The combined effect of drought stress and heat shock on gene expression in tobacco

Abstract

In nature, plants encounter a combination of environmental conditions that may include stresses such as drought or heat shock. Although drought and heat shock have been extensively studied, little is known about how their combination affect plants. We used cDNA arrays, coupled with physiological measurements, to study the effect of drought and heat shock on tobacco (Nicotiana tabacum) plants. A combination of drought and heat shock resulted in the closure of stomata, suppression of photosynthesis, enhancement of respiration, and increased leaf temperature. Some transcripts induced during drought, e.g. those encoding dehydrin, catalase, and glycolate oxidase, and some transcripts induced during heat shock, e.g. thioredoxin peroxidase, and ascorbate peroxidase, were suppressed during a combination of drought and heat shock. In contrast, the expression of other transcripts, including alternative oxidase, glutathione peroxidase, phenylalanine ammonia lyase, pathogenesis-related proteins, a WRKY transcription factor, and an ethylene response transcriptional co-activator, was specifically induced during a combination of drought and heat shock. Photosynthetic genes were suppressed, whereas transcripts encoding some glycolysis and pentose phosphate pathway enzymes were induced, suggesting the utilization of sugars through these pathways during stress. Our results demonstrate that the response of plants to a combination of drought and heat shock, similar to the conditions in many natural environments, is different from the response of plants to each of these stresses applied individually, as typically tested in the laboratory. This response was also different from the response of plants to other stresses such as cold, salt, or pathogen attack. Therefore, improving stress tolerance of plants and crops may require a reevaluation, taking into account the effect of multiple stresses on plant metabolism and defense.

Figure 1

Measurements of photosynthesis and respiration in plants subjected to heat shock, drought stress, and a combination of heat shock and drought stress. Plants were subjected to stresses as described in “Materials and Methods,” and photosynthetic activity and dark respiration were measured with an LI-6400 apparatus (LI-COR, Lincoln, NE). Photosynthetic activity is shown to be suppressed after drought stress or a combination of drought and heat shock, whereas respiration is enhanced after heat shock and a combination of drought and heat shock. A combination of drought and heat shock, therefore, is different from drought or heat shock by having a high rate of respiration and a low rate of photosynthetic activity. Results are presented as mean and sd of five individual measurements.

Figure 2

Stomatal conductance (A) and leaf temperature (B) of plants subjected to heat shock, drought stress, and a combination of heat shock and drought stress. Measurements were performed as described in “Materials and Methods.” The temperature of leaves subjected to a combination of drought and heat shock is shown to be higher than that of plants subjected to heat shock in the absence of drought. This difference may result from the inability of plants, subjected to the stress combination, to cool their leaves by transpiration because their stomata are closed.

Figure 3

Changes in the steady-state level of transcripts encoding stress response and metabolic proteins and enzymes during a combination of drought and heat shock. RNA gel blots were used to assay the steady-state level of selected transcripts during a combination of drought and heat shock. Many of the transcripts shown in this figure have a distinct expression pattern during a combination of drought and heat shock. RNA isolation, blots, and analysis are described in “Materials and Methods.”

Figure 4

Changes in the steady-state level of transcripts encoding stress response and metabolic proteins and enzymes after different environmental stresses. RNA gel blots were used to assay the steady-state level of selected transcripts during different stresses. RNA isolation, blots, and analysis are described in “Materials and Methods.”

Figure 5

Expression of transcripts with homology to stress response cDNAs isolated from the desert plant R. raetam. RNA gel blots were used to study the expression of different transcripts that hybridized to cDNAs isolated from the desert plant R. raetam subjected to a combination of drought and heat shock in its natural environment. Hybridizations were performed at a high stringency (60°C) using full-length R. raetam cDNA clones as described in “Materials and Methods.”