Vascularization, high-volume solution flow, and localized roles for enzymes of sucrose metabolism during tumorigenesis by Agrobacterium tumefaciens

Origin of Pathogens of Grapevine Crown Gall Disease in Hokkaido in Japan as Characterized by Molecular Epidemiology of Allorhizobium vitis Strains

Crown gall is a globally distributed and economically important disease of grapevine and other important crop plants. The causal agent of grapevine crown gall is tumorigenic Allorhizobium vitis (Ti) strains that harbor a tumor-inducing plasmid (pTi). The epidemic of grapevine crown gall has not been widely elucidated. In this study, we investigated the genetic diversity of 89 strains of Ti and nonpathogenic A. vitis to clarify their molecular epidemiology. Multi-locus sequence analysis (MLSA) of the partial nucleotide sequences of pyrG, recA, and rpoD was performed for molecular typing of A. vitis strains isolated from grapevines with crown gall symptoms grown in 30 different vineyards, five different countries, mainly in Japan, and seven genomic groups A to F were obtained. The results of MLSA and logistic regression indicated that the population of genetic group A was significantly related to a range of prefectures and that the epidemic of group A strains originated mainly in Hokkaido in Japan through soil infection. Moreover, group E strains could have been transported by infected nursery stocks. In conclusion, this study indicates that both soil infection and transporting of infected nursery stocks are working as infection source in Hokkaido.

Stationary sieve element proteins

Vascular plants use the phloem to move sugars and other molecules from source leaves to sink organs such as roots and fruits. Within the phloem, enucleate sieve elements provide the low-resistance pipe system that enable bulk flow of sap. In this review, we provide an overview of the highly specific protein machinery that localize to mature sieve elements without entering the phloem translocation stream. Generally, the proteins either maintain the flow, protect the sieve element against pathogens or transmit system wide signals. A notable exception is found in poppy, where part of the opium biosynthesis is compartmentalized in sieve elements. Biosynthesis of sieve element proteins happens either continuously in companion cell or transiently in immature sieve elements before nuclear disintegration. The latter population is translated during differentiation and stays functional without turnover during the entire lifespan of sieve elements. We discuss how protein longevity imposes some interesting restrictions on plants, especially in arborescent monocots with long living sieve elements.

The Multifaceted Roles of Proline in Cell Behavior

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: (i) A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); (ii) an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; (iii) an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); (iv) a neural metabotoxin associated with schizophrenia; (v) a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; (vi) an epigenetic modifier able to promote DNA and histone hypermethylation; (vii) an inducer of proliferation of stem and tumor cells; and (viii) a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

Non-structural Carbohydrate Metabolism in the Flesh of Stone Fruits of the Genus Prunus (Rosaceae) - A Review

Non-structural carbohydrates are abundant constituents of the ripe flesh of all stone fruits. The bulk of their content comprises sucrose, glucose, fructose and sorbitol. However, the abundance of each of these carbohydrates in the flesh differs between species, and also with its stage of development. In this article the import, subcellular compartmentation, contents, metabolism and functions of non-structural carbohydrates in the flesh of commercially cultivated stone fruits of the family Rosaceae are reviewed.

Structural basis of a novel repressor, SghR, controlling Agrobacterium infection by cross-talking to plants

Agrobacterium tumefaciens infects various plants and causes crown gall diseases involving temporal expression of virulence factors. SghA is a newly identified virulence factor enzymatically releasing salicylic acid from its glucoside conjugate and controlling plant tumor development. Here, we report the structural basis of SghR, a LacI-type transcription factor highly conserved in Rhizobiaceae family, regulating the expression of SghA and involved in tumorigenesis. We identified and characterized the binding site of SghR on the promoter region of sghA and then determined the crystal structures of apo-SghR, SghR complexed with its operator DNA, and ligand sucrose, respectively. These results provide detailed insights into how SghR recognizes its cognate DNA and shed a mechanistic light on how sucrose attenuates the affinity of SghR with DNA to modulate the expression of SghA. Given the important role of SghR in mediating the signaling cross-talk during Agrobacterium infection, our results pave the way for structure-based inducer analog design, which has potential applications for agricultural industry.

Arabidopsis sucrose synthase localization indicates a primary role in sucrose translocation in phloem

Sucrose synthase (SuSy) is one of two enzyme families capable of catalyzing the first degradative step in sucrose utilization. Several earlier studies examining SuSy mutants in Arabidopsis failed to identify obvious phenotypic abnormalities compared with wild-type plants in normal growth environments, and as such a functional role for SuSy in the previously proposed cellulose biosynthetic process remains unclear. Our study systematically evaluated the precise subcellular localization of all six isoforms of Arabidopsis SuSy via live-cell imaging. We showed that yellow fluorescent protein (YFP)-labeled SuSy1 and SuSy4 were expressed exclusively in phloem companion cells, and the sus1/sus4 double mutant accumulated sucrose under hypoxic conditions. SuSy5 and SuSy6 were found to be parietally localized in sieve elements and restricted only to the cytoplasm. SuSy2 was present in the endosperm and embryo of developing seeds, and SuSy3 was localized to the embryo and leaf stomata. No single isoform of SuSy was detected in developing xylem tissue of elongating stem, the primary site of cellulose deposition in plants. SuSy1 and SuSy4 were also undetectable in the protoxylem tracheary elements, which were induced by the vascular-related transcription factor VND7 during secondary cell wall formation. These findings implicate SuSy in the biological events related to sucrose translocation in phloem.

Comparison between tumors in plants and human beings: Mechanisms of tumor development and therapy with secondary plant metabolites

BACKGROUND

Human tumors are still a major threat to human health and plant tumors negatively affect agricultural yields. Both areas of research are developing largely independent of each other. Treatment of both plant and human tumors remains unsatisfactory and novel therapy options are urgently needed.

HYPOTHESIS

The concept of this paper is to compare cellular and molecular mechanisms of tumor development in plants and human beings and to explore possibilities to develop novel treatment strategies based on bioactive secondary plant metabolites. The interdisciplinary discourse may unravel commonalities and differences in the biology of plant and human tumors as basis for rational drug development.

