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 INA), hypocotyls of etiolated cucumber seedlings acquired resistance to infection by Colleto-trichum lagenarium caused by the failure of the fungus to penetrate epidermal cell walls. The hypocotyls contained only low levels of class III chitinase and its mRNA prior to infection. This pathogenesis-related (PR) gene was expressed strongly upon infection but only in resistant hypocotyls and soon after germination of the fungal spores. Chitinase was also induced early by an albino mutant strain of C. lagenarium that can barely penetrate the epidermis. Thus, early recognition of the fungus implies signal compounds able to pass, or being generated in, the hydrophobic epidermal surface. As the apoplastic chitinase accumulates timely at the site of a subsequent attack, it may contribute to disease resistance. The mechanism behind the enhanced responsiveness of epidermal cells was studied by gently abrading the cuticle of susceptible hypocotyls to allow permeation of a water-soluble polymeric fungal elicitor. Induction of chitinase occurred only when the elicitor was applied simultaneously with a resistance inducer such as DCIA, salicylic acid (SA) or a benzothiadiazole (BTH). In addition, long-term root pretreatment with DCIA conditioned the hypocotyls for enhanced elicitor responses. These results demonstrate that the above inducers of acquired resistance can affect expression of the cucumber chitinase gene not only as direct inducers. They can also act synergistically with fungal elicitors and, in addition, condition the hypocotyls in a developmental manner for potentiated elicitation.