d-amino acids metabolism reflects the evolutionary origin of higher plants and their adaptation to the environment.

d-amino acids are the d stereoisomers of the common l-amino acids found in proteins. Over the past two decades, the occurrence of d-amino acids in plants has been reported and circumstantial evidence for a role in various processes, including interaction with soil microorganisms or interference with cellular signalling, has been provided. However, examples are not numerous and d-amino acids can also be detrimental, some of them inhibiting growth and development. Thus, the persistence of d-amino acid metabolism in plants is rather surprising, and the evolutionary origins of d-amino acid metabolism are currently unclear. Systemic analysis of sequences associated with d-amino acid metabolism enzymes shows that they are not simply inherited from cyanobacterial metabolism. In fact, the history of plant d-amino acid metabolism enzymes likely involves multiple steps, cellular compartments, gene transfers and losses. Regardless of evolutionary steps, enzymes of d-amino acid metabolism, such as d-amino acid transferases or racemases, have been retained by higher plants and have not simply been eliminated, so it is likely that they fulfil important metabolic roles such as serine, folate or plastid peptidoglycan metabolism. We suggest that d-amino acid metabolism may have been critical to support metabolic functions required during the evolution of land plants.

High Expression of ALTERNATIVE OXIDASE 2 in Latent Axillary Buds Suggests Its Key Role in Quiescence Maintenance in Rosebush.

Most vegetative axes remain quiescent as dormant axillary buds until metabolic and hormonal signals, driven by environmental changes, trigger bud outgrowth. While the resumption of growth activity is well documented, the establishment and maintenance of quiescence is comparatively poorly understood, despite its major importance in the adaptation of plants to the seasonal cycle or in the establishment of their shape. Here, using the rosebush Rosa hybrida 'Radrazz' as a plant model, we highlighted that the quiescent state was the consequence of an internal and active energy control of buds, under the influence of hormonal factors previously identified in the bud outgrowth process. We found that the quiescent state in the non-growing vegetative axis of dormant axillary buds displayed a low energy state along with a high expression of the ALTERNATIVE OXIDASE 2 (AOX2) and the accumulation of the corresponding protein. Conversely, AOX2 expression and protein amount strongly decreased during bud burst as energy status shifted to a high state, allowing growth. Since AOX2 can deviate electrons from the cytochrome pathway in the mitochondrial respiratory chain, it could drastically reduce the formation of ATP, which would result in a low energy status unfavorable for growth activities. We provide evidence that the presence/absence of AOX2 in quiescent/growing vegetative axes of buds was under hormonal control and thus may constitute the mechanistic basis of both quiescence and sink strength manifestation, two important aspects of budbreak.

Revisiting yield in terms of phloem transport to grains suggests phloem sap movement might be homeostatic.

Phloem sap transport, velocity and allocation have been proposed to play a role in physiological limitations of crop yield, along with photosynthetic activity or water use efficiency. Although there is clear evidence that carbon allocation to grains effectively drives yield in cereals like wheat (as reflected by the harvest index), the influence of phloem transport rate and velocity is less clear. Here, we took advantage of previously published data on yield, respiration, carbon isotope composition, nitrogen content and water consumption in winter wheat cultivars grown across several sites with or without irrigation, to express grain production in terms of phloem sucrose transport and compare with xylem water transport. Our results suggest that phloem sucrose transport rate follows the same relationship with phloem N transport regardless of irrigation conditions and cultivars, and seems to depend mostly on grain weight (i.e., mg per grain). Depending on the assumption made for phloem sap sucrose concentration, either phloem sap velocity or its proportionality coefficient to xylem velocity change little with environmental conditions. Taken as a whole, phloem transport from leaves to grains seems to be homeostatic within a narrow range of values and following relationships with other plant physiological parameters across cultivars and conditions. This suggests that phloem transport per se is not a limitation for yield in wheat but rather, is controlled to sustain grain filling.

Biostimulant Properties of Protein Hydrolysates: Recent Advances and Future Challenges.

Over the past decade, plant biostimulants have been increasingly used in agriculture as environment-friendly tools that improve the sustainability and resilience of crop production systems under environmental stresses. Protein hydrolysates (PHs) are a main category of biostimulants produced by chemical or enzymatic hydrolysis of proteins from animal or plant sources. Mostly composed of amino acids and peptides, PHs have a beneficial effect on multiple physiological processes, including photosynthetic activity, nutrient assimilation and translocation, and also quality parameters. They also seem to have hormone-like activities. Moreover, PHs enhance tolerance to abiotic stresses, notably through the stimulation of protective processes such as cell antioxidant activity and osmotic adjustment. Knowledge on their mode of action, however, is still piecemeal. The aims of this review are as follows: (i) Giving a comprehensive overview of current findings about the hypothetical mechanisms of action of PHs; (ii) Emphasizing the knowledge gaps that deserve to be urgently addressed with a view to efficiently improve the benefits of biostimulants for different plant crops in the context of climate change.

