Carotenoid and transcriptome profiles of a novel citrus cultivar ‘Jinlegan’ reveal mechanisms of yellowish fruit formation

Abstract

Citrus fruit coloration is one of the vital quality traits that is determined primarily by the composition and content of carotenoids. Natural citrus fruit pigment mutants are available to study diverse and complex carotenoid metabolism. Here, ‘Jinlegan’ (MT) tangor is a spontaneous bud mutant derived from ‘Shiranuhi’ (WT) with distinctive bright yellow fruit. High performance liquid chromatography (HPLC) analysis revealed that the yellowish MT flavedo and pulp were primarily caused by the decrease in total carotenoid content. The total carotenoid content in MT flavedo was reduced by 75% (79.98 μg/g DW) compared with that in WT (318.40 μg/g DW), including approximately 84%, 80%, and 60% reductions in the contents of β-cryptoxanthin, violaxanthin and zeaxanthin, respectively. The total carotenoid content in MT pulp was 60% lower (10.09 μg/g DW) than that in WT pulp (26.61 μg/g DW), which was mainly due to a 70% and 30% decrease in the contents of β-cryptoxanthin and zeaxanthin, respectively. To explore the molecular mechanism underlying carotenoid variation in MT, RNA-seq analyses were performed on the flavedo and pulp of WT and MT at five developmental stages. The reduced expression of phytoene synthase (CrPSY) and β-carotenehydroxylase 1 (CrBCH1) in the flavedo and pulp of MT at the breaker stage might be the major cause of the reduction in carotenoids. Weighted gene co-expression network analysis (WGCNA) further identified 23 key transcription factors that are closely associated with carotenoid accumulation. This study demonstrated a comprehensive picture of the metabolic and transcriptional alterations of a unique yellowish citrus fruit mutant, which provides new insights into the molecular regulation of carotenoid accumulation in citrus fruit.