Isoxazolin-5-ones and Amino Acids in Root Exudates of Pea and Sweet Pea Seedlings

Abstract

Seeds of Pisum sativum L. cv Finale and Lathyrs odoratus L. cv Spencer were germinated asepticafly in moistened sand in the dark. At several stages, the amino acid composition of the exudate and of the corresponding roots was analyzed. A number of common amino acids, including homoserine, were exuded by the growing seeding root in an early stage and were partly reabsorbed later. A number of uncommon amino acids, including several isoxazolin-5-one derivatives, uracil alanines, ia-yglutamyl-D-alanine, and a-aminoadipic acid were exuded at different rates.The range of compounds exuded by the roots of higher plants includes sugars, amino acids, enzymes, vitamins, nucleotides, growth hormones, terpenoids, flavanones, organic acids, and inorganic ions (22,24). By means of these exudates, the plants can affect the microflora in the rhizosphere by selective stimulation or inhibition of certain bacteria, fungi, or nematodes. There is also evidence that the root exudates of some plants inhibit the germination of some seeds or are toxic to roots of neighboring plants (22). It is generally believed that this root exudation is an uncontrolled process caused by damage to the cells at the root cap, at the sites where lateral roots emerge, and by infection of the root by microorganisms (23).Pea root exudates have been investigated in several laboratories (1,2,5,20,21,25). Some authors mentioned the presence of unknown amino acids (1,2,25). Rovira (20) showed the presence of UV-absorbing substances in 10-d-old and 21-d-old pea root exudates. Fries and Forsman (5) found some low mol wt UVabsorbing substances in the exudate from excised pea roots, besides adenine, guanine, uridine, and cytidine.A group of heterocyclic products, mostly amino acids, was isolated in our laboratory from pea seedlings and sweet pea seedlings: the uracil alanines willardiine and isowillardiine (15) and eight derivatives of the isoxazolin-5-one ring (13). (See Fig. 1 for the chemical structures.) Possibly, these products are the unknown compounds mentioned earlier by other authors (1, 2, 25).A preliminary study has shown the presence of some of these heterocycic products in root exudates of both pea and sweet pea seedlings (11). The differences in amino acid composition of the exudate and the corresponding root extract suggest that in some cases this exudation may be a specific phenomenon. This paper presents more detailed information on the exudation ofisoxazolin-5-one derivatives and other amino acids by intact roots ofetiolated pea and sweet pea seedlings under axenic conditions. MATERIALS AND METHODS Culture Conditions. Pea (Pisum sativum L. cv Finale) and sweet pea (Lathyrus odoratus L. cv Spencer) seeds were surface-sterilized with ethanol and were washed thoroughly with sterile distilled H20. After imbibing in sterile H20 for 16 h, the seeds were germinated in individual autoclaved glass tubes (20 x 2.8 cm), containing sterile quartz sand (5-cm height) moistened with distilled H20. The tubes were incubated at 25°C in the dark.Preparation...

2-Cyanoethyl-isoxazolin-5-one Is a Major Low Molecular Weight Nitrogenous Compound in Sweet Pea (<i>Lathyrus odoratus</i> L.)

“…These findings clearly indicate that 2-CEIX is a major nitrogenous compound in sweet pea. The contents of 2-CEIX were much higher in these organs than in roots as reported by Kuo et al (1982). The contents of 2-CEIX in the stem and leaves were much higher in the present study than in those reported by Ikegami et al (1984).…”

“…Among compounds, which have been reported to be much contained in sweet pea, 2-CEIX (Kuo et al, 1982) has the coincided molecular composition (Fig. 1B), and all NMR spectra were assigned to 1 H and 13 C of this compound, resulting in the identification.…”

“…In other legume plants, including Pisum sativum (Urquhart and Joy, 1981) and Lupinus albus (Jeschke et al, 1986), glutamine and asparagine are major translocated amino acids. Since asparagine is a major amino acid in sweet pea root (Kuo et al, 1982), it might be transported in xylem.…”

Genetic analysis of a region of the Rhizobium meliloti pSym plasmid specifying catabolism of trigonelline, a secondary metabolite present in legumes

“…Furthermore, it has been reported that trigonelline is present in root exudates of legumes (20,23). The concomitant presence of trigonelline in the host plant and of the corresponding catabolic genes in the symbiotic bacterium suggests that trigonelline could be used as a nutrient by Rhizobium strains during rhizosphere colonization and/or plant infection.…”

“…The presence of the trc genes on the same replicon, in a region closely surrounded by symbiosis genes, suggests some relationship between trigonelline catabolism and symbiosis. It is noteworthy that the genes controlling the catabolism of homoserine, which is a secondary metabolite of legumes (23), are similarly located on the pSym plasmid (21). In contrast, the genes controlling the catabolism of calystegines, which are secondary metabolites of nonlegumes, are located on a plasmid not essential for symbiosis (6,41).…”