Ethylene-triggered cell death during aerenchyma formation in roots

This chapter reviews the occurrence, function and formation of aerenchyma, and suggests that programmed cell death is indeed involved, triggered by the gaseous hormone, ethylene. In many species, not just wetland ones, aerenchyma formation takes place by the death of healthy cells in a highly selective and predictable manner, probably by programmed cell death. Aerenchyma formation in the roots and stems of higher plants takes place either shizogenously, when cells separate at the middle lamella, and cavities develop during subsequent expansion growth, or lysigenously, when cells undergo cell death followed by autolysis, leaving gas-filled cavities. Aerenchyma formation was attributed for a long time to the direct effects of severe O2-deficiency on the viability of maize root cortical cells. However, working with stems of sunflower, aerenchyma formation was found by Kawase to be promoted by exogenous ethylene. Ethylene is also involved in many aspects of ‘stress physiology’ in higher plants.