The interaction between Arbuscular mycorrhizal fungi, rhizobia and root-lesion nematodes (Pratylenchus thornei) in mung bean (Vigna radiata)

The interaction between Arbuscular mycorrhizal fungi, rhizobia and root-lesion nematodes (Pratylenchus thornei) in mung bean (Vigna radiata)

There are a limited number of reports on interactions between the beneficial microsymbionts arbuscular mycorrhizal fungi (AMF) and rhizobia which co-occur with the root-lesion nematode Pratylenchus sp. within the roots of legumes. Mung bean (Vigna radiata) is an important summer legume in the sub-tropical grain region of eastern Australia. It is a host of AMF, Pratylenchus thornei and nitrogen (N) fixing Bradyrhizobium bacteria. These microorganisms are dependent on mung bean for photosynthates and their interactions influence host production and nutrition. Nodulation failure in mung bean reduces plant production, nutrition and N budgets in soils and could be explained by a lack of mycorrhizal inoculum in the soil and/or by infestation with P. thornei. Furthermore, AMF colonisation of the roots may alter the population densities of P. thornei in mung bean.

Initially, in this thesis, a systematic review was carried out to clarify the effect of interactions between AMF and Pratylenchus spp., which showed that their interactions depended on the taxonomic order and genus of AMF, along with host plant functional groupings. With this specificity in mind, the interaction of AMF, rhizobia and P. thornei was investigated for mung bean cv. Jade-AU grown in a vertisol with a full factorial of these biological treatments in glasshouse experiments.

In the first study, AMF and rhizobia acted synergistically, increasing nodulation, biological N fixation, nutrition, growth and seed yield. These positive effects were complicated by P. thornei. Nodulation was reduced by P. thornei infestation which negatively impacted N fixation efficiency. However, mycorrhizal colonisation conferred tolerance to P. thornei which was indicated by maintained plant biomass. Unexpectedly, the population density of P. thornei increased in mycorrhizal mung bean, and the population density was positively correlated with concentrations of phosphorus (P), zinc (Zn) and copper (Cu) in the mung bean shoot.

Therefore, investigations were undertaken to elucidate the role of nutrients behind (i) improved nodulation and biological nitrogen fixation when mung bean was co-inoculated with AMF and rhizobia and (ii) increased population densities of P. thornei in mycorrhizal mung bean. A full factorial experiment included treatments of AMF, rhizobia, and P. thornei, with N, P and Zn fertilisers. It was shown that AMF increased (i) nodulation and N fixation to a level equal to or greater than the application of fertiliser P, (ii) concentrations of P and Zn in the shoot, greater than the application of fertiliser alone and, (iii) the concentration of Cu in the shoot. Rhizobia and/or AMF conferred improvements greater than the addition of fertiliser N, including increased nodulation, shoot N concentration, biomass and yield greater than when rhizobia alone was added; and increased biomass and yield when AMF and rhizobia were both added. Increased population densities of P. thornei in mycorrhizal mung bean was again demonstrated but the application of fertilisers N, P and Zn decreased P. thornei. This result suggested the role of other mechanisms of increased susceptibility, such as AMF may decrease concentrations of defensive compounds against P. thornei in the roots or that AMF colonised roots may provide organic compounds that nutritionally stimulate P. thornei reproduction.

The research presented in this thesis contributes to understanding the complex multipartite interactions that occur between microorganisms in the roots mung bean and their impacts on N fixation, nutrition, biomass and yield. The conservation of AMF within farming systems is strongly advocated to promote and protect their valuable role in increasing biological nodulation and N fixation efficiency by rhizobia, and in improved crop nutrition and yield, while reducing fertiliser inputs. However, it is also crucial to understand that AMF may increase population densities of P. thornei. Agronomic practices and plant breeding to promote the synergism between AMF and rhizobia for mung bean yield, while limiting population densities of P. thornei will benefit mung bean production and subsequent crops in long-term sustainable farming systems.

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