Phloem Sap Composition: What Have We Learnt from Metabolomics?

Phloem sap transport is essential for plant nutrition and development since it mediates redistribution of nutrients, metabolites and signaling molecules. However, its biochemical composition is not so well-known because phloem sap sampling is difficult and does not always allow extensive chemical analysis. In the past years, efforts have been devoted to metabolomics analyses of phloem sap using either liquid chromatography or gas chromatography coupled with mass spectrometry. Phloem sap metabolomics is of importance to understand how metabolites can be exchanged between plant organs and how metabolite allocation may impact plant growth and development. Here, we provide an overview of our current knowledge of phloem sap metabolome and physiological information obtained therefrom. Although metabolomics analyses of phloem sap are still not numerous, they show that metabolites present in sap are not just sugars and amino acids but that many more metabolic pathways are represented. They further suggest that metabolite exchange between source and sink organs is a general phenomenon, offering opportunities for metabolic cycles at the whole-plant scale. Such cycles reflect metabolic interdependence of plant organs and shoot-root coordination of plant growth and development.

Ascorbate-glutathione pathways mediated by cytokinin regulate H2O2 levels in light-controlled rose bud burst.

Rosebush (Rosa "Radrazz") plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of hydrogen peroxide (H2O2) present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H2O2 scavenging through the promotion of Ascorbic acid-Glutathione (AsA-GSH)-dependent pathways and the continuous decrease in H2O2 production. Here we showed Respiratory Burst Oxidase Homologs expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H2O2 remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA-GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H2O2 to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine, an inhibitor of GSH synthesis, to solve the sequence of events involving H2O2/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H2O2 scavenging, resulting in initiation of bud outgrowth.

Does Potassium (K+) Contribute to High-Nitrate (NO3-) Weakening of a Plant's Defense System against Necrotrophic Fungi?

In this opinion article, we have analyzed the relevancy of a hypothesis which is based on the idea that in Arabidopsis thaliana jasmonic acid, a (JA)-mediated defense system against necrotrophic fungi is weakened when NO3- supply is high. Such a hypothesis is based on the fact that when NO3- supply is high, it induces an increase in the amount of bioactive ABA which induces the sequestration of the phosphatase ABI2 (PP2C) into the PYR/PYL/RCAR receptor. Consequently, the Ca sensors CBL1/9-CIPK23 are not dephosphorylated by ABI2, thus remaining able to phosphorylate targets such as AtNPF6.3 and AtKAT1, which are NO3- and K+ transporters, respectively. Therefore, the impact of phosphorylation on the regulation of these two transporters, could (1) reduce NO3- influx as in its phosphorylated state AtNPF6.3 shifts to low capacity state and (2) increase K+ influx, as in its phosphorylated state KAT1 becomes more active. It is also well known that in roots, K+ loading in the xylem and its transport to the shoot is activated in the presence of NO3-. As such, the enrichment of plant tissues in K+ can impair a jasmonic acid (JA) regulatory pathway and the induction of the corresponding biomarkers. The latter are known to be up-regulated under K+ deficiency and inhibited when K+ is resupplied. We therefore suggest that increased K+ uptake and tissue content induced by high NO3- supply modifies the JA regulatory pathway, resulting in a weakened JA-mediated plant's defense system against necrotrophic fungi.

Leafamine®, a Free Amino Acid-Rich Biostimulant, Promotes Growth Performance of Deficit-Irrigated Lettuce.

Water deficit causes substantial yield losses that climate change is going to make even more problematic. Sustainable agricultural practices are increasingly developed to improve plant tolerance to abiotic stresses. One innovative solution amongst others is the integration of plant biostimulants in agriculture. In this work, we investigate for the first time the effects of the biostimulant -Leafamine®-a protein hydrolysate on greenhouse lettuce (Lactuca sativa L.) grown under well-watered and water-deficit conditions. We examined the physiological and metabolomic water deficit responses of lettuce treated with Leafamine® (0.585 g/pot) or not. Root application of Leafamine® increased the shoot fresh biomass of both well-watered (+40%) and deficit-irrigated (+20%) lettuce plants because the projected leaf area increased. Our results also indicate that Leafamine® application could adjust the nitrogen metabolism by enhancing the total nitrogen content, amino acid (proline) contents and the total protein level in lettuce leaves, irrespective of the water condition. Osmolytes such as soluble sugars and polyols, also increased in Leafamine®-treated lettuce. Our findings suggest that the protective effect of Leafamine is a widespread change in plant metabolism and could involve ABA, putrescine and raffinose.

