Abstract
Proanthocyanidins (PAs) are importantmetabolites that enhance freezing tolerance of plants. Actinidia arguta, especially freezing-tolerant germplasms, accumulate abundant PAs in dormant shoots and thereby enhance freezing tolerance, but the underlying mechanism is unknown. In this study, we used two A. arguta with contrasting cold-resistant phenotypes,KL and RB, to explore the mechanisms in response to cold tolerance. We determined that a leucoanthocyanidin reductase gene (AaLAR1) was more highly expressed in freezing-tolerant KL than in freezing-sensitive RB. Moreover, overexpressing AaLAR1 in kiwifruit promoted PAs biosynthesis and enhanced cold tolerance. The AaLAR1 promoters of various A. arguta germplasms differ due to the presence of a 60-bp deletion in cold-tolerant genotypes that forms a functional binding site for MYC-type transcription factor. Yeast one-hybrid and two-hybrid, dual-luciferase reporter, bimolecular fluorescence complementationand coimmunoprecipitation assays indicated that the AaMYC2a binds to the MYC-core cis-element in the AaLAR1 promoter with the assistance of AaMYB5a, thereby promoting PAs accumulation in the shoots of cold-tolerant kiwifruit. We conclude that the variation in the AaLAR1 promoter and the AaMYC2a-AaMYB5a-AaLAR1 module shape freezing tolerance in A. arguta. The identification of a key structural variation in the AaLAR1 promoter offers a new target for resistance breeding of kiwifruit.
Published Version
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