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

Ethylene is the main hormone controlling climacteric fruit ripening; however, the mechanisms underlying the developmental transition leading to initiation of the ripening process remain elusive although the presumed role of active hormone interplay has been often 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 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 upregulation results in premature initiation of ripening whereas downregulation extends the time to ripening. Overexpressing 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 regulating auxin transport. The data support a model where 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 main hormonal signals and by RIN and Sl-SAUR69 as modulators of the balance between the two hormones. This article is protected by copyright. All rights reserved.