Expression of pLOX1, a potato LOX gene now identified as a 9-LOX type 1 , was strongly induced in AA-treated and P. infestans-inoculated potato tuber disks and leaves , as was a tomato LOX in AA-treated tomato leaves . LA-treatment did not induce pLOX1 expression or LOX activity. Heat treatment of tuber disks inactivates enzyme activity and abolishes HPETE formation following AA treatment , and EP-induced responses are strongly diminished when LOX activity is inhibited or absent . Nonetheless, definitive experiments with LOX knock-out/knock-down or over expression lines to critically test specific LOX isoforms in EP action have not been reported. While it has been proposed and is quite likely that the 9-oxylipin pathway metabolites of AA may directly act as signal molecules to activate defense responses , AA and/or its metabolites may also induce expression and activity of oxylipin pathway enzymes to form biologically active metabolites from the plant LA and ALA pools. Studies during the past 15 years in solanaceous plants point to the importance of 9-LOX and the 9-oxylipin pathway in defense, and have demonstrated that the 9-LOXs from potato, tobacco, and pepper can utilize AA as a substrate. Many of these studies have investigated defense responses against oomycete pathogens or used elicitor preparations from oomycetes likely containing EP . 9-hydroperoxy fatty acids can be utilized by downstream oxylipin pathway enzymes to form other compounds that have been found to function in defense. In particular, DESs are induced in response to elicitors and pathogen attack in several solanaceous species including potato, tobacco,flood tray and pepper.Recent experiments indicate that treatment of tomato roots with EP induces resistance against P. capsici.

Hydroponically grown tomato plants whose roots were treated with EP and subsequently inoculated with P. capsici experience significantly less rot and collapse at the crowns than plants whose roots were treated with H2O, LA, or ALA, indicating that exposure of tomato roots to EP prior to inoculation with P. capsici reduces susceptibility of the plants to P. capsici . Further experiments demonstrate that roots and crowns display significantly increased lignification responses following root treatment with AA and EPA and subsequent inoculation with P. capsici compared to roots treated with H2O, LA, and ALA. AA-treatment of tomato roots elicits increased expression of 9-LOX and 9-DES genes in tomato roots compared to control treatments . Expression of 9-DES is also increased following inoculation of roots with P. capsici . In conclusion, although EP action in plants is complicated, evidence supports an important role for LOX and likely a 9-oxylipin pathway in the initiation of plant responses. Furthermore, in Arabidopsis an intact JA pathway is required for AA activity, implicating a 13-LOX. Whether DES and divinylethers participate in the plant response to EP observed in solanaceous plants is unresolved, although ongoing research in our laboratory will address this issue. The search for a traditional PRR for EP in plant cells analogous to those for other MAMPs, although intriguing, may not be productive given other mechanisms for rapid uptake of PUFA by plant cells and their entry into oxylipin metabolism.β-linked glucose polysaccharides are the most abundant compo nent of Phytophthora cell walls, comprising more than 80% of the wall dry weight . These include insoluble β-1→4-linked and β-1→3, β-1→6-linked glucans, with the latter by far the more abundant of these polymers. In addition to the abundance of glucose, compositional analyses of cell walls also reveal minor amounts of mannose and glucosamine, as well as protein and lipid similar to levels found in cell walls of fungi. In addition to the insoluble glucans, solu ble β-1→3-linked glucans are present at various developmental stages in the oomycete life cycle. For example these can be found in the germination fluids of cystospores as well as other stages, and during synthesis and remodeling of the wall during growth, thus making them potentially available at the host–pathogen interface during infection .

Laminarans are linear β-1→3-linked glucans that provide the dominant storage carbohydrate in Phytophthora and other oomycetes, as well as other stramenopiles . The β-1→3, β-1→6-linked glucans present a very complex array of possible structures, some with well-established activity in modulating plant innate immunity. The most prominent example is the elicitor activity associated with glucans isolated from cultures and cell walls of the soybean pathogen P. sojae . Albersheim et al. showed that these were potent inducers of the flflavonoid phytoalexin, glyceollin, and related defense reactions in soybean cotyledons. β- glucan oligosaccharide fractions of varying complexity had elicitor activity suggesting a model whereby cell wall fragments released during infection provide the physiological triggers of the plant defense response. The smallest active fragment following partial acid hydrolysis of P. sojae cell walls was purified and shown to be a hexa -D-glucitol. This oligosaccharide and its corresponding unreduced hepta-β-glucoside elicited at concentrations between 10−7 to 10−9 M . Subsequent work by Michael Hahn and coworkers further defined the branched β-1→3, β-1→6 structural motif essential to maximally induce phytoalexin accumulation and found that the hepta-β-glucoside specifically bound to soy bean membranes with high affinity . These investigators provided strong evidence that the binding activity was associated with a membrane protein or glycoprotein. Subsequent efforts by other laboratories identified hydrophobic membrane proteins that bind β-glucans with high affinity from soybean and other legumes . Reconsti tution of the soybean homolog in lipid vesicles strongly bound the hepta-β-glucoside , which could be displaced by glucans with different degrees of polymerization in competitive binding assays. The elicitor activity and high affinity binding of the hepta-β- glucoside and related β-glucans are limited to members of the Fabaceae . Biochemical purification and additional studies indicate the binding proteins from legumes constitute a family of proteins of different sizes , with different carbohydrate active domains, one that binds β-glucans and another with glucanase activity capable of releasing elicitor-active fragments from Phytophthora cell walls .

