Ron Mittler

Summary: The accumulation of hydrogen peroxide (H₂O₂) in plants is typically associated with biotic or abiotic stresses. However, H₂O₂ is continuously produced in cells during normal metabolism. Yet, little is known about how H₂O₂ accumulation will affect plant metabolism in the absence of pathogens or abiotic stress. Here, we report that a deficiency in the H₂O₂‐scavenging enzyme, cytosolic ascorbate peroxidase (APX1), results in the accumulation of H₂O₂ in Arabidopsis plants grown under optimal conditions. Knockout‐Apx1 plants were characterized by suppressed growth and development, altered stomatal responses, and augmented induction of heat shock proteins during light stress. The inactivation of Apx1 resulted in the induction of several transcripts encoding signal transduction proteins. These were not previously linked to H₂O₂ signaling during stress and may belong to a signal transduction pathway specifically involved in H₂O₂ sensing during normal metabolism. Surprisingly, the expression of transcripts encoding H₂O₂ scavenging enzymes, such as catalase or glutathione peroxidase, was not elevated in knockout‐Apx1 plants. The expression of catalase, two typical plant peroxidases, and several different heat shock proteins was however elevated in knockout‐Apx1 plants during light stress. Our results demonstrate that in planta accumulation of H₂O₂ can suppress plant growth and development, interfere with different physiological processes, and enhance the response of plants to abiotic stress conditions. Our findings also suggest that at least part of the induction of heat shock proteins during light stress in Arabidopsis is mediated by H₂O₂ that is scavenged by APX1.

Studying plants growing naturally within their habitat can contribute greatly to our understanding of molecular and biochemical processes involved in the response of plants to changes in environmental conditions. Seasonal and diurnal variations in the expression of several transcripts involved in the defence of plants against abiotic stresses, i.e. transcripts encoding dehydrins, heat shock proteins, and antioxidative enzymes, were followed in Retama raetam plants that grow naturally within an arid dune ecosystem. These were associated with changes in environmental parameters simultaneously recorded at the research sites. It was found that the expression pattern of some transcripts correlated with changes in environmental conditions in the different plants during the year. In contrast, the expression pattern of other transcripts appeared to be plant specific and may be associated with phenotypic variability (plasticity). Transcripts encoding different heat shock proteins were induced in a co‐ordinated manner, suggesting that their corresponding products function as a chaperone network. Measurements of photosynthetic activity revealed a diurnal cycle in R. raetam. Photosynthesis was highest during early morning, declined toward the stressful midday hours and recovered at late afternoon. The present analysis suggests that R. raetam uses a combination of different avoidance strategies in co‐ordination with active defence mechanisms to withstand the stressful conditions that prevail within the desert ecosystem.

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.

Summary: The plant genome is a highly redundant and dynamic genome. Here, we show that double antisense plants lacking the two major hydrogen peroxide‐detoxifying enzymes, ascorbate peroxidase (APX) and catalase (CAT), activate an alternative/redundant defense mechanism that compensates for the lack of APX and CAT. A similar mechanism was not activated in single antisense plants that lacked APX or CAT, paradoxically rendering these plants more sensitive to oxidative stress compared to double antisense plants. The reduced susceptibility of double antisense plants to oxidative stress correlated with suppressed photosynthetic activity, the induction of metabolic genes belonging to the pentose phosphate pathway, the induction of monodehydroascorbate reductase, and the induction of IMMUTANS, a chloroplastic homologue of mitochondrial alternative oxidase. Our results suggest that a co‐ordinated induction of metabolic and defense genes, coupled with the suppression of photosynthetic activity, can compensate for the lack of APX and CAT. In addition, our findings demonstrate that the plant genome has a high degree of plasticity and will respond differently to different stressful conditions, namely, lack of APX, lack of CAT, or lack of both APX and CAT.

Summary: Dormancy is an important developmental program allowing plants to withstand extended periods of extreme environmental conditions, such as low temperature or drought. Seed dormancy, bud dormancy and desiccation tolerance have been extensively studied, but little is known about the mechanisms involved in the dormancy of drought‐tolerant plants, key to the survival of many plant species in arid and semi‐arid environments. Subtractive PCR cloning of cDNAs from Retama raetam, a C₃ drought‐tolerant legume, revealed that dormancy in this plant is accompanied by the accumulation of transcripts encoding a pathogenesis‐related, PR‐10‐like protein; a low temperature‐inducible dehydrin; and a WRKY transcription factor. In contrast, non‐dormant plants subjected to stress conditions contained transcripts encoding a cytosolic small heat‐shock protein, HSP18; an ethylene‐response transcriptional co‐activator; and an early light‐inducible protein. Physiological and biochemical analysis of Rubisco activity and protein in dormant and non‐dormant tissues suggested a novel post‐translational mechanism of regulation that may be controlled by the redox status of cells. Ultrastructural analysis of dormant plants revealed that air spaces of photosynthetic tissues contained an extracellular matrix that may function to prevent water loss. The cytosol of dormant cells appeared to be in a glassy state, limiting metabolic activity. A combination of biochemical, molecular and structural mechanisms, in association with metabolic suppression, may be key to the extreme drought tolerance of R. raetam and its acclimation to the desert ecosystem. These may enable plants to withstand long periods of drought, as well as rapidly to exit dormancy upon rainfall.