Publications

4475

Phy.to.plas'ma. Gr. neut. n.phyton, a plant; Gr. neut. n.plasmasomething formed or molded, a form; N.L. neut. n.Phytoplasma, a form from a plant.Tenericutes / Mollicutes / Acholeplasmatales / Incertae Sedis ‐ Family II /CandidatusPhytoplasmaThe generic name ‘CandidatusPhytoplasma’ has been adopted by specialists to refer to a monophyletic clade of wall‐less phytopathogenic bacteria affiliated with the orderAcholeplasmatalesin the classMollicutes. These small (<1 µm) pleomorphic cells occur within the nutritionally‐rich phloem sap and sieve elements of plants, and in the gut or hemolymph of insects that feed on plants. Sustained culture in cell‐free media has not yet been demonstrated for any phytoplasma, and a recent report of successful axenic culture remains to be independently replicated. Reference strains are maintained in plants by periodic graft inoculation. Sequence analysis of PCR‐amplified rDNA, plus considerations of the plant host range and vector species, provide a basis for phytoplasma strain identification and assignment to phylogenetic groups. Hundreds of plant species are susceptible to infection, with signs including discoloration, stunting, virescence, phyllody, proliferation of flowers or shoots, or sterility. The necessity for strict quarantine regulations to control the spread of phytoplasmas among important plants such as fruit trees, palms, and grapevines illustrates the importance of accurate identification and nomenclature for these organisms.

‘Candidatus Liberibacter solanacearum’ (CLso) haplotype C is associated with disease in carrots and transmitted by the carrot psyllid Trioza apicalis. To identify possible other sources and vectors of this pathogen in Finland, samples were taken of wild plants within and near the carrot fields, the psyllids feeding on these plants, parsnips growing next to carrots, and carrot seeds. For analyzing the genotype of the CLso-positive samples, a multilocus sequence typing (MLST) scheme was developed. CLso haplotype C was detected in 11% of the T. anthrisci samples, in 35% of the Anthriscus sylvestris plants with discoloration, and in parsnips showing leaf discoloration. MLST revealed that the CLso in T. anthrisci and most A. sylvestris plants represent different strains than the bacteria found in T. apicalis and the cultivated plants. CLso haplotype D was detected in 2 of the 34 carrot seed lots tested, but was not detected in the plants grown from these seeds. Phylogenetic analysis by unweighted-pair group method with arithmetic means clustering suggested that haplotype D is more closely related to haplotype A than to C. A novel, sixth haplotype of CLso, most closely related to A and D, was found in the psyllid T. urticae and stinging nettle (Urtica dioica, Urticaceae), and named haplotype U.

AbstractOf late, the presence of Candidatus Phytoplasma trifolii was reported as a serious threat to the pepper crop in Zacatecas, México; therefore, asymptomatic and symptomatic pepper plants were collected from a commercial field among three samplings after the fruit set stage was reached. Total DNA was extracted using the CTAB‐based method and tested for phytoplasma using a nested PCR assay, followed by a BLAST, and restriction fragment length polymorphism (RFLP) analysis of 16S rDNA sequences, which confirmed the presence of phytoplasma group 16SrVI, “Candidatus Phytoplasma trifolii” in the symptomatic plants. As the metabolic pathways of pathogen‐infected plants tend to change, resulting in a biochemical differentiation with the noninfected plants, the polyphenolic compound concentrations were quantified from the vegetative tissues (root, stem, leaves and developed fruit/big bud) and were analysed based on a principal component analysis (PCA). Results revealed that, in general, plants tend to a progressive increase in total phenols, flavonoids, condensed tannins and anthocyanins related to the plants exposure to “Ca. P. trifolii” infection, and PCA demonstrated that almost 90% of the observed variance was explained by the first two components. Hence, the phenolic content of the plants increases as a response of the defence mechanism, which reflects its condition and resistance.

Huanglongbing ( HLB ), caused by ‘ Candidatus Liberibacter asiaticus’ (Las), is a devastating disease of citrus trees in Florida. Previous work showed that the rootstock cultivar Cleopatra mandarin ( Citrus reticulata ) has a higher population of Las in roots than Swingle citrumelo ( C. paradisi  ×  Poncirus trifoliata ). Las reduced fibrous root biomass and sucrose content in Cleopatra mandarin more than in Swingle citrumelo. To understand the mechanisms for susceptibility to Las infection, sucrose and hormone metabolism status were evaluated in Cleopatra mandarin and Swingle citrumelo. In fibrous roots of Cleopatra mandarin, higher expression of genes related to sucrose cleavage was consistent with lower sucrose content compared to noninoculated seedlings at 5 weeks post‐root trimming (wpt). In fibrous roots of Swingle citrumelo, both sucrose content and gene expression related to sucrose cleavage were less disrupted by Las infection compared to Cleopatra mandarin at 5 wpt. Genes associated with salicylic acid ( SA ), ethylene ( ET ) and abscisic acid ( ABA ) synthesis, and ABA signalling, phospholipases D ( PLD ), and phospholipase A2 ( PLA 2) were activated by Las infection at 5 wpt in Cleopatra mandarin. Expression of downstream effectors of SA , i.e. NPR 1, WRKY 70 and PR 1, did not change in Cleopatra mandarin, suggesting inhibition of the response to SA by the elevation of ABA , ET and PLD . In contrast, the up‐regulation of PR 1, lower response of sucrose metabolism genes and down‐regulation of biosynthesis of phytohormones indicates that Swingle citrumelo activates a more effective defence against this biotrophic pathogen than Cleopatra mandarin.