INDUCED RESISTANCE IN SUGAR-BEET AGAINST CERCOSPORA-BETICOLA - INDUCTION BY DICHLOROISONICOTINIC ACID IS INDEPENDENT OF CHITINASE AND BETA-1,3-GLUCANASE TRANSCRIPT ACCUMULATION

Of various chemical and biological compounds known to induce resistanc e in different species of plants, only 2,6-dichloroisonicotinic acid ( INA) possessed the ability to induce resistance in sugar beet (Beta vu lgaris) against Cercospora beticola. Repeated spraying with low concen trations of INA during the week prior to challenge inoculation with th e fungus induced complete local and systemic resistance. None of the t ranscripts encoding specific pathogenesis-related proteins, three chit inases (of classes III and IV) and one beta-1,3-glucanase, were found to accumulate in the INA-treated tissue. However, a somewhat earlier a ccumulation was observed following subsequent inoculation with the fun gus, possibly due to INA-induced potentiation of the plant cells resul ting in more rapid activation of the defence system. All four transcri pts were induced locally by salicylic acid and all but one accumulated in tissue after wounding. Remarkably, one specific transcript, encodi ng a basic class IV chitinase, was found to accumulate to high levels following mechanical shaking. High temperature and drought stress had no apparent effects on transcript accumulation, and one of the transcr ipts, an acidic class IV chitinase, showed no stress induction at all. During infection with C. beticola the genes showed biphasic accumulat ion patterns in the inoculated leaves, with an early one day accumulat ion to high mRNA levels followed by a later steady state of high trans cript accumulation. The major difference between susceptible and parti ally resistant plants was a stronger early transient expression in the latter.