Abstract
Legume biological nitrogen (N) fixation is the most important N source in agroecosystems, but it is also a process requiring a considerable amount of phosphorus (P). Therefore, developing legume varieties with effective N(2) fixation under P-limited conditions could have profound significance for improving agricultural sustainability. We show here that inoculation with effective rhizobial strains enhanced soybean (Glycine max) N(2) fixation and P nutrition in the field as well as in hydroponics. Furthermore, we identified and characterized a nodule high-affinity phosphate (Pi) transporter gene, GmPT5, whose expression was elevated in response to low P. Yeast heterologous expression verified that GmPT5 was indeed a high-affinity Pi transporter. Localization of GmPT5 expression based on β-glucuronidase staining in soybean composite plants with transgenic roots and nodules showed that GmPT5 expression occurred principally in the junction area between roots and young nodules and in the nodule vascular bundles for juvenile and mature nodules, implying that GmPT5 might function in transporting Pi from the root vascular system into nodules. Overexpression or knockdown of GmPT5 in transgenic composite soybean plants altered nodulation and plant growth performance, which was partially dependent on P supply. Through both in situ and in vitro (33)P uptake assays using transgenic soybean roots and nodules, we demonstrated that GmPT5 mainly functions in transporting Pi from roots to nodules, especially under P-limited conditions. We conclude that the high-affinity Pi transporter, GmPT5, controls Pi entry from roots to nodules, is critical for maintaining Pi homeostasis in nodules, and subsequently regulates soybean nodulation and growth performance.
Highlights
Legume biological nitrogen (N) fixation is the most important N source in agroecosystems, but it is a process requiring a considerable amount of phosphorus (P)
Compared with the total P content, soybean fresh weight was much less affected by the changes of P supply (Fig. 2C)
Leaf and root soluble Pi concentrations in high P were 10 and 26 times higher than in low P, respectively, while the Pi concentration in nodules was only increased 1.75 times in high P versus low P (Fig. 2D). This suggests that stabilizing Pi homeostasis in nodules under P-deficient conditions might be important for legume growth and symbiotic N2 fixation
Summary
Legume biological nitrogen (N) fixation is the most important N source in agroecosystems, but it is a process requiring a considerable amount of phosphorus (P). Overexpression or knockdown of GmPT5 in transgenic composite soybean plants altered nodulation and plant growth performance, which was partially dependent on P supply. Through both in situ and in vitro 33P uptake assays using transgenic soybean roots and nodules, we demonstrated that GmPT5 mainly functions in transporting Pi from roots to nodules, especially under P-limited conditions. We conclude that the high-affinity Pi transporter, GmPT5, controls Pi entry from roots to nodules, is critical for maintaining Pi homeostasis in nodules, and subsequently regulates soybean nodulation and growth performance. Developing legume varieties with effective N2 fixation under P-limited conditions will be important to reduce N and P fertilization and enhance agricultural sustainability. To the best of our knowledge, none of the Pi transporters have been reported to be involved in the P nutrition of the legume-rhizobia symbiosis system
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