Bevordering van de overleving van Ensifer meliloti in luzerne inoculanten

The rising population and increased demand for food causes pressure on agriculture to produce crops more abundant and more efficient. Unlike natural ecosystems, where the need in nutrients is low, crops demand more nutrients to produce biomass in a quick manner. Nitrogen is often a limiting factor for growth. To maximize the yield of crops in intensive agriculture, (artificial) fertilizer is added as an external nitrogen source. However, this is not an effective and sustainable solution for the nitrogen deficiency. Alternatively certain micro-organisms, called rhizobia, have the ability to provide plants with nitrogen. Rhizobia are soil bacteria which are capable to live in symbiosis with legumes such as soybean and common bean. The bacteria can colonize roots and nodules form in which they fix atmospheric nitrogen and pass it to the plant. In soil of fields rhizobial populations are often not abundant and the nitrogen fixation capacity is low on average. Thereby a sufficient nitrogen supply to crops due to rhizobia cannot be ensured. For this reason inoculants are used in agriculture; which are bacterial cultures that are added to the plant to promote plant growth/health. Rhizobia selected on their nitrogen fixing capacity can be applied to the root environment in high numbers by the use of inoculants. Although inoculants can be directly added just before sowing the soil, inoculation by seed coating is more efficient as the rhizobia are concentrated around the seeds. However, seed inoculation in not always effective due to inconsistent quality of the inoculants. During inoculant production, coating of the seeds with the inoculant and the storage of the treated seeds, the rhizobia are exposed to adverse environmental conditions. Mainly desiccation causes the rhizobia to die before sowing en thereby lose their positive effect on the plant. In order to benefit from the effect of the rhizobia, seeds need to be sown just after coating; however due to logistic reasons a longer storage period is often preferred. In this research project different techniques were employed to increase the number of viable rhizobia on seeds by making the bacteria more resistant. Different additives were added during the growth of the rhizobia to test if these molecules could prepare and/or protect the bacteria against the harsh conditions the are exposed to during the inoculation process. Furthermore we used the natural diversity of the rhizobia. A collection of strains from different locations was made and used for their potential use in inoculant. Finally evolution was employed to select for natural mutations that increase the tolerance against desiccation of the rhizobia. By implementing these techniques, we could significantly increase the shelf life of the rhizobia on the seeds and obtained insight of mechanisms that are involved in resistance against desiccation.

Publication year:2020

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