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University of Cape Town (2002)

Molecular and physiological study of water-deficit stress on selected Eragrostis species

Ginbot, Zekarias Gebremedhin

Titre : Molecular and physiological study of water-deficit stress on selected Eragrostis species

Auteur : Ginbot, Zekarias Gebremedhin

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) 2002

Eragrostis nindensis and Eragrostis tef are wild and domestic grasses respectively that belong to the subfamily Eragrostideae. E. nindensis is desiccation tolerant while E. tef is desiccation sensitive. The responses of these plants to water-deficit stress were studied using molecular and physiological approaches. A cDNA library of E. nindensis was screened to identify differentially expressed genes during dehydration. Physiological studies included monitoring changes in photosynthesis, respiration, ultrastructure and membrane integrity of plants during dehydration and rehydration. The differential screening of the cDNA library, using a radio-labelled cDNA from hydrated and dehydrated leaves respectively, revealed two genes, referred as Nin-19 and Nin-44, that were differentially expressed in dehydrated leaves of E. nindensis. These genes were sequenced and partially characterized. Nin-19 did not show considerable identity with any known genes and was not studied any further. Nin-44 was identified as a dehydrin-like gene with approximately 99 % identity to seven water-deficit stress responsive genes on a section of about 60 bp near the 3’ end. As its sequence was found to represent a partial insert size, two forward and reverse primers were designed to find the full length through RT-PCR. Despite repeated attempts, no products that could be used in subsequent procedures were achieved using this technique. Hence, further characterization of this gene also could not be performed and different approaches were suggested. The physiological studies showed that E. nindensis is desiccation tolerant but E. tefis not, the latter dying below RWC of about 33 %. Difference among plants in physiological responses became evident after 6 days of dehydration treatment, which resulted in a decline of RWC to 65%, 39% and 33% in E. nindensis, E.tif(R) (red-seeded) and E. tef (W) (white-seeded) plants respectively. A significant decrease in photosynthesis, transpiration, stomatal conductance, and an increase in electrolyte leakage occurred in all species after 6 days of dehydration, but leaves of E. tef (W) did not recover from this level of dehydration when watered. Instead, new leaves were observed to re-grow from the stem nodes. The leaves of red-seeded variety of E. tef did recover fully from RWC of 39 %. After a further 3 days dehydration both varieties of E. tef died. On the other hand, E. nindensis was found to survive extreme water-deficit (-10 % RWC tested here) and recovered full physiological activity when watered. The electrolyte leakage study on these plants indicated major injury on E. tef(W), being intermediate in E. tef(R) and very low in E. nindensis, which coincided with the trend of declining in RWC and other metabolic activities measured. The ultrastructural study on E. tef varieties also showed evidence of the damage caused by dehydration, but the difference among these species was not significant enough to indicate the level of susceptibility of the plants to dehydration damage. The study demonstrated that E. tef varieties are not drought tolerant and showed a considerable difference in their responses to water-deficit stress with each other and with respect to E. nindensis. However, E. tef (R) seems to have a better control over transpiration and some form of repair mechanism operational at least until dehydration to 39 % RWC. This is proposed to be a better performing cereal in conditions of water stress. On the other hand E. nindnsis did not suffer major injury from the dehydration treatment and confirmed to be desiccation tolerant.


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