Hongye Li

Effect of chitosan on incidence of brown rot, quality and physiological attributes of postharvest peach fruit

The effect of chitosan (5.0 and 10.0 mg ml −1) on the incidence of brown rot (caused by Monilinia fructicola), quality attributes and senescence physiology of peaches was investigated. It was found that both concentrations of chitosan reduced the incidence of brown rot significantly and delayed the development of disease compared with the control, but were less effective than the fungicide prochloraz. Chitosan-treated peaches were firmer and had higher titratable acidity and vitamin C content than prochloraz-treated or control peaches. Compared to control (water-treated) peaches, chitosan-treated peaches showed lower respiration rate, less ethylene and malondialdehyde (MDA) production, higher superoxide dismutase (SOD) activity and better membrane integrity. Hence it can be concluded that chitosan has the potential to control brown rot, preserve valuable attributes and prolong the shelf life of postharvest peaches, presumably because of its antifungal property and inhibition of the ripening and senescence process of postharvest peaches. © 2000 Society of Chemical Industry

Genetic Bottlenecks Reduce Population Variation in an Experimental RNA Virus Population

ABSTRACT Genetic bottlenecks are stochastic events that limit genetic variation in a population and result in founding populations that can lead to genetic drift. Evidence of past genetic bottlenecks in numerous biological systems, from mammals to viruses, has been described. In this study, we used an artificial population of Cucumber mosaic virus consisting of 12 restriction enzyme marker-bearing mutants. This population was inoculated onto young leaves of tobacco plants and monitored throughout the course of systemic infection. We show here that the genetic variation in a defined population of an RNA virus is significantly, stochastically, and reproducibly reduced during the systemic infection process, providing clear evidence of a genetic bottleneck.

Analysis of Genetic Bottlenecks during Horizontal Transmission of Cucumber Mosaic Virus

ABSTRACT Genetic bottlenecks may occur in virus populations when only a few individuals are transferred horizontally from one host to another, or when a viral population moves systemically from the infection site. Genetic bottlenecks during the systemic movement of an RNA plant virus population were reported previously (H. Li and M. J. Roossinck, J. Virol. 78:10582-10587, 2004). In this study we mechanically inoculated an artificial population consisting of 12 restriction enzyme marker mutants of Cucumber mosaic virus (CMV) onto young leaves of squash plants and used two aphid species, Aphis gossypii and Myzus persicae, to transmit the virus populations from infected source plants to healthy squash plants. Horizontal transmission by aphids constituted a significant bottleneck, as the population in the aphid-inoculated plants contained far fewer mutants than the original inoculum source. Additional experiments demonstrated that genetic variation in the artificial population of CMV is not reduced during the acquisition of the virus but is significantly reduced during the inoculation period.

Genetic Structure and Population Variability of Tomato Yellow Leaf Curl China Virus

ABSTRACT Geminiviruses have circular single-stranded DNA genomes and are important pathogens in tropical and subtropical regions, but their population diversity and variability are poorly understood. Here, we have investigated variations accumulating in Tomato yellow leaf curl China virus (TYLCCNV), a geminivirus in the genus Begomovirus of the family Geminiviridae. The population variation was analyzed in a naturally infected tomato (Solanum lycopersicom) plant and in Nicotiana benthamiana and tomato plants experimentally infected with a swarm of TYLCCNV DNA clones to provide an identical sequence for initiation of infection. Our results demonstrate that the population of TYLCCNV in a naturally infected tomato plant was genetically heterogeneous and that rapid mutation occurred in the populations amplified from N. benthamiana and tomato plants that had been infected with cloned DNA. This feature of the population of TYLCCNV in these plants consisted of the consensus sequence and a pool of mutants that are not identical but are closely related to the consensus sequence, and it coincides with the quasispecies concept described for many RNA viruses. The mutation frequency was circa 10−4 in N. benthamiana and tomato at 60 days postinoculation, a value comparable to that reported for plant RNA viruses. The quasispecies-like nature of the TYLCCNV populations suggested that TYLCCNV is capable of rapid evolution and adaptation in response to changing agricultural practices.

