During arbuscular mycorrhizal (AM) symbiosis, activation of a symbiosis signaling pathway induces gene expression necessary for accommodation of AM fungi. Here, we focus on pathway components Medicago truncatula INTERACTING PROTEIN OF DOES NOT MAKE INFECTIONS 3 (IPD3) and IPD3 LIKE (IPD3L), which are potential orthologs of Lotus japonicus CYCLOPS, a transcriptional regulator essential for AM symbiosis. In the double mutant ipd3 ipd3l, hyphal entry through the epidermis and overall colonization levels are reduced relative to the wild type but fully developed arbuscules are present in the cortex. In comparison with the wild type, colonization of ipd3 ipd3l is acutely sensitive to higher phosphate levels in the growth medium, with a disproportionate decrease in epidermal penetration, overall colonization, and symbiotic gene expression. When constitutively expressed in ipd3 ipd3l, an autoactive DOES NOT MAKE INFECTIONS 3 induces the expression of transcriptional regulators REDUCED ARBUSCULAR MYCORRHIZA 1 and REQUIRED for ARBUSCULE DEVELOPMENT 1, providing a possible avenue for arbuscule development in the absence of IPD3 and IPD3L. An increased sensitivity of ipd3 ipd3l to GA3 suggests an involvement of DELLA. The data reveal partial redundancy in the symbiosis signaling pathway, which may ensure robust signaling in low-phosphorus environments, while IPD3 and IPD3L maintain signaling in higher-phosphorus environments. The latter may buffer the pathway from short-term variation in phosphorus levels encountered by roots during growth in heterogeneous soil environments.
Keywords: mycorrhizal interactions.