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

Abstract

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.