Diversity of soyabean root nodule bacteria recovered from Zimbabwean soils

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Zimbabwe has a long history of soyabean breeding programmes that have developed many improved soyabean varieties with various disease tolerances; and high yields, up to 5t/ha. Soyabean can depend on symbiotic nitrogen fixation (SNF) with root nodule bacteria for their entire nitrogen requirements. In comparison to use of inorganic nitrogen fertilizers, SNF is comparably inexpensive and environmentally friendly. Effective soyabean root nodule bacteria (RNB) populations in Zimbabwean soils are too erratic to be depended on for economic yields of soyabean. Inoculating soyabean seed with elite root nodule bacteria at planting, is a cheap insurance for effective SNF. For more than thirty years, Zimbabwean inoculants for soyabean, at the Soil Productivity Research Laboratory, where Mazvita works and currently leads inoculant production, carry the elite strain MAR 1491. However, the strain has been shown to exhibit limited saprophytic competence.

Indigenous rhizobia are expected to show greater resilience to the agro-climatic conditions of Zimbabwe, since it is their origin. In the present study, we prospected for indigenous root nodule bacteria from smallholder farms and soyabean breeding facilities, seeking superior adaptation to agro-climatic conditions and higher nitrogen fixation capacity, to recommend for use as new rhizobia inoculant strains.

One hundred and thirty-seven isolates were recovered and characterized for their morphology on culture medium. All isolates were slow-growing and fell in to one of two categories based on the wet or dry colonies. Partial sequences of six genes were compared for their polymorphisms using twenty isolates, and recA gene was found to be most informative. Subsequently, all 137 isolates were submitted to partial recA gene sequencing. This distinguished the isolates into four species of Bradyrhizobium, with only 13 % of isolates identifying with the species that was inoculated, B. diazoefficiens. Sixty percent of all isolates recovered are B. elkanii which was never inoculated into the soils and five percent belongs to B. diazoefficiens species, which has only been reported in Canada and Brazil. There was greater diversity on the smallholder farms than on the soyabean breeding facilities, despite that there were more isolates recovered from the latter. Representatives of the four species were submitted to further tests, including host-range tests and an interesting pattern of effective and ineffective nodulation or lack of nodulation was found with Crotalaria juncea, Vigna radiata and Phaseolus vulgaris.

All isolates were screened for nitrogen fixation, in batches, and in comparison to the elite standard strain MAR 1491, under glasshouse conditions. Isolates were highly variable in their nitrogen fixation, between and across species. The best two isolates from each species were submitted to field testing at three sites in Zimbabwe. B. elkanii isolates had the highest nodulation while the highest nitrogen fixation was generated by B. diazoefficiens and decreased in the order B. japonicum > B. elkanii > B. ottawaense. Isolates NAZ 710, NAZ 629 and NAZ 626 are recommended for use as rhizobia inoculants for soyabean in Zimbabwe. Mazvita has spent the last year putting together a thesis for examination.

Mazvita Chiduwa, pursuing a PhD with Murdoch University, Australia, supervised by Ravi Tiwari, John Howieson, Julie Ardley, Graham O’Hara and Paul Mapfumo, in the final stages of writing up her thesis. (Click here for her 2017 update)