Cell - cell contacts mediated by glycoproteins play a role in growth control. For example contactinhibin N-glycosylated protein is responsible for density dependent inhibition of fibroblasts growth. Cells stop proliferation when contacts between them are close ; they form a monolayer.
Plant Carbohydrates I - Intracellular Carbohydrates | F.A. Loewus | Springer
Contactinhibin is synthesized in highly sialylated, inactive form. After contacts between the cells are established, the glycoprotein is converted to low sialylated, biologically active form. It is placed in the plasma membrane and binds to a specific cell-surface receptor, CiR.
Two outstanding developments, discovery of sugar nucleotides and the advent of chromatography, brought together the insight and a means of probing complexities inherent in plant carbohydrates. These advances, combined with a modern knowledge of enzymes and cellular metabolism, have provided new horizons of investigation for the student of plant physiology.
This volume and its companion Vol. To accommodate the extensive amount of information to be presented, subject matter has been divided, somewhat arbitrarily, into intracellular and extracellular carbohydrates, with the latter defined as carbohydrates occurring in space out- side the plasma membrane plasmalemma.
This classification is not exclusive; rather it is intended to lend a degree of flexibility to the way in which subject matter is arranged between volumes.
The first section of this volume addresses the occurrence, metabolism, and function of monomeric and higher saccharides of fungi, algae, and higher plants. Help Centre. My Wishlist Sign In Join. Loewus Editor , W. Tanner Editor. Be the first to write a review.
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Based on the analysis of the Arabidopsis genome, the array of putative PRRs encoded in plants is much higher than in mammals. Plant PRRs are receptor-like kinases RLKs or receptor-like proteins RLPs , which are localized at the plasma membrane and possess extracellular domain for ligand recognition. In Arabidopsis thaliana , the chitin elicitor receptor kinase 1 CERK1 is the key chitin binding and signaling component Miya et al.
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By using a chimeric approach, Brutus et al. Such formations can improve the ligand recognition, signal transduction or perform a regulatory role Monaghan and Zipfel, Thereafter, eliciting activity of OS has been largely studied Table 1. In particular, laminarin induces defense responses in rice Inui et al. OGAs also induce various defense responses Ferrari et al. Chitin derivatives induce lignification in wheat Barber et al.
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In tomato and Commelina communis , Lee et al. These results unambiguously show that OS can be also perceived by guard cells Figure 1 , thus leading to signaling, defense activation, and also stomatal movements generally a stomatal closure. Experiments performed with grapevine leaves also showed a higher responsiveness of guard cells to OS, compared to epidermal cells Trouvelot et al. Figure 1. Carbohydrates have to penetrate through the hydrophobic cuticle to reach epidermal or guard cells to be perceived by PRR receptors and trigger signaling events and defense reactions immune responses.
They could also enter the leaf along the surfaces of the stomatal pores. Microorganisms living in the phyllosphere secrete enzymes susceptible to hydrolyze OS. Depending on their structure, released fragments may induce defense signaling and responses or not. Numerous papers have therefore described the capacity of oligosaccharides to induce plant defenses. However, few of them have reported induced resistance of plants against pathogens. Laminarin confers protection of grapevine against Plasmopara viticola and Botrytis cinerea and of tobacco against tobacco virus mosaic TVM Klarzynski et al.
Treatment with chitin reduces the susceptibility of rice to Magnaporthe oryzae Tanabe et al. Curiously, the effect of chitin treatment on resistance remains rather mild whereas chitosan induces a strong resistance of different plant species to fungal pathogens Sharp, However, besides its elicitor activity Benhamou et al. The biological activity of oligosaccharides is highly dependent on their degree of polymerization DP and pattern.
Fu et al. The highest activity of chitin was generally reported for heptamers and octamers and little or no activity for shorter oligomers Vander et al. For chitosan, oligomers with a DP ranged between 7 and 10 are usually the most active Hadwiger, Dissimilar size-depending biological response were also reported for OGAs Reymond et al. DP between 10 and 16 are indeed often referred to as optimal size to induce defenses Navazio et al. Whereas they induce defense reactions in tomato or potato Weber et al.
Original Research ARTICLE
Acetylation and methylation also impact OS activity. The chitin acetylation status is crucial for CERK1 binding as AtCERK1 can weakly bind the partially deacetylated chitosan whereas it possesses no affinity for fully deacetylated chitooligomers Petutschnig et al. Chitosan heptaose and octaose do not elicit ROS burst and cell death in rice, suggesting that also rice requires acetylated ligands for immune activation Kaku et al.
For OGAs, acetylation is also important since both acetylated and unacetylated ones induce defense events in wheat whereas only acetylated ones lead to an increase in papilla-associated fluorescence and to a reduction of formed fungal haustoria of Blumeria graminis f. As example, a reduction of B. Conversely, the overexpression of PME in strawberry leads to a reduced degree of methyl esterification of OGA, an increase of defense genes expression and an improved resistance against B. Biomolecules substituted with sulfate groups are involved in major physiological functions in plants and animals.
The presence of sulfate groups and the degree of sulfation DS can modulate the biological activities of oligosaccharides. Laminarin was therefore sulfated to improve its activity, providing PS3 with a DS of 2. Moreover, PS3-enhanced resistance of grapevine herbaceous plantlets against Plasmopara viticola is more efficient than with laminarin Trouvelot et al. Interestingly, PS3 acts by priming in this interaction whereas laminarin directly elicits defense events Trouvelot et al.
A structure-activity study was also conducted for laminarin sulfates having different degrees of sulfation ranging between 0. A DS higher than 0. Moreover, the maximal eliciting activity is obtained for DS of at least 1. Sugars are also involved in plant immunity as signaling molecules Sheen et al. As examples, sucrose induces isoflavonoids synthesis as a defense response against Fusarium oxysporum in lupine Morkunas et al. Galactinol stimulates the accumulation of defense-related gene transcripts in tobacco plants, enhances resistance against Botrytis cinerea and Erwinia carotovora and is a signaling component of the induced systemic resistance caused by Pseudomonas chlororaphis Kim et al.
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Trehalose induces PAL and peroxidase activities associated with partial resistance of wheat against powdery mildew Reignault et al. In Arabidopsis cell suspensions, sucrose or glucose induces the expression of several PR-genes and accumulation of the corresponding proteins PR-2 and PR-5 through a SA-dependent pathway Thibaud et al. As reviewed by Bolouri-Moghaddam and Van Den Ende , other sugars such as psicose or D-allose can stimulate plant immunity and upregulate defense genes expression. It is tempting to think that some sugars could be considered as elicitors. However, plants may respond to changes of extracellular levels of sugars rather than sugars themselves.
How plants perceive sugars is highly complex and needs to be further investigated.
The perception of hexoses is achieved by hexokinases Granot et al. However, hexokinase independent pathways were also reported Lalonde et al. Sucrose and other disaccharides seem to be perceived at the level of the plasma membrane Goddijn and Smeekens, ; Rolland et al. However, sucrose can also be hydrolyzed by apoplastic invertases, providing hexoses that will be perceived by membrane or cytosolic sensors Sheen et al.
Research on sugar transporters is also in progress and will help to decipher the mechanisms associated to sugar perception. Such changes are perceived by plants and allow induction of defenses Herbers et al.