Multiple guests in a single host : interactions across symbiotic and phytopathogenic bacteria in phloem-feeding vectors : a review

Some pathogenic phloem-limited bacteria are a major threat for worldwide agriculture due to the heavy economic losses caused to many high-value crops. These disease agents \u2013 phytoplasmas, spiroplasmas, liberibacters, and Arsenophonus-like bacteria \u2013 are transmitted from plant to plant by phloem-feeding Hemiptera vectors. The associations established among pathogens and vectors result in a complex network of interactions involving also the whole microbial community harboured by the insect host. Interactions among bacteria may be beneficial, competitive, or detrimental for the involved microorganisms, and can dramatically affect the insect vector competence and consequently the spread of diseases. Interference is observed among pathogen strains competing to invade the same vector specimen, causing selective acquisition or transmission. Insect bacterial endosymbionts are another pivotal element of interactions between vectors and phytopathogens, because of their central role in insect life cycles. Some symbionts, either obligate or facultative, were shown to have antagonistic effects on the colonization by plant pathogens, by producing antimicrobial substances, by stimulating the production of antimicrobial substances by insects, or by competing for host infection. In other cases, the mutual exclusion between symbiont and pathogen suggests a possible detrimental influence on phytopathogens displayed by symbiotic bacteria; conversely, examples of microbes enhancing pathogen load are available as well. Whether and how bacterial exchanges occurring in vectors affect the relationship between insects, plants, and phytopathogens is still unresolved, leaving room for many open questions concerning the significance of particular traits of these multitrophic interactions. Such complex interplays may have a serious impact on pathogen spread and control, potentially driving new strategies for the containment of important diseases

Citrus Tristeza Virus

Virus diseases of perennial trees and vines have characteristics not amenable to study using small model annual plants. Unique disease symptoms such as graft incompatibilities and stem pitting cause considerable crop losses. Also, viruses in these long-living plants tend to accumulate complex populations of viruses and strains. Considerable progress has been made in understanding the biology and genetics of Citrus tristeza virus (CTV) and in developing it into a tool for crop protection and improvement. The diseases in tree and vine crops have commonalities for which CTV can be used to develop a baseline. The purpose of this review is to provide a necessary background of systems and reagents developed for CTV that can be used for continued progress in this area and to point out the value of the CTV-citrus system in answering important questions on plant-virus interactions and developing new methods for controlling plant diseases

A novel subspecies of ‘Candidatus Liberibacter africanus’ found on native Teclea gerrardii (Family: Rutaceae) from South Africa

The phloem limited bacterium „Candidatus Liberibacter africanus‟ is associated with citrus greening disease in South Africa. This bacterium has been identified solely from commercial citrus in Africa and the Mascarene islands, and its origin may lie within an indigenous rutaceous host from Africa. Recently, in determining whether alternative hosts of Laf exist amongst the indigenous rutaceous hosts of its triozid vector, Trioza erytreae, three novel subspecies of Laf were identified i.e. „Candidatus Liberibacter africanus subsp. clausenae‟, „Candidatus Liberibacter africanus subsp. vepridis‟ and „Candidatus Liberibacter africanus subsp. zanthoxyli‟ in addition to the formerly identified „Candidatus Liberibacter africanus subsp. capensis‟. The current study expands upon the range of indigenous rutaceous tree species tested for liberibacters closely related to Laf and its subspecies. A collection of 121 samples of Teclea and Oricia species were sampled from Oribi Gorge and Umtamvunu nature reserves in KwaZulu Natal. Total DNA was extracted and the presence of liberibacters from these samples determined using a generic liberibacter TaqMan real-time PCR assay. Liberibacters from positive samples were further characterised through amplification and sequencing of the 16S rRNA, outer-membrane protein (omp) and 50S ribosomal protein L10 (rplJ) genes. A single Teclea gerrardii specimen tested positive for a liberibacter and, through phylogenetic analyses of the three genes sequenced, was shown to be unique, albeit closely related to „Ca. L. africanus‟ and „Ca. L. africanus subsp. zanthoxyli‟. We propose that this newly identified liberibacter be named „Candidatus Liberibacter africanus subsp. tecleae‟.Citrus Research International (CRI), the National Research Foundation-THRIP program (NRF-THRIP) and the Department of Science and Technology (DST)/NRF Centre of Excellence in Tree Health Biotechnology (CTHB).http://link.springer.com/journal/104822018-03-31hb2017Microbiology and Plant Patholog