Effects of chitosan and oligochitosan on growth of two fungal pathogens and physiological properties in pear fruit
Introduction
Postharvest diseases caused by pathogenic fungi result in major losses of fruits and vegetables, and synthetic chemical fungicides are the primary means to application at present (Spadaro & Gullino, 2004). However, synthetic chemical fungicides are potentially harmful on human health and the emergence of pathogens which are resistant to these chemicals (Holmes & Eckert, 1999). Moreover, public concern over the indiscriminate use of synthetic fungicides has been growing. Thus, it is significant to develop new alternatives for disease control (Tian, 2006).
Chitosan, together with its derivatives, has been reported as a promising alternative to control postharvest diseases. Different from other synthetic chemical fungicides, chitosan is a natural polysaccharide with a chemical structure of poly β-(1 → 4)N-acetyl-d-glucosamine (Bautista-Baňos et al., 2006). Chitosan and oligochitosan have broad-spectrum antibacterial activities (Jeon et al., 2001, Liu et al., 2006, No et al., 2002, Zheng and Zhu, 2003). In addition, chitosan and oligochitosan as fungicides are effective in inhibiting spore germination, germ tube elongation and mycelial growth of fungal phytopathogens, such as Alternaria solani (Xu, Zhao, Han, & Du, 2007), Botrytis cinerea (El Ghaouth et al., 1992, Chien and Chou, 2006, Liu et al., 2007), Fusarium (Eweis et al., 2006, Xu et al., 2007), Rhizopus stolonifer (El Ghaouth et al., 1992, Hernández-Lauzardo et al., 2008), Penicillium (Chien and Chou, 2006, Liu et al., 2007), Phytophthora capsici (Xu et al., 2007) and Sclerotium rolfsii (Eweis et al., 2006). Furthermore, chitosan and oligochitosan also act as an exogenous elicitor for fruits and vegetables, and thus protects hosts from further fungal infection. It has been reported that chitosan could boost the activity of defense-related enzymes or pertinent substance in Arabidopsis (Cabrera, Messiaen, Cambier, & Van Cutsem, 2006), tomato fruit (Liu et al., 2007), orange, strawberry and raspberry fruit (Fajardo et al., 1998, Zhang and Quantick, 1998). Oligochitosan has also shown effective to elicit production of hydrogen peroxide (Li et al., 2009, Lin et al., 2005), to increase activities of phenylalanine ammonialyase (PAL) and POD (Vander, Varum, Domard, Eddine, & Moerschbacher, 1998), and to up-regulate gene expression of β-1,3-glucanase and chitinase (Lin et al., 2005). Thanks to their antifungal activity and resistance induction, chitosan and its derivatives have been successfully applied to extend the storage life of fruits and vegetables (Chien and Chou, 2006, El Ghaouth et al., 1992, Liu et al., 2007).
Although chitosan and its derivatives have been considered as versatile biopolymers in agriculture applications, their potential application as antimicrobial compounds, elicitors or preservatories of decay remains to be further explored (Bautista-Baňos et al., 2006, Yang et al., 2009). Little information is known about comparison of inhibitory effect of chitosan and oligochitosan on fungal pathogenicity and disease control at present. Alternaria kikuchiana Tanaka and Physalospora piricola Nose are two kinds of fungal pathogens of pear fruit in storage. Using these two fungi as subject, this study aimed at the difference between chitosan and oligochitosan (1) as fungicides by comparison their half maximal inhibitory concentration (IC50) on spore germination, germ tube elongation and mycelial growth; (2) as elicitor by comparison the inductive effect on the activities of related enzymes of host; (3) as preservatories on decay control of pear fruit.
Section snippets
Chitosan and oligochitosan
Chitosan with 90% deacetylation and viscosity average molecular weight of 350 kDa dissolved at 25 g/L in 1% HCl by stirring at room temperature, and the original solution was then diluted to a series of 0.1, 0.3, 0.5, 0.7, 1.0, 2.0 and 5.0 g/L, and the pH value of each solution was adjusted to 5.5 with NaOH. Oligochitosan with average molecular weight 6 kDa was dissolved in sterile distilled water at 25 g/L, and the original solution was then diluted to a series of the concentrations as the same as
Effects of chitosan or oligochitosan on spore germination and germ tube elongation
As shown in Fig. 1, spore germination and germ tube elongation of A. kikuchiana and P. piricola were significantly inhibited by chitosan or oligochitosan with concentration-dependent mode (P < 0.05). Both chitosan and oligochitosan at 5.0 g/L inhibited completely spore germination of the two fungi after 6 h inoculation. The IC50 of chitosan on the spore germination of A. kikuchiana and P. piricola was 1.85 and 1.81 g/L, respectively (Table 1). Correspondingly, the IC50 of oligochitosan was 1.51 and
Discussion
Chitosan and its derivatives offer a great potential as natural biodegradable substances which have anti-microbial and eliciting activities (Bautista-Baňos et al., 2006, Benhamou, 1996). In the present study, we demonstrated that chitosan and oligochitosan were effective in inhibiting spore germination and mycelial growth of A. kikuchiana and P. piricola. By the comparison of IC50 of chitosan and oligochitosan on spore germination, germ tube elongation and mycelial growth (Table 1), we
Conclusion
The present study reported the different effects of chitosan and oligochitosan on developmental inhibition of A. kikuchiana and P. piricola, on inductive physiological and biochemical change of host and on control of related postharvest diseases. Chitosan and oligochitosan had stronger inhibitory effect on mycelia growth than spore germination, germ tube elongation of both A. kikuchiana and P. piricola by the comparison of IC50. Moreover, P. piricola was more sensitive to chitosan and
Acknowledgements
The research was supported by National Natural Science Foundation of China (30972063) and the Ministry of Science and Technology of China (2006BAD22B03). We thank Professor Guixi Wang and Yuguang Du for their help in this research.
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