Concept: Persian lime
Citrus tristeza virus (CTV) is phloem-restricted in natural citrus hosts. The 23 kDa protein (p23) encoded by the virus is an RNA silencing suppressor and a pathogenicity determinant. Expression of p23, or its N-terminal 157 amino acid fragment comprising the zinc-finger and flanking basic motifs, driven by the constitutive 35S promoter of cauliflower mosaic virus incites CTV-like symptoms and other aberrations in transgenic citrus. To better define the role of p23 in CTV pathogenesis, we compared the phenotypes of Mexican limes transformed with p23-derived transgenes from the severe T36 or the mild T317 CTV isolates under the control of the phloem-specific promoter from commelina yellow mottle virus (CoYMV) or the 35S promoter. Expression of the constructs restricted to the phloem incited a phenotype resembling CTV-specific symptoms (vein clearing and necrosis, and stem pitting), but not the non-specific aberrations (like mature leaf epinasty and yellow pinpoints, growth cease and apical necrosis) observed when p23 was ectopically expressed. Furthermore, vein necrosis and stem pitting in Mexican lime appeared specifically associated with p23 from T36. Phloem-specific accumulation of the p23Δ158-209(T36) fragment was sufficient to incite the same anomalies, indicating that the region comprising the N-terminal 157 amino acids of p23 is responsible (at least in part) for the vein clearing, stem pitting and possibly vein corking in this host.
Citrus bacterial canker is a disease that has severe economic impact on citrus industries worldwide and is caused by a few species and pathotypes of Xanthomonas. X. citri subsp. citri strain 306 (XccA306) is a type A (Asiatic) strain with a wide host range, whereas its variant X. citri subsp. citri strain Aw12879 (Xcaw12879, Wellington strain) is restricted to Mexican lime.
Two main types of triploid limes are produced worldwide. The ‘Tahiti’ lime type (Citrus latifolia) is predominant, while the ‘Tanepao’ type (C. aurantiifolia) is produced to a lesser extent. Both types result from natural interspecific hybridization involving a diploid gamete of C. aurantiifolia ‘Mexican’ lime type (itself a direct interspecific C. micrantha × C. medica hybrid). The meiotic behaviour of a doubled-diploid ‘Mexican’ lime, the interspecific micrantha/medica recombination and the resulting diploid gamete structures were analysed to investigate the possibility that ‘Tahiti’ and ‘Tanepao’ varieties are derived from natural interploid hybridization.
Molds are responsible for postharvest spoilage of citrus fruits. The objective of this study was to evaluate the effect of temperature on growth rate and the time to visible growth of Aspergillus niger strains isolated from citrus fruits. The growth of these strains was studied on agar lime medium (AL) at different temperatures, and growth rate was estimated using the Baranyi and Roberts model (Int. J. Food Microbiol. 23:277-294, 1994). The Rosso et al. cardinal model with inflexion (L. Rosso, J. R. Lobry, S. Bajard, and J. P. Flandrois, J. Theor. Biol. 162:447-463, 1993) was used as a secondary model to describe the effect of temperature on growth rate and the lag phase. We hypothesized that the same model could be used to calculate the time for the mycelium to become visible (tv) by substituting the lag phase (1/λ and 1/λopt) with the time to visible colony (1/tv-opt and 1/tv), respectively, in the Rosso et al.
An assay addressed to identify interactions between Citrus dwarfing viroid (CDVd) and Citrus tristeza virus (CTV) showed that viroid titer was enhanced by the co-infecting CTV in Mexican lime but not in Etrog citron. Since CTV encodes three RNA silencing suppressors (RSS), p23, p20 and p25, an assay using transgenic Mexican limes expressing each RSS revealed that p23, and to a lower extent p25, recapitulated the effect observed in coinfections of CTV and CDVd.
The mechanisms determining the host range of Xanthomonas are still undeciphered, despite much interest in their potential roles in the evolution and emergence of plant pathogenic bacteria. Xanthomonas citri pv. citri (Xci) is an interesting model of host specialization because of its pathogenic variants: pathotype A strains infect a wide range of Rutaceous species, whereas pathotype A*/A(W) strains have a host range restricted to Mexican lime (Citrus aurantifolia) and alemow (Citrus macrophylla). Based on a collection of 55 strains representative of Xci worldwide diversity assessed by amplified fragment length polymorphism (AFLP), we investigated the distribution of type III effectors (T3Es) in relation to host range. We examined the presence of 66 T3Es from xanthomonads in Xci and identified a repertoire of 28 effectors, 26 of which were shared by all Xci strains, whereas two (xopAG and xopC1) were present only in some A*/A(W) strains. We found that xopAG (=avrGf1) was present in all A(W) strains, but also in three A* strains genetically distant from A(W) , and that all xopAG-containing strains induced the hypersensitive response (HR) on grapefruit and sweet orange. The analysis of xopAD and xopAG suggested horizontal transfer between X. citri pv. bilvae, another citrus pathogen, and some Xci strains. A strains were genetically less diverse, induced identical phenotypic responses and possessed indistinguishable T3E repertoires. Conversely, A*/A(W) strains exhibited a wider genetic diversity in which clades correlated with geographical origin and T3E repertoire, but not with pathogenicity, according to T3E deletion experiments. Our data outline the importance of taking into account the heterogeneity of Xci A*/A(W) strains when analysing the mechanisms of host specialization.