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Title: Aquaporins as candidate genes for drought avoidance QTL on rice chromosome 7
Author: Abubakar, Zainab Adamu
ISNI:       0000 0004 5918 0130
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2015
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Rice, being an ancient domesticated crop plant, is the most important and widely consumed cereal after wheat. Oryza sativa is the most consumed of the two species of domesticated rice and a focus of the present study. To understand the mechanisms of crop plant's adaptation to drought, it is a crucial step to identify the key genes that can be used for advanced breeding or engineering transgenic crops with improved drought tolerance without reducing yield or biomass. Mapping populations of 205 recombinant inbred (F6) lines were produced from a cross of varieties Bala and Azucena of the Oryza sativa species by single-seed descent as described in Price et al. (2000). Repeated studies in the Bala and Azucena mapping population under field drought conditions revealed a locus on Chromosome 7 worthy of further consideration. Through bioinformatics examination of the reference genome, it was observed that underneath this QTL lies a cluster of four genes with homology to PIPs (a subfamily of aquaporins), with three being PIP2; 1, 2; 4 and 2; 5 while the fourth, PIP2; 3 appears to be incomplete. Functional differences in the activity of these genes could explain the QTLs detected here, since all could theoretically affect root hydraulic conductance. The overall aim of the thesis was to test the hypothesis that these aquaporin genes are candidate genes for the drought avoidance QTL on chromosome 7 detected in the Bala x Azucena mapping population. Studies were carried out in controlled environments using two recombinant near isogenic lines with Azucena and Bala alleles under different drought scenarios of 21 %, 13 % & 10 % gravimetric water contents (GWC) respectively. Drought reduced root hydraulic conductance in the Azucena allele of the RINIL pair was consistent with a hypothesis that the candidate aquaporin genes which are located underneath the QTL are likely to be causative. In addition, a screen for reduced activity of these candidate genes was conducted. Transgenic seed have been generated in CIRAD, Montpellier, France using RNA interference (RNAi). These transgenics, which should have a down-regulation of the expression of the aquaporin genes, were used to study the role of aquaporin genes in drought tolerance. Many plants were screened and some were identified as lines with differing expression levels of these candidate genes. The data obtained from this study indicated that lower hydraulic flow is associated with reduced PIP2; 1 expression. This was confirmed in well-watered plants using whole root systems in a pressure chamber where root hydraulic conductance was reduced by about 25 %. A mutant was obtained from Korea that has a T-DNA insertion in an exon of PIP2; 4. A screen to identify mutants from the segregating seeds was performed using hydroponics. In the study, growth physiology and root hydraulic flow of rice T-DNA mutants was investigated in order to dissect and evaluate the co-segregation of T-DNA mutants with observed phenotype adequately linked to the biological function of the mutants, and compliment with wild type copy of the gene. Results of the study concluded to establish the possibility of mutagenesis been valid in identifying a T-DNA line of PIP2; 4 that is down regulated in response to rice root water uptake by evaluating root hydraulic flow. Further work is needed to confirm the T-DNA insertion at the gene level. Finally, a drought screen was conducted to identify the functional mechanisms that govern the activity of the candidate aquaporin genes in plants under varying water stress conditions and discover the physiological implications of altered aquaporin activity. The hypothesis tested was that differences either in the natural occurring allelic region around the drought avoidance QTL in the RINILs or RNAi knocking out affects the drought physiology of the rice plants in a way that is compatible with the predicted function of the aquaporins. The observations were in agreement with an idea that PIP2.1 is significant in regulating root water hydraulic conductance and it affects whole plant water relations. It is also consistent with the concept that allelic variation seen in aquaporins that reside in a cluster on Chromosome 7 of the rice genome (including PIP2.1) are excellent candidates for a QTL related to drought avoidance in the same location. results indicate that that the contrasting alleles of the aquaporins PIP2; 1, 2; 4 and/or 2; 5 affect root hydraulic properties and are good physiological candidate genes for drought avoidance QTL on chromosome 7. The research indicates that genetic modulation of aquaporin function in roots is a promising route in the production of crops suited to drought-prone environments.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Aquaporins ; Rice