Abstract text: Plants exhibit different heights and morphologies under varying light conditions, such as differences in stem length. Light plays a crucial role in regulating the thickening and elongation of stem cell walls during stem growth and development. We found that the blue light receptor cryptochrome1 (CRY1) transmits blue light signals to MYC2/MYC4, which directly binds to the NST1 promoter, promoting NST1 gene expression and thus thickening the fiber cell walls while inhibiting their elongation in stems. Conversely, the red-light signaling module formed by the red and far-red light receptors phytochrome B (PhyB) and phytochrome-interacting factor 4 (PIF4) interferes with the binding of MYC2/MYC4 to the NST1 promoter, inhibiting fiber cell wall thickening. Additionally, the PhyB and PIF4 molecular module directly regulates the expression of the transcription factor BR ENHANCED EXPRESSION 2 (BEE2), promoting fiber cell wall elongation. Thus, the molecular regulatory network composed of blue and red light signals synergistically regulates the thickening and elongation of cell walls in stems, shaping the characteristics of fiber cells and the height of the plant.