Ascorbate glutathione-dependent H2O2 scavenging is an important process in axillary bud outgrowth in rosebush.

BACKGROUND AND AIMS: Branching is an important mechanism of plant shape establishment and the direct consequence of axillary bud outgrowth. Recently, hydrogen peroxide (H2O2) metabolism, known to be involved in plant growth and development, has been proposed to contribute to axillary bud outgrowth. However, the involvement of H2O2 in this process remains unclear. METHODS: We analysed the content of H2O2 during bud outgrowth and characterized its catabolism, both at the transcriptional level and in terms of its enzymatic activities, using RT-qPCR and spectrophotometric methods, respectively. In addition, we used in vitro culture to characterize the effects of H2O2 application and the reduced glutathione (GSH) synthesis inhibitor l-buthionine sulfoximine (BSO) on bud outgrowth in relation to known molecular markers involved in this process. KEY RESULTS: Quiescent buds displayed a high content of H2O2 that declined when bud outgrowth was initiated, as the consequence of an increase in the scavenging activity that is associated with glutathione pathways (ascorbate-glutathione cycle and glutathione biosynthesis); catalase did not appear to be implicated. Modification of bud redox state after the application of H2O2 or BSO prevented axillary bud outgrowth by repressing organogenesis and newly formed axis elongation. Hydrogen peroxide also repressed bud outgrowth-associated marker gene expression. CONCLUSIONS: These results show that high levels of H2O2 in buds that are in a quiescent state prevents bud outgrowth. Induction of ascorbate-glutathione pathway scavenging activities results in a strong decrease in H2O2 content in buds, which finally allows bud outgrowth.

Mitochondrial complex I dysfunction increases CO2 efflux and reconfigures metabolic fluxes of day respiration in tobacco leaves.

Mutants affected in complex I are useful to understand the role played by mitochondrial electron transport and redox metabolism in cellular homeostasis and signaling. However, their respiratory phenotype is incompletely described and a specific examination of day respiration (Rd ) is lacking. Here, we used isotopic methods and metabolomics to investigate the impact of complex I dysfunction on Rd in two respiratory mutants of forest tobacco (Nicotiana sylvestris): cytoplasmic male sterile II (CMSII) and nuclear male sterile 1 (NMS1), previously characterized for complex I disruption. Rd was higher in mutants and the inhibition of leaf respiration by light was lower. Higher Rd values were caused by increased (phosphoenol)pyruvate (PEP) metabolism at the expense of anaplerotic (PEP carboxylase (PEPc) -catalyzed) activity. De novo synthesis of Krebs cycle intermediates in the light was larger in mutants than in the wild-type, although numerically small in all genotypes. Carbon metabolism in mutants involved alternative pathways, such as alanine synthesis, and an increase in amino acid production with the notable exception of aspartate. Our results show that the alteration of NADH re-oxidation activity by complex I does not cause a general inhibition of catabolism, but rather a re-orchestration of fluxes in day respiratory metabolism, leading to an increased CO2 efflux.

Three cytosolic glutamine synthetase isoforms localized in different-order veins act together for N remobilization and seed filling in Arabidopsis.

Glutamine synthetase (GS) is central for ammonium assimilation and consists of cytosolic (GS1) and chloroplastic (GS2) isoenzymes. During plant ageing, GS2 protein decreases due to chloroplast degradation, and GS1 activity increases to support glutamine biosynthesis and N remobilization from senescing leaves. The role of the different Arabidopsis GS1 isoforms in nitrogen remobilization was examined using 15N tracing experiments. Only the gln1;1-gln1;2-gln1;3 triple-mutation affecting the three GLN1;1, GLN1;2, and GLN1;3 genes significantly reduced N remobilization, total seed yield, individual seed weight, harvest index, and vegetative biomass. The triple-mutant accumulated a large amount of ammonium that could not be assimilated by GS1. Alternative ammonium assimilation through asparagine biosynthesis was increased and was related to higher ASN2 asparagine synthetase transcript levels. The GS2 transcript, protein, and activity levels were also increased to compensate for the lack of GS1-related glutamine biosynthesis. Localization of the different GLN1 genes showed that they were all expressed in the phloem companion cells but in veins of different order. Our results demonstrate that glutamine biosynthesis for N-remobilization occurs in veins of all orders (major and minor) in leaves, it is mainly catalysed by the three major GS1 isoforms (GLN1;1, GLN1;2, and GLN1;3), and it is alternatively supported by AS2 in the veins and GS2 in the mesophyll cells.