RESULTS

Plant tumors and galls develop upon infection by bacteria (e.g. Agrobacterium tumefaciens and A. vitis, which harbor oncogenic T-DNA) and by insects (e.g. gall wasps, aphids). Plant tumors are benign, i.e. they usually do not ultimately kill their host, but they can lead to considerable economic damage due to reduced crop yields of cultivated plants. Human tumors develop by biological carcinogenesis (i.e. viruses and other infectious agents), chemical carcinogenesis (anthropogenic and non-anthropogenic environmental toxic xenobiotics) and physical carcinogenesis (radioactivity, UV-radiation). The majority of human tumors are malignant with lethal outcome. Although treatments for both plant and human tumors are available (antibiotics and apathogenic bacterial strains for plant tumors, cytostatic drugs for human tumors), treatment successes are non-satisfactory, because of drug resistance and the severe adverse side effects. In human beings, attacks by microbes are repelled by cellular immunity (i.e. innate and acquired immune systems). Plants instead display chemical defense mechanisms, whereby constitutively expressed phytoanticipin compounds compare to the innate human immune system, the acquired human immune system compares to phytoalexins, which are induced by appropriate biotic or abiotic stressors. Some chemical weapons of this armory of secondary metabolites are also active against plant galls. There is a mutual co-evolution between plant defense and animals/human beings, which was sometimes referred to as animal plant warfare. As a consequence, hepatic phase I-III metabolization and excretion developed in animals and human beings to detoxify harmful phytochemicals. On the other hand, plants invented "pro-drugs" during evolution, which are activated and toxified in animals by this hepatic biotransformation system. Recent efforts focus on phytochemicals that specifically target tumor-related mechanisms and proteins, e.g. angiogenic or metastatic inhibitors, stimulators of the immune system to improve anti-tumor immunity, specific cell death or cancer stem cell inhibitors, inhibitors of DNA damage and epigenomic deregulation, specific inhibitors of driver genes of carcinogenesis (e.g. oncogenes), inhibitors of multidrug resistance (i.e. ABC transporter efflux inhibitors), secondary metabolites against plant tumors.

CONCLUSION

The exploitation of bioactive secondary metabolites to treat plant or human tumors bears a tremendous therapeutic potential. Although there are fundamental differences between human and plant tumors, either isolated phytochemicals and their (semi)synthetic derivatives or chemically defined and standardized plant extracts may offer new therapy options to decrease human tumor incidence and mortality as well as to increase agricultural yields by fighting crown galls.

Ecological Conditions and Molecular Determinants Involved in Agrobacterium Lifestyle in Tumors

The study of pathogenic agents in their natural niches allows for a better understanding of disease persistence and dissemination. Bacteria belonging to the Agrobacterium genus are soil-borne and can colonize the rhizosphere. These bacteria are also well known as phytopathogens as they can cause tumors (crown gall disease) by transferring a DNA region (T-DNA) into a wide range of plants. Most reviews on Agrobacterium are focused on virulence determinants, T-DNA integration, bacterial and plant factors influencing the efficiency of genetic transformation. Recent research papers have focused on the plant tumor environment on the one hand, and genetic traits potentially involved in bacterium-plant interactions on the other hand. The present review gathers current knowledge about the special conditions encountered in the tumor environment along with the Agrobacterium genetic determinants putatively involved in bacterial persistence inside a tumor. By integrating recent metabolomic and transcriptomic studies, we describe how tumors develop and how Agrobacterium can maintain itself in this nutrient-rich but stressful and competitive environment.

The biotroph Agrobacterium tumefaciens thrives in tumors by exploiting a wide spectrum of plant host metabolites

Agrobacterium tumefaciens is a niche-constructing biotroph that exploits host plant metabolites. We combined metabolomics, transposon-sequencing (Tn-seq), transcriptomics, and reverse genetics to characterize A. tumefaciens pathways involved in the exploitation of resources from the Solanum lycopersicum host plant. Metabolomics of healthy stems and plant tumors revealed the common (e.g. sucrose, glutamate) and enriched (e.g. opines, γ-aminobutyric acid (GABA), γ-hydroxybutyric acid (GHB), pyruvate) metabolites that A. tumefaciens could use as nutrients. Tn-seq and transcriptomics pinpointed the genes that are crucial and/or upregulated when the pathogen grew on either sucrose (pgi, kdgA, pycA, cisY) or GHB (blcAB, pckA, eno, gpsA) as a carbon source. While sucrose assimilation involved the Entner-Doudoroff and tricarboxylic acid (TCA) pathways, GHB degradation required the blc genes, TCA cycle, and gluconeogenesis. The tumor-enriched metabolite pyruvate is at the node connecting these pathways. Using reverse genetics, we showed that the blc, pckA, and pycA loci were important for aggressiveness (tumor weight), proliferation (bacterial charge), and/or fitness (competition between the constructed mutants and wild-type) of A. tumefaciens in plant tumors. This work highlighted how a biotroph mobilizes its central metabolism for exploiting a wide diversity of resources in a plant host. It further shows the complementarity of functional genome-wide scans by transcriptomics and Tn-seq to decipher the lifestyle of a plant pathogen.

Neovascularization during leafy gall formation on Arabidopsis thaliana upon Rhodococcus fascians infection

Extensive de novo vascularization of leafy galls emerging upon Rhodococcus fascians infection is achieved by fascicular/interfascicular cambium activity and transdifferentiation of parenchyma cells correlated with increased auxin signaling. A leafy gall consisting of fully developed yet growth-inhibited shoots, induced by the actinomycete Rhodococcus fascians, differs in structure compared to the callus-like galls induced by other bacteria. To get insight into the vascular development accompanying the emergence of the leafy gall, the anatomy of infected axillary regions of the inflorescence stem of wild-type Arabidopsis thaliana accession Col-0 plants and the auxin response in pDR5:GUS-tagged plants were followed in time. Based on our observations, three phases can be discerned during vascularization of the symptomatic tissue. First, existing fascicular cambium becomes activated and interfascicular cambium is formed giving rise to secondary vascular elements in a basipetal direction below the infection site in the main stem and in an acropetal direction in the entire side branch. Then, parenchyma cells in the region between both stems transdifferentiate acropetally towards the surface of the developing symptomatic tissue leading to the formation of xylem and vascularize the hyperplasia as they expand. Finally, parenchyma cells in the developing gall also transdifferentiate to vascular elements without any specific direction resulting in excessive vasculature disorderly distributed in the leafy gall. Prior to any apparent anatomical changes, a strong auxin response is mounted, implying that auxin is the signal that controls the vascular differentiation induced by the infection. To conclude, we propose the "sidetracking gall hypothesis" as we discuss the mechanisms driving the formation of superfluous vasculature of the emerging leafy gall.

