The chitin backbone of the LCO may cause it to act, in some way, like an elicitor in disease systems. LCO reduces salicylic acid levels in alfalfa roots, potentially aiding in suppression of legume host defense responses, and thereby ensuring successful infection (Martinez-Abarca et al. 1998). More recently, we have shown that a strong reduction in SA level in the leaf tissues occurred when soybean plants were sprayed with LCO (Prithiviraj et al. 2000b). Nod factors can alter phytoalexin concentrations in plant tissues. Nod Rm-IV(C16:l, S) increases medicarpin content in alfalfa (Savoure et al. 1994). Schmidt et al. (1994) reported an increase in concentrations of the flavonoids daidzen, coumestrol and genistein in root exudates of soybean (Glycine max) following exposure to the appropriate Nod factor (LCO) (Savoure et al. 1994). Nod factors have also been shown to induce the expression of chalcone synthase, chalcone reductase and PR genes (Krause et al. 1997). Co-inoculation of soybean plants with B. japonicum and the mycorrhizal fungus Glomus mossae, enhanced root colonization of the fungus, and similar results were observed when highly purified Nod factors (Nod NGR-V (MeFuc, Ac) from Rhizobium sp. NGR 234 (Xie et al. 1995) were applied.
LCOs have been shown to provoke a range of physiological responses in non-host plants. For instance, LCOs induce rapid and transient alkalination of tobacco (Baier et al. 1999) and tomato cells (Stahlein et al. 1994) in suspension cultures. LCOs can restore cell division and embryo development in a carrot mutant (de Jong et al. 1993). Finally, LCOs caused a resumption of cell division in somatic embryo cultures of Norway spruce (Picea abies) (Egertsdotter, von Arnold 1998), even in the absence of auxin and cytokinin (Daychok et al. 2000).
Induction of nod genes by non-host plants has been reported (Hungria, Stacey 1997; Le Strange et al. 1990), raising a possible explanation for the reported PGPR activity of rhizobia in crops such as rice (Biswas et al. 2000a,b; Prayitno et al. 1999). Increased growth of maize and bean in intercropping systems could be attributed to the reciprocal stimulation of Rhizobium and Azospirillum by root exudates of bean and maize (Hungria, Stacey 1997).
During the last decade we have found that treatment of soybean seeds with genistein-induced cultures of B. japonicum enhanced the germination and emergence of soybean and other crop plants under field conditions, when compared to uninduced cultures of B. japonicum or genistein. This finding was made first in the field and is a classic example of serendipity in biological research. The stimulation of seed germination only by genistein induced B. japonicum cells suggests that the observed effects might be due to the LCO present in the induced cultures. We have recently shown that the major LCO of B. japonicum enhanced the germination and early growth of a variety of crop plants and the model plant Arabidopsis (Prithiviraj et al. 2000c). Similar effects were observed with a number of synthetic LCOs (unpublished results). Presoaking of seeds in LCO solutions induced rapid emergence of soybean, maize and cotton under field conditions. LCO treatments also increased early growth of maize and soybean in pot and hydroponic experiments. Irrigation of maize seedlings with LCO solution doubled variables such as leaf area, plant height, and root and shoot dry weight. When hydroponically grown three-day-old seedlings of soybean and maize were treated with 10"7, 10"9 or 10"11 M LCO the biomass of both soybean and maize was increased. At 10~9 M and 10"7 M LCO, the soybean root biomass was 7-16% larger and roots were 34-44% longer than in the control. LCO treatment also had positive effects on the root growth of soybean and maize, increasing the total length, projected area and surface area.
Enhancement of plant photosynthesis due to B. japonicum soybean associations has been reported. Imsande (1989a,b) reported increased net photosynthesis and grain yield in soybean inoculated with B. japonicum as compared with plants not inoculated but adequately supplemented with N fertilizer. Thus it seems probable that rhizobial associations enhance photosynthesis and that this might be mediated by signal molecules. To test the hypothesis that LCO is responsible for the increased photosynthesis a series of experiments were conducted in the greenhouse and in the field. Spray application of LCO at submicromolar concentrations enhanced the photosynthetic rates of soybean, maize, rice, bean, canola, apple and grapes. On average there was a 10-20% increase in the photosynthetic rate and this was concomitant with an increase in stomatal conductivity and constant or decreased leaf internal CO2 concentration. Under field conditions, spray application of LCO at concentrations of 10"6, 10~8 and 10~I() M resulted in increased soybean grain yields of up to 40%. Taken together, the results of our experiments suggest the possible use of this novel class of signal molecules in improving crop production.
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