The genus Nepenthes represents carnivorous plants with pitcher traps capable of efficient prey capture and digestion. The possible involvement of plant chitinases in this process was studied in Nepenthes khasiana. Two different types of endochitinases were identified in the liquid of closed traps exhibiting substrate specificity for either long chitin polymers or N-acetylglucosamine (GlcNAc) oligomers. Injection of chitin into such closed sterile pitchers induced the appearance of additional endochitinase isoenzymes, with substrate specificity only for long chitin polymers. No significant exochitinase (N-acetyl-beta-glucosaminidase) or chitobiosidase activity could be detected in the non-induced or induced trap liquid. Four genes representing two subgroups of basic chitinases, denoted as Nkchit1b and Nkchit2b, were isolated from the secretory region of N. khasiana pitchers. The main differences between the two subgroups are the presence of a proline-rich hinge region only in NkCHIT1b and a C-terminal putative vacuole targeting extension only in NkCHIT2b, indicating different compartmentalization of the two enzymes. Reverse transcription-polymerase chain reaction (RT-PCR) evaluation of mRNA levels showed that the Nkchit2b genes are constitutively expressed in the secretory cells while transcription of Nkchit1b genes is induced by chitin injection. These results show for the first time the involvement of genes encoding chitinases in prey-trap interaction and their differential expression and activity during prey trapping.
Nepenthes spp. are carnivorous plants that have developed insect capturing traps, evolved by specific modification of the leaf tips, and are able to utilize insect degradation products as nutritional precursors. A chitin-induced antifungal ability, based on the production and secretion to the trap liquid of droserone and 5-O-methyldroserone, is described here. Such specific secretion uniquely occurred when chitin injection was used as the eliciting agent and probably reflects a certain kind of defence mechanism that has been evolved for protecting the carnivory-based provision of nutritional precursors. The pitcher liquid containing droserone and 5-O-methyldroserone at 3:1 or 4:1 molar ratio, as well as the purified naphthoquinones, exerted an antifungal effect on a wide range of plant and human fungal pathogens. When tested against Candida and Aspergillus spp., the concentrations required for achieving inhibitory and fungicidal effects were significantly lower than those causing cytotoxicity in cells of the human embryonic kidney cell line, 293T. These naturally secreted 1,4-naphthoquinone derivatives, that are assumed to act via semiquinone enhancement of free radical production, may offer a new lead to develop alternative antifungal drugs with reduced selectable pressure for potentially evolved resistance.
Isolate ISR398 of Septoria tritici (which produces none to few pycnidia on the wheat cv. Seri 82 and high coverage on cv. Shafir) and isolate ISR8036 (which is virulent on both cultivars) were genetically cotrans-formed using the selectable marker gene hph, which confers resistance to hygromycin B (hygB), and the reporter gene uidA, encoding beta-glucuronidase (GUS). Most of the genetically transformed isolates (98.8%) produced similar pycnidial coverage on seedlings of 'Seri 82' and 'Shafir' as the two wild-type isolates. Southern analysis of 25 randomly selected hygB(R)GUS(+) transformants probed with the uidA sequence revealed multiple insertion sites. GUS activity was determined fluorimetrically by measuring the conversion of 4-methylumbelliferyl beta-D-glucuronide (MUG) to 4-methylumbelliferone (MU). The high GUS-expressing transformants 398D97 and 8036E27 were used to elucidate fungal development within inoculated leaf tissue by using GUS activity to estimate the fungal proteins content in planta. Increase in fungal biomass was recorded in 'Shafir' inoculated with the GUS-expressing transformants 398D97 and 8036E27 following a 12-day latent period. A 15-day latent period was recorded in 'Seri 82' inoculated with 8036E27, whereas an 18-day latent period was recorded on 'Seri 82' inoculated with 398D97 and the two mixtures 398D97 + ISR8036 and ISR398 + 8036E27. The rate of fungal development and the estimated level of fungal proteins at the pycnidia maturation stage was high in leaves of 'Shafir' and moderate to low on 'Seri 82', even in cases in which no significant differences were recorded in pycnidial coverage. An endogenous capacity to hydrolyze beta-1,4-D-glucuronidase was recorded in leaves inoculated with wild-type isolates. The latent periods in MU production of the uidA-expressing transformants mimicked those recorded for the wild-type isolates. However, at all stages, the levels of MU produced in wheat inoculated with wild-type isolates were markedly lower than those produced by GUS-expressing transformants. The mode of interaction (compatible or incompatible) determined the onset of the induction, rate, and level of enzyme production.
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