Dynamic Changes in Ascorbic Acid Content during Fruit Development and Ripening of Actinidia latifolia (an Ascorbate-Rich Fruit Crop) and the Associated Molecular Mechanisms.

Actinidia latifolia is one of the very few kiwifruit genotypes with extremely high ascorbic acid (AsA) content. However, a transcriptome atlas of this species is lacking. The accumulation of AsA during fruit development and ripening and the associated molecular mechanisms are still poorly understood. Herein, dynamic changes in AsA content at six different stages of A. latifolia fruit development and ripening were determined. AsA content of A. latifolia fruit reached 1108.76 ± 35.26 mg 100 g-1 FW at full maturity. A high-quality, full-length (FL) transcriptome of A. latifolia was successfully constructed for the first time using third-generation sequencing technology. The transcriptome comprises 326,926 FL non-chimeric reads, 15,505 coding sequences, 2882 transcription factors, 18,797 simple sequence repeats, 3328 long noncoding RNAs, and 231 alternative splicing events. The genes involved in AsA biosynthesis and recycling pathways were identified and compared with those in different kiwifruit genotypes. The correlation between the AsA content and expression levels of key genes in AsA biosynthesis and recycling pathways was revealed. LncRNAs that participate in AsA-related gene expression regulation were also identified. Gene expression patterns in AsA biosynthesis and metabolism exhibited a trend similar to that of AsA accumulation. Overall, this study paves the way for genetic engineering to develop kiwifruits with super-high AsA content.

Genome-Wide Identification of MYB Transcription Factors and Screening of Members Involved in Stress Response in Actinidia.

MYB transcription factors (TFs) play an active role in plant responses to abiotic stresses, but they have not been systematically studied in kiwifruit (Actinidia chinensis). In this study, 181 AcMYB TFs were identified from the kiwifruit genome, unevenly distributed on 29 chromosomes. The high proportion (97.53%) of segmental duplication events (Ka/Ks values less than 1) indicated that AcMYB TFs underwent strong purification selection during evolution. According to the conservative structure, 91 AcR2R3-MYB TFs could be divided into 34 subgroups. A combination of transcriptomic data under drought and high temperature from four AcMYB TFs (AcMYB2, AcMYB60, AcMYB61 and AcMYB102) was screened out in response to stress and involvement in the phenylpropanoid pathway. They were highly correlated with the expression of genes related to lignin biosynthesis. qRT-PCR analysis showed that they were highly correlated with the expression of genes related to lignin biosynthesis in different tissues or under stress, which was consistent with the results of lignin fluorescence detection. The above results laid a foundation for further clarifying the role of MYB in stress.

Characterization and functional validation of ß-carotene hydroxylase AcBCH genes in Actinidia chinensis.

Carotenoids are the pigment substances of yellow-fleshed kiwifruit, and among them ß-cryptoxanthin has only been detected in the brighter yellow-fleshed variety 'Jinshi 1'. ß-Carotene hydroxylase (BCH) catalyzes the formation of ß-cryptoxanthin and zeaxanthin, but its molecular characteristics and functions have not been fully explained. Here we isolated two ß-carotene hydroxylase genes, AcBCH1 and AcBCH2 from kiwifruit (Actinidia chinensis), and their relative expression levels exhibited a close correlation with the content of ß-cryptoxanthin. AcBCH1 catalyzed the formation of ß-cryptoxanthin when transformed into ß-carotene-accumulating yeast cells. Moreover, silenced expression of AcBCH1 in kiwifruit caused decreases in the contents of zeaxanthin, lutein, and ß-cryptoxanthin, and an increase in ß-carotene content. The content of ß-carotene decreased significantly after the AcBCH1/2 genes were overexpressed in tomato. The content of zeaxanthin increased and ß-carotene decreased in transgenic kiwifruit seedlings. The results will enrich our knowledge of the molecular mechanisms of carotenoid biosynthesis in kiwifruit.

Comparative analysis of flavonoids in white and red table grape cultivars during ripening by widely targeted metabolome and transcript levels.

The flavonoid metabolites were compared between red 'Summer Black' (SB) and white 'Shine Muscat' (SM) table grapes during fruit development based on widely targeted metabolome. A total of 134 flavonoids were identified in two cultivars, including 37 flavones, 33 flavonols, and 11 anthocyanidins, and so on. From young to veraison, the composition and the content of most flavonoids were decreasing in both cultivars but increased at maturation in SB. In general, SB has higher flavonoid compositions and content than SM during the whole fruit development, especially the content of anthocyanin after veraison. While the SM had higher content of flavonols such as quercetin, kaempferol and their derivatives. The expression of anthocyanin-related genes such as UFGT, OMT, GST, MATE, MYBA1, and MYBA2 was remarkably higher in SB than those in SM, which may attribute to higher anthocyanin content, while the higher expression of F3H and FLS resulted higher level of flavonols in SM. These results improve our understanding of flavonoid profiles and molecular mechanism in table grape cultivars.

Methylation of MYBA1 is Associated with the Coloration in "Manicure Finger" Grape Skin.

The "Manicure Finger" grape is notable for its fingerlike berries with a bright red top and yellow base; however, the mechanism underlying this color difference remains unknown. This study showed that the anthocyanin concentration and the expression levels of anthocyanin-related genes in the top skin were notably higher than those in the basal skin. The expression levels of DFR, UFGT, and GST were significantly correlated with the anthocyanin content. The promoters of the two VvUFGT alleles can be activated by VvMYBA1, which was verified by the yeast one-hybrid assay, the dual-luciferase reporter gene assay, and the electrophoretic mobility shift assay. Moreover, the methylation level of the VvMYBA1 promoter (-1488 to -1083 bp) in the top skin was significantly lower than that in the basal skin and was positively correlated with the anthocyanin content. Our data suggest that methylation levels of the VvMYBA1 promoter play a crucial role in regulating grape skin coloration.