Department of Biology, University of Western Ontario, London, ON, Canada.
Amino Acids. 2012 Jun;42(6):2307-18. doi: 10.1007/s00726-011-0973-4. Epub 2011 Jul 29.
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.
天冬酰胺酶催化 L-天冬酰胺降解为 L-天冬氨酸和氨，并参与高等植物源组织中天冬酰胺的分解代谢。拟南芥基因组包含两个基因，ASPGA1 和 ASPGB1，属于不同的天冬酰胺酶亚家族。在严重氮限制的条件下，野生型拟南芥的种子大小略有减小。然而，在 ASPG 基因失活的纯合 T-DNA 插入突变体中没有观察到这种反应。在氮充足的条件下，ASPG 突变体在成熟种子中具有较高水平的游离天冬酰胺。这种表型仅在光照不足的条件下观察到，此时向种子转运的碳氮比很低。与野生型相比，缺失一种或两种天冬酰胺酶的突变体对 L-天冬酰胺作为单一氮源抑制主根伸长和根毛出现更为敏感。这种增强的抑制与双 aspga1-1/-b1-1 突变体根中天冬酰胺的积累增加有关。这表明抑制根生长可能是由天冬酰胺本身或天冬酰胺衍生的代谢物引起的，而不是天冬酰胺酶、天冬氨酸或氨的产物。在萌发过程中，ASPGA1 启动子与β-葡聚糖酶的融合在胚乳细胞中从珠孔端开始表达。表达最初在整个根和下胚轴中很高，但在三天后仅限于根尖，这可能表明向氮异养生长的转变。
