Abstract
Considerable interest has been evoked by the analysis of the regulatory pathway in carbohydrate metabolism and cell growth involving the non-reducing disaccharide trehalose (TRE). TRE is at small concentrations in mesophytes such as Arabidopsis thaliana and Triticum aestivum, excluding a role in osmoregulation once suggested for it. Studies of TRE metabolism, and genetic modification of it, have shown a very wide and more important role of the pathway in regulation of many processes in development, growth, and photosynthesis. It has now been established that rather than TRE, it is trehalose 6-phosphate (T6P) which has such profound effects. T6P is the intermediary in TRE synthesis formed from glucose-6-phosphate and UDP-glucose, derived from sucrose, by the action of trehalose phosphate synthase. The concentration of T6P is determined both by the rate of synthesis, which depends on the sucrose concentration, and also by the rate of breakdown by trehalose-6-phosphate phosphatase which produces TRE. Changing T6P concentrations by genetically modifying the enzymes of synthesis and breakdown has altered photosynthesis, sugar metabolism, growth, and development which affect responses to, and recovery from, environmental factors. Many of the effects of T6P on metabolism and growth occur via the interaction of T6P with the SnRK1 protein kinase system. T6P inhibits the activity of SnRK1, which de-represses genes encoding proteins involved in anabolism. Consequently, a large concentration of sucrose increases T6P and thereby inhibits SnRK1, so stimulating growth of cells and their metabolic activity. The T6P/SnRK1 mechanism offers an important new view of how the distribution of assimilates to organs, such as developing grains in cereal plants, is achieved. This review briefly summarizes the factors determining, and limiting, yield of wheat (particularly mass/grain which is highly conserved) and considers how T6P/SnRK1 might function to determine grain yield and might be altered to increase them. Increasing the potential rate of filling and mass/grain are ways in which total crop yield could be increased with good husbandry which maintains crop assimilation Cereal yields globally are not increasing, despite the greater production required to meet human demand. Careful targeting of T6P is showing much promise for optimization of source/sink for yield improvement and offers yet further possibilities for increasing sink demand and grain size in wheat.
Highlights
Understanding of the integration of the processes of carbon assimilation and of growth of organs in higher plants is advancing rapidly, but mechanisms are complex and incompletely understood
One mechanism linking the two has been identified: it involves trehalose 6-phosphate, the precursor of trehalose acting as a sensor of sucrose concentration (Paul, 2007; Paul et al, 2008), which regulates the activity of a protein kinase SnRK1: this activates or represses gene expression for proteins of basic metabolism
trehalose 6-phosphate (T6P) and TRE usually occur at very small concentrations compared with many other carbohydrates, T6P increases substantially – albeit over the micromolar range – with greater availability of sucrose and larger fluxes of sucrose to organs and correlates strongly with changes in carbohydrates, e.g., starch deposition in the endosperm of wheat grains is associated with T6P in the endosperm of wheat (MartínezBarajas et al, 2011)
Summary
Understanding of the integration of the processes of carbon assimilation and of growth of organs in higher plants is advancing rapidly, but mechanisms are complex and incompletely understood. Genetic transformation of rice with TPS and alteration of TRE concentration has been claimed to increase osmolyte concentrations and so confer “drought tolerance” (Garg et al, 2002; Jeong et al, 2010) but it is recognized that this “drought tolerance” is a result of decreased growth and stomatal conductance, so slowing water loss and the onset of drought (Lawlor, 2013) These effects have been observed in different species, of different ages, and in different organs showing T6P to be a general, probably universal, regulator of plant carbohydrate metabolism, development, and growth. SnRK1 belongs to the SNF1/AMPK group of protein kinases and is the plant homolog of AMP-activated protein kinase in mammals, a sensor maintaining cellular energy homeostasis by regulating anabolic and catabolic processes and balance This group of kinases, and other regulatory mechanisms such as the transcription factor bZIP11 with which they interact (O’Hara et al, 2013) regulate the cell cycle, cell division, apoptosis, and cell and tissue metabolism; modification of the system results in changed growth (Guérinier et al, 2013). Be manipulated to increase yields of crops (Martínez-Barajas et al, 2011)
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