Light-Regulated Reprogramming in Moss: SHMT1 Mediates Blue Light Enhancement of Cell Regeneration

Cellular reprogramming can be induced by external stimuli such as hormones and environmental stresses. No molecular relationships between light conditions and cellular reprogramming have been established. In this study, blue light (BL) was found to accelerate cellular reprogramming using protoplasts of Physcomitrium patens. To better understand the underlying molecular mechanism, single-cell RNA sequencing was performed on protoplasts that were either freshly isolated (CK), grown under BL (B24h) or white light (WL) for 24 h (W24h). The expression level of most reprogramming-related genes in B24h was higher than in W24h, particularly during the early stage of reprogramming. By Spearman correlation analysis between light- and reprogramming-related genes, and qRT-PCR screening, the serine hydroxymethyltransferase 1 (SHMT1) gene was identified. We found that the SHMT1 gene, upregulated by BL during early phase of regeneration, induced the expression of reprogramming-related genes. Protoplasts transiently or stably over-expressing SHMT1 and excised leaves from the stable SHMT1 Overexpression plants all exhibited accelerated regeneration. Mutants lacking SHMT1 function showed the opposite phenotype. Overall, our study revealed for the first time that BL promoted cellular reprogramming via upregulation of SHMT1 in plants.