LAUSR

Main conclusion Cultivar-biased regulation of HSFB4a and HSFA7 mediates heat stress tolerance/sensitivity in tomato. Reduced HSFB4a repressor levels and enhanced HSFA7 activator levels govern thermo-tolerance in tolerant cultivars. Abstract Heat shock factors (HSFs) are at the core of heat stress (HS) response in plants. However, the contribution of HSFs governing the inherent thermo-tolerance 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 -sensitive cultivars (CA4 and Pusa Ruby) of tomato under HS revealed cultivar-biased regulation of an activator ( HSFA7 ) and a repressor ( HSFB4a ) class HSF. HSFA7 exhibited strong upregulation while HSFB4a showed downregulation in tolerant tomato cultivar upon HS. Functional characterization of HSFA7 and HSFB4a in a tolerant–sensitive cultivar pair by virus-induced gene silencing (VIGS)-based silencing and transient overexpression established them as a positive and a negative regulator of HS tolerance, respectively. Promoter: GUS reporter assays and promoter sequence analyses suggest heat-mediated transcriptional control of both the HSF genes in the contrasting cultivars. Moreover, degradome data highlighted HSFB4a is a probable target of microRNA Sly-miR4200. Transient in-planta Sly-MIR4200 -effector: HSFB4a -reporter assays showed miRNA-dependent target down-regulation. Chelation of miRNA by short-tandem-target-mimic of Sly-miR4200 increased target abundance, highlighting a link between Sly-miR4200 and HSFB4a. This miRNA has induced several folds upon HS in the tolerant cultivar where HSFB4a levels are reduced, thus exhibiting the inverse miR:target expression. Thus, we speculate that the alleviation of HSFB4a and increased HSFA7 levels govern thermo-tolerance in the tolerant cultivar by regulating downstream heat stress-responsive genes.