Early Senescence in Older Leaves of Low Nitrate-Grown Atxdh1 Uncovers a Role for Purine Catabolism in N Supply

The nitrogen (N)-rich ureides allantoin and allantoate, which are products of purine catabolism, play a role in N delivery in Leguminosae. Here, we examined their role as an N source in nonlegume plants using Arabidopsis (Arabidopsis thaliana) plants mutated in XANTHINE DEHYDROGENASE1 (AtXDH1), a catalytic bottleneck in purine catabolism. Older leaves of the Atxdhl mutant exhibited early senescence, lower soluble protein, and lower organic N levels as compared with wild-type older leaves when grown with 1 mm nitrate but were comparable to the wild type under 5 mm nitrate. Similar nitrate-dependent senescence phenotypes were evident in the older leaves of allantoinase (Ataln) and allantoate amidohydrolase (Ataah) mutants, which also are impaired in purine catabolism. Under low-nitrate conditions, xanthine accumulated in older leaves of Atxdh1, whereas allantoin accumulated in both older and younger leaves of Ataln but not in wild-type leaves, indicating the remobilization of xanthine-degraded products from older to younger leaves. Supporting this notion, ureide transporter expression was enhanced in older leaves of the wild type in low-nitrate as compared with high-nitrate conditions. Elevated transcripts and proteins of AtXDH and AtAAH were detected in low-nitrate-grown wild-type plants, indicating regulation at protein and transcript levels. The higher nitrate reductase activity in Atxdh1 leaves compared with wild-type leaves indicated a need for nitrate assimilation products. Together, these results indicate that the absence of remobilized purine-degraded N from older leaves of Atxdhl caused senescence symptoms, a result of higher chloroplastic protein degradation in older leaves of low-nitrate-grown plants.