Volume 4 Number 1 2010
CONTENTS AND ABSTRACTS
Sumona Karjee, Neeti Sanan-Mishra, Sunil Kumar Mukherjee (India) Viral Suppressors of RNA Silencing in Plants (pp 1-13)
Review: In eukaryotes, small RNAs play a crucial regulatory role in many processes including development, maintenance of genome stability and antiviral responses. These different but overlapping RNA-guided pathways are collectively termed 'RNA silencing'. In plants, RNA silencing serves as a major line of antiviral defense that is induced by, and targeted against viruses. As a counter-defensive strategy, viruses have evolved to encode suppressor proteins that inhibit various stages of the silencing process. These suppressors are diverse in sequence and structure and appear to be encoded by virtually any type of plant viruses. This review focuses on the novel methods of suppressor screening and revealing the characteristics of RNA silencing suppressors. We have also discussed the mechanism of suppression activity, which principally operate by modifying the accumulation, activity, and/or transmission of siRNAs through either direct interaction with the RNA species or components of the RNA silencing machinery. Finally, the biotechnological applications of silencing suppression have been considered.
Raymond A. Cloyd (USA) Pesticide Mixtures and Rotations: Are these Viable Resistance Mitigating Strategies? (pp 14-18)
Invited Mini-Review: Resistance to pesticides (in this case, insecticides and miticides) is a concern to producers associated with ornamental cropping systems such as greenhouses and nurseries. Resistance typically develops due to continually using insecticides and/or miticides with similar modes of action or chemistries in the presence of common detoxification pathways. Two strategies that have been suggested to mitigate resistance developing in arthropod (insect and mite) pest populations are the implementation of pesticide mixtures or pesticide rotations. However, the use of either strategy is still controversial as it is has not been adequately demonstrated quantitatively that these strategies actually mitigate resistance. Pesticide mixtures involve exposing individuals in an arthropod pest population to each pesticide simultaneously whereas pesticide rotations are the alternating use of pesticides with dissimilar modes of action. There are, however, a number of assumptions pertaining to both pesticide mixtures and rotations that dictate how successful these strategies may be in delaying the onset of resistance including 1) resistance associated with each pesticide is monogenic and independently genetically controlled, 2) individuals in the arthropod pest population with doubly-resistance genes or with multiple resistance mechanisms are rare, 3) a certain frequency or proportion of individuals in the arthropod pest population are left untreated due to the presence of refugia, 4) the pesticides used are equally persistent so that any individuals in the arthropod pest population are not exposed to just one pesticide for an extended length of time, and 5) the pesticides used have different modes of action. Either strategy of delaying or mitigating resistance should be incorporated with alternative pest management tactics such as cultural, sanitation, and biological control, which will reduce continued reliance on pesticides.
Trazilbo J. Paula Júnior, Waldir C. Jesus Junior, Marcelo A. B. Morandi, Wagner Bettiol, Rogério F. Vieira (Brazil), Bernhard Hau (Germany) Interactions between Aerial and Soil-borne Pathogens: Mechanisms and Epidemiological Considerations (pp 19-28)
Review: Aerial and soil-borne pathogens can simultaneously attack different parts of the same plant. The latter can alter the susceptibility of hosts to infection by aerial pathogens and vice versa. Normally the effects are via the host plant. The studies related to the interactions among soil-borne and aerial pathogens generally do not emphasize epidemiological aspects. Most of them are merely descriptive. Even for the most studied interactions, the mechanisms involved are rarely described. Although pathogen interaction phenomenon still remains poorly studied, it is evident in many situations that appropriate strategies of disease management may take into account the possibility of pathogen infection of different parts of the same plant. In this review, we discuss the current literature regarding the mechanisms and epidemiology of interactions between soil-borne and aerial pathogens. Examples of interactions between both soil-borne fungi and aerial pathogens and nematodes and aerial pathogens are given considering effects on the disease or pathogen dynamics and the crop yield loss.
Vamsi Pallam Reddy, Madhurababu Kunta, Shad D. Nelson, John V. da Graça, Mani Skaria (USA) Gene Expression Studies in Sour Orange and C-22 Rootstocks Challenged with the Fungus, Phytophthora nicotianae and the Nematode, Tylenchulus semipenetrans (pp 29-34) Ipublish™ What isIpublish?
