articleDifferential expression of maize chitinases in the presence or absence of Trichoderma harzianum strain T 22 and indications of a novel exo-endo-heterodimeric chitinase activity

Background: The interaction of plants with endophytic symbiotic fungi in the genus Trichoderma alters the plant proteome and transcriptome and results in enhanced plant growth and resistance to diseases. In a previous study, we identified the numerous chitinolytic enzyme families and individual enzymes in maize which are implicated in plant disease resistance and other plant responses. Results: We examined the differential expression of the entire suite of chitinolytic enzymes in maize plants in the presence and absence of T. harzianum. Expression of these enzymes revealed a band of chitinolytic enzyme activity that had greater mass than any known chitinase. This study reports the characterization of this large protein. It was found to be a heretofore undiscovered heterodimer between an exoand an endo-enzyme, and the endo portion differed between plants colonized with T. harzianum and those grown in its absence and between shoots and roots. The heterodimeric enzymes from shoots in the presence and absence of T. harzianum were purified and characterized. The dimeric enzyme from Trichoderma-inoculated plants had higher specific activity and greater ability to inhibit fungal growth than those from control plants. The activity of specific chitinolytic enzymes was higher in plants grown from Trichoderma treated seeds than in control plants. Conclusions: This is the first report of a dimer between endoand exochitinase. The endochitinase component of the dimer changed post Trichoderma inoculation. The dimer originating from Trichoderma inoculated plants had a higher antifungal activity than the comparable enzyme from control plants. Background Trichoderma spp. are opportunistic root colonizing fungal plant symbionts [1] that induce numerous changes in plant gene expression and physiology. Among the phenotypic changes are increased systemic resistance to plant diseases [1-8]; increased growth of plants and roots, including an increase in fertilizer use efficiency and uptake [9-12]; and a generalized increase in resistance to abiotic stresses [1,13]. We recently completed an analysis of the proteome of maize plants in the presence or absence of T. harzianum strain T22 [11,14]. Even though T22 was present only on roots, there were 141 proteins that we identified as upregulated and 50 that were down-regulated in shoots [11], while in roots 20 up-regulated and 11 down-regulated proteins were found [14]. A large portion of the up-regulated proteins were involved in carbohydrate metabolism, while a number of others were involved in photosynthesis or resistance to stress. In addition, starch accumulation in maize plants whose roots were colonized with T22 was greater than in the control [11]. We suggested that Trichoderma induces both increased growth, which is mediated by an increase in photosynthetic and respiratory rates, and systemic induced resistance [14]. In an earlier study we determined that maize root colonization enhanced expression of chitinolytic enzyme activity [15]. This is frequently assumed to be part of induced systemic resistance [8]. However, in our pro* Correspondence: michalsho@volcani.agri. gov.il 1 Department of Horticultural Sciences, Cornell University, Geneva, NY 14456