The PhyB-PIF4 module directly photoregulates BR-ENHANCED EXPRESSION genes to enhance stem fiber cell elongation in Arabidopsis.

Fiber cells are a primary component of vascular tissue in angiosperms, offering mechanical support for most aerial structures, reinforcing vascular transport functions, and enabling vertical growth. Our previous research discovered light as a vital signal in modulating fiber cell-wall thickening and thus length and strength during maturation. Here, we reveal that the red light-sensing Phytochrome B (PhyB)/Phy-Interacting Factor-4 (PIF4) module plays a major photoregulatory role in Arabidopsis stem fiber growth upon activation through its negative influence on the downstream transcription factors encoded by the BR-ENHANCED EXPRESSION (BEE) gene family. Whereas stem fiber cells in the phyB mutant hyperelongate, those in the pif quadruple mutant, which blocks the expression of PIF1-4, are shorter. DNA binding studies showed that this PhyB/PIF module photoregulates BEE2 expression by directly interacting with G-box-like elements within its promoter. Accordingly, fibers in the triple bee123 mutant are shorter, whereas stems overexpressing BEE2 develop longer fibers. BEE2 in particular then encourages fiber cell elongation by directly enhancing expression of genes associated with cell-wall loosening, two being the xyloglucan endotransglycosylases/glucanases XTH8 and XTH33. Our studies identify a key multicomponent photoregulatory cascade in fiber cell elongation, thus offering possible avenues for engineering stem properties by modulating light signals.