Transcriptional reprogramming during Garcinia-type recalcitrant seed germination of Garcinia mangostana

Mangosteen (Garcinia mangostana L.) is an apomictic tropical fruit tree species which asexually produces recalcitrant seeds that are sensitive to desiccation and not viable for long-term storage. It is important to understand seed germination to inform strategy for the propagation and conservation of important plants, especially for recalcitrant species. However, little is known about the molecular physiology of tropical seed germination. Garcinia-type seed germination is unique such that the primary root and shoot emerge from the opposite ends, which is akin to in vivo direct organogenesis. For the first time, time-course transcriptome-wide gene expression during mangosteen seed germination from day 0 to day 7 was performed through RNA-sequencing with RT-qPCR validation using independent samples in triplicates. The analyses of differentially expressed genes (DEGs) and expression profile identified D3 as a key turning point for transcriptional reprogramming with an indication of a transient shift to anaerobic respiration. Particularly, the activation of ethylene signalling through increased expression of genes involved in its biosynthesis and the upregulation of ethylene-responsive transcription factors lead to the induction of direct organogenesis concurrently with seed germination. Abscisic acid (ABA) signalling appeared to play a role in stress response while gibberellin (GA) provides growth potential during mangosteen seed germination. The antagonistic relationship between ABA and GA is conserved in mangosteen as manifested by global gene expression changes based on the Arabidopsis co-expression network analysis. The current transcriptomic study comprehensively describes various molecular aspects of Garcinia-type recalcitrant seed germination and highlights similarities to in vitro somatic embryogenesis but without a differentiated embryo. This study provides insights to improve seed germination for the mass propagation of mangosteen.