Differential regulation of tomato ethylene responsive factor LeERF3b, a putative repressor, and the activator Pti4 in ripening mutants and in response to environmental stresses

Ethylene responsive transcription factors (ERFs) can be grouped into different classes with either gene activator or repressor activity. We have isolated a tomato ERF cDNA clone (LeERF3b) with sequence similarity to class II (repressor class) of the ERF family, which is regulated differently from Pti4 (a tomato ERF domain-containing gene that activates other genes). LeERF3b has similarities to other tomato ERF cDNAs but the DNA or predicted amino acid sequences have significant differences. Northern analysis showed that Pti4 was highly expressed during fruit ripening, whereas LeERF3b accumulated before and declined sharply after the onset of ripening. Furthermore, Pti4 mRNA was significantly reduced in low-ethylene tomato fruit containing an ACC oxidase sense-suppression transgene and also in the ethylene insensitive mutant never ripe (Nr). By contrast, the LeERF3b mRNA was markedly increased in those fruits. Environmental stresses including drought, desiccation and low temperature increased significantly the expression level of LeERF3b, but markedly reduced the level of Pti4 mRNA. Conversely, wounding induced the accumulation of Pti4 mRNA, but had no significant effect on the level of LeERF3b. These opposing patterns of regulation of mRNA accumulation are consistent with the activator function of Pti4 and a repressor function for LeERF3b in ethylene responses.