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Plant Programmed Cell Death and Environmental Constraints Adenylate Homeostasis and Aerenchyma Formation
H. Uchimiya, P. K. Samarajeewa and M. Kawai
Adenylate kinase (ATP:AMP phosphotrans-ferase) is known to supply ADP in energy-producing systems. Adenylate metabolism is directly associated with energy-producing metabolism under conditions of O2 deficiency.
The enzymatic activity of adenylate kinase was stimulated in submerged rice seedlings. This activity was enhanced in every organ of the submerged plants. Treatment with N2 gas had the same effect as submergence, suggesting that O2 depletion is a major cause of the stimulation of adenylate kinase activity. Furthermore, different induction patterns of enzymatic activity were seen in two rice varieties, FR13A and IR42, which are, respectively, tolerant and intolerant of complete submergence.
NaCl-induced adenylate kinase activation was confirmed in the indica rice cultivar IR28 susceptible to salinity stress, but not in the NaCl-tolerant indica cultivar Nona Bokra. In salt sensitive rice plants, adenylate kinase may play some role in adenylate homeostasis for the early stages of rice seedlings subjected to salt stress.
Cellular events which occur before cell collapse were examined in the root cortex of rice during aerenchyma formation. The cell collapse started at a specific position in the mid-cortex. These cells were distinct in shape from those located to the periphery. Cells destined to collapse expanded more radially than in an anti-radial (tangential) direction before death. Furthermore, cell collapse was preceded by acidification and loss of plasma membrane integrity in cells of the mid-cortex. Sequential death of neighboring cells followed a radial path. Further analysis of the role of NaCl in suppression of cortical cell death was confined to the delay of early stages of cell collapse, which was caused by tonoplast disruption and plasma membrane destruction.
Stimulation of Adenylate Kinase in Rice Seedlings under Submergence Stress Introduction
Higher plants are exposed to numerous environmental pressures, such as changes in temperature, dehydration, salinity stress and other adverse soil conditions.1 To survive such stressful conditions, plants have acquired mechanisms by which they adjust their gene expression to adapt to these various environmental constraints. Unlike other graminaceous monocots, rice ( Oryza sativa L.) is extremely tolerant of submergence stress, which is one form of abiotic stress to which land plants are exposed. The biochemical mechanisms which underlie the adaptation of plants to O2 deficiency are still not completely understood, but high fermentative metabolism is important for tolerance of this condition.2-4
Adenylate kinase (ATP:AMP phosphotransferase; EC 18.104.22.168) is known to supply ADP in energy-producing systems. This is a monomeric enzyme that catalyzes the interconversion of adenine nucleotides according to the equation ATP + AMP«2ADP.5 It has been postulated that adenylate metabolism is directly associated with energy-producing metabolism under conditions of O2 deficrtant role in maintaining the energy charge of the adenylate pool.5-6 It would be interesting to examine whether any changes occur in this system in response to submergence stress.
We demonstrated that rice adenylate kinase complemented the mutation in a temperature-sensitive adenylate kinase-deficient mutant strain of Escherichia coli.7 Anti-adenylate kinase antibody reacted with a 27 kDa protein in callus, root, and leaf tissues. A cellular fractionation analysis showed that these proteins were mainly located in the cytosol and mitochondrial fractions. Tissue printing immunoblot analysis revealed that ade-nylate kinase protein was strongly expressed in vascular tissues.8,9 Furthermore, we proposed that sodium chloride stimulated adenylate kinase in several salt-sensitive rice varieties.10 Due to the role of this enzyme in the generation of energy, it is important to investigate the enzymatic activity of adenylate kinase in plants that are known to adapt to O2 deficiency. We examined adenylate kinase activity in rice seedlings that had been subjected to submergence.11
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