Pectins: structure, biosynthesis, and oligogalacturonide-related signaling.

Publisher Summary This chapter discusses the role of leucine–rich repeat (LRR) proteins in plant defenses. LRR proteins are involved in specific protein–protein interactions and are confined predominantly to eukaryotes. Their existence in animals and fungi has been known for some time, but has been described only recently and incompletely for plants. The first plant LRR proteins described were receptor serine/threonine kinases and polygalacturonase–inhibiting proteins. LRR proteins have a significant role in plant defenses. Resistance to a diverse range of pathogens, including nematodes, fungi, bacteria, and viruses involves LRR proteins either as resistance proteins or as proteins required for resistance proteins to function. The limitation of pathogen ingress and aggressiveness may also involve LRR proteins which function as PR proteins. Several LRR proteins, such as FIL2, the AJWL proteins seem to be expressed in reproductive tissues. These proteins might have roles in the defense of reproductive tissues from pathogens or roles in reproduction itself. Pollen self–incompatibility and species incompatibility may also be viewed as forms of defense. It would not be surprising if the parallels between resistance and pollen incompatibility extended to the involvement of LRR proteins in both processes.

The Role of Leucine-Rich Repeat Proteins in Plant Defences

▪ Abstract Polygalacturonase-inhibiting proteins (PGIPs) are extracellular plant proteins capable of inhibiting fungal endopolygalacturonases (PGs). Plants have evolved different PGIPs with specific recognition abilities against the many PGs produced by fungi. The genes encoding PGIPs are organized into families, and different members of each family may encode proteins with nearly identical characteristics but different specificities and regulation. PGIPs are typically induced by pathogen infection and stress-related signals. The recognition ability of PGIPs resides in their LRR (leucine-rich repeat) structure, where solvent-exposed residues in the β-strand/β-turn motifs of the LRRs are determinants of specificity. Manipulation of the primary structure of PGIPs is expected to generate more efficient PGIPs with novel recognition specificities to protect crop plants against pathogens.

The role of polygalacturonase-inhibiting proteins (pgips) in defense against pathogenic fungi

Colletotrichum: A model genus for studies on pathology and fungal-plant interactions.

Oligosaccharins are complex carbohydrates that can function in plants as molecular signals that regulate growth, development, and survival in the environment [3]. Studies of plant-microorganism interactions yielded the first evidence that oligosaccharins could serve as biological signals. Much of this research focused on the synthesis and accumulation of antimicrobial phytoalexins in response to microbial attack. Phytoalexin synthesis and accumulation are observed not only after microbial infection, but also after treatment of plant tissue with cell-free extracts of microbial origin. The active components in these extracts are commonly referred to as ‘elicitors’. The term ‘elicitor’ was originally used to refer to molecules and other stimuli that induce the synthesis and accumulation of phytoalexins in plant cells [130], but is now commonly used for molecules that stimulate any plant defense mechanism [68, 70, 71, 104].

Oligosaccharins: structures and signal transduction.

Polygalacturonase-inhibiting protein (PGIP) is a cell wall-associated protein that specifically binds to and inhibits the activity of fungal endopolygalacturonases. The Phaseolus vulgaris gene encoding PGIP has been cloned and characterized. Using a fragment of the cloned pgip gene as a probe in Northern blot experiments, it is demonstrated that the pgip mRNA accumulates in suspension-cultured bean cells following addition of elicitor-active oligogalacturonides or fungal glucan to the medium. Rabbit polyclonal antibodies specific for PGIP were generated against a synthetic peptide designed from the N-terminal region of PGIP; the antigenicity of the peptide was enhanced by coupling to KLH. Using the antibodies and the cloned pgip gene fragment as probes in Western and Northern blot experiments, respectively, it is shown that the levels of PGIP and its mRNA are increased in P. vulgaris hypocotyls in response to wounding or treatment with salicylic acid. Using gold-labeled goat-anti-rabbit secondary antibodies in EM studies, it has also been demonstrated that, in bean hypocotyls infected with Colletotrichum lindemuthianum, the level of PGIP preferentially increases in those cells immediately surrounding the infection site. The data support the hypothesis that synthesis of PGIP constitutes an active defense mechanism of plants that is elicited by signal molecules known to induce plant defense genes.

Polygalacturonase‐inhibiting protein accumulates in Phaseolus vulgaris L. in response to wounding, elicitors and fungal infection

Microbial pectic enzymes not only macerate plant tissue but also kill plant cells and elicit synthesis and accumulation of phytoalexins (Hahn et al., 1988). We have evidence suggesting that these three physiological activities of pectic enzymes may be regulated by factors such as endo-polygalacturonase inhibiting proteins (PGIP) and pH.

Phytotoxic Effects and Phytoalexin-Elicitor Activity of Microbial Pectic Enzymes

Linear α-l,4-linked oligogalacturonides of chain lengths between 10 and 13 have been shown to elicit phytoalexin accumulation and lignification in plants, but shorter oligogalacturonides do not have this effect (Hahn et al 1981; Nothnagel et al 1983; Jin & West 1984; Davis et al 1986; Robertsen 1986; De Lorenzo et al 1987b; Cervone et al 1989). Phytoalexin elicitor-active oligogalacturonides are thought to be released from the plant cell wall by the action of two groups of microbial pectic-degrading enzymes, the endopolygalacturonases and the endopectate lyases. These enzymes release elicitor-active oligogalacturonides from polygalacturonic acid in vitro but also quickly depolymerize the elicitor-active chains, converting them into shorter, elicitor-inactive molecules (Bruce & West 1982; Nothnagel et al 1983; Cervoneet al 1987). We will discuss two mechanisms by which the action of fungal endopolygalacturonases and bacterial endopectate lyases may be regulated such that the formation of elicitor-active oligogalacturonides is favored.

Y. Ito

Papers

Polygalacturonase‐inhibiting protein accumulates in Phaseolus vulgaris L. in response to wounding, elicitors and fungal infection

Phytotoxic Effects and Phytoalexin-Elicitor Activity of Microbial Pectic Enzymes

Release of Phytoalexin Elicitor-Active Oligogalacturonides by Microbial Pectic Enzymes