GA20ox1 -mediated GA4 production promotes inflorescence stem growth in Arabidopsis via inner-layer cell proliferation

Elongation of flowering stems is crucial for successful sexual reproduction and seed dispersal, yet its cellular mechanisms remain largely unknown. Traditionally, it has been assumed that new cells supplied by meristematic tissues sustain stem growth. However, our stem growth and cell proliferation assays revealed that active cell division also occurred in differentiated cells below the rib zone. Given the essential role of gibberellin (GA) in promoting stem elongation, we compared cell growth between wild-type and GA-deficient mutant plants. In the upper 0.0–1.0 cm of the stem, GA-deficient mutants showed reduced cell proliferation and unexpectedly enlarged cells, despite the well-known role of GA in cell expansion. This finding implies that cell proliferation is the primary driver of GA-dependent stem elongation. To investigate how GA biosynthesis is regulated during stem growth, we conducted phytohormone and gene expression assays using juvenile seedlings, dissected shoot apices, and stems. The data suggested that the production of bioactive GA4 was upregulated in the upper stem through the induction of the GA-biosynthetic gene GIBBERELLIN 20 OXIDASE 1 and its spatial coexpression with the downstream gene GIBBERELLIN 3 OXIDASE 1. Our results provide detailed insight into GA-dependent inflorescence stem elongation, with implications for other eudicots, including crop species.