Hydrogen peroxide, a messenger with too many roles?

Hydrogen peroxide is thought to be involved in many cellular processes in plants as a signal, a mediator, or an effector molecule. These range from developmental processes (programmed cell death, senescence, hormone action, and cell wall biosynthesis), to responses to biotic stimuli (bacteria, virus, fungi, and various herbivores), and abiotic stresses (high light, drought, cold, UV, ozone, mechanical wounding, and nutrient deprivation). In addition, hydrogen peroxide was found to regulate the expression of different stress response genes such as pathogenesis related proteins and heat shock proteins. In their paper Pei et al. 1 present physiological and biochemical evidence for a direct role of hydrogen peroxide in mediating abscisic acid (ABA)-induced stomatal closure in Arabidopsis. Stomata are highly specialized cells of the plant epidermis. They function as guards that regulate the exchange of gas between the atmosphere and the air space within the leaf. The inner leaf air spaces of plants can be viewed as the plant’s equivalent of lungs in animals because they contain the photosynthetically active cells of the mesophyll where the majority of atmospheric CO 2 uptake and fixation occurs. Sugars produced by the mesophyll cells at the inner leaf air spaces are transported by the plant vascular system and support active metabolic processes in non-photosynthetic tissues such as developing young leaves, flowers, and roots. Because plant tissue at the inner leaf spaces is highly sensitive to dehydration, the stomata function as guards that prevent dehydration by closing and preventing water loss during drought. Closure of stomata during plant dehydration is mediated by the plant hormone ABA, and is accompanied by an increase in cytosolic calcium in guard cells.