What would further strengthen the case for these as physiological receptors for β-glucan-triggered immune responses in soybean is evidence that the binding speci- fificity for diverse oligoglucosides matches their bio-activity as elicitors. To our knowledge corresponding knock-out or knock down genetic experiments within legumes to corroborate receptor function have not been reported, although the soybean protein expressed in tomato confers binding of the hepta-β-glucoside .β-1→3-glucans also figure prominently as immune modulators in the potato – P. infestans interaction,arandanos planta although the story here is complicated by their reported action as both enhancers and suppressors of elicitor activity. However, this differential activity has not been reconciled with the degree of biochemical resolution as was done with P. sojae glucans to unambiguously assign enhancer or suppressor activity to the various oligoglucosides within the active fractions. Doke and Tomiyama using a potato protoplast assay showed that water soluble, anionic and non-anionic β-glucans suppressed the elicitor activity of a crude hyphal wall fraction from P. infestans. They suggested a degree of race-specificity in that glucans from compatible races of the pathogen were more active than those of incompatible races in suppressing the HR and ROS induced by the hyphal wall elicitor. The suppressive glucans were partially characterized and shown to have a DP of 17–23 glucose units with β-1→3 and β-1→6 link ages, and were present in the fluids of germinating cystospores . The purified hepta-β-glucoside from P. sojae was neither active as an elicitor nor as a suppressor in potato. A subsequent study showed that water soluble glucans from spore germination fluids of P. capsici have similar effect in suppressing elicitor-induced cell death in pepper and tomato cell suspensions . Race specificity attributed to the glucans in the context of HR suppression is difficult to reconcile with the contemporary paradigm of effector-triggered immunity and resistance -gene action . The model for β-glucans as suppressors is further complicated by their enhancement of EP elicitor activity. β-glucans, although lacking inherent elicitor activity in potato, can strongly enhance the activity of EP. Several lines of evidence suggest the combined action of eliciting and non-eliciting components provide a maximal defense response. Initial evidence came from reconstitution experiments whereby highly elicitor-active, solubilized cell wall fractions were hydrolyzed in base-borohydride, leaving polysaccharides intact but hydrolyzing any esterified fatty acids, which were then removed by solvent extraction. This resulted in complete loss of elicitor activity, which was restored by addition of AA and EPA to the base hydrolyzed wall fractions at their levels initially present . Subsequent fractionation, partial purification and analysis showed that the enhancers were indeed β-1→3-glucans . Preisig and Ku´c further demonstrated that the glucans provide a 10–100 fold enhancement of the activity of AA concentrations that alone are below the thresh old for induction of phytoalexins and related responses. The glucans also revealed elicitor activity of other EPs, particularly 5-eicosatrienoic acids. The most active β-glucan fractions had similar DP as the suppressor glucans, and were then found to suppress the HR induced by incompatible races of P. infestans, suggesting that the enhancers and suppressors could be the same. These classic experiments indicate that members of the Solanaceae have an intriguing system for perceiving specific β- glucans and EP to coordinate a strong resistance response.

The activity of these glucans in modulating immunity in potato, in particular, suggests a receptor-mediated process subject to attenuation by competing ligands as observed in legumes. For example, the suppressive action of the β-glucans against the HR induced by pathogen inoculum or the crude hyphal wall elicitor may have resulted from similarities in oligosaccharin motifs that compete for a putative MAMP receptor. Algal polysaccharides, such as the storage β-glucans laminarin and carrageenan, activate defense responses in some plants, although sulfated carrageenans appear to be far more active than laminarins as elicitors . However, in potato, laminarin neither elicits nor suppresses, providing a negative control treatment in the studies of the more complex β-1→3-linked glucans. Although considerably less active than the β-glucans, N, N’- diacetyl-D-chitobiose, the hapten for the potato lectin, inhibited the HR induced by incompatible races of P. infestans in potato and modestly enhanced the elicitor activ ity of AA . Although other carbohydrates may modulate the plant immune response to some degree, the exceptionally strong biological activity of the oomycete oligosac charins indicates considerable structural specificity in their action.Protection against fungi in vertebrates involves both innate and adaptive immunity . Innate anti-fungal immunity is primarily mediated by diverse pattern-recognition receptors associated with phagocytes, which upon activation ingest and kill or degrade the invading microbe. Carbohydrates associated with the fungal cell wall, in particular, are well positioned to be recognized by these receptors. The adaptive and highly specific immune response to the invader is then engaged following generation of cytokines and chemokines along with the presentation of microbial antigens to lymphocytes. There are multiple pattern-recognition receptors for β-glucans in phagocytes and the molecular details for some of these inter actions have been characterized . These include the transmembrane dectin-1, a natural killer cell-receptor-like C-type lectin found on macrophages, neutrophils and dendritic cells, which specifically recognizes β-1→3- and β-1→6-linked glucans as well as intact yeast cells . Zymosan, a complex cell wall preparation from Saccharomyces cerevisiea used to promote inflammation in experimental models, also stimulates dectin-1 and macrophage activation. Of particular interest in relation to the topic of this review is that zymosan induces cytosolic phospholipase A2 in macrophages that releases AA for conversion into pro-inflammatory prostaglandins and leukotrienes . Intriguing here is the apparent cross-kingdom conservation whereby β-glucans operate in concert with AA metabolites and other signals to orchestrate an innate immune response. The extent to which this analogy and underlying mechanisms trans late to plant–oomycete interactions remains to be determined. Arabidopsis and Solanum species have proteins with C-type lectin motifs with some homology to dectin-1. However, they appear to be rare in plants and their functions are unresolved .Stories and rumors have circulated for years about biotechnology projects in horticulture being shelved because of intellectual property conflicts. In a typical situation, a plant scientist at a university agricultural-experiment station or a smaller seed firm has developed a remarkable new variety using the cutting-edge scientific tools of plant biotechnology. Then, as they or the nursery or the growers’ association with whom they work take the next steps to develop and release the new variety to commercial growers, their efforts are quickly and quietly shut down by a letter from an attorney.