The repressor and co-activator HsfB1 regulates the major heat stress transcription factors in tomato.

Plants code for a multitude of heat stress transcription factors (Hsfs). Three of them act as central regulators of heat stress (HS) response in tomato (Solanum lycopersicum). HsfA1a regulates the initial response, and HsfA2 controls acquired thermotolerance. HsfB1 is a transcriptional repressor but can also act as co-activator of HsfA1a. Currently, the mode of action and the relevance of the dual function of HsfB1 remain elusive. We examined this in HsfB1 overexpression or suppression transgenic tomato lines. Proteome analysis revealed that HsfB1 overexpression stimulates the co-activator function of HsfB1 and consequently the accumulation of HS-related proteins under non-stress conditions. Plants with enhanced levels of HsfB1 show aberrant growth and development but enhanced thermotolerance. HsfB1 suppression has no significant effect prior to stress. Upon HS, HsfB1 suppression strongly enhances the induction of heat shock proteins due to the higher activity of other HS-induced Hsfs, resulting in increased thermotolerance compared with wild-type. Thereby, HsfB1 acts as co-activator of HsfA1a for several Hsps, but as a transcriptional repressor on other Hsfs, including HsfA1b and HsfA2. The dual function explains the activation of chaperones to enhance protection and regulate the balance between growth and stress response upon deviations from the homeostatic levels of HsfB1.

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues.

Male reproductive tissues are more sensitive to heat stress (HS) compared to vegetative tissues, but the basis of this phenomenon is poorly understood. Heat stress transcription factors (Hsfs) regulate the transcriptional changes required for protection from HS In tomato (Solanum lycopersicum), HsfA2 acts as coactivator of HsfA1a and is one of the major Hsfs accumulating in response to elevated temperatures. The contribution of HsfA2 in heat stress response (HSR) and thermotolerance was investigated in different tissues of transgenic tomato plants with suppressed HsfA2 levels (A2AS). Global transcriptome analysis and immunodetection of two major Hsps in vegetative and reproductive tissues showed that HsfA2 regulates subsets of HS-induced genes in a tissue-specific manner. Accumulation of HsfA2 by a moderate HS treatment enhances the capacity of seedlings to cope with a subsequent severe HS, suggesting an important role for HsfA2 in regulating acquired thermotolerance. In pollen, HsfA2 is an important coactivator of HsfA1a during HSR HsfA2 suppression reduces the viability and germination rate of pollen that received the stress during the stages of meiosis and microspore formation but had no effect on more advanced stages. In general, pollen meiocytes and microspores are characterized by increased susceptibility to HS due to their lower capacity to induce a strong HSR This sensitivity is partially mitigated by the developmentally regulated expression of HsfA2 and several HS-responsive genes mediated by HsfA1a under nonstress conditions. Thereby, HsfA2 is an important factor for the priming process that sustains pollen thermotolerance during microsporogenesis.

In the complex family of heat stress transcription factors, HsfA1 has a unique role as master regulator of thermotolerance in tomato.

We generated transgenic tomato plants with altered expression of heat stress transcription factor HsfA1. Plants with 10-fold overexpression of HsfA1 (OE plants) were characterized by a single HsfA1 transgene cassette, whereas plants harboring a tandem inverted repeat of the cassette showed cosuppression (CS plants) by posttranscriptional silencing of the HsfA1 gene connected with formation of small interfering RNAs. Under normal growth conditions, major developmental parameters were similar for wild-type (WT), OE, and CS plants. However, CS plants and fruits were extremely sensitive to elevated temperatures, because heat stress-induced synthesis of chaperones and Hsfs was strongly reduced or lacking. Despite the complexity of the plant Hsf family with at least 17 members in tomato, HsfA1 has a unique function as master regulator for induced thermotolerance. Using transient reporter assays with mesophyll protoplasts from WT tomato, we demonstrated that plasmid-encoded HsfA1 and HsfA2 were well expressed. However, in CS protoplasts the cosuppression phenomenon was faithfully reproduced. Only transformation with HsfA2 expression plasmid led to normal expression of the transcription factor and reporter gene activation, whereas even high amounts of HsfA1 expression plasmids were silenced. Thermotolerance in CS protoplasts was restored by plasmid-borne HsfA2, resulting in expression of chaperones, thermoprotection of firefly luciferase, and assembly of heat stress granules.