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.

Onion plants (Allium cepa L, cv. Downing Yellow Globe) grown in pots and infected by the mycorrhizal fungusGlomus etunicatus Becker and Gerdemann were more drought tolerant than were non-mycorrhizal individials when exposed to several periods of soil water stress separated by periods of high water supply, as shown by greater fresh and dry weights and higher tissue phosphorus levels in the mycorrhizal plants. The tissues of stressed, non-mycorrhizal plants were deficient in P, despite the fact that only non-mycorrhizal plants were fertilized with high levels of P (26 mg P per 440 g soil). Differences in plant water relations (leaf water potentials or transpiration rates) and changes in soil P levels which may have affected plant growth were investigated, and discounted as factors important for the results. The P nutrition of plants has been implicated in the ability of plants to tolerate drought and it was concluded that the ability of the mycorrhizal fungus to maintain adequate P nutrition in the onions during soil water stress was a major factor in the improved drought tolerance. Infection of the root by the fungus was found not to be affected by water stress or P fertilization but fungal reproduction, as determined by spore numbers in the soil, was decreased by water stress and by P fertilization.

SUMMARYThe effects of flavonoid compounds on VA mycorrhiza root colonization and growth of w-hite clover {Trifolium repens L.) plants under growth chamber conditions are reported. The isoflavonoids, formononetin and biochanin A, previously identified from clover roots, stimulated colonization and growtb of clover, while several other flavonoid compounds were inactive when tested at concentrations of 5 mg 1"', The flavone, chr\'sin, wben applied 3t concentrations higher than those tested for formononetin and biochanin A, also increased root colonization and plant growth. The stimulatory effects of the isoflavonoids on p\ant growth were mediated by VA mycorrhiza) fungi and were dependent on concentration, period of growth and soil spore density. Maximum responses were found when 5 mg 1"' solutions were applied to soi! containing 2 to 4 VA mycorrhiza spores g"^ of soil. Tbese results may provide insights on the molecular mechanisms of bost-fungus interaction and for the development of technology to exploit the potential of the indigenous VA mycorrhizai fungi in field soil.

MATERIALS AND METHODSMycorrhizal soybean (Glycine max L. Merr. var. plants have lower resistances to water transport than nonmycorrhizal plants after 4.5 weeks of growth. Although resistances of whole plants differ by 40 %, there were no differences in the resistances of stems plus leaves, indicating that the major effect of the mycorrhizae was to reduce the resistance of the roots. Since the fungitoxicant, p-chloronitrobenzene, had no effect on resistances to water transport, reduced resistances were probably not caused by a direct modification of the transport pathway by the fungus. Differences in resistance between mycorrhizal and nonmycorrhizal soybean were essentially eliminated by the application of nutrients to the soil. Thus, lowered resistances of mycorrhizal roots growing in soil with low levels of nutrients probably resulted from the enhanced nutrient status of the plant brought about by the fungus. Mycorrhizal infection increased growth at both low and high nutrient levels.The vesicular-arbuscular mycorrhizal relationship, one of the most common forms of symbiosis, increases the growth of many host plant species (1-3, 10, 15, 16, 20, 25, 29) including soybean (30, 31). The increased growth has been attributed to an enhanced nutrient uptake by the host plant (2,12,15,(18)(19)(20)26). Mycorrhizal plants take up more phosphorus than nonmycorrhizal controls when relatively unavailable sources of phosphorus are supplied to the roots (12,26). In most cases the addition of more readily available phosphorus eliminates differences in growth and phosphorus uptake (3,12,13,20,26).We have recently shown that VA3 mycorrhizae also decrease the resistance to water transport in whole soybean plants (31). The present study was undertaken to determine where the fungus acts and whether the lowered resistance to water transport can be attributed to increases in the nutrient status of the host brought about by the mycorrhizae.'This work was supported in part by The method used for determining resistance to water transport was that of Boyer (5, 6), which has previously given results that compare well with independent methods (6, 31) and provides consistent determinations of whole plant resistances in mycorrhizal and nonmycorrhizal soybean (31). The method involves measurement of the recovery in water potential of a moderately water-deficient leaf on an intact plant. The time required for recovery, which is determined when no transpiration is occurring, is dependent on the resistance to water transport.For measurement of the recovery of a whole plant, the blade of an intact soybean leaf which was moderately water deficient was sealed in a thermocouple psychrometer chamber that measures the water potential of intact leaves (4). The soil and root system were then submerged in degassed water, excess water was drained away, and the recovery in water potential of the leaf was recorded. After determining that the data conformed to the transfer equation for a plane sheet (5, 9, 1 1), the resistance of the whole plant was calc...

Two isoflavonoids isolated from clover roots grown under phosphate stress were characterized as formononetin (7-hydroxy,4'-methoxy isoflavone) and biochanin A (5,7-dihydroxy,4'-methoxy isoflavone). At 5 ppm, these compounds stimulated hyphal growth in vitro and root colonization of an undescribed vesiculararbuscular mycorrhiza, a Glomus sp. (INVAM-112). The permethylated products of the two compounds were inactive. These findings suggest that the isoflavonoids studied may act as signal molecules in vesiculararbuscular mycorrhiza symbiosis. Vesicular-arbuscular mycorrhizae (VAM) result from a complex sequence of interactions between fungal hyphae and host cells, leading to a functional mutualistic state (6). Plant factors stimulate VAM hyphal growth in vitro and also the precolonization phase of VAM formation (4, 11, 13, 25, 26, 29). It has been suggested (14, 15) that the quantity rather than the presence of specific compounds in the root exudates is responsible for stimulation of fungal growth and VAM root colonization (36). Other studies, however, showed no relationship between root exudation and VAM infection (2). Viable spores of most VAM fungal species readily germinate on distilled water (39), and there is no evidence that they require any specific host factors. However, certain components of root exudates or plant cells may act as signal molecules capable of inducing hyphal growth, branching, differentiation, and host penetration (4, 5, 25). An earlier report from this laboratory (11) indicated the presence of a transient VAM-stimulating factor in exudates from phosphorus-deprived young white clover seedlings. This study also indicated that the quality of the exudate is important in stimulating VAM hyphal growth. Similar results have been found with stressed suspension-cultured legume cells (29) and with cultures of Ri T-DNA-transformed roots (3-5). The studies with transformed roots also indicated that root-inducing factors are required for the fungus to switch from preinfection to a biotrophic state. However, the nature of these highly active plant factors has not been determined to date. In this study, we report the isolation and identification of VAM-stimulatory compounds from clover roots. MATERIALS AND METHODS General analytical methods. 'H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Varian Gemini 300 spectrometer in CD3OD solutions at 25°C. Infrared spectra in KBr were obtained with a Perkin-Elmer 1600 series FT-IR spectrometer, and UV spectra were recorded on a Shimadzu UV-265 spectrometer. Melting points were recorded on a Thomas model 40 micro hot-stage apparatus and are uncorrected. Mass spectra (MS) were obtained on a JEOL model HX-110 mass spectrometer. Vacuum liquid