Mechanisms by Which High Temperatures Suppress Anthocyanin Coloration in Flowers and Fruits, and Discovery of Floricultural Crops that Exhibit High‑Temperature-Tolerant Flower Pigmentation

Anthocyanins are ubiquitously accumulated in diverse plant species and play crucial roles in plant development. In addition, anthocyanin pigmentation is associated with significant characteristics in the flowers and fruits of horticultural products. Notably, anthocyanin biosynthesis and storage are known to be affected by environmental factors. High ambient temperatures often suppress anthocyanin accumulation in flowers and fruits, raising concerns regarding the increase in atmospheric temperatures caused by global warming. Therefore, a comprehensive understanding of the mechanisms underlying the effects of high ambient temperatures on the regulation of anthocyanin biosynthesis and storage is necessary to maintain pigment quality and quantity of the products. In this review, we provide comprehensive information on the role of high-temperature-related signaling pathways in the regulation of anthocyanin biosynthesis. The expression of genes involved in anthocyanin biosynthesis is mainly regulated by R2R3-MYB activators, R2R3-MYB repressors, and R3-MYB negative regulators. The expression levels of R2R3-MYB activators decrease under high-temperature conditions, as observed in many flowers and fruits. The upregulation of R2R3-MYB repressors and R3-MYB negative regulators has also been demonstrated in some plant species under high-temperature conditions. The high-temperature-related signaling pathways have been evaluated mainly in the vegetative organs of Arabidopsis and apple fruits. In these organs, light strongly influences anthocyanin biosynthesis in addition to ambient temperatures. The CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)-ELONGATED HYPOCOTYL 5 (HY5) module and B-box proteins upregulate the R2R3-MYB activators under light conditions, while they downregulate the R2R3-MYB activators under high-temperature conditions. However, the pathways that transduce high-temperature signals in flowers are poorly understood. Unlike in fruits and vegetative organs, light exerts relatively small effects on anthocyanin pigmentation in flowers, suggesting that the COP1-HY5 module-independent pathway could be responsible for the regulation of R2R3-MYB regulators in many flowers. Further research to clarify the related signaling pathways in flowers is needed to find solutions to overcome the problem of color fading caused by high ambient temperatures. In addition, exceptional cases have been reported in which high temperatures do not inhibit or enhance the anthocyanin pigmentation of flowers. Such species can prove helpful in elucidating the mechanisms underlying temperature-mediated regulation of anthocyanin pigmentation and as parental materials for crossbreeding.