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Anthocyanins have long been suggested as having great potential in offering photoprotection to plants facing high light irradiance. Nonetheless, their effective ability in protecting the photosynthetic apparatus from supernumerary photons has been questioned by some authors, based upon the inexact belief that anthocyanins almost exclusively absorb green photons, which are poorly absorbed by chlorophylls. Here we focus on the blue light absorbing features of anthocyanins, a neglected issue in anthocyanin research. Anthocyanins effectively absorb blue photons: the absorbance of blue relative to green photons increases from tri‐ to mono‐hydroxy B‐ring substituted structures, reaching up to 50% of green photons absorption. We offer a comprehensive picture of the molecular events activated by low blue‐light availability, extending our previous analysis in purple and green basil, which we suggest to be responsible for the "shade syndrome" displayed by cyanic leaves. While purple leaves display overexpression of genes promoting chlorophyll biosynthesis and light harvesting, in green leaves it is the genes involved in the stability/repair of photosystems that are largely overexpressed. As a corollary, this adds further support to the view of an effective photoprotective role of anthocyanins. We discuss the profound morpho‐anatomical adjustments imposed by the epidermal anthocyanin shield, which reflect adjustments in light harvesting capacity under imposed shade and make complex the analysis of the photosynthetic performance of cyanic versus acyanic leaves. We offer conclusive evidence that the blue light‐absorbing properties of anthocyanins are responsible for the shade nature of cyanic leaves/individuals and, as a corollary, this strongly supports the view of an effective photoprotective role of anthocyanins. [ABSTRACT FROM AUTHOR]

SUMMARY: In dense canopy, a reduction in red to far‐red (R/FR) light ratio triggers shade avoidance responses (SARs) in Arabidopsis thaliana, a shade avoiding plant. Two red/far‐red (R/FR) light photoreceptors, PHYB and PHYA, were reported to be key negative regulators of the SARs. PHYB represses the SARs under normal light conditions; however, the role of PHYA in the SARs remains elusive. We set up two shade conditions: Shade and strong Shade (s‐Shade) with different R/FR ratios (0.7 and 0.1), which allowed us to observe phenotypes dominated by PHYB‐ and PHYA‐mediated pathway, respectively. By comparing the hypocotyl growth under these two conditions with time, we found PHYA was predominantly activated in the s‐Shade after prolonged shade treatment. We further showed that under s‐Shade, PHYA inhibits hypocotyl elongation partially through repressing the brassinosteroid (BR) pathway. COP1 and PIF4,5 act downstream of PHYA. After prolonged shade treatment, the nuclear localization of COP1 was reduced, while the PIF4 protein level was much lower in the s‐Shade than that in Shade. Both changes occurred in a PHYA‐dependent manner. We propose that under deep canopy, the R/FR ratio is extremely low, which promotes the nuclear accumulation of PHYA. Activated PHYA reduces COP1 nuclear speckle, which may lead to changes of downstream targets, such as PIF4,5 and HY5. Together, these proteins regulate the BR pathway through modulating BES1/BZR1 and the expression of BR biosynthesis and BR target genes. Significance Statement: Phytochrome A (PHYA) is a known negative regulator of the shade avoidance responses, but when and how it functions in shade remains elusive. We set up a strong shade condition, which allowed us to observe PHYA‐dominated responses. We found PHYA begins to inhibit shade‐induced hypocotyl elongation after prolonged strong shade treatment, which promoted PHYA nuclear localization. It subsequently inhibits shade‐induced hypocotyl elongation through repressing the BR pathway, which involves COP1 and PIF4,5. [ABSTRACT FROM AUTHOR]

Photosynthetic performance and the expression of genes involved in light signaling and the biosynthesis of isoprenoids and phenylpropanoids were analysed in green ('Tigullio', TIG) and red ('Red Rubin', RR) basil. The aim was to detect the physiological and molecular response mechanisms to high sunlight. The attenuation of blue-green light by epidermal anthocyanins was shown to evoke shade-avoidance responses with consequential effects on leaf morpho- anatomical traits and gas exchange performance. Red basil had a lower mesophyll conductance, partially compensated by the less effective control of stomatal movements, in comparison with TIG. Photosynthesis decreased more in TIG than in RR in high sunlight, because of larger stomatal limitations and the transient impairment of PSII photochemistry. The methylerythritol 4-phosphate pathway promoted above all the synthesis and de-epoxidation of violaxanthin-cycle pigments in TIG and of neoxanthin and lutein in RR. This enabled the green leaves to process the excess radiant energy effectively, and the red leaves to optimize light harvesting and photoprotection. The greater stomatal closure observed in TIG than in RR was due to enhanced abscisic acid (ABA) glucose ester deglucosylation and reduced ABA oxidation, rather than to superior de novo ABA synthesis. This study shows a strong competition between anthocyanin and flavonol biosynthesis, which occurs at the level of genes regulating the oxidation of the C2-C3 bond in the dihydro-flavonoid skeleton. [ABSTRACT FROM AUTHOR]

Phytochromes are sensory photoreceptors associated with the photomorphogenesis of plants. Rice has 3 phytochromes (phy); phyA, phyB and phyC. To understand the role of phyA under low light (LL) and normal light (NL) environments in plant growth, photosynthesis, biomass and yield, rice phytochrome A mutant (phyA) along with its wild-type (WT) genotype Akitakomachi were grown in field conditions. To induce LL (∼338 μmole photons m−2 s−1) both WT and phyA were shaded by the agro-shade net that reduced light intensity by ∼75%. The NL intensity (∼1355 μmole photons m−2 s−1) served as the experimental control. The plant height decreased by 8%−10% in phyA mutants both in NL and LL. Other developmental parameters, leaf area index, specific leaf weight, and spikelet fertility declined in the mutants in NL and to a larger extent in the LL. phyA had a lower electron transport rate (ETR), which increased its non-photochemical quenching (NPQ). The photosynthetic rate (P N), stomatal conductance(g s), transpiration rate (E), apparent quantum yield (AQE), maximum carboxylation efficiency (Vcmax) and mesophyll conductance (gm) significantly decreased in phyA than WT in LL. Furthermore, the expression of rice phytochrome-interacting factor-like protein (OsPIL1) was relatively downregulated in phyA under LL, resulting in a reduced total chlorophyll (Chl) and Chl b content and an increased Chl a/b ratio. Additionally, a relatively lower upregulation of F v /F m and expression of the isoforms of chlorophyll-a/b-proteins in phyA under LL suggests its inferior light-harvesting capacity compared to WT plants. Reduced P N resulted in a lowered overall carbon budget in phyA that hampered grain yield by 55% more than WT under LL. Our findings demonstrated the critical role of phytochrome A in orchestrating a series of shade-acclimation responses to salvage LL-induced stress and optimize the harvest index in rice. • Compared to wild-type (WT), photosynthetic rate, stomatal conductance, electron transfer rate and quantum yield are significantly reduced in phyA mutant under low light (LL) than normal light (NL) conditions. • Grain yield reduction in phyA mutant (55%) was much higher compared to its WT lines (25%) under LL. • Phytochrome A may have a possible key role in modulating shade-acclimation responses in rice necessary to optimize yield loss under LL conditions. [ABSTRACT FROM AUTHOR]