Dynamic distribution and the role of abscisic acid during seed development of a lady's slipper orchid, Cypripedium formosanum

BACKGROUND AND AIMS

Although abscisic acid (ABA) is commonly recognized as a primary cause of seed dormancy, there is a lack of information on the role of ABA during orchid seed development. In order to address this issue, the localization and quantification of ABA were determined in developing seeds of Cypripedium formosanum.

METHODS

The endogenous ABA profile of seeds was measured by enzyme-linked immunosorbent assay (ELISA). Temporal and spatial distributions of ABA in developing seeds were visualized by immunohistochemical staining with monoclonal ABA antibodies. Fluoridone was applied to test the causal relationship between ABA content and seed germinability.

KEY RESULTS

ABA content was low at the proembryo stage, then increased rapidly from 120 to 150 days after pollination (DAP), accompanied by a progressive decrease in water content and seed germination. Immunofluorescence signals indicated an increase in fluorescence over time from the proembryo stage to seed maturation. From immunogold labelling, gold particles could be seen within the cytoplasm of embryo-proper cells during the early stages of seed development. As seeds approached maturity, increased localization of gold particles was observed in the periplasmic space, the plasmalemma between embryo-proper cells, the surface wall of the embryo proper, and the inner walls of inner seed-coat cells. At maturity, gold particles were found mainly in the apoplast, such as the surface wall of the embryo proper, and the shrivelled inner and outer seed coats. Injection of fluoridone into capsules resulted in enhanced germination of mature seeds.

CONCLUSIONS

The results indicate that ABA is the key inhibitor of germination in C. formosanum. The distinct accumulation pattern of ABA suggests that it is synthesized in the cytosol of embryo cells during the early stages of seed development, and then exported to the apoplastic region of the cells for subsequent regulatory processes as seeds approach maturity.

Plant-Agrobacterium interaction mediated by ethylene and super-Agrobacterium conferring efficient gene transfer

Agrobacterium tumefaciens has a unique ability to transfer genes into plant genomes. This ability has been utilized for plant genetic engineering. However, the efficiency is not sufficient for all plant species. Several studies have shown that ethylene decreased the Agrobacterium-mediated transformation frequency. Thus, A. tumefaciens with an ability to suppress ethylene evolution would increase the efficiency of Agrobacterium-mediated transformation. Some studies showed that plant growth-promoting rhizobacteria (PGPR) can reduce ethylene levels in plants through 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, which cleaves the ethylene precursor ACC into α-ketobutyrate and ammonia, resulting in reduced ethylene production. The whole genome sequence data showed that A. tumefaciens does not possess an ACC deaminase gene in its genome. Therefore, providing ACC deaminase activity to the bacteria would improve gene transfer. As expected, A. tumefaciens with ACC deaminase activity, designated as super-Agrobacterium, could suppress ethylene evolution and increase the gene transfer efficiency in several plant species. In this review, we summarize plant-Agrobacterium interactions and their applications for improving Agrobacterium-mediated genetic engineering techniques via super-Agrobacterium.

Evolution of proline biosynthesis: enzymology, bioinformatics, genetics, and transcriptional regulation

Proline is not only an essential component of proteins but it also has important roles in adaptation to osmotic and dehydration stresses, redox control, and apoptosis. Here, we review pathways of proline biosynthesis in the three domains of life. Pathway reconstruction from genome data for hundreds of eubacterial and dozens of archaeal and eukaryotic organisms revealed evolutionary conservation and variations of this pathway across different taxa. In the most prevalent pathway of proline synthesis, glutamate is phosphorylated to γ-glutamyl phosphate by γ-glutamyl kinase, reduced to γ-glutamyl semialdehyde by γ-glutamyl phosphate reductase, cyclized spontaneously to Δ(1)-pyrroline-5-carboxylate and reduced to proline by Δ(1)-pyrroline-5-carboxylate reductase. In higher plants and animals the first two steps are catalysed by a bi-functional Δ(1) -pyrroline-5-carboxylate synthase. Alternative pathways of proline formation use the initial steps of the arginine biosynthetic pathway to ornithine, which can be converted to Δ(1)-pyrroline-5-carboxylate by ornithine aminotransferase and then reduced to proline or converted directly to proline by ornithine cyclodeaminase. In some organisms, the latter pathways contribute to or could be fully responsible for the synthesis of proline. The conservation of proline biosynthetic enzymes and significance of specific residues for catalytic activity and allosteric regulation are analysed on the basis of protein structural data, multiple sequence alignments, and mutant studies, providing novel insights into proline biosynthesis in organisms. We also discuss the transcriptional control of the proline biosynthetic genes in bacteria and plants.

Agrobacterium tumefaciens responses to plant-derived signaling molecules

As a special phytopathogen, Agrobacterium tumefaciens infects a wide range of plant hosts and causes plant tumors also known as crown galls. The complexity of Agrobacterium-plant interaction has been studied for several decades. Agrobacterium pathogenicity is largely attributed to its evolved capabilities of precise recognition and response to plant-derived chemical signals. Agrobacterium perceives plant-derived signals to activate its virulence genes, which are responsible for transferring and integrating its Transferred DNA (T-DNA) from its Tumor-inducing (Ti) plasmid into the plant nucleus. The expression of T-DNA in plant hosts leads to the production of a large amount of indole-3-acetic acid (IAA), cytokinin (CK), and opines. IAA and CK stimulate plant growth, resulting in tumor formation. Agrobacterium utilizes opines as nutrient sources as well as signals in order to activate its quorum sensing (QS) to further promote virulence and opine metabolism. Intriguingly, Agrobacterium also recognizes plant-derived signals including γ-amino butyric acid and salicylic acid (SA) to activate quorum quenching that reduces the level of QS signals, thereby avoiding the elicitation of plant defense and preserving energy. In addition, Agrobacterium hijacks plant-derived signals including SA, IAA, and ethylene to down-regulate its virulence genes located on the Ti plasmid. Moreover, certain metabolites from corn (Zea mays) also inhibit the expression of Agrobacterium virulence genes. Here we outline the responses of Agrobacterium to major plant-derived signals that impact Agrobacterium-plant interactions.

