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.

Pseudomonas syringae pv. actinidiae (Psa), a bacterial pathogen, causes bacterial canker disease in kiwifruit. To elucidate the local and systemic influences of Psa infection on kiwifruit, comprehensive analyses were conducted by combining metabolomic and physiological approach under Psa-infected treatment and control in leaves, stems and bleeding saps. Our results show that Psa infection stimulated kiwifruit metabolic reprogramming. Levels of sugars, fumarate, and malic acid were decreased in Psa-infected leaves and stems, accompanied by the increased level of amino acids (AAs), which is associated with energy metabolism and defense metabolism. Arg, Gln, and pyroglutamic acid systematically were accumulated in long-distance leaves, which probably confers to systemic acquired resistance (SAR). In situ zymography analysis showed that Psa-infection increased N-acetyl-glucosaminidases (chitinases) activity and was highest at the root tips, indicating Psa- inoculation accelerated the nitrogen (N) cycling in kiwifruit. Moreover, phenolic compounds were negatively and lignin was positively related to kiwifruit Psa-resistance, respectively. Our results first reveal that Psa enhances infection by manipulating carbon (C)/N metabolism and sweet immunity, and that host lignin synthesis is a major physical barrier for restricting bacterial infection. This study provides an insight into the complex remodeling of plant metabolic response to Psa stress.