Genetic mechanisms for size- and light-dependent reproductive maturation in two maple species, <i>Acer amoenum</i> var. <i>amoenum</i> and <i>Acer mono</i> var. <i>glabrum</i>

BACKGROUND AND AIMS: Wild trees have a long prematuration period before flowering. Threshold tree size and light conditions are suspected to trigger the reproductive maturation, though genetic evidence is needed to confirm this. We investigated the genetic mechanisms underlying the onset of reproduction using two wild maple species, Acer amoenum var. amoenum (Aa) and A. mono var. glabrum (Am). METHODS: We performed RNA-seq on leaves and buds sampled from the canopy tops of wild juvenile and mature trees to capture RNA expression during bud formation. We identified genes whose expression levels were highly correlated with FLOWERING LOCUS T (FT) as well as with tree size and/or light conditions; such genes were designated as differentially expressed genes (DEGs). Genes significantly correlated with light were further examined through a light manipulation experiment. KEY RESULTS: Most DEGs were correlated with both tree size and light conditions, demonstrating clear expression changes between saplings and adult trees. In Am, DEG expression patterns in adult trees further relied on tree size and light conditions, unlike Aa, the shade-adapted species. DEGs included mRNA of several flowering pathway transcription factors, including SQUAMOSA promoter-binding protein-like, FRIGIDA-like, and COP1, indicating their involvement in the lifetime reproductive schedule of Acer trees. Some of these genes responded to light manipulation, demonstrating light-dependent adjustments. CONCLUSIONS: Overall, our results reveal a synchronous shift in flowering-related gene expression across tree size, along with interspecific differences correlated with their different light requirements.