Ureide synthesis, accumulation and transport in Arabidopsis plants under salt and osmotic stress

Abstract

The ureides allantoin and allantoic acid play a central role in nitrogen transport in nodulating tropical legumes. However, the complete enzyme set for ureide synthesis and a family of ureide permeases are widely distributed in the plant kingdom suggesting their participation in physiological processes not properly characterized yet. In Arabidopsis, microarrays studies showed an upregulation of ureides synthesis genes (xanthine dehydrogenase, uricase) during abiotic stresses. On the contrary, allantoinase gene expression is strongly reduced after stress suggesting that allantoin may accumulate in the cells. We observed accumulation of allantoin in Arabidopsis plants under osmotic and salt stresses. This effect was exacerbated in plants grown with ammonium as nitrogen source and suppressed in the presence of sucrose as carbon source. The analysis of two independent T-DNA insertion lines, causing knockout of allantoinase (alla-1 and alla-2) showed constitutively elevated concentrations of allantoin, but a noticeable morphological phenotype remained elusive. To determine the physiological relevance of allantoinase gene repression on resistance to stress, transgenic lines were generated on the genotype alla-1, in which the coding sequence of allantoinase was introduced under the control of the stress inducible promoter RD29A. As expected, alla-1 pRD29A::Aln plants were not able to accumulate allantoin under stress conditions. The phenotype of KO and RD29A:ALN plants was analysed under salt and osmotic stress conditions. In addition, the phenotype of a KO mutant of AtUPS5 (ups5) was analysed. AtUPS5 transports allantoin, is expressed in the root cortex and endodermis and its expression increases during salt and osmotic stress, suggesting a rol in long-distance transport of allantoin during stress.