Sucrose and invertases, a part of the plant defense response to the biotic stresses

Sucrose is the main form of assimilated carbon which is produced during photosynthesis and then transported from source to sink tissues via the phloem. This disaccharide is known to have important roles as signaling molecule and it is involved in many metabolic processes in plants. Essential for plant growth and development, sucrose is engaged in plant defense by activating plant immune responses against pathogens. During infection, pathogens reallocate the plant sugars for their own needs forcing the plants to modify their sugar content and triggering their defense responses. Among enzymes that hydrolyze sucrose and alter carbohydrate partitioning, invertases have been reported to be affected during plant-pathogen interactions. Recent highlights on the role of invertases in the establishment of plant defense responses suggest a more complex regulation of sugar signaling in plant-pathogen interaction.

Plant responses to Agrobacterium tumefaciens and crown gall development

Agrobacterium tumefaciens causes crown gall disease on various plant species by introducing its T-DNA into the genome. Therefore, Agrobacterium has been extensively studied both as a pathogen and an important biotechnological tool. The infection process involves the transfer of T-DNA and virulence proteins into the plant cell. At that time the gene expression patterns of host plants differ depending on the Agrobacterium strain, plant species and cell-type used. Later on, integration of the T-DNA into the plant host genome, expression of the encoded oncogenes, and increase in phytohormone levels induce a fundamental reprogramming of the transformed cells. This results in their proliferation and finally formation of plant tumors. The process of reprogramming is accompanied by altered gene expression, morphology and metabolism. In addition to changes in the transcriptome and metabolome, further genome-wide ("omic") approaches have recently deepened our understanding of the genetic and epigenetic basis of crown gall tumor formation. This review summarizes the current knowledge about plant responses in the course of tumor development. Special emphasis is placed on the connection between epigenetic, transcriptomic, metabolomic, and morphological changes in the developing tumor. These changes not only result in abnormally proliferating host cells with a heterotrophic and transport-dependent metabolism, but also cause differentiation and serve as mechanisms to balance pathogen defense and adapt to abiotic stress conditions, thereby allowing the coexistence of the crown gall and host plant.

Soluble and filamentous proteins in Arabidopsis sieve elements

Phloem sieve elements are highly differentiated cells involved in the long-distance transport of photoassimilates. These cells contain both aggregated phloem-proteins (P-proteins) and soluble proteins, which are also translocated by mass flow. We used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to carry out a proteomic survey of the phloem exudate of Arabidopsis thaliana, collected by the ethylenediaminetetraacetic acid (EDTA)-facilitated method. We identified 287 proteins, a large proportion of which were enzymes involved in the metabolic precursor generation and amino acid synthesis, suggesting that sieve tubes display high levels of metabolic activity. RNA-binding proteins, defence proteins and lectins were also found. No putative P-proteins were detected in the EDTA-exudate fraction, indicating a lack of long-distance translocation of such proteins in Arabidopsis. In parallel, we investigated the organization of P-proteins, by high-resolution transmission electron microscopy, and the localization of the phloem lectin PP2, a putative P-protein component, by immunolocalization with antibodies against PP2-A1. Transmission electron microscopy observations of P-proteins revealed bundles of filaments resembling strings of beads. PP2-A1 was found weakly associated with these structures in the sieve elements and bound to plastids. These observations suggest that PP2-A1 is anchored to P-proteins and organelles rather than being a structural component of P-proteins.

The purine-rich DNA-binding protein OsPurα participates in the regulation of the rice sucrose synthase 1 gene expression

The rice sucrose synthase 1 (RSus1) gene is transcriptionally induced by sucrose, and a region within its promoter, at -1117 to -958 upstream of the transcription initiation site, was found to be essential for enhancing the sucrose-induced expression. Further dissection of this region revealed that a group of nuclear proteins interact with a 39-bp fragment named A-3-2 (-1045 to -1007). A protein that specifically and directly interacted with A-3-2 was isolated from the suspension-cultured cells of rice and was subsequently identified as a purine-rich DNA-binding protein. The amino acid sequence of this protein, OsPurα, exhibited 73% identity with the Arabidopsis Purα-1 protein, and its modeled structure resembled the structure of Pur-α in Drosophila. Recombinant OsPurα expressed and purified from Escherichia coli was demonstrated to have DNA-binding activity and to interact with A-3-2 specifically. Moreover, OsPurα was able to enhance sucrose-induced expression of the β-glucuronidase (GUS) reporter gene, which was transcriptionally fused to two copies of a DNA fragment containing A-3-2 and the cauliflower mosaic virus 35S minimal promoter, in vivo. The level of OsPurα bound to A-3-2 was higher in cells cultured in the presence of sucrose; however, the level of OsPurα mRNA in cells was not affected by sucrose. The results of this study demonstrate that OsPurα participates in the regulation of RSus1 expression in response to sucrose; nevertheless, it may require other partner proteins for full function.

The structure of sucrose synthase-1 from Arabidopsis thaliana and its functional implications

Sucrose transport is the central system for the allocation of carbon resources in vascular plants. During growth and development, plants control carbon distribution by coordinating sites of sucrose synthesis and cleavage in different plant organs and different cellular locations. Sucrose synthase, which reversibly catalyzes sucrose synthesis and cleavage, provides a direct and reversible means to regulate sucrose flux. Depending on the metabolic environment, sucrose synthase alters its cellular location to participate in cellulose, callose, and starch biosynthesis through its interactions with membranes, organelles, and cytoskeletal actin. The x-ray crystal structure of sucrose synthase isoform 1 from Arabidopsis thaliana (AtSus1) has been determined as a complex with UDP-glucose and as a complex with UDP and fructose, at 2.8- and 2.85-Å resolutions, respectively. The AtSus1 structure provides insights into sucrose catalysis and cleavage, as well as the regulation of sucrose synthase and its interactions with cellular targets.

A fine control of quorum-sensing communication in Agrobacterium tumefaciens

The bacterial pathogen Agrobacterium tumefaciens produces the quorum-sensing (QS) signal 3-oxo-octanoylhomoserine lactone (OC8HSL) for controlling horizontal transfer of its tumor inducing (Ti) plasmid that carries both the T-DNA and the virulence genes. Over-accumulation of OC8HSL also increases severity of plant symptoms (number of emerging tumors at infection site) by an unknown mechanism. A. tumefaciens strain C58 expresses two lactonases, AttM (BlcC) and AiiB, that cleave OC8HSL and are potential modulators of QS. Recent data highlight the direct contribution of lactonases AttM and AiiB in the control of OC8HSL level and QS-regulated functions such as conjugation of Ti plasmid and seriousness of plant symptoms. Expression of the two lactonases is regulated by different plant signals. A working model of QS in the course of the A. tumefaciens-plant host interaction is proposed and discussed.

