An Investigation of the Bioactivity of Cyanobacterial Exometabolites in Plant Stress Tolerance

Chua, Alysha (2019) An Investigation of the Bioactivity of Cyanobacterial Exometabolites in Plant Stress Tolerance. Doctoral thesis, Waterford Institute of Technology.

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Abstract

As the global population expands and climates rapidly change, reliable access to enough inexpensive, nutritious food is already a major problem. The challenge of enhancing food security cannot be at the expense of environmental damage, therefore sustainable agriculture must be a central tenet. Around the world, N2-fixing inoculants like cyanobacteria are used in sustainable agriculture programmes to enhance yields and mitigate plant responses to stress. Presently, assessment of novel cereal varieties focuses mostly on high yields while screening for stress tolerance is an expensive, time-consuming process. However, it is proposed here that cell death modes, especially programmed cell death (PCD), can be used as a marker of plant stress tolerance. PCD is a normal facet of plant growth and development, but plant cells also activate environmentally-induced PCD as a protective mechanism during stress exposure and it is possible to limit stress-induced PCD to minimise crop yield losses. Here, it is shown that medium conditioned by the cyanobacteria Nostoc muscorum reduces PCD in stressed plant cells, and a further investigation identified proline as the major bioactive N. muscorum-derived compound. To start, a root hair assay (RHA) was used as an in vivo tool to enumerate the overall plant stress response in Arabidopsis thaliana and to characterise the bioactivity of cyanobacteria N. muscorum conditioned medium (CM). Heat stressed A. thaliana exhibited reduced PCD when treated with N. muscorum CM fractions. Proline emerged as a bioactive candidate of interest and was confirmed in N. muscorum CM using the ninhydrin assay and HPLC. Furthermore, proline feeding experiments revealed a similar performance to CM but with marginally lower PCD suppression levels. Confirmation of proline as the main bioactive candidate was attained by treating mutant Arabidopsis lines with impaired proline transporters with exogenous proline and CM fractions. The RHA was also successfully used as a high-throughput screening tool to pinpoint stress-tolerant and susceptible Triticum aestivum and Hordeum vulgare varieties. Heat stress experiments showed that winter and spring barley varieties could be subdivided into their seasonal groups based on their PCD susceptibility. Furthermore, stress-induced PCD levels were used to investigate basal, induced and cross-stress tolerance of eight wheat varieties to heat and salt stress. The RHA identified varieties with high basal tolerance based on their performance after single and combined stress exposure. However, these varieties also had an unexpectedly slower cross-stress tolerance response than their stress-susceptible counterparts, demonstrating slower flexibility against recurrent stress exposure. Finally, to relate findings back to applications in sustainable agricultural practices, preliminary work to encapsulate proline in slow-release microspheres found that the maximum safe dosage of proline was 8 μM in Arabidopsis, 10 μM in barley and 100 μM in wheat; however, proline bioactivity was only effective at a narrow stress range. Collectively, this thesis demonstrates that cyanobacteria-derived proline elevates plant stress tolerance by inhibiting PCD and that by using the RHA, PCD is a convenient marker of plant stress tolerance and susceptibility. This offers preliminary evidence of a novel biofertiliser mechanism for enhancing plant stress tolerance independent of the existing mechanisms cited in the literature.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: Cyanobacterial Exometabolites, Plant Stress Tolerance
Departments or Groups: *NONE OF THESE*
Divisions: School of Science > Department of Computing, Maths and Physics
Depositing User: Derek Langford
Date Deposited: 08 Oct 2019 13:41
Last Modified: 08 Oct 2019 13:41
URI: http://repository.wit.ie/id/eprint/3383

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