Objectives We investigated the effects of phosphorus (P) supply level and inoculation of arbuscular mycorrhizal fungus (AMF) strains on the growth, phosphorus (P) uptake, and the expression of mycorrhiza-induced phosphate transporter and sucrose transporter genes in soybean genotypes with different P efficiency, as well as the relationship of AMF and P efficiency.

Methods A pot experiment was conducted using P-efficient soybean genotype Baxi 10 (BX10) and P-inefficient genotype Bendi No. 2 (BD2) as experimental materials. The treatment was composed of two P supply levels (low P, and high P), and exogenous inoculation of two AMF strains, Rhizophagus irregularis (Ri) or Rhizophagus custos (Rc), taking no AMF (NM) inoculation as the controls. After 6 weeks of treatment, the plant dry weight, P uptake, root-to-shoot ratio, mycorrhizal infection rate, root traits, and the expression of mycorrhiza-induced phosphate transporter genes and sucrose transporter gene were analyzed.

Results Compared with low P, high P significantly increased the shoot and root dry weights as well as P uptake in soybean, reduced the mycorrhizal growth response (MGR) and mycorrhizal P response (MPR) of both genotypes of soybean. Under low P condition, both Ri and Rc significantly increased the shoot and root dry weights and P uptake of both the soybean genotypes, increased the total root length, root surface area, and root volume of the P-efficient genotype BX10. The expression of the phosphate transporter gene GmPT9, which is primarily responsible for P uptake via the mycorrhizal pathway, was significantly upregulated by AMF inoculation in both soybean genotypes as well. The expression of mycorrhiza-induced GmPT8 was also significantly upregulated in P-inefficient genotype BD2. However, there was no significant difference in the expression of GmPT4, which is primarily responsible for P uptake via root pathway, between the treatments with and without AMF inoculation. The root dry weight and root-to-shoot ratio of BX10 were significantly higher than those of BD2 under both low P and high P conditions without AMF inoculation, and the average root diameter of BX10 was significantly higher than BD2 under high P conditions without AMF inoculation. Under low P conditions with AMF inoculation, the total root length, root surface area, and root volume of BX10 were significantly higher than BD2. Under high P conditions with AMF inoculation, the root surface area and root volume of BX10 were also significantly higher than BD2. Therefore, the shoot and root dry weights and P uptake of BX10 inoculated with Ri were significantly higher than BD2. However, the expression levels of GmPT8, GmPT9, and GmSWEET6 in BD2 roots were significantly higher than those in BX10 under low P conditions with Ri inoculation.

Conclusion High P supply significantly promoted plant growth and improved plant P uptake in two soybean genotypes, whereas the MGR and MPR of the two genotypes were all higher in low P than in high P. Inoculation with AMF further increased the shoot and root dry weights as well as P uptake in BX10 at high P, but did not have that effect in BD2. Under low P condition, AMF inoculation could simultaneously activate both root and mycorrhizal pathways to uptake P, and thereby significantly increased the shoot and root dry weights as well as P uptake in both soybean genotypes. The promotion of root growth by AMF inoculation in BX10 was greater than in BD2. The relative expression levels of mycorrhiza-induced phosphate and sugar transporter genes could not directly reflect the amount of P and sugar allocation in the mycorrhizal symbiotic system.