A Study on the Cell Layer Patterns of a Citrus Periclinal Chimera Reveals β-Cryptoxanthin Regulation in Citrus Fruits

As a dietary provitamin A carotenoid, β-cryptoxanthin is more bioavailable than common carotenoids. However, β-cryptoxanthin is a minor carotenoid in most crops, except for certain citrus species. A chimera (OCC) is identified, which grows from the graft junction between mandarin (OOO) and grapefruit (CCC), and it exhibits a unique β-cryptoxanthin profile. OCC is a periclinal chimera with the L1 cell layer from OOO and L2 and L3 layers from CCC. The flavedo, pulp, and segment membranes of OCC are generated from all three cell layers, but the proportions of these cell layers differ; OOO:CCC (L1:L2/L3) is ≈1:4 in the flavedo, 1:1 in the pulp, and 1:3 in the segment membrane. A nucleus-localized transcriptional activator, MYB107, is identified, which regulates β-cryptoxanthin variation between OCC and its donors. MYB107 expression is closely associated with β-cryptoxanthin accumulation, and its overexpression in citrus calli and fruits enhance carotenoid biosynthesis and upregulate carotenogenic genes, whereas MYB107 expression interference demonstrates the opposite effects. MYB107 directly binds to and activates the promoters of β-carotene hydroxylase (BCH), positively regulating β-cryptoxanthin biosynthesis. These findings provide new insights into β-cryptoxanthin accumulation patterns in citrus, highlighting the potential to improve the nutritional and aesthetic value of citrus fruits.