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

## TL;DR

A citrus chimera's cell layer patterns reveal how β-cryptoxanthin, a beneficial carotenoid, is regulated in citrus fruits.

## Contribution

The study identifies MYB107 as a key regulator of β-cryptoxanthin biosynthesis in citrus fruits.

## Key findings

- MYB107 is a nucleus-localized transcriptional activator that regulates β-cryptoxanthin accumulation.
- MYB107 overexpression enhances carotenoid biosynthesis and upregulates carotenogenic genes.
- MYB107 directly activates the promoter of β-carotene hydroxylase (BCH), promoting β-cryptoxanthin biosynthesis.

## Abstract

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.

Researchers have identified a citrus chimera with distinctive tissue origins and carotenoid profiles. It is discovered that fruit tissues develop from all three cell layers but in different proportions. A key transcription factor, MYB107, regulates β‐cryptoxanthin production by directly activating carotenoid biosynthesis genes, explaining why some citrus fruits develop deep orange coloration while others remain lighter.