The phosphate transporters LjPT4 and MtPT4 mediate early root responses to phosphate status in non mycorrhizal roots

Arbuscular mycorrhizal (AM) symbiosis improves host plant phosphorous (P) status and elicits the expression of AM-inducible phosphate transporters (PTs) in arbuscule-containing cells, where they control arbuscule morphogenesis and P release. We confirmed such functions for LjPT4 in mycorrhizal Lotus japonicus. Promoter-GUS experiments showed LjPT4 transcription not only in arbusculated cells but also in root tips, in the absence of the fungus: here LjPT4 transcription profile depended on the phosphate level. In addition, quantitative RT-PCR confirmed the expression of Lotus and Medicago truncatula PT4 in the tips of non-mycorrhizal roots. Starting from these observations, we hypothesized that AM-inducible PTs may have a regulatory role in plant development, irrespective of the fungal presence. Firstly, we focused on root development responses to different phosphate treatments in both plants demonstrating that phosphate starvation induced a higher number of lateral roots. By contrast, Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching. Phosphate starvation-induced genes and a key auxin receptor, MtTIR1, showed an impaired expression in mtpt4 plants. We suggest PT4 genes as novel components of the P-sensing machinery at the root tip level, independently of AM fungi.