Blue and red light-enrichments induce dwarfism in Hydrangea macrophylla through molecular regulation of gibberellin pathway and of cell wall extensibility, without deleterious effects on photosynthetic parameters

Hydrangea macrophylla is a popular ornamental plant, and controlling its growth is essential for producing compact branched pot plants with abundant inflorescences. Plant growth regulators (PGR) enable growth control, but environmental concerns and their phasing out require eco-friendly alternatives for dwarfing plants. Light spectrum manipulation has shown potential for growth control in numerous species, but photomorphogenic responses vary greatly with genotypes and cultural conditions. Two light spectra were applied on two H. macrophylla cultivars, Nanping and Wudu, to reduce stem elongation: one enriched in blue light (B-enr) and one enriched in red and blue lights (RB-enr). Both spectra significantly reduced stem elongation in both cultivars, with an efficiency comparable to daminozide, a commonly used PGR. Transcriptional analysis revealed that B-enr, RB-enr and daminozide treatments up-regulated genes related to cell wall modification (HmPGX3, HmPE1, HmEXPB2, HmCESA5 and HmKNAT7) and gibberellin metabolism and signaling pathways, while down-regulating genes associated with secondary growth (HmXYP1, HmSAMS1 and HmCAD1). These results suggest that light spectra impacted the same regulatory pathway of stem elongation as daminozide. However, unlike daminozide, both light spectra enhanced photosynthetic processes, increasing chlorophyll index, stomatal conductance and maximum photosystem II efficiency. This study describes two effective light spectra as alternatives to chemical dwarfing in H. macrophylla and provides novel insights into the molecular mechanisms influencing growth responses to light quality. It also brings with first evidence in a plant species of a light control over KNAT7, a key regulator of cell expansion and secondary cell wall thickening.