Autophagy in Plants: Both a Puppet and a Puppet Master of Sugars

Autophagy is a major pathway that recycles cellular components in eukaryotic cells both under stressed and non-stressed conditions. Sugars participate both metabolically and as signaling molecules in development and response to various environmental and nutritional conditions. It is therefore essential to maintain metabolic homeostasis of sugars during non-stressed conditions in cells, not only to provide energy, but also to ensure effective signaling when exposed to stress. In both plants and animals, autophagy is activated by the energy sensor SnRK1/AMPK and inhibited by TOR kinase. SnRK1/AMPK and TOR kinases are both important regulators of cellular metabolism and are controlled to a large extent by the availability of sugars and sugar-phosphates in plants whereas in animals AMP/ATP indirectly translate sugar status. In plants, during nutrient and sugar deficiency, SnRK1 is activated, and TOR is inhibited to allow activation of autophagy which in turn recycles cellular components in an attempt to provide stress relief. Autophagy is thus indirectly regulated by the nutrient/sugar status of cells, but also regulates the level of nutrients/sugars by recycling cellular components. In both plants and animals sugars such as trehalose induce autophagy and in animals this is independent of the TOR pathway. The glucose-activated G-protein signaling pathway has also been demonstrated to activate autophagy, although the exact mechanism is not completely clear. This mini-review will focus on the interplay between sugar signaling and autophagy

The Arabidopsis GolS1 promotor as a potential biosensor for heat stress and fungal infection?

Thesis (MSc)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Galactinol (Gol) has classically been considered to serve as a galactose donor during the biosynthesis of raffinose family oligosaccharides (RFOs). These sucrosyl oligosaccharides have been well characterised in their roles in carbon translocation and storage and, abiotic stress protection in plants. However, recent findings have demonstrated Gol to be an efficient free radical scavenger and it has also been suggested to act as signalling molecule during induced systemic resistance (ISR), upon pathogen infection. Collectively, these findings centres to the involvement of only a single galactinol synthase gene (GolS, synthesising Gol) in Arabidopsis (AtGolS1, At2g47180). The AtGolS1 isoform has been shown to be transcriptionally up-regulated during heat stress and Botrytis cinerea infection. Further, it is also responsive to jasmonic acid, a key component of the ISR pathway. Here we targeted the AtGolS1 promotor containing well defined heat shock transcription factor elements and a single putative jasmonate binding element, to develop a dual-functional biosensor with the ability to detect both heat stress and Botrytis cinerea infection. We created transgenic Arabidopsis lines where the reporter genes β-glucuronidase (GUS) and the green florescent protein (GFP) were under the control of the AtGolS1 promotor. Using the native AtGolS1 gene as a point of reference, we confirmed that the reporter genes were transcriptionally responsive to both heat stress and methyl jasmonate treatment in transgenic Arabidopsis. Under the same experimental conditions, both GUS assays and GFP imaging correlated with these transcriptional responses. Finally, we infected the transgenic lines with Botrytis cinerea infections to analyse reporter activity. Transcript analysis of transgenic lines clearly showed an increase in transcript abundance for both the native AtGolS1 and the reporter genes in reponse to B. cinerea infection. Similarly, reporter assays revealed a distinct difference in activity between infected and uninfected plants from 24h to 96h after Botrytis cinerea infection. These results provide sufficient proof-of-concept for the AtGolS1 promotor to be used as a dual functional biosensor for both heat stress and fungal infection.AFRIKAANS OPSOMMING: Galaktinol (Gol) is aanvanklik beskou as ʼn galaktose skenker tydens die biosintese van raffinose familie van oligosakkariede (RFO). Hierdie sukrosiel oligosakkariede is goed gekenmerk vir hul funksies in koolstof translokasie en storing, sowel as die beskerming teen abiotiese stres in plante. Onlangse bevindinge het Gol geklassifiseer as 'n doeltreffende vry radikaal werwer, en is voorgestel om op te tree as 'n sein molekule tydens geïnduseerde sistemiese weerstand (ISR), tydens patogeen infeksie. Gesamentlik plaas hierdie bevindinge klem op die betrokkenheid van 'n enkele galaktinol sintase geen (GolS, sintetiseer Gol) in Arabidopsis (AtGolS1, At2g47180). Dit is voorheen bewys dat die AtGolS1 isoform transkripsioneel op-gereguleer word tydens hitte-stres en Botrytis cinerea infeksie. Verder is dit ook sensitief vir jasmijnsuur, 'n belangrike komponent van die ISR pad. Gedurende hierdie studie het ons die AtGolS1 promotor geteiken, wat die goed gedefinieërde hitte-skok transkripsie faktor bindings elemente en 'n enkele vermeende jasmijnsuur bindings element bevat, om 'n dubbele-funksionele biosensor te ontwikkel met die vermoë om beide hitte-stres en Botrytis cinerea infeksie op te spoor. Ons het transgeniese Arabidopsis lyne gegenereer waar die rapporteerder gene β--glukuronidase (GUS) en die groen fluoressent proteïen (GFP) onder die beheer van die AtGolS1 promotor is. Deur gebruik te maak van die inheemse AtGolS1 geen as 'n verwysingspunt, het ons bevestig dat die rapporteerder gene op ‘n transkriptionele vlak reageer op beide hitte-stres en metiel jasmijnsuur behandeling in transgeniese Arabidopsis. Onder dieselfde eksperimentele kondisies het beide GUS toetse en GFP fotografie gekorreleer met die transkripsie analise. Ten slotte, het ons die transgeniese lyne aan Botrytis cinerea infeksies blootgetel om die rapporteerder aktiwiteit te ontleed. Transkripsie analise van transgeniese lyne het ʼn duidelik toename in transkripsie vlakke getoon vir beide die plaaslike AtGolS1 geen en die rapporteerder gene in reaksie op B. cinerea infeksie. Eenders, het rapporteerder toetse 'n duidelike toename in aktiwiteit tussen geïnfekteerde en ongeïnflekteerde plante getoon vanaf 24 h tot 96 h na Botrytis cinerea infeksie. Hierdie resultate bied voldoende bewys-van-konsep vir die AtGolS1 promotor om gebruik te word as 'n dubbele funksionele biosensor vir beide hitte-stres en swam infeksie

