Light-controlled flavonoid biosynthesis in fruits

Abstract

Light is one of the most important environmental factors affecting flavonoid biosynthesis in plants. The absolute dependency of light to the plant development has driven evolvement of sophisticated mechanisms to sense and transduce multiple aspects of the light signal. Light effects can be categorized in photoperiod (duration), intensity (quantity), direction and quality (wavelength) including UV-light. Recently, new information has been achieved on the regulation of light-controlled flavonoid biosynthesis in fruits, in which flavonoids have a major contribution on quality. This review focuses on the effects of the different light conditions on the control of flavonoid biosynthesis in fruit producing plants. An overview of the currently known mechanisms of the light-controlled flavonoid accumulation is provided. R2R3 MYB transcription factors are known to regulate by differential expression the biosynthesis of distinct flavonoids in response to specific light wavelengths. Despite recent advances, many gaps remain to be understood in the mechanisms of the transduction pathway of light-controlled flavonoid biosynthesis. A better knowledge on these regulatory mechanisms is likely to be useful for breeding programs aiming to modify fruit flavonoid pattern.

Keywords: MYBs; UV; anthocyanins; berries; flavonols; fruits; light; proanthocyanidins.

Figure 1

The spectrum of solar radiation reaching from gamma rays to radio waves with closer view on visible wavelengths and plant photoreceptors absorbing specific wavelength regions. Cry, cryptochromes; Phy, phytochromes; Phot, phototropins; UV, ultraviolet; UVR8, UV-B photoreceptor.

Figure 2

Summer solar radiant flux spectra of two European locations (Tromsø, Norway, latitude 69°N, longitude 18°E; Trento, Italy, latitude 46°N, longitude 11°E) under clear sky and midday conditions. Tromsø UV-B_{(280–320 nm)}: 0.71 μjoule cm⁻; UV-A_{(320–400 nm)}: 12.37 μjoule cm⁻²: PAR_{(400–700 nm)}: 1389 μmoles photones m⁻² s⁻¹. Trento UV-B_{(280–320 nm)}: 2.29 μjoule cm⁻²; UV-A_{(320–400 nm)}: 45.5 μjoule cm⁻²: PAR_{(400–700 nm)}: 3670 μmoles photones m⁻² s⁻¹. IR, infrared; PAR, photosynthetic active radiation.

Figure 3

COP1 acts as a central repressor in light signaling pathway by interacting directly with photoreceptors to mediate different light-regulated plant developmental processes. (A) In darkness, nuclear localized COP1 targets positive regulators, such as transcription factors HY5 and R2R3 MYBs, for ubiquitination and subsequent protein degradation through a 26S proteasome pathway. (B) In visible light, interaction with activated photoreceptors repress function of COP1 that is subsequently exported from nucleus allowing nuclear-localized transcription factors to accumulate and induce gene expression in light-regulated processes. The expression of structural flavonoid genes is directly regulated by R2R3 MYB transcription factors which may be regulated by bZIP transcription factor such as HY5. During the process, photoreceptors are ubiquitinated by COP1 and targeted for degradation (Lau and Deng, 2012).

Figure 4

Proposed mechanism for signaling pathway affecting flavonoid biosynthesis under UV-B radiation. UV-B radiation is strongly absorbed by tryptophan (Trp) amino acid residues in the dimeric form of UVR8 photoreceptor leading to the monomerization of UVR8. Monomeric UVR8 and COP1 form a complex that accumulates in the nucleus of the cells. The UVR8-COP1-SPA complex stabilizes bZIP transcription factor HY5 promoting the activity of different R2R3 MYBs for the transcription of specific flavonoid biosynthesis genes (Favory et al., ; Christie et al., ; Jenkins, ; Li et al., 2014).