Abstract
BackgroundFruit development is controlled by plant hormones, but the role of hormone interactions during fruit ripening is poorly understood. Interactions between ethylene and the auxin indole-3-acetic acid (IAA) are likely to be crucial during the ripening process, since both hormones have been shown to be implicated in the control of ripening in a range of different fruit species.ResultsGrapevine (Vitis vinifera L.) homologues of the TRYPTOPHAN AMINOTRANSFERASE RELATED (TAR) and YUCCA families, functioning in the only characterized pathway of auxin biosynthesis, were identified and the expression of several TAR genes was shown to be induced by the pre-ripening application of the ethylene-releasing compound Ethrel. The induction of TAR expression was accompanied by increased IAA and IAA-Asp concentrations, indicative of an upregulation of auxin biosynthesis and conjugation. Exposure of ex planta, pre-ripening berries to the ethylene biosynthesis inhibitor aminoethoxyvinylglycine resulted in decreased IAA and IAA-Asp concentrations. The delayed initiation of ripening observed in Ethrel-treated berries might therefore represent an indirect ethylene effect mediated by increased auxin concentrations. During berry development, the expression of three TAR genes and one YUCCA gene was upregulated at the time of ripening initiation and/or during ripening. This increase in auxin biosynthesis gene expression was preceded by high expression levels of the ethylene biosynthesis genes 1-aminocyclopropane-1-carboxylate synthase and 1-aminocyclopropane-1-carboxylate oxidase.ConclusionsIn grape berries, members of both gene families involved in the two-step pathway of auxin biosynthesis are expressed, suggesting that IAA is produced through the combined action of TAR and YUCCA proteins in developing berries. The induction of TAR expression by Ethrel applications and the developmental expression patterns of auxin and ethylene biosynthesis genes indicate that elevated concentrations of ethylene prior to the initiation of ripening might lead to an increased production of IAA, suggesting a complex involvement of this auxin and its conjugates in grape berry ripening.
Highlights
Fruit development is controlled by plant hormones, but the role of hormone interactions during fruit ripening is poorly understood
The similar effects of Ethrel and auxins on the ripening behaviour of grapes combined with the increasing evidence for complex interactions between ethylene and indole-3-acetic acid (IAA) is suggestive of a link between these two plant hormones in the control of ripening
In order to investigate the possibility of an ethylene/auxin interaction in grape berry ripening, pre-veraison Shiraz berries were treated with either Ethrel or a Control solution in two consecutive seasons
Summary
Fruit development is controlled by plant hormones, but the role of hormone interactions during fruit ripening is poorly understood. The Ethrel-induced delay in the onset of sugar and anthocyanin accumulation resembles the ripening-delaying effects caused by the pre-veraison application of auxins [18,19,20,21,22,23], which have emerged as another important factor in fruit ripening in both climacteric and non-climacteric fruit (reviewed by [6,7]) In climacteric fruit, such as tomato (Solanum lycopersicum Mill.) [24,25], or banana (Musa paradisiaca L.) [26] and in nonclimacteric fruit like strawberry [27,28] and grape [29,30,31,32] the concentrations of IAA before and during ripening were generally found to be low. A similar pattern of IAA-Asp accumulation has been found in tomato which suggests a more complex role for auxins in fruit ripening that requires further investigation [29]
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