The selection of a sugar for transport and storage of carbon in plants

Abstract

Sugars perform two vital functions in plants: as compatible solutes protecting the cell against osmotic stress, and as mobile source of immediate and longterm energy requirement for growth and development. The two sugars that occur commonly in nature are sucrose and trehalose. Sucrose comprises of one glucose and one fructose molecule; trehalose comprises of two glucose molecules. Trehalose occurs in significant amounts in insects and fungi which greatly outnumber the plants. Surprisingly, in plants trehalose has been found in barely detectable amounts, if at all, raising the question 'why did nature select sucrose as the mobile energy source and as storage sugar for the plants'? Modelling revealed that when attached to the ribbon-shaped β-1,4-glucan a trehalose molecule is shaped like a hook. This suggests that the β-1,4-glucan chains with attached trehalose will fail to align to form inter-chain hydrogen bonds and coalesce into a cellulose microfibril, as a result of which in trehaloseaccumulating plant cells the cell wall will tend to become leaky. Thus in plants an evolutionary selection was made in favour of sucrose as the mobile energy source. Genetic engineering of plant cells for production of trehalose for combating abiotic stresses is fraught with risk of damage to the cell wall.