Red light-induced citrus fruit colouration is attributable to increased
carotenoid metabolism regulated by the NAC transcription factor FcrNAC22
Abstract
Carotenoids play vital roles in the colouration of plant tissues and
organs, particularly fruits; however, the regulation of carotenoid
metabolism in fruits during ripening is largely unknown. Here, we show
that red light (RL) promotes fruit colouration by inducing accelerated
degreening and carotenoid accumulation in kumquat fruits. Transcriptome
profiling reveals that a NAC family transcription factor, FcrNAC22 is
specifically induced in RL-irradiated fruits. FcrNAC22 localizes to the
nucleus, and its gene expression is up-regulated as fruits change
colour. Results from dual luciferase, yeast one-hybrid (Y1H) assays and
electrophoretic mobility shift assays (EMSAs) indicate that FcrNAC22
directly binds to and activates the promoters of three genes (i.e.
FcrLCYB1, FcrBCH2 and FcrNCED5) encoding key enzymes in carotenoid
metabolic pathway. Moreover, FcrNAC22 overexpression in citrus and
tomato fruits as well as in citrus callus enhances expression of a dozen
of carotenoid biosynthetic genes including the aforementioned three,
accelerates plastid conversion into chromoplasts, and promotes colour
change. Knock down of FcrNAC22 expression in transient transformed
citrus fruits attenuates fruit coloration induced by red light. Taken
together, our results demonstrate that FcrNAC22 is an important
transcription factor that mediates RL-induced fruit colouration via
upregulation of carotenoid metabolism in citrus and perhaps in other
plants.