Abstract
The plant growth regulator prohexadione-Ca acts as an inhibitor of 2-oxoglutarate-dependent dioxygenases in gibberellin biosynthesis, thereby reducing longitudinal shoot growth. Additionally, the compound lowers the incidence and severity of fire blight (Erwinia amylovora) and scab (Venturia inaequalis) in apples and pears. Other diseases may also be restricted. The movement of E. amylovora is inhibited in plants treated with prohexadione-Ca, thereby reducing the spread of the disease. An induced change in flavonoid metabolism is assumed to be the cause of lowered pathogen incidence: 2-Oxoglutarate-dependent flavanone 3-hydroxylase, another dioxygenase, is also blocked by prohexadione-Ca. As a result, a novel pathway leading to 3-deoxycatechins and derived compounds is induced and several flavonoids and other phenylpropanoids are newly formed, both in apples and in pears. Concomitantly, the content of distinct, regularly-occurring compounds of this type is increased. Infection with E. amylovora also leads to an increase in regular phenylpropanoids. Some of these compounds inhibit in vitro growth of E. amylovora. Therefore, it may be argued that prohexadione-Ca triggers defence reactions in apple and pear shoots by qualitatively and quantitatively changing their spectrum of flavonoids and other phenylpropanoids.