Ethylene-sensitive and insensitive regulation of transcription factor expression during in vitro tomato sepal ripening

Tomato (Solanum lycopersicum, formerly Lycopersicon esculentum) cv. VFNT Cherry sepals, when cultured in vitro between 16 degrees C and 22 degrees C, change their genetic programme to that of ripening fruit. Previously regulation of a number of transcription factors and a putative G-protein-coupled receptor that may be involved in tomato fruit ripening and cool-temperature sepal morphogenesis had been revealed. Many of those genes such as TAG1, TM4, TM6, AP2-like (LeAP2FR), YABBY2-like (LeYAB2), and LeCOR413-PM1 have not been investigated for ethylene regulation. Ethylene-independent, regulated transcripts may be part of an early signalling process induced or de-repressed by cool temperature that causes a switch in the genetic programme of the sepals. In this paper, ethylene regulation of a number of these and other putative signalling factors are investigated during cool-temperature-induced sepal morphogenesis. 1-Methylcyclopropene was used to block ethylene-induced gene expression by interrupting the ethylene signal transduction pathway that occurs in ripening tomato fruits and presumably in ripening sepals. Transcripts of several putative transcription factors previously shown to be up-regulated during cool-temperature-induced sepal morphogenesis (TAG1, TM4, LeAP2FR) were only slightly or not induced in 1-methylcyclopropene-treated sepals, indicating either direct or indirect ethylene regulation. Two genes, VAHOX1, a homeobox domain leucine-zipper-encoding gene, and LeYAB2, a putative zinc-finger transcription factor-encoding gene, increased in treated and untreated sepals indicating regulation by cool temperatures independently of ethylene.