Characterization of SUMO proteases and other proteins involved in water stress responses in Triticum aestivum L.
Date
2021-03
Authors
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Currently, approximately 4.5 billion people in developing countries consider wheat (Triticum
aestivum L.) as a staple crop as it is a key source of daily calories. Therefore, it is ranked the
second most important grain crop in the developing world. Meanwhile, climate change associated with
severe drought conditions and the rising global mean temperatures has resulted in sporadic soil
water shortages. Water deficit due to drought creates concerns because, historically, drought is
the leading cause of yield loss in wheat. Thus, a more comprehensive understanding of the
mechanisms underlying wheat drought responses is imperative. One of the latest avenues in
plant-drought response is SUMOylation, which is a post-translational modification. SUMOylation is
responsible for affecting essential proteins during water deficit stress changing their
functionality, thus contributing to the plant succumbing to drought. SUMO proteases can counter the
process by acting directly within the SUMOylation pathway. Therefore, SUMO proteases are an ideal
target for manipulating stress-responses. In this Ph.D. study, novel findings have confirmed the
overarching hypothesis “that soil water deficit stress influences an array of physiological,
metabolic, and proteomic mechanisms mitigated by reducing levels of SUMOylation, consequently
delaying protein turn over, thereby increasing drought tolerance in the cereal crop wheat.” Proof
for this hypothesis has been that overexpression of an A. thaliana cysteine protease (Overly
Tolerant To Salt-1, OTS1) (At1g60220), in wheat leads to improved plant growth under drought
conditions. These overexpressing plants had an improved stomatal conductance and photosynthesis
rate (Fv/Fm), accompanied by a higher total chlorophyll content than the controls. More
importantly, these overexpressing plants had a reduced level of SUMOylated proteins with delayed
senescence under drought conditions, allowing these plants to survive up to 14 days without water
(with a final soil water content ± 15%). This finding further suggests that SUMO proteases may
influence an array of mechanisms in wheat to the advantage of the crop to be more tolerant to water
deficit stress caused by drought. This is the first report to elucidate SUMOylation effects in the
hexaploid crop wheat. Furthermore, this Ph.D. work is also in agreement with various other studies
that showed water deficit stress constrains almost every part of developing plants, inducing
morphological and physiological changes, and cellular biochemical alterations as an adaptative
response. To substantiate the aforementioned, random mutagenesis was also applied to produce two
new wheat mutants, RYNO3926, and BIG8-1, with both expressing water deficit stress tolerance. While
the BIG8-1 mutant survived three weeks without water, the
RYNO3936 mutant could only endure two weeks without any water but rapidly recovered fully, despite
leaves being completely dry/dehydrated after exposure to water deficit stress. Qualitative
proteomic analysis further revealed that both the mutant and control have an improve regulation of
SUMOylation when compared to non-mutant plants and in addition to having more
ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Quantitative proteomic analysis also
revealed that RYNO3936 mutant plants expressed a large number of proteins to endure drought
conditions such as the abscisic stress-ripening protein, cold-induced protein, cold-responsive
protein, dehydrin, Group 3 late embryogenesis, and a lipoprotein (LAlv9) belonging to the family of
lipocalins. Moreover, BIG8-8 mutant uniquely expressed ABC transporter permeases and Xylanase
inhibitor protein during severe water deficit stress. Collectively the research supports the idea
that there is a multifaceted nature of how plants react to drought, which is a non-linear response,
as it involves multiple pathways related to genomics, transcriptomics, proteomics, and phenomics
responses. Though this dissertation does not argue a comparative analysis of which of the lines is
more superior, it does advocate that each developed line will thrive better with specific dryland
conditions, adding to the
knowledge for future breeding programs to improve drought tolerance.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhDAgric)--Stellenbosch University, 2021.