Arbuscular Mycorrhizal Fungus Improves Rhizobium–Glycyrrhiza Seedling Symbiosis under Drought Stress

Zhipeng Hao,Xin Zhang,Xuelian Jiang,Zhaoxiang Wu,Baodong Chen,Wei Xie

doi:10.3390/agronomy9100572

Abstract

Rhizobia and arbuscular mycorrhizal (AM) fungi can potentially alleviate the abiotic stress on the legume Glycyrrhiza (licorice), while the potential benefits these symbiotic microbes offer to their host plant are strongly influenced by environmental factors. A greenhouse pot experiment was conducted to investigate the effects of single and combined inoculation with a rhizobium Mesorhizobium tianshanense Chen and an AM fungus Rhizophagus irregularis Walker & Schuessler on Glycyrrhiza uralensis Fisch. seedling performance under different water regimes. Drought stress inhibited rhizobium nodulation but increased mycorrhizal colonization. Furthermore, co-inoculation of rhizobium and AM fungus favored nodulation under both well-watered and drought stress conditions. Glycyrrhiza seedling growth showed a high mycorrhizal dependency. The seedlings showed a negative growth dependency to rhizobium under well-watered conditions but showed a positive response under drought stress. R. irregularis-inoculated plants showed a much higher stress tolerance index (STI) value than M. tianshanense-inoculated plants. STI value was more pronounced when plants were co-inoculated with R. irregularis and M. tianshanense compared with single-inoculated plants. Plant nitrogen concentration and contents were significantly influenced by inoculation treatments and water regimes. R. irregularis inoculation significantly increased plant shoot and root phosphorus contents. AM fungus inoculation could improve Glycyrrhiza plant–rhizobium symbiosis under drought stress, thereby suggesting that tripartite symbiotic relationships were more effective for promoting plant growth and enhancing drought tolerance.

Highlights

The legume Glycyrrhiza is widely used as a medicinal herb and as an industry material due to the large amount of glycyrrhizin, an important bioactive triterpenoid saponin derived from licorice roots and stolons [1,2]
R. irregularis inoculated plants showed mycorrhizal colonization, which was significantly increased by 8.7% and 33.2% under drought stress compared with the well-watered conditions in plants inoculated without and with M. tianshanense, respectively
The co-inoculation of M. tianshanense with mycorrhizal fungus favored the nodulation with a nodule number that is 6.1-fold and nodule dry weight that is 6.4-fold higher compared with single inoculation with M. tianshanense under well-watered conditions

Summary

Introduction

The legume Glycyrrhiza (licorice) is widely used as a medicinal herb and as an industry material due to the large amount of glycyrrhizin, an important bioactive triterpenoid saponin derived from licorice roots and stolons [1,2]. Glycyrrhiza plants are cultivated to restore degraded ecosystems, in arid and semi-arid regions [3]. Drought stress and nutrient deficiency are the two main factors limiting the growth and production of Glycyrrhiza seedling cultivation [4]. Plants have evolved a series of mechanisms to cope with these stresses. Agronomy 2019, 9, 572 the mutualism of plant and microbe could affect plant growth, nutrient uptake, and resistance to abiotic and biotic stresses, including drought stress [5]. Rhizobia and arbuscular mycorrhizal (AM) fungi could form symbiosis with legumes and improve plant mineral nutrition, especially nitrogen (N) and phosphorus (P) nutrition [6].

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Discussion

Conclusion