Abstract
The cuticle covering plants' aerial surfaces is a unique structure that plays a key role in organ development and protection against diverse stress conditions. A detailed analysis of the tomato colorless-peel y mutant was carried out in the framework of studying the outer surface of reproductive organs. The y mutant peel lacks the yellow flavonoid pigment naringenin chalcone, which has been suggested to influence the characteristics and function of the cuticular layer. Large-scale metabolic and transcript profiling revealed broad effects on both primary and secondary metabolism, related mostly to the biosynthesis of phenylpropanoids, particularly flavonoids. These were not restricted to the fruit or to a specific stage of its development and indicated that the y mutant phenotype is due to a mutation in a regulatory gene. Indeed, expression analyses specified three R2R3-MYB–type transcription factors that were significantly down-regulated in the y mutant fruit peel. One of these, SlMYB12, was mapped to the genomic region on tomato chromosome 1 previously shown to harbor the y mutation. Identification of an additional mutant allele that co-segregates with the colorless-peel trait, specific down-regulation of SlMYB12 and rescue of the y phenotype by overexpression of SlMYB12 on the mutant background, confirmed that a lesion in this regulator underlies the y phenotype. Hence, this work provides novel insight to the study of fleshy fruit cuticular structure and paves the way for the elucidation of the regulatory network that controls flavonoid accumulation in tomato fruit cuticle.
Highlights
Most aerial plant surfaces are covered with a cuticle, a heterogeneous layer composed mainly of cutin and wax lipids
The y Mutant Phenotype The tomato y mutant, examined here as part of our effort to characterize the unique features of reproductive organ surfaces, was described long ago as carrying a monogenic recessive colorless fruit peel mutation [27,29]
Unlike the fruit of the CHSsilenced plants described by Schijlen et al [7], y fruits are not parthenocarpic and an analysis of their surface by electron microscopy did not reveal any significant difference from the wt fruit
Summary
Most aerial plant surfaces are covered with a cuticle, a heterogeneous layer composed mainly of cutin and wax lipids. Embedded in the cutin matrix are cuticular waxes, which are complex mixtures of very-long-chain fatty-acid derivatives [1] In many species these include triterpenoids and other secondary metabolites, such as sterols, alkaloids and phenylpropanoids, including flavonoids. The flavonoid naringenin chalcone (NarCh) accumulates up to 1% dry weight of the tomato fruit cuticle: it is the yellow pigment that accumulates in wild-type (wt) fruit peel, and it is the first intermediate in the biosynthesis of flavonols. It is produced by chalcone synthase (CHS) from p-coumaroyl-CoA and malonylCoA and subsequently converted into naringenin (Nar) by chalcone isomerase (CHI) [2]. The contribution of NarCh to the characteristic features of the fruit cuticle was examined by Luque et al [3], who suggested that it plays an important role in controlling water transport across the polymer matrix
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