The Tomato Hoffman's Anthocyaninless Gene Encodes a bHLH Transcription Factor Involved in Anthocyanin Biosynthesis That Is Developmentally Regulated and Induced by Low Temperatures.

Zhengkun Qiu,Zejun Huang,Xiaoxuan Wang,Jianchang Gao,Yanmei Guo,Yongchen Du,Keqiang Wu

doi:10.1371/journal.pone.0151067

Abstract

Anthocyanin pigments play many roles in plants, including providing protection against biotic and abiotic stresses. Many of the genes that mediate anthocyanin accumulation have been identified through studies of flowers and fruits; however, the mechanisms of genes involved in anthocyanin regulation in seedlings under low-temperature stimulus are less well understood. Genetic characterization of a tomato inbred line, FMTT271, which showed no anthocyanin pigmentation, revealed a mutation in a bHLH transcription factor (TF) gene, which corresponds to the ah (Hoffman's anthocyaninless) locus, and so the gene in FMTT271 at that locus was named ah. Overexpression of the wild type allele of AH in FMTT271 resulted in greater anthocyanin accumulation and increased expression of several genes in the anthocyanin biosynthetic pathway. The expression of AH and anthocyanin accumulation in seedlings was shown to be developmentally regulated and induced by low-temperature stress. Additionally, transcriptome analyses of hypocotyls and leaves from the near-isogenic lines seedlings revealed that AH not only influences the expression of anthocyanin biosynthetic genes, but also genes associated with responses to abiotic stress. Furthermore, the ah mutation was shown to cause accumulation of reactive oxidative species and the constitutive activation of defense responses under cold conditions. These results suggest that AH regulates anthocyanin biosynthesis, thereby playing a protective role, and that this function is particularly important in young seedlings that are particularly vulnerable to abiotic stresses.

Highlights

Anthocyanins, the plant pigments responsible for red, purple and blue colors in flowers and fruits, act as visual cues to attract insects that pollinate and help disperse seeds [1]
Many of the genes that encode enzymes involved in anthocyanin biosynthesis have been well characterized, and they can be divided into two groups: the early biosynthetic genes (EBGs; including CHS, encoding chalcone synthase; CHI, chalcone isomerase; and flavonoid 3-hydroxylase (F3H), flavanone 3-hydroxylase) that are common to different flavonoid sub-pathways, and the late biosynthetic genes (LBGs; F3’5’H, encoding flavonoid 3’5’-hydroxylase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanidin synthase; 3GT, flavonoid 3-O-glucosyltransferase; Reverse transcription (RT), anthocyanin rhamnosyltransferase; AAC, anthocyanin acyltransferase; 5GT, flavonoid 5-O-glucosyltransferasese; GST, glutathione S-transferase), which contribute to anthocyanin and proanthocyanidin biosynthesis [10, 11]
Since anthocyanin biosynthesis in tomato can be induced by exposure to low temperatures, we investigated whether AH expression changed in response to this stress

Summary

Introduction

Anthocyanins, the plant pigments responsible for red, purple and blue colors in flowers and fruits, act as visual cues to attract insects that pollinate and help disperse seeds [1]. They are synthesized in vegetative tissues and have been referred to as ‘Nature’s Swiss Army Knife’ due to their diverse roles in protecting against biotic stress and abiotic stresses, including those caused by insects, phytopathogens, drought, UV irradiation and low temperatures [2,3,4,5]. Many of these TFs have been identified in several model species, including maize (Zea mays), petunia (Petunia×hybrida) and Arabidopsis thaliana [11,12,13,14,15,16,17,18]

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