Spatiotemporal profiles of gene activity in stamen delineate nucleo-cytoplasmic interaction in a male sterile somatic cybrid citrus

Cytoplasmic male sterility (CMS) has long been used to produce seedless fruits in perennial woody crops like citrus. A male sterile somatic cybrid citrus (G1+HBP) was generated from protoplast fusion between a CMS callus parent ‘Guoqing No. 1’ Satsuma mandarin (Citrus unshiu, G1) and a fertile mesophyll parent Hirado Buntan pummelo (Citrus grandis, HBP). To uncover the male sterile mechanism of G1+HBP, we compared the transcriptome profiles of stamen organ and cell types at five stages between G1+HBP and HBP, including the initial stamen primordia, enlarged stamen primordia, pollen mother cells, tetrads and microspores captured by laser microdissection (LMD). The stamen organ and cell types showed distinct gene expression profiles. A majority of genes involved in stamen development were differentially expressed, especially CgAP3.2 that was downregulated in enlarged stamen primordia and upregulated in tetrads of G1+HBP compared with HBP. The jasmonic acid (JA) and auxin related biological processes were enriched among the differential expression genes (DEGs) of stamen primordia, and the content of JA biosynthesis metabolites was higher in flower buds and anthers of G1+HBP. In contrast, the content of auxin biosynthesis metabolites was lower in G1+HBP. The mitochondrial tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS) processes were enriched among the DEGs in stamen primordia, meiocytes and microspores, indicating the dysfunction of mitochondria in stamen organ and cell types of G1+HBP. Taken together, the malfunction of mitochondria-nuclear interaction might cause disorder in stamen development, and thus lead to male sterility in the citrus cybrid.