Fig. 8.

A model integrating DELLAs and AM symbiosis. In Pi-deprived plants, DELLA transcript levels are high (Fig. S3) (71), and DELLA promotes arbuscule formation and hyphal growth in the cortex (Figs. 1 and 3–5) through the activation of as yet unknown transcription factors (TFs). Arbuscule development and the resulting symbiotic Pi transport leads to an increase in Pi levels in the root (94), resulting in a decrease in DELLA transcript levels (Fig. S3) (71). Genes of GA biosynthesis and GA inactivation are induced during symbiosis, indicating complex regulation of bioactive GA levels (Fig. 2D) (35). GA levels rise in the roots (40), leading to the degradation of DELLA protein. A decrease in DELLA (transcripts and protein) regulates arbuscule development through negative feedback. In plants with a high Pi status, low levels of DELLA proteins result in minimal arbuscule formation.