PSII Activity Was Inhibited at Flowering Stage with Developing Black Bracts of Oat

Abstract

The color of bracts generally turns yellow or black from green during cereal grain development. However, the impact of these phenotypic changes on photosynthetic physiology during black bract formation remains unclear. Two oat cultivars (Avena sativa L.), ‘Triple Crown’ and ‘Qinghai 444’, with yellow and black bracts, respectively, were found to both have green bracts at the heading stage, but started to turn black at the flowering stage and become blackened at the milk stage for ‘Qinghai 444’. Their photosynthetic characteristics were analyzed and compared, and the key genes, proteins and regulatory pathways affecting photosynthetic physiology were determined in ‘Triple Crown’ and ‘Qinghai 444’ bracts. The results show that the actual PSII photochemical efficiency and PSII electron transfer rate of ‘Qinghai 444’ bracts had no significant changes at the heading and milk stages but decreased significantly (p < 0.05) at the flowering stage compared with ‘Triple Crown’. The chlorophyll content decreased, the LHCII involved in the assembly of supercomplexes in the thylakoid membrane was inhibited, and the expression of Lhcb1 and Lhcb5 was downregulated at the flowering stage. During this critical stage, the expression of Bh4 and C4H was upregulated, and the biosynthetic pathway of p-coumaric acid using tyrosine and phenylalanine as precursors was also enhanced. Moreover, the key upregulated genes (CHS, CHI and F3H) of anthocyanin biosynthesis might complement the impaired PSII activity until recovered at the milk stage. These findings provide a new insight into how photosynthesis alters during the process of oat bract color transition to black.

Features of Activity of the Phenylpropanoid Biosynthesis Pathway in Melanin-Accumulating Barley Grains

“…Besides effect of melanogenesis on phenylpropanoid metabolism, downregulation of genes associated with photosynthesis in stage 2 was revealed in the BLP line compared to Bowman. This finding is consistent with a recent report on oat bracts where the inhibition of photosynthesis was demonstrated at the flowering stage during melanin pigmentation development ( Liu et al, 2021 ). In barley, underexpression of the genes related to photosynthesis was observed in the study at the late milk stage, before appearance of melanins in the grain.…”

Transcriptional convergence after repeated duplication of an amino acid transporter gene leads to the independent emergence of the black husk/pericarp trait in barley and rice

“…In addition to its evolutionary significance, the identification of HvBlp in this study also marked a significant advance in our understanding of the genetic and metabolic pathways of melanin biosynthesis in plants. In addition to HvBlp and OsBh4 , a homologous gene to OsBh4 has also been shown to be upregulated in black‐coloured oats (Liu et al ., 2021 ). It would be interesting for future studies to examine the genetic basis and evolutionary origin of the black husk/pericarp trait in this species.…”

“…The putative function of HvBlp and OsBh4 as potential tyrosine transporters is consistent with a recent transcriptome and metabolome study (Glagoleva et al ., 2022 ), which highlighted the involvement of phenylpropanoid biosynthesis pathway in melanin production in black‐coloured barley. Similar reports have been made for melanin synthesis in Zanthoxylum bungeanum (Fei et al ., 2021 ) and oat (Liu et al ., 2021 ). Indeed, tyrosine has been shown as the precursor of all phenylpropanoids biosynthesis, which branched into different pathways for the production of flavonoids, melanins and other types of phenolic acids (Barros and Dixon, 2020 ; Cao et al ., 2021 ; Fei et al ., 2021 ; Singh et al ., 2021 ).…”