Priming with γ-Aminobutyric Acid against Botrytis cinerea Reshuffles Metabolism and Reactive Oxygen Species: Dissecting Signalling and Metabolism

The stress-inducible non-proteinogenic amino acid γ-aminobutyric acid (GABA) is known to alleviate several (a)biotic stresses in plants. GABA forms an important link between carbon and nitrogen metabolism and has been proposed as a signalling molecule in plants. Here, we set out to establish GABA as a priming compound against Botrytis cinerea in Arabidopsis thaliana and how metabolism and reactive oxygen species (ROS) are influenced after GABA treatment and infection. We show that GABA already primes disease resistance at low concentrations (100 µM), comparable to the well-characterized priming agent β-Aminobutyric acid (BABA). Treatment with GABA reduced ROS burst in response to flg22 (bacterial peptide derived from flagellum) and oligogalacturonides (OGs). Plants treated with GABA showed reduced H2O2 accumulation after infection due to increased activity of catalase and guaiacol peroxidase. Contrary to 100 µM GABA treatments, 1 mM exogenous GABA induced endogenous GABA before and after infection. Strikingly, 1 mM GABA promoted total and active nitrate reductase activity whereas 100 µM inhibited active nitrate reductase. Sucrose accumulated after GABA treatment, whereas glucose and fructose only accumulated in treated plants after infection. We propose that extracellular GABA signalling and endogenous metabolism can be separated at low exogenous concentrations

UDP-Glucose: A Potential Signaling Molecule in Plants?

This perspective paper focuses on the most recent results suggesting a potential role for UDP-Glucose as a signaling molecule in plants. In animals, UDP-Glucose is well-established as an extracellular signaling molecule that is sensed by G-protein coupled receptors, activating several downstream defense mechanisms. Recent studies have shown that abnormal growth occurred in both vegetative and reproductive tissue of plants with reduced UDP-Glucose levels, and this could be rescued by exogenous UDP-Glucose. In plants with increased biomass accumulation, the genes involved in UDP-Glucose production were up-regulated. However, excessive endogenous accumulation of UDP-Glucose induced programmed cell death (PCD), and this could also be obtained by exogenous UDP-Glucose application. Plants with decreased UDP-glucose were insensitive to pathogen induced PCD. We speculate that UDP-Glucose acts as an extracellular signaling molecule in plants, and that it may be perceived as a damage-associated molecular pattern

Spermine and Spermidine Priming against <i>Botrytis cinerea</i> Modulates ROS Dynamics and Metabolism in Arabidopsis

Polyamines (PAs) are ubiquitous small aliphatic polycations important for growth, development, and environmental stress responses in plants. Here, we demonstrate that exogenous application of spermine (Spm) and spermidine (Spd) induced cell death at high concentrations, but primed resistance against the necrotrophic fungus Botrytis cinerea in Arabidopsis. At low concentrations, Spm was more effective than Spd. Treatments with higher exogenous Spd and Spm concentrations resulted in a biphasic endogenous PA accumulation. Exogenous Spm induced the accumulation of H2O2 after treatment but also after infection with B. cinerea. Both Spm and Spd induced the activities of catalase, ascorbate peroxidase, and guaiacol peroxidase after treatment but also after infection with B. cinerea. The soluble sugars glucose, fructose, and sucrose accumulated after treatment with high concentrations of PAs, whereas only Spm induced sugar accumulation after infection. Total and active nitrate reductase (NR) activities were inhibited by Spm treatment, whereas Spd inhibited active NR at low concentrations but promoted active NR at high concentrations. Finally, γaminobutyric acid accumulated after treatment and infection in plants treated with high concentrations of Spm. Phenylalanine and asparagine also accumulated after infection in plants treated with a high concentration of Spm. Our data illustrate that Spm and Spd are effective in priming resistance against B. cinerea, opening the door for the development of sustainable alternatives for chemical pesticides

