Arbuscular mycorrhizal symbiosis alters the expression of PHT1 phosphate transporters in roots and nodules of P-starved soybean plants

Roots acquire phosphorus (P) as orthophosphate (Pi) through phosphate transporters of the PHT1 family with different affinities to Pi, a process significantly influenced by arbuscular mycorrhizal (AM) symbiosis. However, scarce P availability may constrain nodulation and performance in legumes. Soybean ( Glycine max ) is an ideal model to study tripartite symbiosis between roots, AM fungi and rhizobia. To evaluate AM influence on GmPHT1 expression in roots and nodules, Bradyrhizobium elkanii -inoculated soybean plants were exposed to low Pi concentration (50 µM) via nutrient solution and inoculated (+ AM) or not (− AM) with the AM fungus Glomus macrocarpum . Control treatment consisted of non-inoculated plants grown under sufficient P conditions (500 µM; − AM + P). Plants were collected at the flowering and grain filling stages. Under P-starvation, mycorrhizal plants showed low intraradical colonization and did not differ in terms of biomass, nodulation and P content from the non-mycorrhizal plants, indicating strong P-limitation and no AM-related growth promotion. However, the expression profile of PHT1 transporters in roots and nodules was effectively altered by mycorrhization. P-starvation induced the expression of several GmPHT1 genes in roots and nodules, while AM symbiosis repressed GmPHT1;6 , 7 and 10 in roots and GmPHT1;3 , 5 , 7 , 8 , and 10 in nodules. Therefore, even under low levels of root colonization, AM symbiosis significantly modulated the pattern of PHT1 expression under P-starvation. GmPHT1 expression profile suggests different pathways of Pi acquisition in mycorrhizal and non-mycorrhizal plants during P-starvation, however, the low mycorrhizal colonization was not able to deliver adequate P nutrition to the plant.