Objectives To identify soybean xyloglucan endotransglycosylases/hydrolases (XTHs) gene family, analyze their expression pattern and responses to low phosphorus (P) stress, and preliminarily explore the role of soybean XTH38 in root growth.

Methods The tested crops were soybean cultivar Yuechun 03-3 (YC03-3), and the wild type of Arabidopsis thaliana called Columbia (Columbia-0, Col-0) ecotype. A total of 61 XTH genes were identified in soybean by bioinformatics methods, and evolutionary analysis of soybean XTH family members was performed. Using the hydroponics method, two nutrient solutions of high P (HP, control) and low P (LP) were set, and soybean seedlings were treated for 14 days. The roots and leaves of soybean seedlings were sampled, and the expression level of 5 GmXTHs in the seedlings was analyzed by quantitative real time PCR. The transgenic method revealed the function of GmXTH38 in regulating the main root growth and lateral root formation of Arabidopsis thaliana under nutrient stress conditions such as LP, low iron (LFe) and high iron (HFe).

Results Bioinformatics analysis revealed that soybean XTH family had 61 members and 3 different subgroups, among which GmXTH38 was in the same subgroup as AtXTH9 and AtXTH23. Quantitative real time PCR was used to explore responses of GmXTH to low P stress. We found that GmXTH family members had different expression patterns in soybean organs or tissues, among which GmXTH28, GmXTH38, GmXTH41 and GmXTH52 were induced by LP. GmXTH38 in soybean roots and leaves were induced by LP. In contrast to HP, the activities of GmXTH38 promoter in Arabidopsis seedling roots and leaves was enhanced in LP. Ectopic overexpression of GmXTH38 revealed that under full nutrient conditions, namely high P or middle iron, overexpression of GmXTH38 inhibited primary root growth, number of lateral roots, and decreased density of lateral root. Under LP, LFe or HFe stress, compared with the wild Col-0, the overexpression of GmXTH38 in Arabidopsis resulted in shortened primary root, decrease in lateral roots and density of lateral roots. In contrast to Col-0, overexpression of GmXTH38 resulted in higher sensitivity of lateral root formation to low P. Overexpression of GmXTH38 resulted in a more pronounced decrease in Arabidopsis density of lateral root under LFe or HFe stress. Overexpression of GmXTH38 increased the sensitivity of Arabidopsis primary root growth to LFe or HFe.

Conclusions A total of 61 XTH genes (GmXTH1–GmXTH61) were identified in soybean genome. GmXTH38 was significantly induced by low P in roots and leaves. Overexpression of GmXTH38 in Arabidopsis inhibits primary root growth and lateral root formation in full nutrient conditions. Overexpression of GmXTH38 in Arabidopsis alters root growth in response to phosphorus or iron stress.