Excised maize (Zea mays L.) root tips were used to monitor the effects of prolonged
glucose starvation on
nitrogen metabolism. Following root-tip excision,
sugar content was rapidly exhausted, and
protein content declined to 40 and 8% of its initial value after 96 and 192 h, respectively. During
starvation the contents of free
amino acids changed.
Amino acids that belonged to the same "synthetic family" showed a similar pattern of changes, indicating that their content, during
starvation, is controlled mainly at the level of their common biosynthetic steps.
Asparagine, which is a good marker of
protein and
amino-acid degradation under stress conditions, accumulated considerably until 45 h of
starvation and accounted for 50% of the
nitrogen released by protein degradation at that time. After 45 h of
starvation,
nitrogen ceased to be stored in
asparagine and was excreted from the cell, first as
ammonia until 90-100 h and then, when
starvation had become irreversible, as
amino acids and aminated compounds. The study of
asparagine metabolism and
nitrogen-assimilation pathways throughout
starvation showed that: (i)
asparagine synthesis occurred via
asparagine synthetase (EC 6.3.1.1) rather than
asparagine aminotransferase (EC 2.6.1.14) or the β-cyanoalanine pathway, and
asparagine degradation occurred via
asparaginase (EC 3.5.1.1); and (ii) the enzymic activities related to
nitrogen reduction and assimilation and
amino-acid synthesis decreased continuously, whereas
glutamate dehydrogenase (EC 1.4.1.2-4) activities increased during the reversible period of
starvation. Considered together, metabolite analysis and enzymic-activity measurements showed that
starvation may be divided into three phases: (i) the acclimation phase (0 to 30-35 h) in which the root tips adapt to transient
sugar deprivation and partly store the
nitrogen released by protein degradation, (ii) the survival phase (30-35 to 90-100 h) in which the root tips expel the
nitrogen released by protein degradation and
starvation may be reversed by
sugar addition and (iii) the cell-disorganization phase (beyond 100 h) in which all metabolites and enzymic activities decrease and the root tips die.