Walls around tumours - why plants do not develop cancer

In plants, as in animals, most cells that constitute the organism limit their reproductive potential in order to provide collective support for the immortal germ line. And, as in animals, the mechanisms that restrict the proliferation of somatic cells in plants can fail, leading to tumours. There are intriguing similarities in tumorigenesis between plants and animals, including the involvement of the retinoblastoma pathway as well as overlap with mechanisms that are used for stem cell maintenance. However, plant tumours are less frequent and are not as lethal as those in animals. We argue that fundamental differences between plant and animal development make it much more difficult for individual plant cells to escape communal controls.

Proline metabolism and its implications for plant-environment interaction

Proline has long been known to accumulate in plants experiencing water limitation and this has driven studies of proline as a beneficial solute allowing plants to increase cellular osmolarity during water limitation. Proline metabolism also has roles in redox buffering and energy transfer and is involved in plant pathogen interaction and programmed cell death. Some of these unique roles of proline depend on the properties of proline itself, whereas others depend on the "proline cycle" of coordinated proline synthesis in the chloroplast and cytoplasm with proline catabolism in the mitochondria. The regulatory mechanisms controlling proline metabolism, intercellular and intracellular transport and connections of proline to other metabolic pathways are all important to the in vivo functions of proline metabolism. Connections of proline metabolism to the oxidative pentose phosphate pathway and glutamate-glutamine metabolism are of particular interest. The N-acetyl glutamate pathway can also produce ornithine and, potentially, proline but its role and activity are unclear. Use of model systems such as Arabidopsis thaliana to better understand both these long studied and newly emerging functions of proline can help in the design of next-generation experiments testing whether proline metabolism is a promising metabolic engineering target for improving stress resistance of economically important plants.

LeFRK2 is required for phloem and xylem differentiation and the transport of both sugar and water

It has been suggested that LeFRK2, the major fructose-phosphorylating enzyme in tomato plants, may be required for stem xylem development. Yet, we do not know if this enzyme affects the development of individual vessels, whether it affects water conductance, or whether it affects phloem development and sugar transport. Here, we show that suppression of LeFRK2 results in a significant reduction in the size of vascular cells and slows fiber maturation. The vessels in stems of LeFRK2-antisense plants are narrower than in WT plants and have thinner secondary cell walls. Although the cambium produces rounded secondary vessels, these vessels become deformed during the early stages of xylem maturation. Water conductance is then reduced in stems, roots, and leaves, suggesting that LeFRK2 influences xylem development throughout the entire vascular system. Interestingly, the build-up of positive xylem pressure under static (no-flow) conditions was also decreased. Suppression of LeFRK2 reduced the length and width of the sieve elements, as well as callose deposition. To examine the effect of LeFRK2 suppression on phloem transport, we created triple-grafted plants in which a portion of the wild-type stem was replaced with an antisense interstcok, and compared the contents of the transported sugar, sucrose, in the different portions of these stems. Sucrose contents above and within the LeFRK2-antisense interstock were significantly higher than those below the graft. These results show that the antisense interstock restricted the downward movement of sucrose, suggesting that LeFRK2 is required for both phloem and xylem development.

Agrobacterium tumefaciens promotes tumor induction by modulating pathogen defense in Arabidopsis thaliana

Agrobacterium tumefaciens causes crown gall disease by transferring and integrating bacterial DNA (T-DNA) into the plant genome. To examine the physiological changes and adaptations during Agrobacterium-induced tumor development, we compared the profiles of salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and auxin (indole-3-acetic acid [IAA]) with changes in the Arabidopsis thaliana transcriptome. Our data indicate that host responses were much stronger toward the oncogenic strain C58 than to the disarmed strain GV3101 and that auxin acts as a key modulator of the Arabidopsis-Agrobacterium interaction. At initiation of infection, elevated levels of IAA and ET were associated with the induction of host genes involved in IAA, but not ET signaling. After T-DNA integration, SA as well as IAA and ET accumulated, but JA did not. This did not correlate with SA-controlled pathogenesis-related gene expression in the host, although high SA levels in mutant plants prevented tumor development, while low levels promoted it. Our data are consistent with a scenario in which ET and later on SA control virulence of agrobacteria, whereas ET and auxin stimulate neovascularization during tumor formation. We suggest that crosstalk among IAA, ET, and SA balances pathogen defense launched by the host and tumor growth initiated by agrobacteria.

Proline antagonizes GABA-induced quenching of quorum-sensing in Agrobacterium tumefaciens

Plants accumulate free L-proline (Pro) in response to abiotic stresses (drought and salinity) and presence of bacterial pathogens, including the tumor-inducing bacterium Agrobacterium tumefaciens. However, the function of Pro accumulation in host-pathogen interaction is still unclear. Here, we demonstrated that Pro antagonizes plant GABA-defense in the A. tumefaciens C58-induced tumor by interfering with the import of GABA and consequently the GABA-induced degradation of the bacterial quorum-sensing signal, 3-oxo-octanoylhomoserine lactone. We identified a bacterial receptor Atu2422, which is implicated in the uptake of GABA and Pro, suggesting that Pro acts as a natural antagonist of GABA-signaling. The Atu2422 amino acid sequence contains a Venus flytrap domain that is required for trapping GABA in human GABA(B) receptors. A constructed atu2422 mutant was more virulent than the wild type bacterium; moreover, transgenic plants with a low level of Pro exhibited less severe tumor symptoms than did their wild-type parents, revealing a crucial role for Venus flytrap GABA-receptor and relative abundance of GABA and Pro in host-pathogen interaction.

Regulation of photosynthesis by sugars in sugarcane leaves

In sugarcane, increased sink demand has previously been shown to result in increased photosynthetic rates that are correlated with a reduction in leaf hexose concentrations. To establish whether sink limitation of photosynthesis is a result of sugar accumulation in the leaf, excision and cold-girdling techniques were used to modify leaf sugar concentrations in pot-grown sugarcane. In excised leaves that were preincubated in darkness for 3h, sucrose accumulation was reduced but accumulated again upon transfer to the light, while hexose concentrations remained lower than in controls (7.7 micromol mg(-1)FW versus 18.6 micromol mg(-1)FW hexose in controls). These results were associated with a 66% and 59% increase in photosynthetic assimilation (A) and electron transport rate (ETR), respectively, compared to controls maintained in the light. Similar increases in photosynthesis were observed when dark-treated leaves were supplied with 5mM sorbitol, but not when supplied with 5mM sucrose. Further analyses of (14)C-labeled sugars indicated rapid turnover between sucrose and hexose. Cold-girdling (5 degrees C) increased sucrose and hexose levels and resulted in a decline of photosynthetic rates over 5d (48% and 35% decline in assimilation rate and ETR, respectively). These sugar-induced changes in photosynthesis were independent of changes in stomatal conductance. This study demonstrates that the down-regulation of photosynthesis in response to culm sugar accumulation reported previously could be due to the knock-on effect of accumulation of sugar in leaf tissue, and supports the contention that hexose, rather than sucrose, is responsible for the modulation of photosynthetic activity.