Modelling temporal variation of parameters used in two photosynthesis models: influence of fruit load and girdling on leaf photosynthesis in fruit-bearing branches of apple.

Background and Aims: Several studies have found seasonal and temporal variability in leaf photosynthesis parameters in different crops. This variability depends upon the environment, the developmental stage of the plant and the presence or absence of sinks. Girdling involves the removal of the bark and phloem down to the youngest xylem all around the stem and prevents export of photoassimilates out of the stem. The load of developing fruits has often been reported to influence the individual net leaf photosynthesis rate (Pn) in tree crops. In this study, we chose (1) to model the key parameters of photosynthesis models of leaves (Pgmax, Rd, α and θ) as a function of time and using these two means (girdling and low fruit load) to alter the source-sink balance and (2) to compare three models: the rectangular and non-rectangular hyperbola model by Thornley, as well as the non-rectangular hyperbola model by Marshall and Biscoe. Methods: Six-year-old fruit-bearing branches of 10-year-old apple trees were used to study and model the seasonal variation of photosynthetic parameters in leaves of vegetative shoots, as a function of global fruit load (at the branch level), with or without girdling, during the growing season of 2015. Three treatments were applied: control, low load (LL) or low load + girdling (LLG). For each fruit-bearing branch, light-response curves of Pn for two leaves of vegetative shoots were measured at two different positions, proximal and distal. Key Results: The model of Marshall and Biscoe was the most accurate for the simulation of Pn in fruit-bearing branches of apple trees with time (season) and the three treatments applied. Conclusion: The present study proposed a way to model the photosynthesis rate by temporal and environmental variables only. A proper validation of this model will be necessary to extend its utilization and appreciate its predictive capacity fully.

Direct assessment of the metabolic origin of carbon atoms in glutamate from illuminated leaves using 13 C-NMR.

Glutamate (Glu) is the cornerstone of nitrogen assimilation and photorespiration in illuminated leaves. Despite this crucial role, our knowledge of the flux to Glu de novo synthesis is rather limited. Here, we used isotopic labelling with 13 CO2 and 13 C-NMR analyses to examine the labelling pattern and the appearance of multi-labelled species of Glu molecules to trace the origin of C-atoms found in Glu. We also compared this with 13 C-labelling patterns in Ala and Asp, which reflect citrate (and thus Glu) precursors, that is, pyruvate and oxaloacetate. Glu appeared to be less 13 C-labelled than Asp and Ala, showing that the Glu pool was mostly formed by 'old' carbon atoms. There were modest differences in intramolecular 13 C-13 C couplings between Glu C-2 and Asp C-3, showing that oxaloacetate metabolism to Glu biosynthesis did not involve C-atom redistribution by the Krebs cycle. The apparent carbon allocation increased with carbon net photosynthesis. However, when expressed relative to CO2 fixation, it was clearly higher at low CO2 while it did not change in 2% O2 , as compared to standard conditions. We conclude that Glu production from current photosynthetic carbon represents a small flux that is controlled by the gaseous environment, typically upregulated at low CO2 .

Cytokinins Are Initial Targets of Light in the Control of Bud Outgrowth.

