Ethylene is involved in vascular cavity formation in pea (Pisum sativum) primary roots

A lengthy cavity usually forms in the vascular cylinders of pea (Pisum sativum) primary roots in response to sudden flooding at 25°C. This is thought to be a form of aerenchyma. Ethylene has been shown to mediate inducible aerenchyma in maize, therefore the role of ethylene in the formation of cavities in pea roots was examined. Pea seedlings grown for 4 d in 2 L beakers in vermiculite moistened below field capacity – conditions that do not favor cavity formation – were flooded with solutions containing ethylene inhibitors (AOA, EGTA, and STS). Pea seeds were germinated and grown in suitable containers (0.8-1.0 L) for 4 d at 25°C in the dark in similar vermiculite. These were then exposed to various concentrations of ethylene for 1 d, or they were flooded and endogenous ethylene was measured periodically by gas chromatography. Observations of roots exposed to exogenous ethylene were made by light microscope. All three inhibitors of ethylene suppressed cavity formation in flooded roots. Exogenous ethylene exposure caused cavities to frequently form in a dose-dependent manner in unflooded roots and caused an increase in mean cortical cell size and number. Flooding increased the rate of ethylene release into the air space above the medium surface. These results indicate ethylene mediates vascular cavity formation and add to the evidence that vascular cavities are the result of programmed cell death and may function as a type of aerenchyma.