Differential vascularization of nematode-induced feeding sites

Sedentary nematodes are destructive plant pathogens that cause significant yield losses. In the roots of their host plants, cyst nematodes (CNs) and root-knot nematodes (RKNs) induce different, highly specialized feeding sites--syncytia or giant cells (GCs), respectively--to optimize nutrient uptake. We compared the mechanisms by which nutrients are delivered from the model host plant, Arabidopsis, to GCs induced by the RKN Meloidogyne incognita or to syncytia induced by the CN Heterodera schachtii. From previous work, syncytia were known to be symplastically connected to newly formed host phloem composed of sieve elements (SEs) and companion cells. Here we studied the formation of plasmodesmata (PD) during GC and syncytia development by monitoring a viral movement protein that targets branched PD and the development of host phloem during GC formation by applying confocal laser scanning microscopy and immunocytochemistry. Analyses of plants expressing soluble or membrane-anchored green fluorescent protein in their phloem demonstrated symplastic isolation of GCs. GCs were found to be embedded in a tissue that consists exclusively of SEs. These de novo-formed SEs, contained nuclei and were interconnected by secondary PD. A similar interconnection of SEs was observed around syncytia. However, these secondary PD were also present at the SE-syncytium interface, demonstrating the postulated symplastic connection. Our results show that CNs and RKNs, despite their close phylogenetic relatedness, employ fundamentally different strategies to withdraw nutrients from host plants.

Cytokinin deficiency causes distinct changes of sink and source parameters in tobacco shoots and roots

Cytokinin deficiency causes pleiotropic developmental changes such as reduced shoot and increased root growth. It was investigated whether cytokinin-deficient tobacco plants, which overproduce different cytokinin oxidase/dehydrogenase enzymes, show changes in different sink and source parameters, which could be causally related to the establishment of the cytokinin deficiency syndrome. Ultrastructural analysis revealed distinct changes in differentiating shoot tissues, including an increased vacuolation and an earlier differentiation of plastids, which showed partially disorganized thylakoid structures later in development. A comparison of the ploidy levels revealed an increased population of cells with a 4C DNA content during early stages of leaf development, indicating an inhibited progression from G2 to mitosis. To compare physiological characteristics of sink leaves, source leaves and roots of wild-type and cytokinin-deficient plants, several photosynthetic parameters, content of soluble sugars, starch and adenylates, as well as activities of enzymes of carbon assimilation and dissimilation were determined. Leaves of cytokinin-deficient plants contained less chlorophyll and non-photochemical quenching of young leaves was increased. However, absorption rate, photosynthetic capacity (F(v)/F(m) and J(CO2 max)) and efficiency (Phi CO(2 app)), as well as the content of soluble sugars, were not strongly altered in source leaves, indicating that chlorophyll is not limiting for photoassimilation and suggesting that source strength did not restrict shoot growth. By contrast, shoot sink tissues showed drastically reduced contents of soluble sugars, decreased activities of vacuolar invertases, and a reduced ATP content. These results strongly support a function of cytokinin in regulating shoot sink strength and its reduction may be a cause of the altered shoot phenotype. Roots of cytokinin-deficient plants contained less sugar compared with wild-type. However, this did not negatively affect glycolysis, ATP content, or root development. It is suggested that cytokinin-mediated regulation of the sink strength differs between roots and shoots.

Cloning and characterization of a soluble acid invertase-encoding gene from muskmelon

Soluble acid invertase (S-AIV; EC 3.2.1.26) is thought to play a critical role in sucrose hydrolysis in muskmelon (Cucumis melo L.) fruit. A full-length cDNA clone encoding S-AIV was isolated from muskmelon by RT-PCR and RACE. The clone, designated as CmS-AIV1, contains 2178 nucleotides with an open reading frame of 1908 nucleotides. The deduced 636 amino acid sequence showed high identities with other plant soluble acid invertases. Northern blot analysis indicated that CmS-AIV1 was expressed in flowers and fruit, but was not detected in roots, stems or leaves. Moreover, the mRNA accumulation of CmS-AIV1 showed its maximum level at 10 days after pollination (DAP) and decreased gradually during fruit development until its minimum level at mature fruit. Interestingly, the sucrose content was very low in fruit before 20 DAP but increased dramatically between 20 and 30 DAP during fruit development. In contrast to sucrose content, the activities of S-AIV was very high in fruit before 20 DAP and decreased apparently between 20 and 30 DAP, suggesting that sucrose metabolism may be linked to the CmS-AIV1 transcript level in muskmelon fruit.

A central role of abscisic acid in drought stress protection of Agrobacterium-induced tumors on Arabidopsis

Crown gall tumors induced by Agrobacterium tumefaciens represent a sink that has to be provided with nutrients and water by the host plant. The lack of an intact epidermis or cuticle results in uncontrolled loss of water. However, neither the tumor nor the host plant displays wilting. This phenomenon points to drought adaptation in both tumors and the crown gall host plant. To understand the underlying molecular mechanisms of protection against desiccation the gene expression pattern of Arabidopsis (Arabidopsis thaliana) tumors was integrated with the profile of stress metabolites: Arabidopsis tumors accumulated high amounts of abscisic acid (ABA), the ethylene precursor aminocyclopropyl carboxylic acid, osmoprotectants, and form a suberized periderm-like protective layer. Suberization of the outer tumor cell layers most likely is mediated by ABA since external application of ABA induced suberization of Arabidopsis roots. However, the expression level of the classical marker genes, known to respond to drought stress and/or ABA, was lower in tumors. Instead another set of drought and/or ABA-inducible genes was more highly transcribed. Elevated transcription of several ABA-dependent aquaporin genes might indicate that ABA controls the water balance of the tumor. The retarded tumor growth on abi and aba mutant plants underlined the importance of a tumor-specific ABA signaling pathway. Taken together, we propose that ABA is an important signal for protection of tumors against desiccation and thus supports tumor development.

