Defining the convergence of ethylene and auxin signaling in tomato

Abstract The role of the classical plant hormones in the coordination of plant growth and development has long been recognized, as has the fact that their signaling pathways interact or ‘cross-talk’ (Muday et al., 2012). In this vein, a considerable research effort has been placed into understanding how ethylene and auxin interact, either synergistically or antagonistically, to control a variety of plant developmental processes. In the model plant Arabidopsis, most of these studies have focused on root formation and hypocotyl elongation, whereas in tomato the importance of this interaction has also been investigated during fruit development and ripening (Chaabouni et al., 2009). Several lines of evidence reinforce this: (1) it has been shown that mutations which affect the synthesis and distribution of signaling of auxin display abnormal ethylene responses (Pickett et al., 1990); (2) ethylene and auxin mutually regulate the transcription of key genes associated with the biosynthesis of both hormones (Muday et al., 2012); and (3) ethylene promotes auxin transport from the meristem to the root elongation zone triggering ethylene-mediated root growth inhibition (Pickett et al., 1990; Ruzicka et al., 2007; Stepanova et al., 2007). However, in spite of the fact that these phenomena are well characterized at the physiological level until now relatively little has been known concerning the molecular mechanisms which underlie them. In this issue of New Phytologist, Liu et al. (2018; pp. 631–640) used a combination of reverse genetic approaches, and transactivation and electrophoretic mobility shift assays to elucidate that ERF.B3 integrates ethylene and auxin signaling by effecting the expression of the auxin signaling component IAA27. These results provide compelling evidence that the interaction of ERF.B3 and IAA27 represents an important integrative hub which, at least partially, mediates ethylene–auxin cross-talk in tomato. In order to place this finding in a proper biological context it is important to provide some brief background information concerning work on the individual hormones before returning to the data reported by Liu et al. (2018)