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The tomato heat stress transcription factor A1a (Sly-HSFA1a) acts as the master regulator of the heat stress (HS) by directly regulating the transcription of Sly-HSFA2. However, it is unclear whether the activation of Sly-HSFA2 alone is sufficient to trigger the entire transcriptional cascade downstream of Sly-HSFA1a. Therefore, the present study aims to delineate the Sly-HSFA1a governed downstream HSFs cascade regulating the tomato heat stress response. The study identified several HSFs with common and specific roles in different HS regimes as well as in HS memory. Furthermore, the study established Sly-HSFA7, Sly-HSFA6b, Sly-HSFA4c, Sly-HSFB1 and Sly-HSFB2b as new downstream targets of Sly-HSFA1a during heat stress by using virus-induced-gene-silencing (VIGS) of Sly-HSFA1a. Moreover, the silencing of downstream target Sly-HSFA7 and Sly-HSFB1 revealed the orchestration of downstream transcriptional cascade of HSFs regulated individually or in a synergistic manner by Sly-HSFA1a and Sly-HSFA7 along with co-activator Sly-HSFB1. This complex transcriptional cascade of HSFs sheds light on regulatory mechanisms that enable tomato plants to respond to various heat stress conditions to maintain cellular homeostasis.

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.

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)