A metabolic pathway leading to mannosylfructose biosynthesis in Agrobacterium tumefaciens uncovers a family of mannosyltransferases

A metabolic pathway for biosynthesis of the nonreducing disaccharide mannosylfructose (beta-fructofuranosyl-alpha-mannopyranoside), an important osmolyte in Agrobacterium tumefaciens, was discovered. We have identified and functionally characterized two ORFs that correspond to genes (named mfpsA and mfppA) encoding the rare enzymes mannosylfructose-phosphate synthase and mannosylfructose-phosphate phosphatase, an associated phosphohydrolase. The mfpsA and mfppA genes are arranged in an operon structure, whose transcription is up-regulated by NaCl, resulting in the accumulation of mannosylfructose in the cells. Not only is the biosynthesis of mannosylfructose mechanistically similar to that of sucrose, but the corresponding genes for the biosynthesis of both disaccharides are also phylogenetic close relatives. Importantly, a protein phylogeny analysis indicated that mannosylfructose-phosphate synthase defines a unique group of mannosyltransferases.

Regulation of invertase: a 'suite' of transcriptional and post-transcriptional mechanisms

Recent evidence indicates that several mechanisms can alter invertase activity and, thus, affect sucrose metabolism and resource allocation in plants. One of these mechanisms is the compartmentalisation of at least some vacuolar invertases in precursor protease vesicles (PPV), where their retention could control timing of delivery to vacuoles and hence activity. PPV are small, ER-derived bodies that sequester a subset of vacuolar-bound proteins (such as invertases and protease precursors) releasing them to acid vacuoles in response to developmental or environmental signals. Another newly-identified effector of invertases is wall-associated kinase 2 (WAK2), which can regulate a specific vacuolar invertase in Arabidopsis (AtvacINV1) and alter root growth when osmolyte supplies are limiting. WAKs are ideally positioned to sense changes in the interface between the cell wall and plasma membrane (such as turgor), because the N-terminus of each WAK extends into the cell wall matrix (where a pectin association is hypothesised) and the C-terminus has a cytoplasmic serine/threonine kinase domain (signalling). Still other avenues of invertase control are provided by a diverse group of kinases and phosphatases, consistent with input from multiple sensing systems for sugars, pathogens, ABA and other hormones. Mechanisms of regulation may also vary for the contrasting sugar responses of different acid invertase transcripts. Some degree of hexokinase involvement and distinctive kinetics have been observed for the sugar-repressed invertases, but not for the more common, sugar-induced forms examined thus far. An additional means of regulation for invertase gene expression lies in the multiple DST (Down STream) elements of the 3' untranslated region for the most rapidly repressed invertases. Similar sequences were initially identified in small auxin-up RNAs (SAUR) where they mediate rapid mRNA turnover. Finally, the invertase inhibitors, cell wall- and vacuolar inhibitors of fructosidase (CIF and VIF, respectively) are indistinguishable by sequence alone from pectin methylesterase inhibitors (PMEI); however, recent evidence suggests binding specificity may be determined by flexibility of a short, N-terminal region. These recently characterised processes increase the suite of regulatory mechanisms by which invertase - and, thus, sucrose metabolism and resource partitioning - can be altered in plants.

An integrated view of gene expression and solute profiles of Arabidopsis tumors: a genome-wide approach

Transformation of plant cells with T-DNA of virulent agrobacteria is one of the most extreme triggers of developmental changes in higher plants. For rapid growth and development of resulting tumors, specific changes in the gene expression profile and metabolic adaptations are required. Increased transport and metabolic fluxes are critical preconditions for growth and tumor development. A functional genomics approach, using the Affymetrix whole genome microarray (approximately 22,800 genes), was applied to measure changes in gene expression. The solute pattern of Arabidopsis thaliana tumors and uninfected plant tissues was compared with the respective gene expression profile. Increased levels of anions, sugars, and amino acids were correlated with changes in the gene expression of specific enzymes and solute transporters. The expression profile of genes pivotal for energy metabolism, such as those involved in photosynthesis, mitochondrial electron transport, and fermentation, suggested that tumors produce C and N compounds heterotrophically and gain energy mainly anaerobically. Thus, understanding of gene-to-metabolite networks in plant tumors promotes the identification of mechanisms that control tumor development.

Determination of structural requirements and probable regulatory effectors for membrane association of maize sucrose synthase 1

Sucrose (Suc) synthase (SUS) cleaves Suc to form UDP glucose and fructose, and exists in soluble and membrane-associated forms, with the latter proposed to channel UDP glucose to the cellulose-synthase complex on the plasma membrane of plant cells during synthesis of cellulose. However, the structural features responsible for membrane localization and the mechanisms regulating its dual intracellular localization are unknown. The maize (Zea mays) SUS1 isoform is likely to have the intrinsic ability to interact directly with membranes because we show: (1) partial membrane localization when expressed in Escherichia coli, and (2) binding to carbonate-stripped plant microsomes in vitro. We have undertaken mutational analyses (truncations and alanine substitutions) and in vitro microsome-binding assays with the SUS1 protein to define intrinsic membrane-binding regions and potential regulatory factors that could be provided by cellular microenvironment. The results suggest that two regions of SUS1 contribute to membrane affinity: (1) the amino-terminal noncatalytic domain, and (2) a region with sequence similarity to the C-terminal pleckstrin homology domain of human pleckstrin. Alanine substitutions within the pleckstrin homology-like domain of SUS1 reduced membrane association in E. coli and with plant microsomes in vitro without reducing enzymatic activity. Microsomal association of wild-type SUS1 displayed cooperativity with SUS1 protein concentration and was stimulated by both lowering the pH and adding Suc. These studies offer insight into the molecular level regulation of SUS1 localization and its participation in carbon partitioning in plants. Moreover, transgenics with active SUS mutants altered in membrane affinity may be of technological utility.

