The RIN-regulated Small Auxin-Up RNA SAUR69 is involved in the unripe-to-ripe phase transition of tomato fruit via enhancement of the sensitivity to ethylene

Ethylene is the main hormone controlling climacteric fruit ripening; however, the mechanisms underlying the developmental transition leading to the initiation of the ripening process remain elusive, although the presumed role of active hormone interplay has often been postulated. To unravel the putative role of auxin in the unripe-to-ripe transition, we investigated the dynamics of auxin activity in tomato fruit and addressed the physiological significance of Sl-SAUR69, previously identified as a RIN target gene, using reverse genetics approaches. Auxin signalling undergoes dramatic decline at the onset of ripening in wild-type fruit, but not in the nonripening rin mutant. Sl-SAUR69 exhibits reduced expression in rin and its up-regulation results in premature initiation of ripening, whereas its down-regulation extends the time to ripening. Overexpression of Sl-SAUR69 reduces proton pump activity and polar auxin transport, and ectopic expression in Arabidopsis alters auxin transporter abundance, further arguing for its active role in the regulation of auxin transport. The data support a model in which Sl-SAUR69 represses auxin transport, thus generating auxin minima, which results in enhanced ethylene sensitivity. This defines a regulation loop, fed by ethylene and auxin as the main hormonal signals and by RIN and Sl-SAUR69 as modulators of the balance between the two hormones.