Bud outgrowth is controlled by environmental and endogenous factors. Through the use of the photosynthesis inhibitor norflurazon and of masking experiments, evidence is given here that light acts mainly as a morphogenic signal in the triggering of bud outgrowth and that initial steps in the light signaling pathway involve cytokinins (CKs). Indeed, in rose (Rosa hybrida), inhibition of bud outgrowth by darkness is suppressed solely by the application of CKs. In contrast, application of sugars has a limited effect. Exposure of plants to white light (WL) induces a rapid (after 3-6 h of WL exposure) up-regulation of CK synthesis (RhIPT3 and RhIPT5), of CK activation (RhLOG8), and of CK putative transporter RhPUP5 genes and to the repression of the CK degradation RhCKX1 gene in the node. This leads to the accumulation of CKs in the node within 6 h and in the bud at 24 h and to the triggering of bud outgrowth. Molecular analysis of genes involved in major mechanisms of bud outgrowth (strigolactone signaling [RwMAX2], metabolism and transport of auxin [RhPIN1, RhYUC1, and RhTAR1], regulation of sugar sink strength [RhVI, RhSUSY, RhSUC2, and RhSWEET10], and cell division and expansion [RhEXP and RhPCNA]) reveal that, when supplied in darkness, CKs up-regulate their expression as rapidly and as intensely as WL Additionally, up-regulation of CKs by WL promotes xylem flux toward the bud, as evidenced by Methylene Blue accumulation in the bud after CK treatment in the dark. Altogether, these results suggest that CKs are initial components of the light signaling pathway that controls the initiation of bud outgrowth.

Nitrogen metabolism in plants under low oxygen stress.

More frequent flooding and waterlogging events due to more heavy precipitation are expected worldwide in the context of climate change. Accordingly, adaptation of plants to oxygen limitation at both cellular and whole plant levels should be investigated thoroughly, that derived knowledge could be taken into account in breeding programs and agronomical practices for saving plant fitness, growth and development even when oxygen availability is low. In the present review, we highlight current knowledge on essential aspects of low oxygen stress-induced changes in nitrogen metabolism. The involvement of two possible pathways for NO production either via the reaction catalyzed by nitrate reductase or at Complex III or IV of the mitochondrial electron transport chain, thus contributing to ATP synthesis via the so-called nitrite-NO respiration, is discussed. NO is proposed to be scavenged by non-symbiotic hemoglobin (Hb) in a Hb/NO cycle, in which NAD(P)H is oxidized for the conversion of NO into NO3(-). The investigation of an additional adaptation to the decrease in oxygen availability via transcriptional and posttranslational regulation of amino acid synthesis pathways, using publicly available transcriptome and translatome data for Arabidopsis thaliana and rice is also discussed.

Cloning and characterization of a novel fructan 6-exohydrolase strongly inhibited by sucrose in Lolium perenne.

MAIN CONCLUSION: The first 6-fructan exohydrolase (6-FEH) cDNA from Lolium perenne was cloned and characterized. Following defoliation, Lp6 - FEHa transcript level unexpectedly decreased together with an increase in total FEH activity. Lolium perenne is a major forage grass species that accumulates fructans, mainly composed of ß(2,6)-linked fructose units. Fructans are mobilized through strongly increased activities of fructan exohydrolases (FEHs), sustaining regrowth following defoliation. To understand the complex regulation of fructan breakdown in defoliated grassland species, the objective was to clone and characterize new FEH genes in L. perenne. To find FEH genes related to refoliation, a defoliated tiller base cDNA library was screened. Characterization of the recombinant protein was performed in Pichia pastoris. In this report, the cloning and enzymatic characterization of the first 6-FEH from L. perenne is described. Following defoliation, during fructan breakdown, Lp6-FEHa transcript level unexpectedly decreased in elongating leaf bases (ELB) and in mature leaf sheaths (tiller base) in parallel to increased total FEH activities. In comparison, transcript levels of genes coding for fructosyltransferases (FTs) involved in fructan biosynthesis also decreased after defoliation but much faster than FEH transcript levels. Since Lp6-FEHa was strongly inhibited by sucrose, mechanisms modulating FEH activities are discussed. It is proposed that differences in the regulation of FEH activity among forage grasses influence their tolerance to defoliation.

Nitrogen Nutrition Modulates the Response to Alternaria brassicicola Infection via Metabolic Modifications in Arabidopsis Seedlings.

Little is known about the effect of nitrogen nutrition on seedling susceptibility to seed-borne pathogens. We have previously shown that seedlings grown under high nitrate (5 mM) conditions are less susceptible than those grown under low nitrate (0.1 mM) and ammonium (5 mM) in the Arabidopsis-Alternaria brassicicola pathosystem. However, it is not known how seedling metabolism is modulated by nitrogen nutrition, nor what is its response to pathogen infection. Here, we addressed this question using the same pathosystem and nutritive conditions, examining germination kinetics, seedling development, but also shoot ion contents, metabolome, and selected gene expression. Nitrogen nutrition clearly altered the seedling metabolome. A similar metabolomic profile was observed in inoculated seedlings grown at high nitrate levels and in not inoculated-seedlings. High nitrate levels also led to specific gene expression patterns (e.g., polyamine metabolism), while other genes responded to inoculation regardless of nitrogen supply conditions. Furthermore, the metabolites best correlated with high disease symptoms were coumarate, tyrosine, hemicellulose sugars, and polyamines, and those associated with low symptoms were organic acids (tricarboxylic acid pathway, glycerate, shikimate), sugars derivatives and ß-alanine. Overall, our results suggest that the beneficial effect of high nitrate nutrition on seedling susceptibility is likely due to nutritive and signaling mechanisms affecting developmental plant processes detrimental to the pathogen. In particular, it may be due to a constitutively high tryptophan metabolism, as well as down regulation of oxidative stress caused by polyamine catabolism.

