Regulation of photosynthesis by sugar sensing in C3 and C4 plants

Abstract

Photosynthesis provides the plant with energy through the conversion of sunlight into sugars by using carbon dioxide and water. When photosynthesis is limited, plants remobilize stored resources to maintain essential function and growth. If growth is limited, reduced sink strength feeds back on photosynthesis. This dynamic relationship between photosynthesis, growth, and sugar production is integral for plant fitness and is coordinated by several sugar sensing pathways and metabolites. These include the protein kinases Target of Rapamycin (TOR), hexokinase (HXK), and SNF (Sucrose non-fermenting-1) -related kinase 1 (SnRK1), and the sugar sensing metabolite trehalose-6-phosphate (T6P). Except for HXK, regulation of photosynthesis by these sensors remains poorly understood, especially in C4 plants. Work on the C4 grass Setaria viridis has shown that C4 plants may have a lower feedback inhibition in response to high light and sugar levels. Furthermore, evidence shows that the sugar sensors, specifically HXK behave differently than what is known in C3 plants, pointing to a sugar sensing response under low light in C4 plants. The evolution from C3 to C4 photosynthesis involved a suite of anatomical and biochemical modifications, occurring through gene modifications, duplications, and regulatory changes. To adjust for these changes in metabolite fluxes, sugar sensors may have also evolved. In C3 plants, TOR complex 1 (TORC1) is known to regulate autophagy, meristem activation, and glucose sensing in roots to stimulate growth. However, the function of TORC1 in photosynthetic tissue, and more specifically the regulation of photosynthesis and photo assimilates is less known. This has proven difficult as inhibiting TORC1 through targeting of TOR, the catalytic subunit of the complex, is embryo lethal. Some progress has been achieved with inducible promoters, however studying the effects of the lack of the TORC1 photosynthetic tissue has proven nearly impossible. Tissue specific promoters could prove to be another approach to studying the specific effect of genes. The PhD project addresses these critical knowledge gaps to better understand the role of sugar sensing in higher plants, especially C4 plants, and identify novel ways for improving crop productivity. The overall aim was to investigate how these sensors differentially regulate growth and photosynthesis in C3 and C4 model plants.

Date of Thesis submission	2023
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Oula Ghannoum (Supervisor)