SummaryDespite the recent discovery that trehalose synthesis is widespread in higher plants very little is known about its physiological signi®cance. Here we report on an Arabidopsis mutant (tps1), disrupted in a gene encoding the ®rst enzyme of trehalose biosynthesis (trehalose-6-phosphate synthase). The tps1 mutant is a recessive embryo lethal. Embryo morphogenesis is normal but development is retarded and stalls early in the phase of cell expansion and storage reserve accumulation. TPS1 is transiently up-regulated at this same developmental stage and is required for the full expression of seed maturation marker genes (2S2 and OLEOSN2). Sucrose levels also increase rapidly in seeds during the onset of cell expansion. In Saccharomyces cerevisiae trehalose-6-phosphate (T-6-P) is required to regulate sugar in¯ux into glycolysis via the inhibition of hexokinase and a de®ciency in TPS1 prevents growth on sugars (Thevelein and Hohmann, 1995). The growth of Arabidopsis tps1±1 embryos can be partially rescued in vitro by reducing the sucrose level. However, T-6-P is not an inhibitor of AtHXK1 or AtHXK2. Nor does reducing hexokinase activity rescue tps1±1 embryo growth. Our data establish for the ®rst time that an enzyme of trehalose metabolism is essential in plants and is implicated in the regulation of sugar metabolism/embryo development via a different mechanism to that reported in S. cerevisiae.

In resurrection plants and yeast, trehalose has a function in stress protection, but the absence of measurable amounts of trehalose in other plants precludes such a function. The identification of a trehalose biosynthetic pathway in angiosperms raises questions on the function of trehalose metabolism in nonresurrection plants. We previously identified a mutant in the Arabidopsis trehalose biosynthesis gene AtTPS1. Plants homozygous for the tps1 mutation do not develop mature seeds . AtTPS1 expression analysis and the spatial and temporal activity of its promoter suggest that this gene is active outside the seed-filling stage of development as well. A generally low expression is observed in all organs analyzed, peaking in metabolic sinks such as flower buds, ripening siliques, and young rosette leaves. The arrested tps1/tps1 embryonic state could be rescued using a dexamethasone-inducible AtTPS1 expression system enabling generation of homozygous mutant plants. When depleted in AtTPS1 expression, such mutant plants show reduced root growth, which is correlated with a reduced root meristematic region. Moreover, tps1/tps1 plants are retarded in growth and remain generative during their lifetime. Absence of Trehalose-6-Phosphate Synthase 1 in Arabidopsis plants precludes transition to flowering.Trehalose (a-D-glucopyranosyl-[1,1] -a-D-glucopyranoside) accumulation has been observed in a variety of species, most typically in anhydrobionts, which are able to survive complete dehydration. However, trehalose was not detected in plant species, with the exception of stress-induced trehalose accumulation in resurrection plants such as Selaginella lepidophylla (Crowe et al., 1992). Recent isolation of trehalose biosynthetic genes from Arabidopsis (Blazquez et al., 1998;Vogel et al., 1998;Mü ller et al., 2001) has led to the identification of a trehalose biosynthesis pathway in many, if not all, angiosperms. The first trehalose biosynthetic step is catalyzed by trehalose-6-P synthase (TPS), which converts Glc-6-P and UDP-Glc into trehalose-6-P (T-6-P). The second step is catalyzed by T-6-P phosphatase (TPP), which hydrolyzes T-6-P and releases trehalose. This biosynthesis pathway is similar in plants and yeast (Saccharomyces cerevisiae), where this metabolism was first described (Cabib and Leloir, 1958). Trehalose is readily hydrolyzed into two Glc units by trehalase (TRE), which is present in all organs of Arabidopsis and Glycine max (Mü ller et al., 2001).The nonreducing disaccharide trehalose is highly resistant to nonenzymatic hydrolysis (Paiva and Panek, 1996) and is known to stabilize proteins during dehydration (Crowe et al., 1992). However, trehalose levels in nonresurrecting plants are barely detectable and insufficient for this function. It was proposed that trehalose metabolism may play a regulatory role in these plant species (Goddijn and Smeekens, 1998). The ability to utilize available sugars depends on trehalose biosynthesis and is most likely linked to the T-6-P intermediate Schluepmann et al., 2003). Differ...

Jasmonic acid (JA) is a critical hormonal regulator of plant growth and defense. To advance our understanding of the architecture and dynamic regulation of the JA gene regulatory network, we performed a high-resolution RNA-seq time series of methyl JA-treated Arabidopsis thaliana at 15 time points over a 16-h period. Computational analysis showed that methyl JA (MeJA) induces a burst of transcriptional activity, generating diverse expression patterns over time that partition into distinct sectors of the JA response targeting specific biological processes. The presence of transcription factor (TF) DNA binding motifs correlated with specific TF activity during temporal MeJA-induced transcriptional reprogramming. Insight into the underlying dynamic transcriptional regulation mechanisms was captured in a chronological model of the JA gene regulatory network. Several TFs, including MYB59 and bHLH27, were uncovered as early network components with a role in pathogen and insect resistance. Analysis of subnetworks surrounding the TFs ORA47, RAP2.6L, MYB59, and ANAC055, using transcriptome profiling of overexpressors and mutants, provided insights into their regulatory role in defined modules of the JA network. Collectively, our work illuminates the complexity of the JA gene regulatory network, pinpoints and validates previously unknown regulators, and provides a valuable resource for functional studies on JA signaling components in plant defense and development.

The phytohormone jasmonic acid (JA) is a critical regulator of plant growth and defense. To significantly advance our understanding of the architecture and dynamics of the JA gene regulatory network, we performed high-resolution RNA-Seq time series analyses of methyl JAtreated Arabidopsis thaliana. Computational analysis unraveled in detail the chronology of events that occur during the early and later phases of the JA response. Several transcription factors, including ERF16 and bHLH27, were uncovered as early components of the JA gene regulatory network with a role in pathogen and insect resistance. Moreover, analysis of subnetworks surrounding the JA-induced transcription factors ORA47, RAP2.6L, and ANAC055 provided novel insights into their regulatory role of defined JA network modules. Collectively, our work illuminates the complexity of the JA gene regulatory network, pinpoints to novel regulators, and provides a valuable resource for future studies on the function of JA signaling components in plant defense and development.

Highlights d 42% of the tested root microbiota are able to quench local root immune responses d Beneficial Pseudomonas can suppress root immunity by lowering environmental pH d Suppression of immunity facilitates root colonization by these beneficial microbes