35
Calcium as a Messenger in Stress Signal
Transduction
A. S. N. Reddy and Vaka Subba Reddy
Colorado State University, Fort Collins, Colorado
697I. INTRODUCTION
Plant growth in the natural environment is often adversely affected by a number of factors. These include
environmental factors such as low temperature, heat, drought, wind, ultraviolet light, anoxia, and high
salinity and biological factors such as pathogens (bacteria, viruses, and fungi). Abiotic and biotic factors
that limit growth and development of plants and eventually productivity are considered stress factors.
Crop losses due to these various abiotic and biotic stresses are in the billions of dollars annually. It has
been estimated that stress factors (abiotic and biotic) depress the yield of agronomically important crops
in the United States by 78%, of which about 70% is due to unfavorable environmental conditions [1,2].
Plants possess built-in mechanisms to cope with the abiotic and biotic stress factors. Plant scientists have
been studying the effects of various stresses on plants to better elucidate the mechanisms by which plants
respond to stress signals. It is hoped that knowledge derived from the increased understanding of plant re-
sponses to biotic and abiotic stresses would eventually help in developing new plant varieties that are re-
sistant to these stress factors. Advances in molecular and cellular biology are offering a variety of new
approaches to investigate plant responses to stresses.
There has been increasing interest in understanding the biochemical and molecular basis of plant
stress tolerance and in the identification of genes involved in stress tolerance. Several genes that are in-
volved in biotic stress tolerance have been identified and used to obtain transgenic plants with enhanced
resistance to these stresses [3–8]. Because of the complex nature of plant responses to abiotic stresses, the
information about the biochemical and molecular mechanisms that contribute to resistance is limited.
Studies suggest that the regulation of expression of specific genes as well as changes in the levels of cer-
tain osmolytes are an integral part of plant adaptation to stressful environmental conditions [2–7,9–11].
Effects of different stresses on various physiological processes and gene expression are reviewed in other
chapters of this book, hence will not be covered here. The mechanisms by which stress signals induce
changes in gene expression and affect biochemical pathways are beginning to be understood.
Plants, unlike other organisms, are sessile and, therefore, have developed mechanisms to sense and
respond to the stress signals so that they can adapt to or tolerate adverse environmental conditions. How-
ever, different plant species differ in their ability to adapt to environmental variables. The mechanisms by
which plant cells perceive and transduce stress signals are beginning to be elucidated. In animal cells,
messengers such as cyclic nucleotides and calcium play vital roles in signal transduction pathways. The