SlTWD1 Affects Leaf Growth Primarily via Cell Expansion and Contributes to Leaf and Fruit Size Control in Tomato.

Leaf growth is a major determinant of plant architecture and productivity. TWISTED DWARF1 (TWD1/FKBP42) is a conserved immunophilin involved in plant growth and development, but its function in tomato remains unclear. Here, we identified and characterized SlTWD1, the tomato homolog of TWD1. SlTWD1 is highly conserved among land plants and functionally complemented the developmental defects of an Arabidopsis thaliana cr-twd1 mutant. SlTWD1 was broadly expressed in vegetative and reproductive tissues. CRISPR/Cas9-mediated disruption of SlTWD1 caused severe dwarfism and reduced leaf and fruit size, whereas overexpression of SlTWD1 or a C-terminally truncated variant moderately promoted leaf growth. Cellular analyses showed that loss of SlTWD1 markedly reduced epidermal cell size, while overexpression increased cell size, indicating that SlTWD1 promotes leaf growth primarily through cell expansion. Expression changes in auxin- and cell cycle-related genes were also associated with SlTWD1 perturbation. These findings indicate that SlTWD1 functions as an evolutionarily conserved regulator of plant growth that contributes to leaf and fruit size control in tomato.