Culm sheath: A key site for photoassimilate and IAA synthesis during the growth of young culms in Fargesia yunnanensis

The rapid growth of bamboo involves photosynthetic assimilates and hormonal regulation, with the role of culm sheaths remaining poorly understood. This study investigated the hypothesis that the culm sheath is a key site for both photoassimilate production and IAA biosynthesis, thereby coordinately regulating the growth of young culms in F. yunnanensis . By applying exogenous sucrose to culm sheath blades to mimic enhanced photoassimilate supply, we integrated physiological measurements, targeted hormone metabolomics, and transcriptome profiling. Sucrose application to culm sheath blades significantly promoted internode elongation in young bamboo shoots by increasing tissue water pressure, while upregulating cell wall loosening-related gene expression to accelerate cellular expansion. Concurrently, sucrose enhanced soluble sugar content through elevated sucrose-degrading enzyme activity, thereby promoting glycolytic and TCA cycle metabolic flux to provide energetic support for rapid growth. Crucially, we discovered that culm sheath blades are not merely suppliers of carbohydrates but also active sites for IAA biosynthesis. Sucrose application markedly increased the expression levels of key IAA biosynthesis genes in culm sheath blades while promoting IAA transport to developing internodes through upregulated transporter gene expression. The application of IAA biosynthesis inhibitors to the culm sheath blade significantly reduced the shoot elongation rate by 33.3% and culm IAA levels by 12.1%. In contrast, application to the culm sheath body resulted in more substantial inhibition, with a 46.7% reduction in shoot elongation rate and a 27.3% decrease in culm IAA levels. In contrast, sucrose inhibited stress-related hormone (ABA and JA) biosynthesis, alleviating their growth-restrictive effects. This study demonstrates that culm sheath blades supply both photosynthetic assimilates and IAA, thereby regulating culm elongation through photoassimilate-mediated IAA biosynthesis. • Culm sheaths served as crucial sites for both photosynthesis and IAA synthesis. • These sheaths governed internode elongation by mediating hormone synthesis. • Sucrose upregulated IAA biosynthesis while suppressing stress-related hormone production.