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Mohammed, Umar (2017) The improvement of drought tolerance in rice. PhD thesis, University of Nottingham.

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Abstract

The unpredictability of spatio-temporal drought patterns in the field and complexity of the physiological stress response mechanism involved have made it difficult to identify the essential traits for improved productivity under drought stress without compensating yield performance. (Serraj et al., 2011). Drought (severe water limitation) limits rice productivity, particularly in upland and rainfed systems which are prone to frequent drought and other abiotic stresses. Vulnerability in such areas is likely to worsen in the future. It is therefore critical to develop genetic improvement strategies to focus on improving productivity in such regions. The development of improved root system architecture, shoot morphology and whole plant physiological processes in order to generate drought tolerant rice cultivars

It is in this context that this thesis used a multidisciplinary approach to improving drought tolerance in rice. One of the major phases was to study a diversity panel of African rice lines (Oryza sativa L) selected for being particularly well adapted to upland (aerobic) cultivation and was compared to a set of typical lowland (Asian in origin) cultivars. Plants were grown in plastic columns in expanded clay and exposed to temporary drought at key growth stages. In particular, the characterisation of these African accessions has improved our understanding of the interaction between photosynthetic response and how their roots interact with their environment and also in response to the above ground physiology. For example, a high leaf gas exchange CO2 assimilation, stomatal conductance and transpiration identified in two upland genotypes, N8 and N7 with high leaf water use efficiency as a result of a low stomatal conductance and high photosynthesis. The N8 also showed well-developed root system with significantly higher root depth and length, while the N7 has relatively higher central metaxylem area and higher stomatal density. A strong correlation was observed between root morphological properties (e.g. root average diameter, root length and root central metaxylem area) was observed, providing real evidence that optimal root morphology is needed to support high photosynthesis through increased water access and possibly higher hydraulic conductance across diverse genotypes.

Another reverse genetic approach used transformation of key genes in stomatal patterning. The overexpression of the rice homolog EPIDERMAL PATTERNING FACTOR (EPF) resulted in the alteration of stomatal density and size. OsEPF2 exhibited reduced stomatal density, and OsEPFL9 showed increased stomatal density. As a result of reduces numbers of stomata, the EPF2OE showed significantly improved intrinsic water use efficiency and drought tolerance through reduced stomatal conductance and transpiration whilst maintaining a high relative rate of photosynthesis. The OsEPF2 unlike in Arabidopsis (Franks et al., 2015) has shown significantly low stomatal conductance without loss in carbon gain. Unexpectedly the OsEPF2 lines also showed enhancement of lysigenous aerenchyma when growing in both stagnant and aerated hydroponic media, in comparison to the WT. This has a vast implication in the improvement of respiratory losses potentially in non flooded and flooded conditions.

The creation of transgenic lines with the Ethylene Responsive Factor (ERF) group VII motif signature in the GUS N-terminus has been used to show the activity of the ERFVII under drought conditions. The result revealed GUS expression under abiotic stresses; notably hypoxia, submergence and drought. Therefore, the motif of the ERFVII could be used as a sensor for drought in rice and can also be used in the understanding of the role of protein modification in genes related to submergence and drought tolerance such as the SUBMERGENCE 1 (SUB1A)through the principles of N-end Rule Pathway.

In conclusion, this thesis has demonstrated that multiple approaches can be used toward the improvement of water use efficiency and drought responses in rice crops. A reduction in stomatal density can improve water use efficiency in water limited conditions and should be considered in upland or other similar agroecosystems where yield is determined by low rainfall and soil water conservation is critical. This may also be applicable to irrigated rice but there is a possibility of a limitation to the maximum rate of photosynthesis by stomatal conductance. The use of existing natural genetic variation in African genotypes has been shown to be another promising route, especially because drought tolerance is a complex multi-gene trait. It is concluded that both the single gene approach and breeding using existing tolerant lines should be explored as routes to maintaining food security in coming decades. It has also demonstrated clearly that coordination of above and below ground processes is required.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Murchie, Erik Swarup, Ranjan
Subjects:	S Agriculture > SB Plant culture
Faculties/Schools:	UK Campuses > Faculty of Science > School of Biosciences
Item ID:	47616
Depositing User:	Mohammed, Umar
Date Deposited:	12 Mar 2019 11:40
Last Modified:	14 Dec 2020 04:30
URI:	https://eprints.nottingham.ac.uk/id/eprint/47616

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