Xylem transport and shoot accumulation of lumichrome, a newly recognized rhizobial signal, alters root respiration, stomatal conductance, leaf transpiration and photosynthetic rates in legumes and cereals

* Root respiration, stomatal conductance, leaf transpiration and photosynthetic rates were measured in phytotron and field-grown plants following the application of 5 or 10 nM lumichrome, 10 nM ABA (abscisic acid) and 10 ml of 0.2 OD600 infective rhizobial cells. * Providing soybean and cowpea roots with their respective homologous rhizobia and/or purified lumichrome increased the concentration of this molecule in xylem sap and leaf extracts. Relative to control, rhizobial inoculation and lumichrome application significantly increased root respiration in maize, decreased it in lupin, but had no effect on the other test species. * Applying either lumichrome (10 nM), infective rhizobial cells or ABA to roots of plants for 44 h in growth chambers altered leaf stomatal conductance and transpiration in cowpea, lupin, soybean, Bambara groundnut and maize, but not in pea or sorghum. Where stomatal conductance was increased by lumichrome application or rhizobial inoculation, it resulted in increased leaf transpiration relative to control plants. Treating roots of field plants of cowpea with this metabolite up to 63 d after planting showed decreased stomatal conductance, which affected CO2 intake and reduction by Rubisco. * The effect of rhizobial inoculation closely mirrored that of lumichrome application to roots, indicating that rhizobial effects on these physiological activities were most likely due to lumichrome released into the rhizosphere.     

The rhizosphere signal molecule lumichrome alters seedling development in both legumes and cereals

The stimulatory role of lumichrome, a rhizosphere metabolite, was assessed on the growth of legume and cereal seedlings. At a very low nanomolar concentration (5 nm), lumichrome elicited growth promotion in cowpea, soybean, sorghum, millet and maize, but not in common bean, Bambara groundnut and Sudan grass. In soybean and cowpea only, 5 nm lumichrome caused early initiation of trifoliate leaf development, expansion in unifoliate and trifoliate leaves, increased stem elongation and, as a result, an increase in shoot and plant total biomass relative to control. Lumichrome (5 nm) also increased leaf area in maize and sorghum, and thus raised shoot and total biomass but there was no effect on the leaf area of the other cereals. Root growth was also stimulated in sorghum and millet by the supply of 5 nm lumichrome. By contrast, the application of a higher dose of lumichrome (50 nm) depressed development of unifoliate leaves in soybean, the second trifoliate leaf in cowpea, and shoot biomass in soybean. The 50 nm concentration also consistently decreased root development in cowpea and millet, but had no effect on the other species. These data show that lumichrome is a rhizosphere signal molecule that affects seedling development in both monocots and dicots.     

Ueber den abbau von riboflavin in pflanzlichen zellsuspensionskulturen

The degradation of (2-¹⁴C)-riboflavin was investigated in plant cell suspension cultures of Cicer arietinum, Glycine max, Phaseolus aureus, Nicotiana glauca, Petroselinum crispum und Triticum aestivum. All cultures showed degradation of the isoalloxazine system with decreasing magnitude in the order shown above. Riboflavin is not degraded via lumichrome. (2-¹⁴C)lumichrome was not degraded but rather excreted from the cells into the nutrient medium after conversion to a water soluble compound. Riboflavin, but not lumichrome, was incorporated into the coenzymes FMN and FAD.