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Extending Soybean Lifespan

Final Report Summary - SOYLIFE (Extending Soybean Lifespan)

Plant performance and the predictability of crop yield are severely hampered by environmental factors. In particular, abiotic stresses threaten food security world-wide adversely affecting the lives of millions. The SoyLife project addressed this problem by combining skills and expertise in protease-protease inhibitor technology and its applications for crop improvement (Prof. K. Kunert, University of Pretoria, South Africa; incoming EU fellow) with plant stress physiology and metabolism (Prof. C. Foyer, Centre of Plant Sciences at Leeds University, UK; host lab) to enhance the lifespan and yield of legume crops. The aim of the project was to provide new and useful insights into the factors limiting lifespan of the leaves and the nitrogen-fixing root nodules of soybean, which is currently the 4th most important crop worldwide.
This project combined state of the art ‘omics’ technologies and whole plant physiology, to determine whether the ectopic expression of naturally occurring cysteine protease inhibitors in soybean, would change protein turnover and hence the lifespan and productivity of soybean plants during the natural senescence process or the accelerated senescence of organs that is induced be exposure to environmental abiotic stresses, such as drought and low nitrogen. These studies have demonstrated that that ectopic expression of a rice cysteine protease inhibitor, called oryzacystatin (OC-I) enhances the productivity and stress tolerance of soybean and also in the model plant species Arabidopsis thaliana by inhibiting endogenous cysteine protease activities. Most importantly, the ectopic expression of OC-I led to an increase in the protein content of the soybean seeds. It also resulted in alterations in the shoot and root morphology of the soybean plants such that the transformed lines had a much higher number of axillary shoot branches, particularly at the later stages of vegetative growth. Moreover, the leaves of the transformed soybean lines had a longer life span retaining the photosynthetic pigment chlorophyll for longer after flowering that the wild type plants. The OC-I expressing plants were less susceptible to water loss when exposed to drought conditions and they were also able to maintain higher rates of photosynthesis than the wild type.
These studies demonstrated that the effect of ectopic expression of OC-I on shoot and root traits was linked to changes in the metabolism and signaling of a group of plant hormones called strigolactones. Strigolactones are important carotenoid-derived hormones that interact with other plant hormones such as auxin to regulate a range of plant processes such as shoot and root branching, stress tolerance and symbiotic interactions with other organisms.
Research undertaken in this project also showed that plants grown under low soil nitrogen had increased levels of transcripts encoding vacuolar processing enzymes (VPE). However, the low nitrogen-induced increases in VPE mRNAs was significantly lower in the transgenic lines expressing OC-I than the wild type plants. The ectopic expression of OC-I led to enhanced abundance of transcripts unique endogenous cysteine protease inhibitor at the later stages of low nitrogen-induced leaf senescence.
The SoyLife project also established a new RNAseq database for soybean root nodules at Leeds University with data on the transcript profiles of nitrogen-fixing nodules at different stages of development and following exposure to drought stress. These new datasets allow the identification of transcripts encoding most if not all the components of the protease and protease inhibitor systems in soybean as well as other transcripts whose abundance is changed during development or exposure to stress in soybean nodules. Changes in key transcripts selected during the course of the project were confirmed by quantitative real-time PCR.
A field study was undertaken to analyse of the architectural and morphological traits of the root systems (root phenotyping) of different soybean cultivars was undertaken in order to select for drought tolerance traits. Root architecture was determined together with shoot parameters under well watered and drought conditions in the field in three soybean cultivars (A5409RG, Jackson and Prima 2000). A5409RG is a drought-sensitive cultivar with a shallow root phenotype. In contrast, Jackson that is a drought-escaping cultivar with a deep rooting phenotype. Prima 2000 is a semi-drought-tolerant cultivar with an intermediate root phenotype. Prima 2000 had the greatest shoot biomass and grain yield under drought stress, having abundant root nodules even under drought. A positive correlation was observed between nodule size, above-ground biomass and seed yield under well-watered and drought conditions. These findings show that there is a strong association between root and nodule parameters and whole plant productivity. This study highlights the potential application of simple root phenotypic markers in future screening studies for drought-tolerance in soybean.

Taken together, the findings of these studies have greatly increased our knowledge of the role of the cysteine protease and its naturally occurring protease inhibitor system in plant performance, particularly in relation to factors that limit legume leaf and nodule lifespan under drought stress. This project has also allowed the testing of gene function and established a proof of concept that has the potential to underpin the production of a new generation of crop plants that are tailored to perform better than current varieties over a wider range of environmental conditions. The observed effects of the plant protease inhibitor system on root and shoot branching is also potentially important in terms of application.
The extensive new RNAseq dataset obtained for soybean root nodules has provided new scientific knowledge that has enabled us to identify new selectable markers for plant performance in stress conditions. In addition, simple root phenotypic markers have been established for drought-tolerance screening in soybean.
To results and outcomes of the SoyLife project have been disseminated to the wider scientific community by presentations at national and international meetings and seminars at a range of academic national and international institutions.
Research outcomes of the SoyLife project have made a substantial contribution to the ERA, particularly in the FP7 priority area concerning plant health within an optimised agricultural production system, which is a key priority within the Knowledge-Based Bio-Economy (KBBE) programme of FP7. This project also has direct socio-economic impacts with wider outreach and capacity building activities, particularly in Africa. The fellow has returned to the University of Pretoria in South Africa, where he now with colleagues to transfer the knowledge and advances in technology gained during the project. These efforts will assists current programs designed to increase crop production in Africa, ensuring that the less developed world will benefit from the findings, technologies and advances gained through the SoyLife project.

Contact details:

Prof. Christine Foyer Prof. Karl Kunert
Centre for Plant Sciences Department Plant Science
Faculty of Biological Sciences University of Pretoria
University of Leeds Hatfield 0083
LS2 9JT T: +27 12.420 3770
T: +44 113 343 1421 E: karl.kunert@up.ac.za
E: c.foyer@leeds.ac.uk