Ethylene and ripening inhibitor modulate expression of SlHSP17.7A, B class I small heat shock protein genes during tomato fruit ripening

Authors: UPADHYAY, RAKESH - Purdue University; TUCKER, MARK - Retired ARS Employee; Mattoo, Autar

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2020
Publication Date: 6/30/2020
Citation: Upadhyay, R., Tucker, M., Mattoo, A.K. 2020. Ethylene and ripening inhibitor modulate expression of SlHSP17.7A, B class I small heat shock protein genes during tomato fruit ripening. Frontiers in Plant Science. 11:1-15.

Interpretive Summary: Fruits and vegetables are sources of dietary micronutrients, vitamins and antioxidants for daily human consumption. Ripening of tomatoes leads to production of nutrients. In order to be able to add a high dose of nutrients to tomato fruit, it is necessary to determine what factors in fruit control their production and accumulation. Ripening in tomato is primarily controlled by the hormone ethylene, which also leads to fruit softening, and its short shelf life. Although a plethora of genes and proteins have been implicated in tomato fruit ripening, developing fruits to have longer shelf life without compromising the nutrient content has yet to be achieved. A master regulator known as RIN (Ripening Inhibitor) gene along with other regulators has been unearthed. We have tested the role of ethylene and RIN gene in tomato responses to heat shock protein genes. Our intent is to determine and unravel genetic switches that would be useful for developing a longer lasting, nutrient rich and heat tolerant tomato germplasm. Toward that goal, we report here characterization of two novel tomato heat shock protein genes which are expressed at the transition of the mature green tomato fruit into the ripening process. We show here the regulation of small heat shock protein genes as regulated by the RIN transcription factor at the ripening initiation junction. Using different tomato genetic backgrounds, we demonstrate that ethylene hormone and RIN transcription factor crosstalk to regulate these heat shock protein genes. These findings are novel and should help develop new strategies to produce tomato germplasm with prolonged postharvest shelf life along with resistance to heat. This research is of interest to tomato breeders, scientists, plant physiologists and genetic experts.

Technical Abstract: Heat shock proteins (HSPs) are ubiquitous and highly conserved in nature. Heat stress upregulates their gene expression and now it is known that they are also developmentally regulated. We have studied regulation of small HSP genes during ripening of tomato fruit. In this study, we identify two small HSP genes, SlHSP17.7A and SlHSP17.7B, localized on tomato Chr.6 and Chr.9, respectively. Each gene encodes proteins constituting 154 amino acids and has characteristic domains as in other sHSP genes. We found that SlHSP17.7A and SlHSP17.7B gene expression is low in the vegetative tissues as compared to that in the fruit. These sHSP genes are characteristically expressed in a fruit-ripening fashion, being upregulated during the ripening transition of mature green to breaker stage. Their expression patterns mirror that of the rate-limiting ethylene biosynthesis gene ACC (1-aminocyclopropane-1-carboxylic acid) synthase, SlACS2, and its regulator SlMADS-RIN. Exogenous application of ethylene to either mature green tomato fruit or tomato leaves suppressed the expression of both the SlHSP17.7A, B genes. Notably and characteristically, a transgenic tomato line silenced for SlACS2 gene and whose fruits produce ~50% less ethylene in vivo, had higher expression of both the sHSP genes at the fruit ripening transition stages [breaker (BR) and BR+3) than the control fruit. Moreover, differential gene expression of SlHSP17.7A versus SlHSP17.7B gene was apparent in the tomato ripening mutants - rin/rin, nor/nor and Nr/Nr, with the expression of SlHSP17.7A being significantly reduced but that of SlHSP17.7B significantly upregulated as compared to the wildtype (WT). These data indicate that ethylene negatively regulates transcriptional abundance of both these sHSPs. Transient overexpression of the ripening regulator SlMADS-RIN in WT and ACS2-AS mature green tomato fruits suppressed the expression of SlHSP17.7A but not that of SlHSP17.7B. Thus, ethylene directly or in tune with SlMADS-RIN regulates the transcript abundance of both these sHSP genes.