Regulation of C4 photosynthesis by sugar sensing

Abstract

The process of photosynthesis, sugar allocation, export of sugars by photosynthetic tissues and their import by sink tissues plays a vital role in plant growth. Photosynthesis and sink demand are tightly coordinated through metabolic (substrate, allosteric) feedbacks and signalling (sugar, hormones) mechanisms. Sugar signalling integrates sugar production with plant development and environmental cues. Few studies on sugar signalling have directly focussed on the photosynthetic tissues due to the complexity of primary metabolism. In C3 plants, sugar accumulation in source leaves, due to source-sink imbalance, negatively feeds back on photosynthetic gene expression and plant productivity. We have a limited understanding of the molecular mechanisms underlying those feedback regulations, especially in C4 plants, which are becoming increasingly important for global food and bioenergy security. In addition, most transgenic approaches have utilised constitutive promoters which affect gene expression in all plant tissues. This study addresses these critical knowledge gaps by seeking to elucidate the interplay between sugar signalling pathways and the regulation of C4 photosynthesis. This thesis adopted a transgenic approach using a leaf mesophyll-cell specific promoter to investigate how two sugar sensors (Hexokinase (HXK) and Sucrose non-fermenting 1 (Snf1) related protein kinases (SnRK1)) affect photosynthesis and plant growth of the model C4 grass, Setaria viridis. In summary, this thesis involved the generation of S. viridis mutants overexpressing the sugar signalling genes (SvHXK and SvSnRK1α) in mesophyll cells under the control of ZmPEPCpro promoter, which demonstrated that the overexpression of genes in the photosynthetic tissue was possible and generated healthy looking plants. The phenotypic characterisation of SvHXK6 concluded that they may play a role in sugar signalling of the C4 source leaf, by eliciting reduced photosynthesis rates and stomatal conductance of S. viridis. It was also concluded that SvHXK6 may also play a role in biomass allocation in C4 plants by affecting biomass allocation to the leaves of S. viridis. The phenotypic characterisation of SvSnRK1α.3 concluded that it was involved in suppressing plant growth and stomatal conductance, and the latter effect was possibly mediated through interaction with its counterpart SnRK2. This novel finding needs to be further investigated. It was also concluded that the reduced stomatal conductance by overexpression of SvHXK6 and SvSnRK1α.3 may possibly be related to their expression under ZmPEPCpro as the C4 isoform of PEPC is expressed in guard cells and plays a central role in stomatal closure.

Date of Thesis submission	2022
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Oula Ghannoum (Supervisor), Robert Furbank (Supervisor) & Lily Chen (Supervisor)