Transcriptome-metabolome convergence reveals that BsPP2C17 and BsPP2C27 of Bletilla striata (Orchidaceae)...

Orchidaceae seeds are tiny and undeveloped, commonly facing difficulties in germination under natural conditions. Therefore, investigating seed germination of orchids is critically important. In this study, Bletilla striata seed germination was completed, the germination process was divided into six stages through morphological and histocytological observation, including ungerminated seeds (BS0), imbibition (BS1), formation of rhizoids (BS2), protocorm (BS3), radicle (BS4) and plumule (BS5). LC-MS analysis showed that ABA concentration increased and then decreased, while auxins and cytokinins concentration gradually decreased, and various carbohydrates contents gradually increased. Exogenous ABA treatment showed that ABA significantly inhibited the germination of B. striata. RNA-seq analysis found 323 genes were significantly differentially expressed during protocorms formation (p < 0.01), with more than 4-fold change. Two genes in ABA signaling pathway, BsPP2C17 and BsPP2C27, were upregulated by 30-fold and 60-fold, respectively. Heterologous expression in Arabidopsis showed that they could respectively revert the insensitivity to ABA of atap2c4 and atpp2c5 during seed germination, and the overexpression lines Col-OE-BsPP2C17 and Col-OE-BsPP2C27 exhibited hypersensitivity to ABA signals. These demonstrated that BsPP2C17 and BsPP2C27 were involved in the regulation of Arabidopsis seed germination as positive regulators of ABA signaling. These results provided a new perspective for the study of seed germination in Orchidaceae.