Effect of Rhizobium Symbiosis on Low-Temperature Tolerance and Antioxidant Response in Alfalfa (Medicago sativa L.).

Xu-Yang Sha,Jin-Cai Geng,Yu-Shi Liu,Yi-Xin Zhao,Pei-Zhi Yang,Tian-Ming Hu

doi:10.3389/fpls.2019.00538

Xu-Yang Sha, Jin-Cai Geng + Show 4 more

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https://doi.org/10.3389/fpls.2019.00538

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Abstract

Low temperature-induced stress is a major environmental factor limiting the growth and development of plants. Alfalfa (Medicago sativa L.) is a legume well known for its tolerance of extreme environments. In this study, we sought to experimentally investigate the role of rhizobium symbiosis in alfalfa’s performance under a low-temperature stress condition. To do this, alfalfa “Ladak+” plants carrying active nodules (AN), inactive nodules (IN), or no nodules (NN) were exposed to an imposed low temperature stress and their survivorship calculated. The antioxidant defense responses, the accumulation of osmotic regulation substances, the cell membrane damage, and the expression of low temperature stress-related genes were determined in both the roots and the shoots of alfalfa plants. We found that more plants with AN survived than those with IN or NN under the same low temperature-stress condition. Greater activity of oxidation protective enzymes was observed in the AN and IN groups, conferring higher tolerance to low temperature in these plants. In addition, rhizobia nodulation also enhanced alfalfa’s ability to tolerate low temperature by altering the expression of regulatory and metabolism-associated genes, which resulted in the accumulation of soluble proteins and sugars in the nodulated plants. Taken together, the findings of this study indicate that rhizobium inoculation offers a practical way to promote the persistence and growth potential of alfalfa “Ladak+” in cold areas.

Highlights

Alfalfa (Medicago sativa L.) is a widely cultivated forage crop of substantial economic value that possesses excellent agricultural traits, in that its roots can fix atmospheric nitrogen molecules with the help of symbiotic rhizobia (Hou et al, 2013; Quan et al, 2016; Zhang W. et al, 2017)
When the seedlings reached a height of 10 cm, they were randomly divided into three groups: (I) active nodules (AN): alfalfa plants inoculated with the Rhizobium meliloti strain Dormal to form AN, supplemented with 1/4 strength nitrogen-free Hoagland solution (Hoagland and Arnon, 1950) daily; (II) inactive nodules (IN): alfalfa inoculated with the same Rhizobium meliloti strain Dormal as the AN group, but watered daily with 1/4 strength Hoagland solution (Wang et al, 2016); (III) no nodules (NN): alfalfa without rhizobia inoculation, watered daily with 1/4 strength Hoagland nutrient solution
No significant difference in aboveground biomass was observed among the three groups alfalfa (Supplementary Figure S2)

Summary

Introduction

Alfalfa (Medicago sativa L.) is a widely cultivated forage crop of substantial economic value that possesses excellent agricultural traits, in that its roots can fix atmospheric nitrogen molecules with the help of symbiotic rhizobia (Hou et al, 2013; Quan et al, 2016; Zhang W. et al, 2017). The endogenous nitrogen pool accumulated in the root system may enhance the cold-tolerance ability. Rhizobium Symbiosis and Low Temperature of this plant (Dhont et al, 2006). Little research has investigated the effect of rhizobium symbiosis on alfalfa’s tolerance of low temperatures. Earlier work demonstrated that inoculated rhizobia improved the productivity and survival of legumes under low temperature conditions (Prévost et al, 1999, 2003). Elevated CO2 has been shown to stimulate rhizobiuminoculation alfalfa growth and to reduce this plant’s freezing tolerance (Bertrand et al, 2007)

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