Meristems encompass stem/progenitor cells that sustain postembryonic growth of all plant organs. How meristems are activated and sustained by nutrient signalling remains enigmatic in photosynthetic plants. Combining chemical manipulations and chemical genetics at the photoautotrophic transition checkpoint, we reveal that shoot photosynthesis-derived glucose drives target-of-rapamycin (TOR) signalling relays through glycolysis and mitochondrial bioenergetics to control root meristem activation, which is decoupled from direct glucose sensing, growth-hormone signalling, and stem-cell maintenance. Surprisingly, glucose-TOR signalling dictates transcriptional reprogramming of remarkable gene sets involved in central and secondary metabolism, cell cycle, transcription, signalling, transport and folding. Systems, cellular and genetic analyses uncover TOR phosphorylation of E2Fa transcription factor for an unconventional activation of S-phase genes, and glucose-signalling defects in e2fa root meristems. Our findings establish pivotal roles of glucose-TOR signalling in unprecedented transcriptional networks wiring central metabolism and biosynthesis for energy and biomass production, and integrating localized stem/progenitor-cell proliferation through inter-organ nutrient coordination to control developmental transition and growth.
Glutathione plays a pivotal role in protecting plants from environmental stresses, oxidative stress, xenobiotics, and some heavy metals. Arabidopsis plants treated with cadmium or copper responded by increasing transcription of the genes for glutathione synthesis, ␥ -glutamylcysteine synthetase and glutathione synthetase, as well as glutathione reductase. The response was specific for those metals whose toxicity is thought to be mitigated through phytochelatins, and other toxic and nontoxic metals did not alter mRNA levels. Feeding experiments suggested that neither oxidative stress, as results from exposure to H 2 O 2 , nor oxidized or reduced glutathione levels were responsible for activating transcription of these genes. Jasmonic acid also activated the same suite of genes, which suggests that it might be involved in the signal transduction pathway for copper and cadmium. Jasmonic acid treatment increased mRNA levels and the capacity for glutathione synthesis but did not alter the glutathione content in unstressed plants, which supports the idea that the glutathione concentration is controlled at multiple levels. INTRODUCTIONGlutathione (GSH), the tripeptide ␥ -glutamylcysteinylglycine, is the major source of non-protein thiols in most plant cells (Bergmann and Rennenberg, 1993). GSH plays a central role in protecting plants from environmental stresses, including oxidative stress due to the generation of active oxygen species, xenobiotics, and some heavy metals.GSH is involved in quenching reactive oxygen species (Larson, 1988; Alscher, 1989; Foyer et al., 1994b). The ascorbate/GSH cycle reduces H 2 O 2 to water (Foyer and Halliwell, 1976; Alscher, 1989). Ascorbate is also important in maintaining ␣ -tocopherol in the reduced state and therefore links GSH to the dominant free radical scavenger in membranes (Hess, 1994). Plants detoxify many organic contaminants by conjugating them or their metabolites to GSH for storage or further metabolism (Lamoureux et al., 1994). These reactions are catalyzed by glutathione S -transferases (GSTs). Plants are protected from some metals, with cadmium and copper being the most studied, by a group of ␥ -glutamylcysteine ( ␥ -EC) peptides, the phytochelatins (PCs). These molecules have the general structure ( ␥ -Glu-Cys) 2-11 -Gly. They are formed by the polymerization of GSH catalyzed by the transpeptidase phytochelatin synthase (Grill et al., 1985(Grill et al., , 1987 Chen et al., 1997). The PCs bind metals in the cytosol, and the PC metal complex is sequestered in the vacuole (Rauser, 1990).GSH is synthesized from glutamate, cysteine, and glycine by a two-step ATP-dependent reaction (Meister and Anderson, 1983). The first reaction forms ␥ -EC from glutamate and cysteine by the enzyme ␥ -EC synthetase (Hell and Bergmann, 1990), which is encoded by gsh1 (May and Leaver, 1995). GSH is then synthesized by the ligation of ␥ -EC and glycine in the reaction catalyzed by the enzyme GSH synthetase, which is encoded by gsh2 (Wang and Oliver, 1996). When GSH is oxidized as part of ...
We have achieved routine transformation of maize (Zea mays) using an Agrobacterium tumefaciens standard binary (non-super binary) vector system. Immature zygotic embryos of the hybrid line Hi II were infected with A. tumefaciens strain EHA101 harboring a standard binary vector and cocultivated in the presence of 400 mg L Ϫ1 l-cysteine. Inclusion of l-cysteine in cocultivation medium lead to an improvement in transient -glucuronidase expression observed in targeted cells and a significant increase in stable transformation efficiency, but was associated with a decrease in embryo response after cocultivation. The average stable transformation efficiency (no. of bialaphos-resistant events recovered per 100 embryos infected) of the present protocol was 5.5%. Southern-blot and progeny analyses confirmed the integration, expression, and inheritance of the bar and gus transgenes in R 0 , R 1 , and R 2 generations of transgenic events. To our knowledge, this represents the first report in which fertile, stable transgenic maize has been routinely produced using an A. tumefaciens standard binary vector system.
Drought is one of the most important environmental constraints limiting plant growth and agricultural productivity. To understand the underlying mechanism of drought tolerance and to identify genes for improving this important trait, we conducted a gain-of-function genetic screen for improved drought tolerance in Arabidopsis thaliana. One mutant with improved drought tolerance was isolated and designated as enhanced drought tolerance1. The mutant has a more extensive root system than the wild type, with deeper roots and more lateral roots, and shows a reduced leaf stomatal density. The mutant had higher levels of abscisic acid and Pro than the wild type and demonstrated an increased resistance to oxidative stress and high levels of superoxide dismutase. Molecular genetic analysis and recapitulation experiments showed that the enhanced drought tolerance is caused by the activated expression of a T-DNA tagged gene that encodes a putative homeodomain-START transcription factor. Moreover, overexpressing the cDNA of the transcription factor in transgenic tobacco also conferred drought tolerance associated with improved root architecture and reduced leaf stomatal density. Therefore, we have revealed functions of the homeodomain-START factor that were gained upon altering its expression pattern by activation tagging and provide a key regulator that may be used to improve drought tolerance in plants.
Jasmonic acid (JA) is well known to promote lateral root formation but the mechanisms by which JA signalling is integrated into the pathways responsible for lateral root formation, and how it interacts with auxin in this process remains poorly understood. Here, we report that the highly JA-responsive ethylene response factor 109 (ERF109) mediates cross-talk between JA signalling and auxin biosynthesis to regulate lateral root formation in Arabidopsis. erf109 mutants have fewer lateral roots under MeJA treatments compared with wild type whereas ERF109 overexpression causes a root phenotype that resembles those of auxin overproduction mutants. ERF109 binds directly to GCC-boxes in the promoters of ASA1 and YUC2, which encode two key enzymes in auxin biosynthesis. Thus, our study reveals a molecular mechanism for JA and auxin cross-talk during JA-induced lateral root formation.
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