CHITINASE IN CUCUMBER HYPOCOTYLS IS INDUCED BY GERMINATING FUNGAL SPORES AND BY FUNGAL ELICITOR IN SYNERGISM WITH INDUCERS OF ACQUIRED-RESISTANCE

After root pretreatment with 2,6-dichloroisonicotinic acid (DCIA or IN A), hypocotyls of etiolated cucumber seedlings acquired resistance to infection by Colleto-trichum lagenarium caused by the failure of the f ungus to penetrate epidermal cell walls. The hypocotyls contained only low levels of class III chitinase and its mRNA prior to infection. Th is pathogenesis-related (PR) gene was expressed strongly upon infectio n but only in resistant hypocotyls and soon after germination of the f ungal spores. Chitinase was also induced early by an albino mutant str ain of C. lagenarium that can barely penetrate the epidermis. Thus, ea rly recognition of the fungus implies signal compounds able to pass, o r being generated in, the hydrophobic epidermal surface. As the apopla stic chitinase accumulates timely at the site of a subsequent attack, it may contribute to disease resistance. The mechanism behind the enha nced responsiveness of epidermal cells was studied by gently abrading the cuticle of susceptible hypocotyls to allow permeation of a water-s oluble polymeric fungal elicitor. Induction of chitinase occurred only when the elicitor was applied simultaneously with a resistance induce r such as DCIA, salicylic acid (SA) or a benzothiadiazole (BTH). In ad dition, long-term root pretreatment with DCIA conditioned the hypocoty ls for enhanced elicitor responses. These results demonstrate that the above inducers of acquired resistance can affect expression of the cu cumber chitinase gene not only as direct inducers. They can also act s ynergistically with fungal elicitors and, in addition, condition the h ypocotyls in a developmental manner for potentiated elicitation.