Comprehensive physiological, cytological, and transcriptional regulatory analyses reveal the coloration mechanism of culm yellow-slot mutation in moso bamboo (Phyllostachys edulis)

The green tissue in the culm of bamboo, engaging in photosynthesis, supplies carbohydrates to support the rapid growth of its shoots, yet the molecular basis for the chlorosis of bamboo culm tissue has not been elucidated. In contrast to the wild-type moso bamboo (Phyllostachys edulis f. pubescens, MB) with integral-green internodes at culms, the mutant one (P. edulis f. luteosulcata, YM) has typical bicolor internodes with yellow slot and green wall. By pigment quantification and chloroplast ultrastructure investigation, we found that the chlorosis in YM slot was caused by chlorophyll a deficiency, defective stroma thylakoids and misalignment of grana thylakoids. Based on transcriptomic comparisons across yellow and green tissues, the expression of plentiful genes involved in chloroplast components were significantly deceased in the tissue of YM slot, including genes associated with chloroplast development and chlorophyll metabolism. Analysis of differentially expressed genes and a multilayered hierarchical gene regulatory network both suggest that the decreased expression of BBX15s (B-BOX ZINC FINGER PROTEIN 15s) is likely the underlying cause of thylakoid dysgenesis and chlorophyll deficiency in the YM slot. This was further corroborated by Yeast one-hybrid (Y1H) assays and transgenic validation experiments, which demonstrated that BBX15s can directly regulate the transcription of genes related to chloroplast development and chlorophyll synthesis. This study provides insight into the mechanism of moso bamboo culm chloroplast development and serves as a productive basis for further exploration of bamboo culm coloration.