Integrated cytological, physiological, and transcriptome analysis of the bud mutant of jujube (Ziziphus jujuba Mill.) with non-lethal albino phenotype

Although the underlying mechanisms of the albino phenotype have been deciphered in seedlings of model plants, leaf albinism of bud mutants in woody species has been less extensively studied. In this study, a bud mutant of jujube (Ziziphus jujuba Mill.) with non-lethal albino phenotype was characterized. Compared with the normal leaves, the albino leaves showed lower chlorophyll and carotenoid contents, and higher POD activity and proline content to respond to oxidative damage. The impaired chloroplasts with ruptured thylakoid membranes and vacuoles with excess autophagosomes were observed in albino leaves. The decreased photosynthetic capacity in albino leaves was manifested as negative net photosynthetic rate and reduced chlorophyll fluorescence. The molecular mechanism of the albino leaves during development was explored through transcriptome analysis. A total of 3213, 3348, 8072 differentially expressed genes (DEGs) were identified in leaf buds, young leaves and mature leaves of normal plant and albino mutant, respectively. Further analysis indicated that most DEGs related to the biosynthesis of chlorophyll and carotenoid, chloroplast development, and photosynthesis were down-regulated in albino leaf buds, which were likely the main factors responsible for albino phenotype at early stage. Contrarily, DEGs involved in chloroplast development, including PPR, FtsH, mTERF, were mainly up-regulated in albino mature leaves, suggesting different regulatory mechanism for defective chloroplast structure at various stages. In addition, DEGs were significantly enriched in biosynthesis of phenylpropanoids and flavonoids in young and mature leaves of the albino mutant Moreover, transcription factor family genes, involving bHLH, MYB_related, NAC, ERF, B3, and WRKY, were identified as DEGs at three stages, indicating the promoted secondary metabolism production and senescence of the albino leaves. These findings provided new insights into the molecular mechanism of leaf albinism in woody plants.