Polyamine Content and Action in Roots of Zea mays L. in Relation to Aerenchyma Development

Roots of Zea mays L. developed more aerenchyma (cortical gas-filled space) when partially deficient in oxygen (3 kPa) than when supplied with air (20·8 kPa oxygen) in association with faster production of ethylene (ethene). The possibility was tested that the additional ethylene production resulted from decreases in spermidine (spd) and spermine (spm) which share, with ethylene, a common precursor, S-adenosylmethionine. However, no decreases in spd and spm were seen in root tissue up to 4 d-old. Removing oxygen completely also had little effect on spd and spm, but strongly suppressed both ethylene production and aerenchyma formation. Partial oxygen shortage (3 kPa) increased the concentration of putrescine (put), the precursor of spd and spm. This increase was not a response to the extra ethylene formed by such roots since ethylene treatment did no reproduce the effect. Application of inhibitors of put biosynthesis, difluoromethylarginine and difluoromethylornithine, led to increased aerenchyma formation. Exogenous put inhibited the development of aerenchyma while stimulating rather than inhibiting ethylene production, when tested in either air or 3 kPa oxygen. Thus, put appears to limit aerenchyma formation by suppressing ethylene action rather than its production.