RESUMEN
Flavonoids constitute the main nutraceuticals in the leaves of tea plants (Camellia sinensis). To date, although it is known that drought stress can negatively impact the biosynthesis of flavonoids in tea leaves, the mechanism behind this phenomenon is unclear. Herein, we report a protein phosphorylation mechanism that negatively regulates the biosynthesis of flavonoids in tea leaves in drought conditions. Transcriptional analysis revealed the downregulation of gene expression of flavonoid biosynthesis and the upregulation of CsMPK4a encoding a mitogen-activated protein kinase in leaves. Luciferase complementation and yeast two-hybrid assays disclosed that CsMPK4a interacted with CsWD40. Phosphorylation assay in vitro, specific protein immunity, and analysis of protein mass spectrometry indicated that Ser-216, Thr-221, and Ser-253 of CsWD40 were potential phosphorylation sites of CsMPK4a. Besides, the protein immunity analysis uncovered an increased phosphorylation level of CsWD40 in tea leaves under drought conditions. Mutation of the three phosphorylation sites generated dephosphorylated CsWD403A and phosphorylated CsWD403D variants, which were introduced into the Arabidopsis ttg1 mutant. Metabolic analysis showed that the anthocyanin and proanthocyanidin content was lower in ttg1:CsWD403D transgenic plants than ttg1::CsWD403A transgenic and wild type plants. The transient overexpression of CsWD403D downregulated the anthocyanidin biosynthesis in tea leaves. The dual-fluorescein protein complementation experiment showed that CsWD403D did not interact with CsMYB5a and CsAN2, two key transcription factors of procyanidins and anthocyanidins biosynthesis in tea plant. These findings indicate that the phosphorylation of CsWD40 by CsMPK4a downregulates the flavonoid biosynthesis in tea plants in drought stresses.
RESUMEN
BACKGROUND: Laccase (LAC) is the pivotal enzyme responsible for the polymerization of monolignols and stress responses in plants. However, the roles of LAC genes in plant development and tolerance to diverse stresses are still largely unknown, especially in tea plant (Camellia sinensis), one of the most economically important crops worldwide. RESULTS: In total, 51 CsLAC genes were identified, they were unevenly distributed on different chromosomes and classified into six groups based on phylogenetic analysis. The CsLAC gene family had diverse intron-exon patterns and a highly conserved motif distribution. Cis-acting elements in the promoter demonstrated that promoter regions of CsLACs encode various elements associated with light, phytohormones, development and stresses. Collinearity analysis identified some orthologous gene pairs in C. sinensis and many paralogous gene pairs among C. sinensis, Arabidopsis and Populus. Tissue-specific expression profiles revealed that the majority of CsLACs had high expression in roots and stems and some members had specific expression patterns in other tissues, and the expression patterns of six genes by qRTâPCR were highly consistent with the transcriptome data. Most CsLACs showed significant variation in their expression level under abiotic (cold and drought) and biotic (insect and fungus) stresses via transcriptome data. Among them, CsLAC3 was localized in the plasma membrane and its expression level increased significantly at 13 d under gray blight treatment. We found that 12 CsLACs were predicted to be targets of cs-miR397a, and most CsLACs showed opposite expression patterns compared to cs-miR397a under gray blight infection. Additionally, 18 highly polymorphic SSR markers were developed, these markers can be widely used for diverse genetic studies of tea plants. CONCLUSIONS: This study provides a comprehensive understanding of the classification, evolution, structure, tissue-specific profiles, and (a)biotic stress responses of CsLAC genes. It also provides valuable genetic resources for functional characterization towards enhancing tea plant tolerance to multiple (a)biotic stresses.