GmSPX8, a nodule-localized regulator confers nodule development and nitrogen fixation under phosphorus starvation in soybean

Abstract

Background Biological nitrogen fixation (BNF) is an important nitrogen source for legume plants, and highly efficient nitrogen fixation requires sufficient phosphorus (P). However, the mechanism of maintaining nitrogen fixation of the legume nodules under low P concentration remains largely unknown. Results A nodule-localized SPX protein, GmSPX8, was discovered by transcriptome and functional analysis of its role in N2 fixation was characterized in soybean nodules. GmSPX8 was preferentially expressed in nodules and its expression was gradually increased during nodule development. And also the expression pattern was investigated using reporter gene β-glucuronidase (GUS) driven by the promoter of GmSPX8. GmSPX8 was greatly induced and the GUS activity was increased by 12.2% under P deficiency. Overexpression of GmSPX8 in transgenic plants resulted in increased nodule number, nodule fresh weight and nitrogenase activity by 15.0%, 16.0%, 42.5%, subsequently leading to increased N and P content by 17.0% and 19.0%, while suppression of GmSPX8 showed significantly impaired nodule development and nitrogen fixation efficiency under low P stress. These data indicated that GmSPX8 conferred nodule development and nitrogen fixation under low P condition. By yeast two-hybrid screening, GmPTF1 was identified as a potential interacting protein of GmSPX8, which was further confirmed by BiFC, Y2H and pull down assay. Transcript accumulation of GmPTF1 and its downstream genes such as GmEXLB1 and EXPB2 were increased in GmSPX8 overexpressed transgenic nodules, and in the presence of GmSPX8, the transcriptional activity of GmPTF1 in yeast cells and tobacco leaves was greatly enhanced. Conclusions In summary, these findings contribute novel insights towards the role of GmSPX8 in nodule development and nitrogen fixation partly through interacting with GmPTF1 in soybean under low P condition.

Milestones in understanding transport, sensing, and signaling of the plant nutrient phosphorus

“…Overexpressing either GmSPX5 or GmSPX8 increases the number and the fresh weight of nodules, resulting in higher N and P content in nodules than in the wild type ( Zhuang et al . 2021 ; Xing et al . 2022 ).…”

Understandings and future challenges in soybean functional genomics and molecular breeding

“…The SPX member SPX5 interacts with the transcription factor NF‐YC4 and enhances NF‐YC4 binding to the asparagine synthetase‐related gene ASL6 promoter to activate its transcription, which enhances soybean nodule development, particularly under Pi‐sufficient conditions (Zhuang et al, 2021). SPX8 interacts with PTF1 to promote nodule development and N 2 fixation under low‐P conditions (Xing et al, 2022). The β‐expansin INS1 promotes nodule development, plant biomass, and N‐ and P‐use efficiency under low‐P stress (Yang et al, 2021b).…”

“…Phosphorus is a non‐renewable macronutrient essential for plant growth, especially for legumes (e.g., soybean), which require high amounts of P to sustain symbiotic N fixation (Xue et al, 2017; Xing et al, 2022). Low‐P stress inhibits soybean growth and limits yield, including seed number, grain weight per plant, biomass, and root architecture (Hacisalihoglu et al, 2018; Yang et al, 2019; Miao et al, 2022).…”