Dual action of the active oxygen species during plant stress responses

Adaptation to environmental changes is crucial for plant growth and surviva l. However, the molecular and biochemical mechanisms of adaptation are stil l poorly understood and the signaling pathways involved remain elusive. Act ive oxygen species (AOS) have been proposed as a central component of plant adaptation to both biotic and abiotic stresses. Under such conditions, AOS may play two very different roles: exacerbating damage or signaling the ac tivation of defense responses. Such a dual function was first described in pathogenesis but has also recently been demonstrated during several abiotic stress responses. To allow for these different roles, cellular levels of A OS must be tightly controlled. The numerous AOS sources and a complex syste m of oxidant scavengers provide the flexibility necessary for these functio ns. This review discusses the dual action of AOS during plant stress respon ses.