miRNA Regulation Contributes to Increased Photosynthetic Capacity in Autotetraploid Pak Choi

Polyploidy typically confers a stronger growth advantage compared to diploid ancestors, attributed to enhanced photosynthesis in polyploid plants. However, the molecular basis of the enhanced photosynthesis remains poorly understood. This study aimed to compare the diploid and autotetraploid Pak choi (Brassica rapa ssp. chinensis), an important leafy vegetable widely consumed in Asia, at physiological, cellular, and molecular levels. Our findings revealed that autotetraploid Pak choi leaves were thicker with larger intercellular space and cell size, in addition to higher numbers of thylakoids and basal granular stacks. Photosynthetic data demonstrated significantly higher net photosynthesis (Pn) in autotetraploid plants compared to diploids. Transcriptome and miRNA sequencing revealed that the differentially expressed genes were significantly enriched in the thylakoid pathway of photosynthesis, suggesting that thylakoids may play a key role in enhancing autotetraploid photosynthetic ability. Furthermore, integrated analysis indicated that the increased photosynthesis was associated with elevated expression of BcLhcbs due to downregulation of novel-miRNA117 and Bc-miR403-5p. The downregulation of BcLhcb1, BcLhcb2, and BcLhcb4 induces a reduction in net photosynthesis in Pak choi, where silenced plants manifest visibly stunted growth in comparison to their normal counterparts. To ascertain the functional roles of BcLhcbs in diverse plant species, we engineered Arabidopsis plants with overexpression of BcLhcb1, BcLhcb2, and BcLhcb4. This overexpression confers enhanced growth and increased biomass to Arabidopsis, attributed to augmented starch granules and a tightly organized arrangement of thylakoid grana. Subsequently, we observed a diminished growth trend in Arabidopsis upon the overexpression of novel-miRNA117 and Bc-miR403-5p, mirroring the phenotype observed in silenced Pak choi. This underscores the conservancy of the regulatory influence of novel-miRNA117 on BcLhcb1 and BcLhcb4, as well as Bc-miR403-5p on BcLhcb2. These discoveries significantly contribute to a more profound comprehension of growth advantages associated with polyploidy.