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Title: The role of ABA in regulating the movemement of Chlamydomonas reinhardtii in response to stress
Author: Alhijab, Layla
ISNI:       0000 0004 7967 5165
Awarding Body: University of the West of England
Current Institution: University of the West of England, Bristol
Date of Award: 2019
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Plant hormones such as auxin, gibberellin, cytokinin, ethylene, and abscisic acid play key roles in growth and developmental processes in higher plants such as Arabidopsis thaliana. However, the effect of these hormones on algal physiology is still unclear. Our results suggest that these plant hormones do not affect the growth rate of C. reinhardtii. We examined the effect of these hormones in the movement of C. reinhardtii. C.reinhardtii cultures were transferred to a glass measuring cylinder then treated with 50μM of ABA, ACC, H2O2, NAA and combinations of ABA and ACC in either the light or dark. All these hormones except ABA did not alter the phototropic response of C.reinhardtii. We show that exogenous ABA significantly altered the HCO3- uptake of C.reinhardtii in a light-intensity-dependent manner. In high light ABA enhanced HCO3- uptake, while under low light uptake was diminished. Algae were sampled at different time points over 24h in a cyclic 16h photoperiod and were treated with, or without 50 μM ABA in either the light or dark and their position in the water column was monitored by measuring the A750 at different depths. The actual position attained by the algae in the water column correlated with the time at which they were sampled in the cycle; in general, ABA-induced upward movement of the algae. The algae also showed a differential, light-dependent directional taxis response to a fixed ABA source, moving horizontally towards the source in the light and away in the dark. Morever, plants possess glycine-rich RNA-binding proteins (class IV GRPs) that are involved in stress responses that are regulated by ABA. Our sequence analysis revealed that Chlamydomonas appears to possess only a single class IV GRP gene, which we named CrGRP1; it encodes a flagellum-associated RNA-binding protein. We show that CrGRP1 is expressed in a circadian rhythm and in response to abiotic stresses. The expression of CrGRP1 was also assessed during the algal movement experiments and appeared to decline to the greatest extent when ABA-induced the greatest upward movement of the algae. To determine the potential role of the CrGRP1 gene in ABA and light-mediated movement, recombinant pChlamy4 constructs were made to overexpress of knock-down CrGRP1. Ongoing experiments suggest that CrGRP1 negatively regulates the ABA-induced upward movement of the algae, presumably by binding mRNAs required for this response. We suggest that the development of this response mechanism in motile algae may have been an important step in the evolution of terrestrial plants and anticipate that these results will initiate novel algal research that may help to clarify the involvement of ABA in plant tropisms.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available