Drought is considered to be a major threat to soybean production worldwide and yet our current understanding of the effects of drought on soybean productively is largely based on studies on above-ground traits. Although the roots and root nodules are important sensors of drought, the responses of these crucial organs and their drought tolerance features remain poorly characterized. The symbiotic interaction between soybean and rhizobia facilitates atmospheric nitrogen fixation, a process that provides essential nitrogen to support plant growth and development. Symbiotic nitrogen fixation is important for sustainable agriculture, as it sustains plant growth on nitrogen-poor soils and limits fertilizer use for crop nitrogen nutrition. Recent developments have been made in our understanding of the drought impact on soybean root architecture and nodule traits, as well as underpinning transcriptome, proteome and also emerging metabolome information, with a view to improve the selection of more drought-tolerant soybean cultivars and rhizobia in the future. We conclude that the direct screening of root and nodule traits in the field as well as identification of genes, proteins and also metabolites involved in such traits will be essential in order to gain a better understanding of the regulation of root architecture, bacteroid development and lifespan in relation to drought tolerance in soybean.

The symbiotic interaction between soybean plants and rhizobacteria can be severely affected by drought, which results in a reduction in symbiotic nitrogen fixation and ultimately decreased yields. The aim of our research was to determine whether symbiotically efficient rhizobia that can better tolerate soil water deficits can improve nodule performance in plants subjected to drought. Firstly, rhizobial strains were selected that exhibited differences in tolerance to salt (NaCl) or water deficit (PEG 6000). Sinorhizobium fredii strain SMH12 showed the highest tolerance to these treatments while Bradyrhizobium diazoefficiens strain WB74‐1 showed the lowest tolerance. Greenhouse‐grown Prima 2000 soybean plants were then inoculated with either SMH12 or WB74‐1 and subjected to two water deficit regimes. Different nodule and plant growth traits were determined, including nodule number, dry weight, water potential, and the accumulation of malondialdehyde and ureide. Plants inoculated with SMH12 had significantly more nodules under water deficit conditions than those inoculated WB74‐1, despite having lower root and shoot biomass. SMH12‐inoculated plants had higher nodule water potentials and lower malondialdehyde contents than the WB74‐1‐inoculated plants. These results demonstrate that inoculation of soybean plants with the more water deficit‐tolerant S. fredii strain improved nodule characteristics when plants were grown under water deficit conditions. However, these improved nodule characteristics do not always directly translate into better plant growth.

Plants are an effective and inexpensive host for the production of commercially interesting heterologous recombinant proteins. The Escherichia coli-derived glutathione reductase was transiently expressed as a recombinant model protein in the cytosol of tobacco plants using the technique of leaf agro-infiltration. Proteolytic cysteine protease activity progressively increased over time when glutathione reductase accumulated in leaves. Application of cysteine protease promoter-GUS fusions in transgenic tobacco identified a cysteine protease NtCP2 expressed in mature leaves and being stress responsive to be expressed as a consequence of agro-infiltration. Transgenic tobacco plants constitutively expressing the rice cysteine protease inhibitor oryzacystatin-I had significantly lower cysteine protease activity when compared to non-transgenic tobacco plants. Lower cysteine protease activity in transgenic plants was directly related to higher glutathione reductase activity and also higher glutathione reductase amounts in transgenic plants. Overall, our work has demonstrated as a novel aspect that transgenic tobacco plants constitutively expressing an exogenous cysteine protease inhibitor have the potential for producing more recombinant protein very likely due to reduced activity of endogenous cysteine protease activity.

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