Comparative anatomy of gall development on Gypsophila paniculata induced by bacteria with different mechanisms of pathogenicity

Galls induced on Gypsophila paniculata by Pantoea agglomerans pv. gypsophilae (Pag) and Agrobacterium tumefaciens (At), bacteria with different mechanisms of pathogenicity, were compared morphologically and anatomically. The pathogenicity of Pag is dependent on the presence of an indigenous plasmid that harbors hrp gene cluster, genes encoding Hop virulence proteins and biosynthetic genes for auxin (IAA) and cytokinins (CKs), whereas that of At involves host transformation. The Pag-induced gall was rough, brittle and exhibited limited growth, in contrast to the smooth, firm appearance and continuous growth of the At-induced gall. Anatomical analysis revealed the presence of cells with enlarged nuclei and multiple nucleoli, giant cells and suberin deposition in Pag that were absent from At-induced galls. Although circular vessels were observed in both gall types, they were more numerous and the vascular system was more organized in At. An aerenchymal tissue was observed in the upper part of the galls. Ethylene emission from Pag galls, recorded 6 days after inoculation, was eight times as great as that from non-infected controls. In contrast, a significant decrease in ethylene production was observed in Gypsophila cuttings infected with Pag mutants deficient in IAA and CK production. The results presented are best accounted for by the two pathogens having distinct pathogenicity mechanisms that lead to their differential recognition by the host as non-self (Pag) and self (At).

Molecular characterization and expression of four cDNAs encoding sucrose synthase from green bamboo Bambusa oldhamii

Bamboo is distinguished by its rapid growth. To investigate sucrose metabolism in this plant, we cloned the cDNAs encoding sucrose synthase (SuS) from Bambusa oldhamii and investigated their expression in growing shoots and leaves. Four cDNA clones, BoSus1, BoSus2, BoSus3 and BoSus4, were isolated by screening a cDNA library from etiolated bamboo shoots. Recombinant BoSuS proteins were produced in Escherichia coli and purified by immobilized metal affinity chromatography and ultrafiltration. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine the abundance of the transcript of each gene. BoSus1 and BoSus3 may be duplicate or homeologous genes, the sequences of which show high identity. Similarly, BoSus2 shows high identity with BoSus4. Kinetic analysis showed that the two BoSuS isoforms of each type had similar michaelis constant (Km) values for sucrose, but different values for UDP. The four genes were expressed in various bamboo organs but were differentially regulated. The increase in the abundance of their mRNA paralleled the growth rate of the bamboo. The results suggest that, in bamboo, SuS is encoded by at least four genes, each with a specific role in providing substrates for the polysaccharide biosynthesis and/or energy production necessary to support the rapid growth of this species.

Reduction of polar auxin transport in tobacco by the tumorigenic Agrobacterium tumefaciens AK-6b gene

The plant-tumorigenic 6b (AK-6b) gene of Agrobacterium tumefaciens strain AKE10 induces morphological alterations to tobacco plants, Nicotiana tabacum. To investigate the molecular mechanisms underlying these processes, we generated transgenic tobacco harboring the AK-6b gene under the control of a dexamethazone-inducible promoter. Upon induction, transgenic tobacco seedlings exhibited distinct classes of aberrant morphologies, most notably adventitious outgrowths and stunted epicotyls. Histological analysis revealed massive proliferation and altered venation in the newly established outgrowths. Prominent vascular development suggested that auxin metabolism or signaling had been altered. Indeed, basipetal auxin transport in the hypocotyls of the transgenic seedlings was reduced by 50-80%, whereas intracellular auxin contents were only slightly reduced. Analysis of cell extracts by HPLC revealed a large accumulation of phenolic compounds, including the flavonoid kaempferol-3-rutinoside, in transgenic plants compared with wild-type seedlings. As some naturally occurring flavonoids have been shown to affect auxin transport, we suggest that the AK-6b gene expression impairs auxin transport via modulation of phenylpropanoid metabolism, and ultimately results in the observed morphological alterations.

Root-synthesized cytokinin in Arabidopsis is distributed in the shoot by the transpiration stream

To clarify how root-synthesized cytokinins (CKs) are transported to young shoot organs, CK distribution patterns were analysed in free-CK-responsive ARR5::GUS transformants of Arabidopsis thaliana (L.) Heynh. together with free plus bound CKs using specific CK monoclonal antibodies. Plants were subjected to two different growth conditions, completely protected from any air movement, or exposed to gentle wind 3 h before harvesting. In wind-protected plants the strongest ARR5::GUS expression was found in the root cap statocytes, spreading upwards in the vascular cylinder. This pattern in roots was congruent with that found by CK immunolocalization. Shoots of wind-protected plants displayed either no or only low ARR5::GUS expression in the stem vascular bundles, nodal ramifications, and the bases of flower buds; shoot vascular bundles showed patterns of acropetally decreasing staining and the apical parts of buds and leaves were free from ARR5::GUS expression. In wind-exposed plants ARR5::GUS expression was considerably increased in shoots, also in basal-to-apical decreasing gradients. Immunolabelled shoots showed differential staining, with the strongest label in the vascular bundles of stems, leaves, and buds. The fact of the apparent absence of free CK in the buds of wind-protected plants and the typical upward decreasing gradients of free and conjugated CKs suggest that the bulk of the CK is synthesized in the root cap, exported through the xylem and accumulates at sites of highest transpiration where cuticles do not yet exist or do not protect against water loss.

Sucrose metabolism: regulatory mechanisms and pivotal roles in sugar sensing and plant development

Sucrose cleavage is vital to multicellular plants, not only for the allocation of crucial carbon resources but also for the initiation of hexose-based sugar signals in importing structures. Only the invertase and reversible sucrose synthase reactions catalyze known paths of sucrose breakdown in vivo. The regulation of these reactions and its consequences has therefore become a central issue in plant carbon metabolism. Primary mechanisms for this regulation involve the capacity of invertases to alter sugar signals by producing glucose rather than UDPglucose, and thus also two-fold more hexoses than are produced by sucrose synthase. In addition, vacuolar sites of cleavage by invertases could allow temporal control via compartmentalization. In addition, members of the gene families encoding either invertases or sucrose synthases respond at transcriptional and posttranscriptional levels to diverse environmental signals, including endogenous changes that reflect their own action (e.g. hexoses and hexose-responsive hormone systems such as abscisic acid [ABA] signaling). At the enzyme level, sucrose synthases can be regulated by rapid changes in sub-cellular localization, phosphorylation, and carefully modulated protein turnover. In addition to transcriptional control, invertase action can also be regulated at the enzyme level by highly localized inhibitor proteins and by a system that has the potential to initiate and terminate invertase activity in vacuoles. The extent, path, and site of sucrose metabolism are thus highly responsive to both internal and external environmental signals and can, in turn, dramatically alter development and stress acclimation.