Cytological and Transcriptomic Profiling Reveals the Developmental Basis of Stem Diameter Variation in Luffa

Stem diameter is an important agronomic trait determining plant architecture, lodging resistance and yield formation, yet its cytological and molecular mechanisms in the major cucurbit crop Luffa remain largely unclear. Herein, two Luffa inbred lines, S1174 (Luffa acutangula, thin stem) and P93075 (Luffa cylindrica, thick stem), were analyzed. Dynamic phenotyping uncovered a sigmoidal growth pattern with three key developmental stages: initiation (0 d), rapid growth (6 d), and maturation (20 d), during which P93075 consistently exhibited a significantly larger stem diameter than S1174 from 6 d onward. Anatomical analysis showed that P93075 displayed greater parenchyma cell size and vascular bundle area than S1174, suggesting that stem diameter variation is closely associated with parenchyma cell expansion and vascular bundle enlargement. Transcriptome profiling further identified 2611 differentially expressed genes (DEGs), mainly enriched in cell wall organization, phenylpropanoid biosynthesis, and hormone signaling pathways. Through temporal expression patterns of shared DEGs across developmental phases and co-expression network analysis, LacEXT3, encoding a cell wall extensin protein, was prioritized as a promising candidate gene associated with stem radial growth. Consistent with its markedly higher expression in P93075, heterologous overexpression of LacEXT3 in Arabidopsis resulted in increased stem diameter accompanied by reduced plant height. Overall, this study provides new insights into the developmental, cytological, and transcriptomic landscape underlying stem diameter variation in Luffa and offers a valuable resource for future investigation of the molecular mechanisms of stem thickening.