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 resistance in different species of plants, only 2,6-dichloroisonicotinic acid (INA) possessed the ability to induce resistance in sugar beet (Beta vulgaris) against Cercospora beticola. Repeated spraying with low concentrations of INA during the week prior to challenge inoculation with the fungus induced complete local and systemic resistance. None of the transcripts encoding specific pathogenesis-related proteins, three chitinases (of classes III and IV) and one beta-1,3-glucanase, were found to accumulate in the INA-treated tissue. However, a somewhat earlier accumulation was observed following subsequent inoculation with the fungus, possibly due to INA-induced potentiation of the plant cells resulting in more rapid activation of the defence system. All four transcripts were induced locally by salicylic acid and all but one accumulated in tissue after wounding. Remarkably, one specific transcript, encoding 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 transcripts, an acidic class IV chitinase, showed no stress induction at all. During infection with C. beticola the genes showed biphasic accumulation patterns in the inoculated leaves, with an early one day accumulation to high mRNA levels followed by a later steady state of high transcript accumulation. The major difference between susceptible and partially resistant plants was a stronger early transient expression in the latter.