CRISPR/Cas9-Mediated <i>SlMYC2</i> Mutagenesis Adverse to Tomato Plant Growth and MeJA-Induced Fruit Resistance to <i>Botrytis cinerea</i>

Abstract

Methyl jasmonate (MeJA), a natural phytohormone, played a critical role not only in plant growth but also in plant defense response to biotic and abiotic stresses. MYC2, a basic helix-loop-helix transcription factor, is a master regulator in MeJA signaling pathway. In the present work, <i>slmyc2</i> mutants were generated by the clustered regularly interspaced short palindromic repeats and associated Cas9 protein (CRISPR/Cas9) system to investigate the role of <i>SlMYC2</i> in tomato plant growth and fruit disease resistance induced by exogenous MeJA. The results showed that <i>slmyc2</i> mutants possessed a higher number of flowers and a lower fruit setting rate in comparison with wild-type plants. In addition, the fruit shape of s<i>lmyc2</i> mutant was prolate, while the control fruits were oblate. Knockout of <i>SlMYC2</i> significantly decreased the activities of disease defensive and antioxidant enzymes, as well as the expression levels of pathogen-related (PR) genes (<i>SlPR-1</i> and <i>SlPR-STH2</i>) and the key genes related to jasmonic acid (JA) biosynthesis and signaling pathway including <i>allene oxide cyclase</i> (<i>SlAOC</i>), <i>lipoxygenase D</i> (<i>SlLOXD</i>), <i>SlMYC2</i>, and <i>coronatine insensitive 1</i> (<i>SlCOI1</i>), and consequently aggravated the disease symptoms. By contrast, the disease symptoms were largely reduced in MeJA-treated fruit that possessed higher activities of these enzymes and expression levels of genes. However, the induction effects of MeJA on fruit disease resistance and these enzymes' activities and genes' expressions were significantly attenuated by knockout of <i>SlMYC2</i>. Therefore, the results indicated that <i>SlMYC2</i> played positive regulatory roles not only in the growth of tomato plants but also in MeJA-induced disease resistance and the antioxidant process in tomato fruits.