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Summary The footprint of tomato cultivation, a cool region crop that exhibits heat stress (HS) sensitivity, is increasing in the tropics/sub‐tropics. Knowledge of novel regulatory hot spots from varieties growing in the Indian sub‐continent climatic zones could be vital for developing HS‐resilient crops. Comparative transcriptome‐wide signatures of a tolerant (CLN1621L) and sensitive (CA4) cultivar pair shortlisted from a pool of varieties exhibiting variable thermo‐sensitivity using physiological‐, survival‐ and yield‐related traits revealed redundant to cultivar‐specific HS regulation. The antagonistically expressing genes encode enzymes and proteins that have roles in plant defence and abiotic stresses. Functional characterization of three antagonistic genes by overexpression and silencing established Solyc09g014280 (Acylsugar acyltransferase) and Solyc07g056570 (Notabilis) that are up‐regulated in tolerant cultivar, as positive regulators of HS tolerance and Solyc03g020030 (Pin‐II proteinase inhibitor), that are down‐regulated in CLN1621L, as negative regulator of thermotolerance. Transcriptional assessment of promoters of these genes by SNPs in stress‐responsive cis‐elements and promoter swapping experiments in opposite cultivar background showed inherent cultivar‐specific orchestration of transcription factors in regulating transcription. Moreover, overexpression of three ethylene response transcription factors (ERF.C1/F4/F5) also improved HS tolerance in tomato. This study identifies several novel HS tolerance genes and provides proof of their utility in tomato thermotolerance.

Heat shock factors (HSFs) are at the core of heat stress (HS) response in plants. However, the contribution of HSFs governing the inherent thermotolerance mechanism in tomato from sub-tropical hot climates is poorly understood. With the above aim, comparative expression profiles of the HSF family in a HS tolerant (CLN1621L) and a sensitive cultivar (CA4) of tomato under HS revealed cultivar-biased regulation of an activator (HSFA7a) and repressor (HSFB4a) class HSF. Functional characterization of HSFA7a that was strongly up-regulated in the tolerant cultivar by VIGS-based silencing and overexpression established it as a positive regulator of HS-tolerance. While knock-down and overexpression analyses of HSFB4a that was down-regulated in CLN1621L in HS, showed it as a negative regulator of thermotolerance. Promoter:GUS reporter assays and promoter sequence analyses suggest heat-mediated transcriptional control of both the HSF genes in the contrasting cultivars. Moreover, we show HSFB4a is also regulated post-transcriptionally by microRNA Sly-miR4200 using degradome, short-tandem-target-mimic of Sly-miR4200 and transient in-planta Sly-miR4200-effector:HSFB4a-reporter assays. This miRNA is induced several folds upon HS in the tolerant variety thereby reducing HSFB4a levels. We thus propose that the alleviation of HSFB4a repressor governs thermotolerance in the tolerant cultivar by regulating downstream heat stress responsive genes.

The footprint of tomato cultivation, a cool region crop that exhibits heat stress (HS) sensitivity, is increasing in the tropics/sub-tropics. Knowledge of novel regulatory hot-spots from varieties growing in the Indian sub-continent climatic zones could be vital for developing HS-resilient crops. Comparative transcriptome-wide signatures of a tolerant (CLN1621L) and sensitive (CA4) cultivar-pair short-listed from a pool of varieties exhibiting variable thermo-sensitivity using physiological, survival and yield-related traits revealed redundant to cultivar-specific HS-regulation with more up-regulated genes for CLN1621L than CA4. The anatgonisiticly-expressing genes include enzymes; have roles in plant defense and response to different abiotic stresses. Functional characterization of three antagonistic genes by overexpression and TRV-VIGS silencing established Solyc09g014280 (Acylsugar acyltransferase) and Solyc07g056570 (Notabilis), that are up-regulated in tolerant cultivar, as positive regulators of HS-tolerance and Solyc03g020030 (Pin-II proteinase inhibitor), that is down-regulated in CLN1621L, as negative regulator of thermotolerance. Transcriptional assessment of promoters of these genes by SNPs in stress-responsivecis-elements and promoter swapping experiments in opposite cultivar background showed inherent cultivar-specific orchestration of transcription factors in regulating transcription. Moreover, overexpression of three ethylene response transcription factors (ERF.C1/F4/F5) also improved HS-tolerance in tomato. This study identifies several novel HS-tolerance genes and provides proof of their utility in tomato-thermotolerance.HighlightNovel heat stress regulatory pathways uncovered by comparative transcriptome profiling between contrasting tomato cultivars from Indian sub-continent for improving thermotolerance. (20/30)