Original Research Paper: Citrus rootstocks differ in response to soil-borne pests and pathogens exhibiting morphological and molecular changes. Such changes were assessed in two citrus rootstocks, sour orange and C-22, inoculated with the fungus, Phytophthora nicotianae and the citrus nematode, Tylenchulus semipenetrans. These pathogens induce responses and as a result, different genes are expressed. The cDNA-amplified fragment length polymorphism (cDNA-AFLP) was used to identify the differentially expressed genes. Eight differential fragments corresponding to the changes in host gene expression were cloned and sequenced. A homology search at the National Center for Biotechnology Information (NCBI) Genbank database showed one of the clones derived from C-22 rootstock responded to citrus nematode has similarity to the Citrus tristeza virus (CTV) resistance gene locus in Poncirus trifoliata with 89% identity and an E value of -75. This suggests that a gene supporting nematode resistance in citrus could be related to CTV. The AFLP fingerprint profiles were characterized on agarose and poly(NAT)® Elchrom gels for resolution and clarity. Better results were obtained with poly(NAT)®, and the result was assessed for different profiles generated by different primer combinations. Sour orange and C-22 rootstocks were also evaluated for changes in shoot length, shoot weight and root weight in response to P. nicotianae and T. semipenetrans. Sour orange seedlings showed tolerance to P. nicotianae, compared to C-22, based on percentage reduction in shoot length, shoot weight and root weight; however, differences were not significant.
Hayfa Jabnoun-Khiareddine, Mejda Daami-Remadi, Fakher Ayed, Mohamed El Mahjoub (Tunisia) Evaluation of Several Indigenous Microorganisms and Some Bio-Fungicides for Biocontrol of Potato Verticillium Wilt (pp 35-44)
Original Research Paper: Verticillium wilt development was completely suppressed with treatments based on indigenous Trichoderma harzianum, T. virens, Gliocladium catenulatum, Penicillium sp. and rhizobacteria incorporated into the culture substrate 15 days prior inoculation with pathogen. The vascular discoloration extent noted on all potato (Solanum tuberosum L.) ‘Spunta’ plants treated biologically was statistically comparable to that recorded on non-inoculated and untreated plants which were symptomless. Potato plants treated with G. roseum, G. catenulatum, T. harzianum, T. virens, Penicillium sp. and indigenous rhizobacteria showed a significant increase, by more than 55%, of their height in comparison to the untreated and inoculated control. Roots and stem fresh weights increase, compared to the untreated and inoculated control, with some indigenous antagonistic treatments ranging between 49-109 and 28-102%, respectively. Plants treated with Penicillium sp. and indigenous rhizobacteria showed an increase of 44-46% of their tuber weight compared to the untreated and inoculated control. The LDI recorded with bio-fungicides based treatments was reduced by more than 65% in comparison to the inoculated and untreated plants. Wilt development was completely suppressed with BiofolarTM, based on citric acid and mint oil, which was incorporated into infested culture substrate. All plants treated with bio-fungicides showed a significantly increased height of about 12 to 28% compared to the untreated and inoculated control. With T. harzianum (Biocont-TTM) and Pythium oligandrum (Polyversum®) based bio-fungicides, the roots, stem and tuber fresh weights recorded were 28-86, 48-81 and 28-57% higher than that noted on untreated and inoculated control plants, respectively.
Mejda Daami-Remadi, Rafik Bouallègue, Hayfa Jabnoun-Khiareddine, Mohamed El Mahjoub (Tunisia) Comparative Aggressiveness of Tunisian Colletotrichum coccodes Isolates on Potato Assessed via Black Dot Severity, Plant Growth and Yield Loss (pp 45-53)
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Original Research Paper: Black dot of potato has become a serious disease in Tunisia. Colletotrichum coccodes, the causal agent, was found to be widely dispersed throughout the major potato-growing areas. Disease incidence varied depending on regions surveyed and potato organs sampled. The aggressiveness of local C. coccodes isolates was tested on potato cv. ‘Spunta’ plants based on disease severity records on the below-ground plant parts, plant growth and expected yield. Black dot severity, noted 60 days post-planting, was influenced by C. coccodes isolates used for inoculation and a variation in pathogen aggressiveness was recorded. The sclerotial density was higher on roots and stolons than on the below-ground stems. C. coccodes aggressiveness was directly associated with reduced growth, i.e. fresh and dry weights, of the below- and above-ground plant parts. For all fungal treatments or isolates combined, no significant correlation was found between black dot severity and tuber yield although 47% of yield loss was recorded subsequent to inoculation with some Tunisian C. coccodes isolates. Therefore, as shown in the present study, this pathogen may by itself influence potato growth and disease intensity and should be considered as a primary-disease-causing organism.
