , 2020 ; Gámez-Arcas et al, 2022 ).…”

Enhanced Yield of Pepper Plants Promoted by Soil Application of Volatiles From Cell-Free Fungal Culture Filtrates Is Associated With Activation of the Beneficial Soil Microbiota

“…Recent studies have shown that this capacity also extends to VCs emitted by phytopathogens and microorganisms that do not normally interact mutualistically with plants ( Ezquer et al, 2010 ; Bitas et al, 2015 ; Sánchez-López et al, 2016 ; Cordovez et al, 2017 ; Li et al, 2018 ; García-Gómez et al, 2019 , 2020 ; Moisan et al, 2019 ), although several lines of evidence indicate that the mechanisms involved in some plants’ responses to VCs emitted by beneficial and pathogenic microorganisms are different ( Hernández-Calderón et al, 2018 ; García-Gómez et al, 2020 ). In Arabidopsis , enhanced growth and photosynthesis promoted by air application of fungal VCs is associated with increases in levels of active forms of cytokinins (CKs), photosynthetic pigments and transitory starch in leaves, together with reductions in abscisic acid (ABA) contents, and changes in the transcriptome and proteome through mechanisms involving signaling of redox-activated photosynthesis and long-distance communication between roots and the aerial part of the plant ( Zhang et al, 2008 ; Sánchez-López et al, 2016 ; Ameztoy et al, 2019 , 2021 ; García-Gómez et al, 2020 ).…”