What Do We Know About Stress Sensors in Plants?

Salinity stress shares some important physical–chemical characteristics with other abiotic stresses such as desiccation and cold stress. All of these stresses impose an osmotic gradient on plant cells that impedes the movement of water into or retention inside the plasma membrane. Also, these stresses all mediate gene expression changes and other responses through a transient Ca⁺⁺ influx causing elevated cytosolic Ca⁺⁺ levels (Xiong et al., 2002a). As such, genes that control the impact of the response of plants to one of these stresses also affect responses to other factors (Qi and Spalding, 2004). Although we will not discuss here genes that have been identified in screens for response to desiccation or cold stress, it is relevant to note that in all of these screens relatively few genes encoding putative stress sensing (receptor) proteins have been identified. It is interesting to note also that an important class of environmental sensors, two component Hiks, has been found to be involved in thermosensing in prokaryotic cyanobacteria and in Bacillus subtilis (Suzuki et al., 2000; Urao et al., 1999). The well-studied yeast model system has also been utilized to identify the important salinity/osmotic sensor SLN1. Shinozaki et al. have identified in Arabidopsis an Hik, AtHK1, which is able to complement the yeast SLN1 mutant, and is thereby implicated as an osmosensor in plants (Urao et al., 1999). The inability to detect mutations in abiotic stress sensors, at first thought, appears puzzling since these signaling components are the first molecules involved in the plant’s adaptive responses to stress and should therefore have large, easily detectable, influences on adaptive phenotypes such as reducing injury and death, and reduced growth. However, it may be just for this reason of high importance to survival under stress that sensor signal components would likely be highly redundant. This may not only result from the occurrence of more than one gene with the same sensing function but more likely derives from the overlapping function of sensors with some but not complete specificity for particular environmental cues.