Antibacterial activity and mechanism of action of chitosan solutions against apricot fruit rot pathogen Burkholderia seminalis

The in vitro antibacterial activity and mechanism of action of two kinds of acid-soluble chitosan and one water-soluble chitosan against apricot fruit rot pathogen Burkholderia seminalis was examined in this study. Results showed that water-soluble chitosan displayed limited antibacterial activity at four tested concentrations. However, two kinds of acid-soluble chitosan solution at 2.0 mg/mL had strong antibacterial activity against B. seminalis although weak antibacterial activity was observed at a concentration lower than 1 mg/mL. The antibacterial activity of acid-soluble chitosan may be due to membrane disruption, cell lysis, abnormal osmotic pressure, and additional chitosan coating around the bacteria based on integrity of cell membranes test, out membrane permeability assays and transmission electron microscopy observation. In addition, biofilm biomass were markedly reduced after treating with two kinds of acid-soluble chitosan at concentrations of 2.0 and 1.0 mg/mL for 3 and 12 h, indicating the importance of biofilm formation in the antibacterial mechanism of chitosan. Overall, the results clearly indicated that two kinds of acid-soluble chitosan had a potential to control the contamination of apricot fruits caused by B. seminalis.

PdCYP51B, a new putative sterol 14α-demethylase gene of Penicillium digitatum involved in resistance to imazalil and other fungicides inhibiting ergosterol synthesis

Penicillium digitatum, causing green mold decay, is the most destructive postharvest pathogen of citrus fruits worldwide. The phenotypes and genotypes of 403 isolates of P. digitatum, collected from packing houses and supermarkets in Zhejiang, China, during 2000 to 2010, were characterized in terms of their imazalil sensitivity. The frequency of detected imazalil-resistant (IMZ-R) isolates increased from 2.1% in 2000 to 60–84% during 2005–2010. Only 6.5% and 4.5% of the collected IMZ-R isolates belong to the previously described IMZ-R1 and IMZ-R2 genotypes, respectively. To determine the resistance mechanism of the predominant and novel IMZ-R isolates of P. digitatum (termed IMZ-R3), genes PdCYP51B and PdCYP51C, homologous to the sterol 14α-demethylase encoded gene PdCYP51, were cloned from six IMZ-R3 and eight imazalil-sensitive (IMZ-S) isolates of P. digitatum. A unique 199-bp insertion was observed in the promoter region of PdCYP51B in all IMZ-R3 isolates examined but in none of the tested IMZ-S isolates. Further analysis by PCR confirmed that this insertion was present in all IMZ-R3 isolates but absent in IMZ-S, IMZ-R1, and IMZ-R2 isolates. Transcription levels of PdCYP51B in three IMZ-R3 isolates were found to be 7.5- to 13.6-fold higher than that in two IMZ-S isolates of P. digitatum. Introduction of another copy of PdCYP51Bs (from IMZ-S) into an IMZ-S isolate decreased the sensitivity of P. digitatum to 14α-demethylation inhibitors (DMIs) only to a small extent, but introduction of a copy of PdCYP51BR (from IMZ-R3) dramatically increased the resistance level of P. digitatum to DMIs. Regarding PdCYP51C, no consistent changes in either nucleotide sequence or expression level were correlated with imazalil resistance among IMZ-R and IMZ-S isolates. Based on these results, we concluded that (1) the CYP51 family of P. digitatum contains the PdCYP51B and PdCYP51C genes, in addition to the known gene PdCYP51A (previously PdCYP51); (2) PdCYP51B is involved in DMI fungicide resistance; and (3) overexpression of PdCYP51B resulting from a 199-bp insertion mutation in the promoter region of PdCYP51B is responsible for the IMZ-R3 type of DMI resistance in P. digitatum.

