Plastid development of albino viviparous propagules in the woody mangrove species of Kandelia obovata.

The process of plastids developing into chloroplasts is critical for plants to survive. However, this process in woody plants is less understood. Kandelia obovata Sheue, Liu & Yong is a viviparous mangrove species; the seeds germinate on the maternal tree, and the hypocotyls continue to develop into mature propagules. We identified rare albino propagules through field observation among normal green and brown ones. Toward unveiling the propagule plastid development mechanism, albino propagule leaves only have etioplasts, low photosynthesis rates, and drastically reduced chlorophyll a/b and carotenoid contents, but with increased superoxide dismutase activities. To identify candidate genes controlling propagule plastid development, a genome-wide association study (GWAS) was performed between the albino and green propagules. Twenty-five significant single nucleotide polymorphisms (SNPs) were associated with albino propagule plastid development, the most significant SNPs being located on chromosomes 1 and 5. Significant differentially expressed genes were identified in porphyrin and chlorophyll metabolisms, carotenoid and flavonoid biosynthesis by combining transcriptome and GWAS data. In particular, KoDELLAs, encoding a transcription factor and KoCHS, encoding chalcone synthase, may be essential to regulate the albino propagules plastid development through weakened chlorophyll and flavonoid biosynthesis pathways while promoting chlorophyll degradation. Our results provide insights into genetic mechanisms regulating propagule plastid development in woody plants.