Environmental regulation of CO2 concentrating mechanisms in C4 grasses with different biochemical subtypes

Abstract

The CO2 concentrating mechanism (CCM) endows C4 plants with high CO2 assimilation rate and high light saturation which in turn leads to an agricultural and ecological importance that is disproportionately high relative to their small taxonomic representation. The CCM is achieved by a series of anatomical and biochemical adaptations which allow the collaborative operation of two photosynthetic cycles, C4 and C3, across the outer mesophyll (MC) and inner bundle-sheath cells (BSC) to saturate Rubisco by CO2 in BSC. The BSC membrane is not completely impermeable to CO2, and some CO2 leaks out into the surrounding MC which increases the energetic cost of C4 photosynthesis. Therefore, leakiness (¤ò), defined as the rate of CO2 leakage out of BSC into the MC as a fraction of the rate of PEP carboxylation (Vp) is an important measure of CCM efficiency and coordination. Leakiness is determined by the bundle-sheath conductance and CO2 gradient between MC and BSC which in turn depends on PEPC and Rubisco activity. In addition, C4 photosynthesis has been traditionally grouped into three classical subtypes depending on the major C4 acid decarboxylase in the BSC: NADP malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and PEP carboxykinase (PEP-CK). Each subtype is distinguished by anatomical and biochemical features. Our understanding of the influence of C4 and C3 carboxylases activity on CCM coordination in different C4 subtypes is sketchy. Further, the relationship between in vivo and in vitro measures of C4 and C3 carboxylases in C4 photosynthesis is unclear. Temperature is a key environmental factor for the regulation of enzyme activity while light is the source of energy driving photosynthetic reactions. Consequently, both temperature and light can affect CCM coordination by various means. Accordingly, the overarching aim of this PhD project was to compare the CCM coordination of a diverse range of C4 grasses belonging to different biochemical subtypes (NADP-ME, PEP-CK, NAD-ME) under high temperature (warming) and low light (shade). This was achieved by addressing the following specific objectives: (i) investigate the effect of C4/C3 cycle carboxylase activity (in vivo and in vitro) on leakiness (¤ò); (ii) determine the relationship between in vivo and in vitro carboxylase activity; and (iii) study short- and long-term acclimation to temperature and shade in C4 grasses with different 2 biochemical subtypes. This project demonstrated that in vivo and in vitro measures of maximal C3 and C4 cycle carboxylases are not correlated to each other and do not correlate with leakiness (¤ò). In addition, this project established contrasting short-term responses to low light and high temperature of C4 photosynthesis. While increased leaf temperature enhanced CO2 assimilation rate (A) without affecting CCM coordination, low light reduced A and impaired CCM coordination in most of the C4 grasses. In the long term, C4 photosynthesis acclimated greatly to growth at high temperature in NADP-ME species, whereas NAD-ME species showed strong photosynthetic acclimation under shade. Finally, this project confirmed that NAD-ME species are favoured by growth at high temperature while disadvantaged by growth under shade.

Date of Award	2016
Original language	English