Trehalose-6-phosphate (T6P) is a proposed signaling molecule in plants, yet how it signals was not clear. Here, we provide evidence that T6P functions as an inhibitor of SNF1-related protein kinase1 (SnRK1; AKIN10/AKIN11) of the SNF1-related group of protein kinases. T6P, but not other sugars and sugar phosphates, inhibited SnRK1 in Arabidopsis (Arabidopsis thaliana) seedling extracts strongly (50%) at low concentrations (1-20 mM). Inhibition was noncompetitive with respect to ATP. In immunoprecipitation studies using antibodies to AKIN10 and AKIN11, SnRK1 catalytic activity and T6P inhibition were physically separable, with T6P inhibition of SnRK1 dependent on an intermediary factor. In subsequent analysis, T6P inhibited SnRK1 in extracts of all tissues analyzed except those of mature leaves, which did not contain the intermediary factor. To assess the impact of T6P inhibition of SnRK1 in vivo, gene expression was determined in seedlings expressing Escherichia coli otsA encoding T6P synthase to elevate T6P or otsB encoding T6P phosphatase to decrease T6P. SnRK1 target genes showed opposite regulation, consistent with the regulation of SnRK1 by T6P in vivo. Analysis of microarray data showed up-regulation by T6P of genes involved in biosynthetic reactions, such as genes for amino acid, protein, and nucleotide synthesis, the tricarboxylic acid cycle, and mitochondrial electron transport, which are normally down-regulated by SnRK1. In contrast, genes involved in photosynthesis and degradation processes, which are normally up-regulated by SnRK1, were down-regulated by T6P. These experiments provide strong evidence that T6P inhibits SnRK1 to activate biosynthetic processes in growing tissues.

Trehalose metabolism and signaling is an area of emerging significance. In less than a decade our views on the importance of trehalose metabolism and its role in plants have gone through something of a revolution. An obscure curiosity has become an indispensable regulatory system. Mutant and transgenic plants of trehalose synthesis display wide-ranging and unprecedented phenotypes for the perturbation of a metabolic pathway. Molecular physiology and genomics have provided a glimpse of trehalose biology that had not been possible with conventional techniques, largely because the products of the synthetic pathway, trehalose 6-phosphate (T6P) and trehalose, are in trace abundance and difficult to measure in most plants. A consensus is emerging that T6P plays a central role in the coordination of metabolism with development. The discovery of trehalose metabolism has been one of the most exciting developments in plant metabolism and plant science in recent years. The field is fast moving and this review highlights the most recent insights.

The plant Polycomb-group (Pc-G) protein CURLY LEAF (CLF) is required to repress targets such as AGAMOUS (AG) and SHOOTMERISTEMLESS (STM). Using chromatin immunoprecipitation, we identify AG and STM as direct targets for CLF and show that they carry a characteristic epigenetic signature of dispersed histone H3 lysine 27 trimethylation (H3K27me3) and localised H3K27me2 methylation. H3K27 methylation is present throughout leaf development and consistent with this, CLF is required persistently to silence AG. However, CLF is not itself an epigenetic mark as it is lost during mitosis. We suggest a model in which Pc-G proteins are recruited to localised regions of targets and then mediate dispersed H3K27me3. Analysis of transgenes carrying AG regulatory sequences confirms that H3K27me3 can spread to novel sequences in a CLF-dependent manner and further shows that H3K27me3 methylation is not sufficient for silencing of targets. We suggest that the spread of H3K27me3 contributes to the mitotic heritability of Pc-G silencing, and that the loss of silencing caused by transposon insertions at plant Pc-G targets reflects impaired spreading.