Arabidopsis mutants lacking asparaginases develop normally but exhibit enhanced root inhibition by exogenous asparagine

Abstract

Asparaginase catalyzes the degradation of L-asparagine to L-aspartic acid and ammonia, and is implicated in the catabolism of transported asparagine in sink tissues of higher plants. The Arabidopsis genome includes two genes, ASPGA1 and ASPGB1, belonging to distinct asparaginase subfamilies. Conditions of severe nitrogen limitation resulted in a slight decrease in seed size in wild-type Arabidopsis. However, this response was not observed in a homozygous T-DNA insertion mutant where ASPG genes had been inactivated. Under nitrogen-sufficient conditions, the ASPG mutant had elevated levels of free asparagine in mature seed. This phenotype was observed exclusively under conditions of low illumination, when a low ratio of carbon to nitrogen was translocated to the seed. Mutants deficient in one or both asparaginases were more sensitive than wild-type to inhibition of primary root elongation and root hair emergence by L-asparagine as a single nitrogen source. This enhanced inhibition was associated with increased accumulation of asparagine in the root of the double aspga1-1/-b1-1 mutant. This indicates that inhibition of root growth is likely elicited by asparagine itself or an asparagine-derived metabolite, other than the products of asparaginase, aspartic acid or ammonia. During germination, a fusion between the ASPGA1 promoter and beta-glucuronidase was expressed in endosperm cells starting at the micropylar end. Expression was initially high throughout the root and hypocotyl, but became restricted to the root tip after three days, which may indicate a transition to nitrogen-heterotrophic growth.

Unexpectedly low nitrogen acquisition and absence of root architecture adaptation to nitrate supply in a Medicago truncatula highly branched root mutant

“…Indeed, the upregulation of root N transporters in TR185 compared with WT could be characteristic of the N-starvation status in the mutant. Importantly, the decreased GLN and ASN root content in TR185 compared with WT could explain its highly branched root architecture in agreement with Gifford et al (2008) and Ivanov et al (2012). Grafting experiments revealed that the highly branched root phenotype in TR185 was transmissible from shoots and not from roots (Fig.…”

“…More recently, an additional signalling pathway has been pointed out, in which LR emergence is controlled by N-assimilation products. According to Gifford et al (2008), GLN is the predominant signal regulating repression of LR emergence, but an inhibition of root growth by ASN was also shown by Ivanov et al (2012) and its possible role as an N-satiety signal suggested. In the current study, molecular and developmental analyses converge to indicate that the mutant could perceive a permanent N-starvation signal, which induced modification of root N acquisition and architecture compared with WT.…”

Impact of the Disruption of ASN3-Encoding Asparagine Synthetase on Arabidopsis Development

“…It was found that germinating asn3-1 seeds expressed wild-type ASPGA1 and ASPGB1 mRNA levels ( Figure 5). However, a promoter analysis of ASPGA1::GUS demonstrated an expression in seed epidermal cells that began 24 h after sowing [21], suggesting an implication of asparaginase in asparagine hydrolysis. Moreover, the imbibed asn3-1 seeds contained wild-type levels of AGT1 mRNA ( Figure 5).…”

Nitrogen-metabolism related genes in barley - haplotype diversity, linkage mapping and associations with malting and kernel quality parameters

“…For nitrate reductase in barley two isozymes and two genes were described [29,31]. Also for asparaginase in the Arabidopsis genome two genes ASPGA1 and ASPGB1 were found [32] and three genes for aspartate aminotransferase were described in rice [33]. While two genes encoding ferredoxin-dependent glutamate synthase with differential expression patterns are present in Arabidopsis [34], in wheat ten differentially expressed glutamine synthetase sequences were classified [30].…”