280 Biochemical Society Transactions (2005) Volume 33, part 1 The role of trehalose-6-phosphate synthase in Arabidopsis embryo development L.D. G ´ omez, S. Baud and I.A. Graham 1 CNAP, Department of Biology, University of York, Heslington, York YO10 5YW, U.K. Abstract We previously showed that trehalose-6-phosphate synthase 1 (TPS1), which catalyses the first step in trehalose synthesis, is essential for embryo maturation in Arabidopsis [Eastmond, van Dijken, Spielman, Kerr, Tissier, Dickinson, Jones, Smeekens and Graham (2002) Plant J. 29, 225–235]. The tps1 mutant embryos develop more slowly than wild type. Patterning in the tps1 embryos appears normal but they do not progress past the torpedo stage to cotyledon stage, which is when storage reserves start to accumulate in the expanding cotyledons. Our initial data led to the hypothesis that trehalose metabolism plays a key role in regulating storage reserve accumulation by allowing the embryo to respond to the dramatic increase in sucrose levels that occurs at the torpedo stage of embryo development. More recent data demonstrate that while the tps1 mutant is blocked in the developmental progression of embryos from torpedo to cotyledon stage the expression of genes involved in the accumulation of storage reserves proceeds in a similar fashion to wild type. Thus it appears that induction of metabolic processes required for accumulation of storage reserves in tps1 occurs independently of the developmental stage and instead follows a temporal programme similar to wild-type seeds in the same silique. Introduction Seed development in Arabidopsis is generally divided into three phases. In the first, known as embryo morphogenesis, a series of programmed cell divisions results in the basic cell patterning of the mature plant being established. The torpedo stage embryo represents the end of this phase at which stage the root and shoot meristems are formed and small un- expanded cotyledons are evident [1]. The second phase is dominated by the expansion of the cotyledons and hypo- cotyl and the associated accumulation of storage reserve com- pounds. A significant amount of cell division as well as cell expansion occurs during this phase with the number of cotyledon cells increasing substantially from torpedo through to mid-late cotyledon stage [2]. During the final phase, the mature seed is subjected to desiccation after which the embryo reaches a stage of quiescence [3]. This developmental programme is underpinned by coordinate regulation of gene expression including for example those involved in storage reserve accumulation during the second phase of development [4]. A large number of embryo-lethal Arabidopsis mutants that fail to produce viable seed due to disruption in essen- tial embryo development (EMB) genes have been identified [5]. These mutants range from those disrupted in basic house- keeping functions to specific signalling processes essential for embryo development. Until recently trehalose metabolism was not thought to play any significant role in development in higher plants but a number of lines of evidence have led Key words: Arabidopsis, embryo development, reserve accumulation, trehalose-6-phosphate, TPS1. Abbreviations used: TPP, trehalose phosphate phosphatase; TPS, trehalose-6-P synthase; T-6-P, trehalose-6-P. 1 To whom correspondence should be addressed (email iag1@york.ac.uk). to a revision of this view [6,7]. Key among these was the discovery that trehalose-6-P synthase 1 (TPS1) is required for progression past the torpedo stage of embryo development [7] leading to it being classified as an EMB gene [5]. The non-reducing disaccharide trehalose is widely dis- tributed in nature, playing a role in stress protection and carbohydrate storage. Since the demonstration that mani- pulation of the trehalose biosynthetic pathway results in pro- found effects on plant growth and development in ma- ture plants [6], the function of the trehalose pathway in higher plants has been a matter of intense research. Although in a few desiccation-tolerant plants trehalose is present in con- centrations compatible with a role in stress protection, in most plants the amounts (0.15 mg g −1 dry weight) are too low to support such a role [8]. The biosynthesis of trehalose occurs via a phosphorylated intermediate, trehalose-6-P (T-6-P), with two steps catalysed by the enzymes trehalose- 6-P synthase (TPS) and trehalose phosphate phosphatase (TPP). In Saccharomyces cereviseae the TPS1 protein and T-6-P play a key role in regulating the flux of carbon into glycolysis through the regulation of hexokinase [9,10]. The demonstration that the E. coli otsA gene, which encodes a protein with the same catalytic activity as TPS1 but which is significantly divergent at the amino acid level, indicates that the Arabidopsis tps1 mutant phenotype is due to the lack of T-6-P rather than an inherent property of the TPS1 protein. A reverse genetics approach was used for the isolation of transposon insertions in the first and second exons of the AtTPS1 gene (tps1-2 and tps1-1 mutants, respectively) [7]. A third allele with a T-DNA insertion in the last exon of the gene has also recently been identified in the SAIL collection [11]. All of these tps1 alleles are recessive and show C  2005 Biochemical Society