Colletotrichum species associated with cultivated citrus in China

There have been considerable advances in the understanding of species concepts in the genus Colletotrichum. This has lead to the need to carry out fresh surveys of Colletotrichum species associated with important hosts. Colletotrichum species are associated with Citrus plants as saprobes, important pre-harvest and post-harvest pathogens, as well as endophytes. In this study, a total of 312 Colletotrichum strains were isolated from leaves, shoots and fruits of cultivated Citrus and Fortunella species with or without disease symptoms across the main citrus production areas in China. The morphology of all strains were studied and multilocus (ACT, TUB2, CAL, GAPDH, GS, ITS) phylogeny established. Strains were from four important species complexes of Colletotrichum, namely C. gloeosporioides species complex, C. boninense species complex, C. acutatum species complex and a final group including C. truncatum, which was rare on Citrus species. The species belonging to the C. gloeosporioides species complex comprised C. gloeosporioides and C. fructicola, the C. boninense complex comprised C. karstii and a new species C. citricola and the C. acutatum complex included a new species, C. citri. The ability of strains to cause anthracnose on citrus fruits was tested by inoculation and strains of Colletotrichum gloeosporioides, C. fructicola and C. truncatum were pathogenic.

Antibacterial activity of two chitosan solutions and their effect on rice bacterial leaf blight and leaf streak

BACKGROUND Bacterial leaf blight and leaf streak are the two most damaging bacterial diseases of rice. However, few bactericidal chemicals are available for controlling both diseases. The antibacterial properties of two kinds of chitosan with different molecular weights and degrees of N-deacetylation and their effect on rice bacterial leaf blight and leaf streak were evaluated. RESULTS Results showed that the two kinds of chitosan solution possess a strong antibacterial activity against both rice bacterial pathogens and significantly reduced disease incidence and severity by comparison with the control under greenhouse conditions. However, the interaction between chitosan and rice pathogens was affected by the type and concentration of chitosan, the bacterial species and the contact time between chitosan and bacteria. The direct antibacterial activity of chitosan may be attributed to both membrane lysis and the destruction of biofilm. In addition, both chitosan solutions significantly increased the activities of phenylalanine ammonia lyase, peroxidase and polyphenol oxidase in rice seedlings following inoculation of two rice pathogens by comparison with the control. CONCLUSION The role of chitosan in protection of rice against bacterial pathogens has been shown to involve direct antibacterial activity and indirect induced resistance.

The pH signaling transcription factor PacC is required for full virulence in Penicillium digitatum

Penicillium digitatum is the most important postharvest pathogen of citrus fruits. Along disease progression, the infected citrus peel tissue is acidified due to the accumulation of organic acids. So far, relatively little is known about the environmental factors that regulate pathogenicity in this fungus. In this study, the role of the pH signaling transcription factor PacC in the pathogenesis of P. digitatum was investigated. We identified the pacC ortholog (PdpacC) in P. digitatum and found that its transcript levels were elevated under alkaline conditions (pH ≥ 7) in vitro, as well as during the infection of citrus fruits in spite of the low pH (about 3.0 to 3.5) of the macerated tissue. Na+ and pectin also induced the expression of PdpacC. Disruption of PdpacC resulted in impaired mycelial growth under neutral or alkaline pH conditions and on synthetic medium supplemented with pectin as the sole carbon source, and attenuated virulence towards citrus fruits. Introducing the full length of PdpacC into the ΔPdpacC mutant restored all these phenotypes. The expression of the polygalacturonase gene Pdpg2 and pectin lyase gene Pdpnl1 in P. digitatum was upregulated in the wild type strain but not or weakly upregulated in the ΔPdpacC mutant during infection. Disruption of Pdpg2 also resulted in attenuated virulence of P. digitatum towards citrus fruits. Collectively, we conclude that PdPacC plays an important role in pathogenesis of P. digitatum via regulation of the expression of cell wall degradation enzyme genes, such as Pdpg2 and Pdpnl1.

In vitro inhibition of Botryosphaeria dothidea and Lasiodiplodia theobromae, and chemical control of gummosis disease of Japanese apricot and peach trees in Zhejiang Province, China

Abstract Chemical fungicides were applied to suppress pathogen mycelial growth and conidial germination in the laboratory, and to control a gummosis disease in Japanese apricot and peach orchards caused by Botryosphaeria dothidea and Lasiodiplodia theobromae (Syn. = Botryodiplodia theobromae). Field applications were made by spraying or painting after lesion-scraping. Thiophanate-methyl consistently inhibited mycelial growth and conidial germination and controlled disease development in Japanese apricot trees. Recover No. 843 (humic acid plus other chemical elements) provided the best disease control on both tree types in the field and improved plant tissue regrowth in the area of the lesion. The discrepancies between smaller and larger lesions on trees of various ages were also discussed.