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

Abstract

SummaryAsparagine 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.