Comparative analysis of morphology, cytology, and mitochondrial transcriptome of sterile male flowers of rubber tree

Rubber tree [Hevea brasiliensis (Willd. ex A. Juss.) Mull. Arg.] is the exclusive commercial source of natural rubber. Male-sterile (MS) germplasm has proven to be valuable for rubber tree hybrid breeding, but its male sterility mechanism remains unclear. Here, morphological observations of a new MS clone Reyan93-114 showed shriveled anthers, short filaments, and withered male flowers. Transmission electron microscopy revealed the tapetal cells of the anthers were hypertrophic and vacuolated. Microspore protoplasts reduced in size, and the microspore wall had obvious defects: the incomplete intine and the insufficiently deposited exine. Organelles had disappeared in both tapetal cells and microspores. Comparative mitochondrial transcriptome analysis identified three differentially expressed genes in the MS clones: orf126, orf51, and atp8. In particular, orf126 and orf51 were unique insertions in the MS mitochondrial genomes. The former was an unknown open reading frame, and the latter was a fusion gene that contained a portion of atp9. These two genes should be studied as candidate male sterility genes in rubber tree. These results clarified the cytological causes of male sterility and expanded to understand the relationship between mitochondrial genes and male sterility in rubber tree.