Asparaginase gene expression is regulated in a complex spatial and temporal pattern in nitrogen‐sink tissues

Abstract

Summary Asparagine is an important nitrogen transport compound in higher plants. Mobilized asparagine is deamidated by the enzyme asparaginase to aspartate and ammonia, which are major sources of organic nitrogen for plant growth and development. Little is known about the role of asparagine catabolism during plant development as the enzyme is difficult to assay, although it is known that asparaginase activity is located in tissues undergoing rapid growth and development, particularly in young leaves and developing seeds. In tobacco plants containing an asparaginase‐GUS gene fusion, GUS expression was specifically located in young developing tissues such as the apical meristem and expanding leaves. In mature plants, GUS activity was found predominantly in various tissues of the developing seed. In lupin, where up to 80% of the fixed nitrogen is exported as asparagine to the developing seed, GUS activity was also located in developing tissues such as the apical meristem, and in specific tissue types in the developing seed previously shown to contain asparaginase activity. All of the tissues in which asparaginase expression occurred have previously been shown to contain localized high levels of asparagine. This study therefore demonstrates an important role for transcriptional control of an asparaginase gene in regulating asparaginase levels in N‐sink tissues.

Coordination of PsAS1 and PsASPG expression controls timing of re-allocated N utilization in hypocotyls of pine seedlings

“…The transaminase activity (EC 2.6.1.14) has been detected in leaves of soybean (Streeter 1977), pea (Ireland and Joy 1981) and lupin (Atkins et al 1983). Several studies support the view that ASPG is a key enzyme involved in mobilizing N from asparagine in tissues demanding high amounts of N, such as developing seeds and young leaves (Sodek et al 1980;Ireland and Joy 1981;Grant and Bevan 1994), whereas asparagine aminotransferase seems to have minor importance in these tissues (Ireland and Joy 1981;Atkins et al 1983).…”

“…These data suggest that once the AS activity decreases, there is an increase in the consumption of asparagine in the hypocotyl. The expression of ASPG genes has often been associated with tissues undergoing active division and protein synthesis (Lough et al 1992a, b;Grant and Bevan 1994). These tissues use the asparagine provided from sites of synthesis as a primary N source.…”

Changes in the cellular localization of cytosolic glutamine synthetase protein in vascular bundles of rice leaves at various stages of development

“…Recent observations by Razal et al (1996), using 15N_NM R analyses, indicated that NH + released by PAL was first incorporated into the amide nitrogen of glutamine in potato discs. Also, GS1 may function to assimilate NH2 derived from the reaction of asparaginase which is known to be expressed in young developing tissues of transgenic tobacco plants (Grant and Bevan 1994). Asparagine, as well as glutamine, is also the major form of nitrogen-transport compound in phloem solutes of rice plants (Hayashi and Chino 1990).…”

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

“…Expression analysis of a fusion between the promoter of a K ? -independent ASPG from Lupinus angustifolius and b-glucuronidase (GUS) revealed a close association with sink tissues having localized high levels of Asn, including the apical meristem, expanding leaves and developing seed (Grant and Bevan 1994). In L. angustifolius root, ASPG generates ammonia to support nodule development, and later on ASPG transcript and activity levels are suppressed upon nodule maturation (Vincze et al 1994).…”