Appropriate boron application promotes cotton fiber elongation by regulating the metabolism of cell wall biosynthesis, osmolyte accumulation, and cell wall loosening and remodeling

Boron is an essential trace element for sustaining normal growth and development in cotton. Appropriate boron application promotes fiber length, but its underlying mechanism remains unclear. To explore this, a soil column experiment with three boron fertilizer rates (0 kg B ha −1 , as a control; 2.3 kg B ha −1 , optimal boron quantity; 4.6 kg B ha −1 , boron excess) was conducted. Results indicated that compared with the control, optimal boron application increased the maximum fiber elongation velocity, thereby promoting fiber elongation. More specifically, optimal boron application enhanced cellulose synthase (CesA) activity as well as the expression of GhCesA3/6 , GhGAUT1/8/10 , and GhXXT1/2 , which facilitated the biosynthesis of cellulose, pectin, and hemicellulose in the primary cell wall, respectively, thereby promoting fiber elongation. Meanwhile, optimal boron application increased the accumulation of sucrose, hexose, and K + , consequently promoting cell turgor pressure and fiber elongation. Moreover, optimal boron application augmented exo-β-1,4-glucanase activity to accelerate cellulose hydrolysis, increased pectin methylesterase (PME) activity and GhPME1/2 expression to promote pectin de-esterification, and upregulated GhXTH6/7/8 expression to mediate hemicellulose cleavage during the process of primary cell wall loosening and remodeling. Simultaneously, optimal boron application upregulated the expression of GhEXPA1/2/4 encoding expansins. These changes facilitated fiber primary cell wall loosening and remodeling to promote fiber length. This study is the first to uncover the physiological mechanisms of boron regulating fiber length, laying a foundation for efficient boron utilization in cotton production. • Boron promoted fiber length by increasing the maximum fiber elongation velocity. • Boron enhanced the biosynthesis of cellulose, pectin, and hemicellulose in fibers. • Boron increased sucrose, hexose, and K + accumulation in cotton fibers. • Boron facilitated fiber cell wall loosening and remodeling to promote fiber length.