Ross M. Rainbird

Evidence for a Purine Pathway of Ureide Synthesis in N2-fixing Nodules of cowpea [Vigna unguiculata (L.) WALP.]

Summary The principal nitrogenous solutes produced from N 2 -fixation and exported from the nodules of cowpea [ Vigna unguiculata (L.) WALP.] were allantoin and allantoic acid. The following evidence suggests that these ureides were derived from purines. 1. Nodules were particularly active in enzymes of purine oxidation (xanthine dehydrogenase, uricase, allantoinase). 2. These enzymes were found largely restricted to the cytosol of the bacteroid containing cells (ie in close proximity to the sites of N 2 fixation) an-d increased in activity with nodule development. 3. Allopurinol (4-hydroxypyrazolo [3,4-d] pyrimidine) was an effective inhibitor of the xanthine dehydrogenase extracted from cowpea nodules and low concentrations (0.08-0.31 mM) applied to the root systems of intact plants in liquid culture rapidly (within 1 hr) inhibited ureide export, reduced the nodule pools of ureides and caused a concomitant accumulation of xanthine in nodules. 4. Glycine-2, 14 C supplied to slices of nodule tissue was more readily incorporated into allantoin and allantoic acid than glucose-U, 14 C or acetate-1,2, 14 C.

Purification and some properties of urate oxidase from nitrogen-fixing nodules of cowpea.

Urate oxidase (urate: oxygen oxidoreductase, EC 1.7.3.3) was purified 166-fold from nitrogen-fixing root nodules of cowpea Vigna unguiculata [L.] Walp. The purified enzyme showed a specific activity of 5.7 mumol urate oxidised/min per mg protein, a molecular mass of 100 kdaltons, pH optimum between 9 and 10, isoelectric point at PH 6.8, Km(urate) = 18 muM and Km(oxygen) = 29 muM. A number of metal complexing and chelating reagents were inhibitory, as were divalent cations, including Cu2+. Iron stimulated the enzyme. Low concentrations of ammonia, glutamine and xanthine were also inhibitory. The regulation of urate oxidase in relation to the assimilation of fixed nitrogen in legume nodules is discussed.

Internodes of grain legumes — New location for xylem parenchyma transfer cells

SummaryXylem parenchyma transfer cells were observed in the primary and secondary vascular tissue of stem internodes of 21 in 28 species of grain legumes. Their structural features were similar to those of other transfer cells. The relationships of these cells to transfer cells at nodes were investigated. Non-nodulated seedlings ofPhaseolus vulgaris L. formed internode transfer cells if provided mineral nutrients through their roots, but not if grown in distilled water or fed nutrients entirely through their leaves. Wall ingrowths formed in parenchyma of primary xylem ofPhaseolus just before full extension of an internode. The significance of this new location for transfer cells was discussed.