Fructans Prime ROS Dynamics and Botrytis cinerea Resistance in Arabidopsis

Naturally derived molecules can be used as priming or defense stimulatory agents to protect against biotic stress. Fructans have gained strong interest due to their ability to induce resistance in a number of crop species. In this study, we set out to establish the role of fructan-induced immunity against the fungal pathogen Botrytis cinerea in Arabidopsis thaliana. We show that both inulin- and levan-type fructans from different sources can enhance Arabidopsis resistance against B. cinerea. We found that inulin from chicory roots and levan oligosaccharides from the exopolysaccharide-producing bacterium Halomonas smyrnensis primed the NADPH-oxidase-mediated reactive oxygen species (ROS) burst in response to the elicitors flg22, derived from the bacterial flagellum, and oligogalacturonides (OGs), derived from the host cell wall. Neither induced a direct ROS burst typical of elicitors. We also found a primed response after infection with B. cinerea for H2O2 accumulation and the activities of ascorbate peroxidase and catalase. Sucrose accumulated as a consequence of fructan priming, and glucose and sucrose levels increased in fructan-treated plants after infection with B. cinerea. This study shows that levan-type fructans, specifically from bacterial origin, can prime plant defenses and that both inulin and levan oligosaccharide-mediated priming is associated with changes in ROS dynamics and sugar metabolism. Establishing fructan-induced immunity in Arabidopsis is an important step to further study the underlying mechanisms since a broad range of biological resources are available for Arabidopsis

Fructans Prime ROS Dynamics and Botrytis cinerea Resistance in Arabidopsis

Naturally derived molecules can be used as priming or defense stimulatory agents to protect against biotic stress. Fructans have gained strong interest due to their ability to induce resistance in a number of crop species. In this study, we set out to establish the role of fructan-induced immunity against the fungal pathogen Botrytis cinerea in Arabidopsis thaliana. We show that both inulin- and levan-type fructans from different sources can enhance Arabidopsis resistance against B. cinerea. We found that inulin from chicory roots and levan oligosaccharides from the exopolysaccharide-producing bacterium Halomonas smyrnensis primed the NADPH-oxidase-mediated reactive oxygen species (ROS) burst in response to the elicitors flg22, derived from the bacterial flagellum, and oligogalacturonides (OGs), derived from the host cell wall. Neither induced a direct ROS burst typical of elicitors. We also found a primed response after infection with B. cinerea for H2O2 accumulation and the activities of ascorbate peroxidase and catalase. Sucrose accumulated as a consequence of fructan priming, and glucose and sucrose levels increased in fructan-treated plants after infection with B. cinerea. This study shows that levan-type fructans, specifically from bacterial origin, can prime plant defenses and that both inulin and levan oligosaccharide-mediated priming is associated with changes in ROS dynamics and sugar metabolism. Establishing fructan-induced immunity in Arabidopsis is an important step to further study the underlying mechanisms since a broad range of biological resources are available for Arabidopsis.status: Published onlin

Sweet Immunity: The Effect of Exogenous Fructans on the Susceptibility of Apple (Malus × domestica Borkh.) to Venturia inaequalis

There is an urgent need for novel, efficient and environmentally friendly strategies to control apple scab (Venturia inaequalis), for the purpose of reducing overall pesticide use. Fructans are recently emerging as promising &ldquo;priming&rdquo; compounds, standing out for their safety and low production costs. The objective of this work was to test a fructan-triggered defense in the leaves of apple seedlings. It was demonstrated that exogenous leaf spraying can reduce the development of apple scab disease symptoms. When evaluated macroscopically and by V. inaequalis-specific qPCR, levan-treated leaves showed a significant reduction of sporulation and V. inaequalis DNA in comparison to mock- and inulin-treated leaves, comparable to the levels in fosetyl-aluminum-treated leaves. Furthermore, we observed a significant reduction of in vitro mycelial growth of V. inaequalis on plates supplemented with levans when compared to controls, indicating a direct inhibition of fungal growth. Variations in endogenous sugar contents in the leaves were followed during priming and subsequent infection, revealing complex dynamics as a function of time and leaf ontogeny. Our data are discussed in view of the present theories on sugar signaling and fructan-based immunity, identifying areas for future research and highlighting the potential use of fructans in apple scab management in orchards