A Receptor on Soybean Membranes for a Fungal Elicitor of Phytoalexin Accumulation

Abstract

Soybean membrane preparations specifically bound I'4Cjmycolami-naran, a branched 6-1,3-glucan produced by Phytophthora sp. which elicits production of the phytoalexin glyceollin in soybean tissues. A Scatchard plot of the binding data disclosed the presence of a single affinity class of binding sites with a Kd value of 11.5 micromolar for the glucan. To assess the physiologic importance of mycolaminaran binding in phytoalexin elicitation, several derivatives of mycolaminaran were prepared. Reduced mycolaminaran had slightly greater elicitor activity and binding affinity than the native substance, while periodinated mycolaminaran was virtually devoid of either elicitor activity or binding capability. Phosphorylated mycolaminaran, on the other hand, gave values for both elicitor activity and membrane binding which were intermediate between the native and periodinated preparations. No other tested carbohydrates competed with the binding of ['4Cjmycolaminaran. Soybean membrane preparations contained #-1,3-endoglucanase activity that degraded mycolaminaran and reduced both its efficiency as a phytoalexin elicitor and its membrane binding at temperatures above 0°C.Once ["Cjmycolaminaran bound to membranes, however, it was not appreciably susceptible to glucanase attack and could not be displaced with excess unlabeled ligand. Taken collectively, the observations suggest that the membrane binding sites are mycolaminaran-specific receptors which are physiologically involved in the initiation of phytoalexin production in soybean cotyledons. Because the binding of mycolaminaran to membranes was abolished by heat and proteolytic enzymes, the receptor is probably a protein(s) or glycoprotein(s).Phytoalexins are low mol wt antibiotic compounds that are inducibly formed in higher plant tissues after infection by microorganisms and appear to be responsible for disease resistance (6,11). The accumulation of phytoalexins in plants can also be stimulated by certain molecules of pathogen origin, termed elicitors, and these have been used to study the molecular basis of phytoalexin production and the expression of disease resistance (18,19 One method to assess critically the possible presence of receptors on cell membranes is direct binding assays using a homogeneous labeled ligand and isolated membranes. This approach has not been previously used in elicitor studies, probably due to difficulty in obtaining homogeneous, labeled carbohydrate elicitors. Phytophthora spp., however, produce a well characterized intracellular glucan, called mycolaminaran, that elicits glyceollin production in soybean tissues (9) and is unique because of its high degree of homogeneity (15)(16)(17). Mycolaminaran is composed of a linear polymer of about 30 glucose residues in (B 1,3-linkages, with two branching units connected through C-3 and C-6. The exact location of branch points and average chain length of the branches are yet to be experimentally elucidated.

Phytosulfokine-α, a sulfated pentapeptide, stimulates the proliferation of rice cells by means of specific high- and low-affinity binding sites

“…However, we could not detect PSKs' activity in conditioned medium derived from cell cultures of dicotyledonous plants, such as tobacco and Zinnia elegans, suggesting that the growth factor(s) of dicots have different structure(s) from PSKs. This is in line with cross-feeding experiments, which also pointed out incompatibility of factors between monocots and dicots (8,9 M for high-affinity type), but 4 to 10,000 times higher than those for plant signaling molecules, such as the fungal glucan phytoalexin elicitor (23)(24)(25) and lipo-oligosaccharidic NodRm factors (26).…”

Endocytosis of 1,3-?-glucans by broad bean cells at the penetration site of the cowpea rust fungus (haploid stage)

“…Therefore, it is reasonable to suggest that at least a part of the 1,3-]3-glucans in these organelles might either have a structure similar to the fungal wall material or may even have originated from the fungal wall. Fungal 1,3-[3-glucans have been described as active elicitors in phytoalexin synthesis (Yoshikawa et al 1983;Cosio et al 1988;Aldington and Fry 1993).…”

Evidence for specific, high‐affinity binding sites for a proteinaceous elicitor in tobacco plasma membrane

“…However, mechanisms underlying elicitor perception on the plant cell surface and subsequent intracellular transmission of this signal are poorly understood. Evidence for the existence of elicitor high-affinity binding sites was initially obtained with carbohydrate elicitors [3,4,5,6]. High-affinity binding sites were also observed with glycoprotein elicitors, the carbohydrate moiety of which was the structure required for the interaction with the binding sites [7].…”