Hayfa Jabnoun-Khiareddine, Mejda Daami-Remadi, Mohamed El Mahjoub (Tunisia) Pathogenicity and Aggressiveness of Verticillium dahliae Races from Non-Tomato Hosts towards Tomato Cultivars Possessing or not the Ve Gene (pp 54-61)
Original Research Paper: Race 2 strains are widely distributed, vary in aggressiveness and have rapidly supplanted race 1 strains in many tomato production areas in Tunisia and around the world. Based on the presence or not of Verticillium wilt symptoms on resistant (cv. ‘Rio Grande’) and susceptible (cv. ‘Ventura’) tomato cultivars, 8 V. dahliae isolates were typed as race 1, 7 isolates as race 2 and 1 isolate was non-pathogenic to tomato. Isolates from potato, artichoke, melon and olive belonged to V. dahliae race 2. Within the two V. dahliae races issued from non-tomato hosts, great variations in pathogenicity and aggressiveness were noted, as measured by leaf damage and plant height recorded 60 days post inoculation. All V. dahliae races isolates from tomato were aggressive to highly aggressive on eggplant cv. ‘Bonica F1’ but were non-pathogenic towards pepper cv. ‘Chergui F1’.
Madhurababu Kunta (USA), Hongqin Miao (China), Toshiro Shigaki, Jose Luis Perez, Mani Skaria (USA) Ganoderma Infecting Citrus in Texas is a Unique Taxon within the G. lucidum Complex: Evidence from the ITS Region (pp 62-64)
Short Communication: Decline and death of citrus trees on sour orange rootstock in Texas is attributed to a member of the Ganoderma lucidum species-complex. Internal transcribed spacer (ITS) of this taxon showed high identities with several Ganoderma nucleotide sequences previously deposited in the National Center for Biotechnology Information (NCBI) GenBank database. The putative ITS phylogeny generated by comparison with twenty closest Ganoderma species showed that a Texas citrus Ganoderma isolate described here is a unique taxon. ITS sequence can be used for rapid clinical diagnosis of this fungus in multiple crops.
Adeline Su Yien Ting, Tan Siew Hoon, Wai Mei Kay, Chee Li Ern (Malaysia) Characterization of Actinobacteria with Antifungal Potential against Fusarium Crown-Rot Pathogen (pp 65-69)
Research Note: Actinobacteria isolates recovered from soil were investigated for their antifungal properties towards Fusarium spp., the causal agent of Fusarium crown rot disease in bananas. We isolated 33 isolates of Actinobacteria from soil and established that all of the isolates produced antifungal compounds able to inhibit the growth of Fusarium spp. in vitro,although with varying degrees of inhibition. Characterization based on cultural, morphological and biochemical characteristics revealed that 16 different genera of Actinobacteria were isolated. The most dominant genus from our diverse sample pool was Streptomyces with 8 isolates, followed by Nocardiopsis, Actinomadura and Saccharothrix, with 4, 3 and 3 isolates, respectively. Isolates with the most antifungal potential belonged to Streptomyces spp. (isolate 20C1), followed by another isolate of Streptomyces spp. (isolate 15Br2) and Saccharomonospora spp. (isolate 21A2), with 96, 90 and 78% inhibition, respectively. These isolates can be potentially developed as biocontrol agents for field application as crown rot disease is initiated in the field. Future applications may also include investigations into harvesting and applying the metabolites produced by these isolates as biofungicide introduced at the post-harvest stage.
Ankala Basappa Vedamurthy, Subbaramiah Sundar Rajan, Birur Eshwarappa Ravi Shankara, Davangere Shivakumar Sowmya, Pasura Subbaiah Sujan Ganapathy (India) Bioefficacy Studies of Trichoderma viride on Soil-Borne Pathogens (pp 70-74)
Research Note: Trichoderma viride was isolated from the rhizosphere of tomato (Lycopersicon esculentum Mill., cultivar ‘Pusa Ruby’) plants from sick plots in and around Bangalore containing soil-borne fungal pathogens such as Rhizoctonia solani, Sclerotinia sclerotiorum and Fusarium oxysporum which cause damping off, stem rot and wilt, respectively. The efficacy of T. viride was evaluated in vitro and in vivo. The results of in vitro studies showed that T. viride inhibited the mycelial growth of R. solani, S. sclerotiorum and F. oxysporum on PDA. F. oxysporum was greatly inhibited by T. viride in the in vitro studies compared to S. sclerotiorum and R. solani. So, the efficacy of T. viride in the management of wilt was carried out in a pot experiment. In the evaluation of the efficacy of T. viride in in vivo, seed treatment followed by soil application with the T. viride effectively increased the shoot and root length and shoot and root weight and reduced the wilt caused by F. oxysporum compared to untreated controls in tomato.