ESSENTIAL ROLE OF UREASE IN GERMINATION OF NITROGEN-LIMITED ARABIDOPSIS-THALIANA SEEDS

In Arabidopsis thaliana, urease transcript levels increased sharply be tween 2 and 4 d after germination (DAG) and were maintained at maximal levels until at least 8 DAG. Seed urease specific activity declined u pon germination but began to increase in seedlings 2 DAG, reaching app roximately 75% of seed activity by 8 DAG. Urea levers showed a small t ransient increase 1 DAG and then approximately paralleled urease activ ity, reaching maximal levels at approximately 9 DAG. Urease inhibition with phenylphosphorodiamidate resulted in a 2- to 4-fold increase in urea levels throughout seedling development. Arginine pools (0-8 DAG) 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 0 to 6 DAG. Allopurinol, a xanthine dehydrogenase inhibitor, had no effect on urea levels up to 3 DAG but reduced the u rea pool by 30 to 40% during the interval 5 to 8 DAG, suggesting that purine degradation contributed to the urea pool well after germination , if at all. In aged Arabidopsis seeds, there was a correlation betwee n phenylphosphorodiamidate inactivation of urease and germination inhi bition, the latter overcome by NH4NO3 or amino acids. Since urease act ivity, urea precursor, and urea increase in young seedlings, and since urease inactivation results in a nitrogen-reversible inhibition of ge rmination, we propose that urease recycles urea-nitrogen in the seedli ng.