Rhizobacteria and Arbuscular Mycorrhizal Fungi of Oil Crops (Physic Nut and Sacha Inchi): A Cultivable-Based Assessment for Abundance, Diversity, and Plant Growth-Promoting Potentials

Janjira Wiriya,Neung Teaumroong,Rungroch Sungthong,Saisamorn Lumyong,Chakrapong Rangjaroen

doi:10.3390/plants9121773

Abstract

Nowadays, oil crops are very attractive both for human consumption and biodiesel production; however, little is known about their commensal rhizosphere microbes. In this study, rhizosphere samples were collected from physic nut and sacha inchi plants grown in several areas of Thailand. Rhizobacteria, cultivable in nitrogen-free media, and arbuscular mycorrhizal (AM) fungi were isolated and examined for abundance, diversity, and plant growth-promoting activities (indole-3-acetic acid (IAA) and siderophore production, nitrogen fixation, and phosphate solubilization). Results showed that only the AM spore amount was affected by plant species and soil features. Considering rhizobacterial diversity, two classes—Alphaproteobacteria (Ensifer sp. and Agrobacterium sp.) and Gammaproteobacteria (Raoultella sp. and Pseudomonas spp.)—were identified in physic nut rhizosphere, and three classes; Actinobacteria (Microbacterium sp.), Betaproteobacteria (Burkholderia sp.) and Gammaproteobacteria (Pantoea sp.) were identified in the sacha inchi rhizosphere. Considering AM fungal diversity, four genera were identified (Acaulospora, Claroideoglomus, Glomus, and Funneliformis) in sacha inchi rhizospheres and two genera (Acaulospora and Glomus) in physic nut rhizospheres. The rhizobacteria with the highest IAA production and AM spores with the highest root-colonizing ability were identified, and the best ones (Ensifer sp. CM1-RB003 and Acaulospora sp. CM2-AMA3 for physic nut, and Pantoea sp. CR1-RB056 and Funneliformis sp. CR2-AMF1 for sacha inchi) were evaluated in pot experiments alone and in a consortium in comparison with a non-inoculated control. The microbial treatments increased the length and the diameter of stems and the chlorophyll content in both the crops. CM1-RB003 and CR1-RB056 also increased the number of leaves in sacha inchi. Interestingly, in physic nut, the consortium increased AM fungal root colonization and the numbers of offspring AM spores in comparison with those observed in sacha inchi. Our findings proved that AM fungal abundance and diversity likely rely on plant species and soil features. In addition, pot experiments showed that rhizosphere microorganisms were the key players in the development and growth of physic nut and sacha inchi.

Highlights

Physic nut (Jatropha curcas L.) and sacha inchi (Plukenetia volubilis L.) (Figure 1) belong to the same family, Euphorbiaceae, but different subfamilies—Crotonoideae for physic nut and Acalyphoideae for sacha inchi
The total counts of rhizobacteria dwelling on the surfaces of 50 g roots of physic nut or sacha inchi grown at different geographical locations (Table 1) were in a range of 106 colony-forming units (CFUs) (Figure 2)
It seems that soil fertility and physicochemical properties are the critical factors influencing the bacterial abundance in physic nut rhizospheres

Summary

Introduction

Physic nut (Jatropha curcas L.) and sacha inchi (Plukenetia volubilis L.) (Figure 1) belong to the same family, Euphorbiaceae, but different subfamilies—Crotonoideae for physic nut and Acalyphoideae for sacha inchi. Known as “sacha peanut, mountain peanut, or Inca peanut,” is a native species of South America and is being increasingly cultivated in northern Thailand and the Greater Mekong Sub-region [12]. This crop is produced mainly for human consumption rather than bioenergy purposes as its seeds are rich in protein, fatty acids (e.g., omega-3 and omega-6), and oil (30–60%) [12,13,14]. Sacha inchi oil has been renowned mostly for its nutritive and pharmaceutical values [12,13,14], and only a few studies reveal its potential for biofuel production [15,16]

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Discussion

Conclusion