AUTHOR=Upadhyay Rakesh K. , Tucker Mark L. , Mattoo Autar K. 

TITLE=Ethylene and RIPENING INHIBITOR Modulate Expression of SlHSP17.7A, B Class I Small Heat Shock Protein Genes During Tomato Fruit Ripening

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00975

DOI=10.3389/fpls.2020.00975

ISSN=1664-462X

ABSTRACT=<p>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, <italic>SlHSP17.7A</italic> and <italic>SlHSP17.7B</italic>, 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 <italic>SlHSP17.7A</italic> and <italic>SlHSP17.7B</italic> 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, <italic>SlACS2,</italic> and its regulator <italic>SlMADS-RIN</italic>. Exogenous application of ethylene to either mature green tomato fruit or tomato leaves suppressed the expression of both the <italic>SlHSP17.7A, B</italic> genes. Notably and characteristically, a transgenic tomato line silenced for <italic>SlACS2</italic> gene and whose fruits produce ~50% less ethylene <italic>in vivo</italic>, had higher expression of both the <italic>sHSP</italic> genes at the fruit ripening transition stages [breaker (BR) and BR+3] than the control fruit. Moreover, differential gene expression of <italic>SlHSP17.7A versus SlHSP17.7B</italic> gene was apparent in the tomato ripening mutants—<italic>rin/rin</italic>, <italic>nor/nor</italic>, and <italic>Nr/Nr</italic>, with the expression of <italic>SlHSP17.7A</italic> being significantly reduced but that of <italic>SlHSP17.7B</italic> significantly upregulated as compared to the wild type (WT). These data indicate that ethylene negatively regulates transcriptional abundance of both these sHSPs. Transient overexpression of the ripening regulator <italic>SlMADS-RIN</italic> in WT and ACS2-AS mature green tomato fruits suppressed the expression of <italic>SlHSP17.7A</italic> but not that of <italic>SlHSP17.7B</italic>. Thus, ethylene directly or in tune with <italic>SlMADS-RIN</italic> regulates the transcript abundance of both these sHSP genes.</p>