Physiological and metabolic consequences of autophagy deficiency for the management of nitrogen and protein resources in Arabidopsis leaves depending on nitrate availability.

Autophagy is present at a basal level in all plant tissues and is induced during leaf ageing and in response to nitrogen (N) starvation. Nitrogen remobilization from the rosette to the seeds is impaired in autophagy mutants. This report focuses on the role of autophagy in leaf N management and proteolysis during plant ageing. Metabolites, enzyme activities and protein contents were monitored in several autophagy-defective (atg) Arabidopsis mutants grown under low and high nitrate conditions. Results showed that carbon (C) and N statuses were affected in atg mutants before any senescence symptoms appeared. atg mutants accumulated larger amounts of ammonium, amino acids and proteins than wild type, and were depleted in sugars. Over-accumulation of proteins in atg mutants was selective and occurred despite higher endopeptidase and carboxypeptidase activities. Specific over-accumulation of the ribosomal proteins S6 and L13 subunits, and of catalase and glutamate dehydrogenase proteins was observed. atg mutants also accumulated peptides putatively identified as degradation products of the Rubisco large subunit and glutamine synthetase 2 (GS2). Incomplete chloroplast protein degradation resulting from autophagy defects could explain the higher N concentrations measured in atg rosettes and defects in N remobilization. It is concluded that autophagy controls C : N status and protein content in leaves of Arabidopsis.

The Key Role of Glutamate Dehydrogenase 2 (GDH2) in the Control of Kernel Production in Maize (Zea mays L.).

The agronomic potential of glutamate dehydrogenase 2 (GDH2) in maize kernel production was investigated by examining the impact of a mutation on the corresponding gene. Mu-insertion homozygous and heterozygous mutant lines lacking GDH2 activity were isolated and characterized at the biochemical, physiological and agronomic levels. In comparison to the wild type and to the homozygous ghd2 mutants, the heterozygous gdh2 mutant plants were characterized by a decrease in the root amino acid content, whereas in the leaves an increase of a number of phenolic compounds was observed. On average, a 30 to 40% increase in kernel yield was obtained only in the heterozygous gdh2 mutant lines when plants were grown in the field over two years. The importance of GDH2 in the control of plant productivity is discussed in relation to the physiological impact of the mutation on amino acid content, with primary carbon metabolism mostly occurring in the roots and secondary metabolism occurring in the leaves.

Insight into the role of sugars in bud burst under light in the rose.

Bud burst is a decisive process in plant architecture that requires light in Rosa sp. This light effect was correlated with stimulation of sugar transport and metabolism in favor of bud outgrowth. We investigated whether sugars could act as signaling entities in the light-mediated regulation of vacuolar invertases and bud burst. Full-length cDNAs encoding two vacuolar invertases (RhVI1 and RhVI2) were isolated from buds. Unlike RhVI2, RhVI1 was preferentially expressed in bursting buds, and was up-regulated in buds of beheaded plants exposed to light. To assess the importance of sugars in this process, the expression of RhVI1 and RhVI2 and the total vacuolar invertase activity were further characterized in buds cultured in vitro on 100 mM sucrose or mannitol under light or in darkness for 48 h. Unlike mannitol, sucrose promoted the stimulatory effect of light on both RhVI1 expression and vacuolar invertase activity. This up-regulation of RhVI1 was rapid (after 6 h incubation) and was induced by as little as 10 mM sucrose or fructose. No effect of glucose was found. Interestingly, both 30 mM palatinose (a non-metabolizable sucrose analog) and 5 mM psicose (a non-metabolizable fructose analog) promoted the light-induced expression of RhVI1 and total vacuolar invertase activity. Sucrose, fructose, palatinose and psicose all promoted bursting of in vitro cultured buds under light. These findings indicate that soluble sugars contribute to the light effect on bud burst and vacuolar invertases, and can function as signaling entities.