Inoculation of one true leaf of cucumber (Cucumis sativus L.) plants with Pseudomonas syringae pathovar syringae results in the systemic appearance of salicylic acid in the phloem exudates from petioles above, below, and at the site of inoculation. Analysis of phloem exudates from the petioles of leaves 1 and 2 demonstrated that the earliest increases in salicylic acid occurred 8 hours after inoculation of leaf 1 in leaf 1 and 12 hours after inoculation of leaf 1 in leaf 2. Detaching leaf I at intervals after inoculation demonstrated that leaf I must remain attached for only 4 hours after inoculation to result in the systemic accumulation of salicylic acid. Because the levels of salicylic acid in phloem exudates from leaf I did not increase to detectable levels until at least 8 hours after inoculation with P. s. pathovar syringae, the induction of increased levels of salicylic acid throughout the plant are presumably the result of another chemical signal generated from leaf I within 4 hours after inoculation. Injection of salicylic acid into tissues at concentrations found in the exudates induced resistance to disease and increased peroxidase activity. Our results support a role for salicylic acid as an endogenous inducer of resistance, but our data also suggest that salicylic acid is not the primary systemic signal of induced resistance in cucumber.Inoculation of one leaf of cucumber plants and other cucurbits with necrotic lesion-inducing pathogens (7-9) or necrosis/chlorosis-inducing chemicals (3, 4) results in the expression of systemic resistance against disease caused by a number of pathogens. The onset of resistance has been correlated with the initial appearance of necrotic lesions and generally begins to develop 3 to 4 d after the resistance-inducing inoculation (7-9).We have recently demonstrated that systemic resistance can be induced in cucumber within 24 h by inoculating leaf with the HR2-inducing bacterium Pseudomonas syringae pv syringae (17 demonstrated that this leaf must remain attached for only 6 h to result in the systemic expression of enhanced peroxidase activity and a small, but detectable, increase in the level of systemic disease resistance. Allowing the first leaf to remain on the plant for up to 12 h after inoculation with P. s. pv syringae resulted in a further increase in the level of systemic resistance as compared with plants that had the inoculated first leaf detached 6 h after inoculation. TnS mutants of P. s. pv syringae that had lost the ability to induce the HR were also unable to induce systemic resistance and peroxidase activity.Dean and Kuc (1, 2) have provided strong evidence that the systemic signal(s) for induced resistance was generated in and mobilized out of the leaves that were initially inoculated ("source" leaves) with resistance-inducing pathogens. Metraux et al. ( 12) recently reported that cucumber plants inoculated with either Colletotrichum lagenarium or tobacco necrosis virus on one leaf had higher levels of salicylic acid (an exogenous inducer ofres...

Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

Inoculation of leaves of Arabidopsis thaliana (L.) Heynh. with the wheat pathogen, Pseudomonas syringae pv syringae, resulted in the expression of the hypersensitive reaction and in phytoalexin accumulation. No phytoalexin accumulation was detected after infiltration of leaves with a mutant of P. s. syringae deficient in the ability to elicit a hypersensitive reaction; with the crucifer pathogen, Xanthomonas campestris pv campestris; or with 10 milimolar potassium phosphate buffer (pH 6.9). Phytoalexin accumulation was correlated with the restricted in vivo growth of P. s. syringae. A phytoalexin was purified by a combination of reverse phase flash chromatography, thin layer chromatography, followed by reverse phase high performance liquid chromatography. The Arabidopsis phytoalexin was identified as 3-thiazol-2'-yl-indole on the basis of ultraviolet, infrared, mass spectral, 1H-nuclear magnetic resonance, and '3C-nuclear magnetic resonance data.Phytoalexins are low mol wt, antimicrobial compounds of plant origin that accumulate after inoculation with a plant pathogen (14). A number of observations support the hypothesis that phytoalexins play a role in the defense response of plants to pathogens. Phytoalexins are absent in healthy tissues and accumulate after infection by fungal (18,24,30) or bacterial (9, 1 1) pathogens in monocotyledonous plants (24) as well as in dicotyledonous plants (9,18,30). Phytoalexins have been demonstrated to accumulate rapidly at the site of attempted infection in sufficient quantities to inhibit the in vitro growth of fungi (18,30) and bacteria (9). Virulence of the fungus Nectria haematococca on pea is correlated with the ability to detoxify the phytoalexin pisatin (28). Furthermore, transformation of Cochliobolus heterostrophus with the gene encoding pisatin demethylase allowed this maize patho-

Numerous reports have been published on the occurrence, isolation, and characterization of phenolic compounds in plant-soil systems. The low molecular weight phenolics are of great interest because of their effects as allelopathic compounds and plant growth regulators, and they have traditionally been considered as defense molecules in plant-pathogen interactions. More recently, their role as signal molecules in plantmicrobe systems has become evident. Specific molecules can act either as inducers for virulent genes in plantpathogen systems, such as Agrobacterium, or as inducers and "repressors" in Rhizobium-legumc and probably other symbiosis. The overall regulation role of these compounds in compatible and incompatible host-microbe interactions is discussed in this review.

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