Changes in the activity of the alternative oxidase in Orobanche seeds during conditioning and their possible physiological function.

The appearance of the activity of the cyanide insensitive, alternative oxidase (AOX), pathway of oxygen uptake was followed in seeds of Orobanche aegyptiaca during conditioning. The pathway becomes operative during conditioning, up to day three as determined by inhibition of oxygen uptake of the seeds by propyl gallate. At the same time an increasing percentage of oxygen uptake is insensitive to cyanide and an increased oxygen uptake, responsive to propyl gallate, is induced by brief salicylic acid treatment of seeds. By day six of conditioning, these responses decrease and the AOX pathway could not be detected in germinating seeds, after treatment with a germination stimulant. These results were confirmed by following the reaction of extracts of fractions enriched with mitochondria from the conditioned seeds, using a specific antibody against AOX. Treatment of the seeds with inhibitors of AOX during conditioning significantly inhibited their subsequent germination. Addition of hydrogen peroxide after 4 and 7 days of conditioning resulted in reduced germination. In addition treatment of seed with propyl or octyl gallate during conditioning reduced the infection of tomato plants by Orobanche seeds and the development of tubercles of the parasite on the host roots. These results together indicate that the operation of AOX during conditioning has a significant function on the subsequent germination behaviour and pathogenicity of the root parasite. Some potential practical applications of these findings are discussed.