Regulation of synchronism by abscisic-acid-responsive small noncoding RNAs during somatic embryogenesis in larch ( Larix leptolepis )

Low levels of abscisic acid (ABA) are associated with embryo asynchronism during somatic embryogenesis. Small non-coding RNAs (sncRNAs) are emerging as key regulators of embryogenesis. To investigate the roles of sncRNAs in regulating the synchronism of somatic embryogenesis in Larix leptolepis , we examined the endogenous “sncRNAome” in synchronous and asynchronous embryos. Non-redundant sRNAs 24 nt in length were overrepresented in both synchronous and asynchronous embryos, while the majority of redundant sRNAs were 24 nt long in synchronous embryos and 21 nt long in asynchronous embryos. Many more non-redundant unknown sRNAs were expressed in synchronous embryos than in asynchronous embryos, indicating that many sRNAs might participate in the regulation of synchronism during larch somatic embryogenesis. However, the proportion of sRNAs that were miRNAs was higher in asynchronous embryos than in synchronous embryos, and their expression was much higher in asynchronous embryos. Although the proportion of sRNAs that matched repeat regions (ra-siRNA) was higher in asynchronous embryos than in synchronous embryos, the ratio of 24-nt ra-siRNAs to total sRNAs was much lower in asynchronous embryos than in synchronous embryos. Further analysis suggested that miRNAs exemplified by miR156 might participate in the regulation of embryo synchronism in larch, which causes a large number of transcripts encoding differentiation-promoting factors to be down regulated. Therefore, we speculate that sncRNAs induced by ABA inhibit the precocious expression of differentiation-promoting factors, thereby regulating the synchronism of somatic embryogenesis.