Essential Role of Urease in Germination of Nitrogen-Limited Arabidopsis thaliana Seeds

Abstract

In Arabidopsis thaliana, urease transcript levels increased sharply between 2 and 4 d after germination (DAC) and were maintained at maximal levels until at least 8 DAC. Seed urease specific activity declined upon germination but began to increase in seedlings 2 DAC, reaching approximately 75% of seed activity by 8 DAC. Urea levels showed a small transient increase 1 DAC and then approximately paralleled urease activity, reaching maximal levels at approximately 9 DAC. Urease inhibition with phenylphosphorodiamidate resulted in a 2-to 4-fold increase in urea levels throughout seedling development. Arginine pools (0-8 DAC) changed approximately in parallel with the urea pool. Consistent with arginine being a major source of urea, arginase activity increased 10-fold in the interval O to 6 DAC. Allopurinol, a xanthine dehydrogenase inhibitor, had no effect on urea levels up to 3 DAC but reduced the urea pool by 30 to 40% during the interval 5 to 8 DAC, suggesting that purine degradation contributed to the urea pool well after germination, if at all. In aged Arabidopsis seeds, there was a correlation between phenylphosphorodiamidate inactivation of urease and germination inhibition, the latter overcome by NH,NO, or amino acids. Since urease activity, urea precursor, and urea increase in young seedlings, and since urease inactivation results in a nitrogen-reversible inhibition of germination, we propose that urease recycles urea-nitrogen in the seedling.

Isolation of the Ornithine-δ-Aminotransferase cDNA and Effect of Salt Stress on Its Expression inArabidopsis thaliana1

“…Conversion of A. thaliana seed-storage proteins to seedling de novo synthesized proteins potentially involves Arg breakdown, which is consistent with the 10-fold increase in seedling arginase activity 6 d after germination (Zonia et al, 1995). The products of this hydrolysis are urea and Orn.…”

“…The products of this hydrolysis are urea and Orn. Urea appears to have a function in recycling nitrogen into ammonium via urease (Zonia et al, 1995). The massively de novo synthesized Orn is transformed into glutamate semialdehyde via the ␦-OAT enzyme, but this does not appear to influence the Pro levels (Polacco and Holland, 1993).…”

Arginase, Arginine Decarboxylase, Ornithine Decarboxylase, and Polyamines in Tomato Ovaries (Changes in Unpollinated Ovaries and Parthenocarpic Fruits Induced by Auxin or Gibberellin)

“…Increases in arginase activity are important in seed germination, in which a high concentration of Arg is initially present (Zonia et al, 1995). Arg is usually a major nitrogen storage compound, consisting of approximately 65% of the free amino acid nitrogen in young pea cotyledons (De Ruiter and Kolloffel, 1983), so any enzymes involved in its metabolism should be regulated to control its catabolism or its accumulation as a free amino acid or in reserve proteins (De Ruiter and Kolloffel, 1982).…”

Absorção e assimilação de uréia pela bromélia epífita com tanque Vriesea gigantea

“…De forma geral, ela permite que os organismos utilizem a ureia, tanto proveniente no meio externo quanto a gerada endogenamente, e a transforme em amônio, o qual é utilizado diretamente como fonte de nitrogênio para o crescimento (Mobley e Hausinger, 1989;. Duas isoenzimas já foram identificadas em plantas: a urease embrioespecífica, uma proteína abundante em sementes de muitas espécies de plantas (Bailey e Boulter, 1971;Polacco e Holland, 1993;Zonia et al, 1995) e a urease ubíqua, encontrada em pequenas quantidades em todos os tecidos vegetais (Hogan et al, 1983;Polacco e Holland, 1993).…”

“…Aumentos na quantidade de ureia e na atividade da urease durante o desenvolvimento de plântulas de Arabidopsis thaliana, por exemplo, corroboram com o papel da urease na reciclagem da ureia proveniente do catabolismo das reservas nitrogenadas (Zonia et al, 1995), indicando, assim, uma importante função no metabolismo nitrogenado de plântulas.…”