The Arabidopsis ELP3/ELO3 and ELP4/ELO1 genes enhance disease resistance in Fragaria vesca L.

BACKGROUND: Plant immune response is associated with a large-scale transcriptional reprogramming, which is regulated by numerous transcription regulators such as the Elongator complex. Elongator is a multitasking protein complex involved in diverse cellular processes, including histone modification, DNA methylation, and tRNA modification. In recent years, Elongator is emerging as a key regulator of plant immune responses. However, characterization of Elongator's function in plant immunity has been conducted only in the model plant Arabidopsis thaliana. It is thus unclear whether Elongator's role in plant immunity is conserved in higher plants. The objective of this study is to characterize transgenic woodland strawberry (Fragaria vesca L.) overexpressing the Arabidopsis Elongator (AtELP) genes, AtELP3 and AtELP4, and to determine whether F. vesca carries a functional Elongator complex. METHODS: Transgenic F. vesca and Arabidopsis plants were produced via Agrobacterium-mediated genetic transformation and characterized by morphology, PCR, real-time quantitative PCR, and disease resistance test. The Student's t test was used to analyze the data. RESULTS: Overexpression of AtELP3 and AtELP4 in F. vesca impacts plant growth and development and confers enhanced resistance to anthracnose crown rot, powdery mildew, and angular leaf spot, which are caused by the hemibiotrophic fungal pathogen Colletotrichum gloeosporioides, the obligate biotrophic fungal pathogen Podosphaera aphanis, and the hemibiotrophic bacterial pathogen Xanthomonas fragariae, respectively. Moreover, the F. vesca genome encodes all six Elongator subunits by single-copy genes with the exception of FvELP4, which is encoded by two homologous genes, FvELP4-1 and FvELP4-2. We show that FvELP4-1 complemented the Arabidopsis Atelp4/elo1-1 mutant, indicating that FvELP4 is biologically functional. CONCLUSIONS: This is the first report on overexpression of Elongator genes in plants. Our results indicate that the function of Elongator in plant immunity is most likely conserved in F. vesca and suggest that Elongator genes may hold potential for helping mitigate disease severity and reduce the use of fungicides in strawberry industry.

All five host-range variants of Xanthomonas citri carry one pthA homolog with 17.5 repeats that determines pathogenicity on citrus, but none determine host-range variation.

Citrus canker disease is caused by five groups of Xanthomonas citri strains that are distinguished primarily by host range: three from Asia (A, A*, and A(w)) and two that form a phylogenetically distinct clade and originated in South America (B and C). Every X. citri strain carries multiple DNA fragments that hybridize with pthA, which is essential for the pathogenicity of wide-host-range X. citri group A strain 3213. DNA fragments that hybridized with pthA were cloned from a representative strain from all five groups. Each strain carried one and only one pthA homolog that functionally complemented a knockout mutation of pthA in 3213. Every complementing homolog was of identical size to pthA and carried 17.5 nearly identical, direct tandem repeats, including three new genes from narrow-host-range groups C (pthC), A(w) (pthAW), and A* (pthA*). Every noncomplementing paralog was of a different size; one of these was sequenced from group A* (pthA*-2) and was found to have an intact promoter and full-length reading frame but with 15.5 repeats. None of the complementing homologs nor any of the noncomplementing paralogs conferred avirulence to 3213 on grapefruit or suppressed avirulence of a group A* strain on grapefruit. A knockout mutation of pthC in a group C strain resulted in loss of pathogenicity on lime, but the strain was unaffected in ability to elicit an HR on grapefruit. This pthC- mutant was fully complemented by pthA, pthB, or pthC. Analysis of the predicted amino-acid sequences of all functional pthA homologs and nonfunctional paralogs indicated that the specific sequence of the 17th repeat may be essential for pathogenicity of X. citri on citrus.

Xanthomonas citri: breaking the surface.

UNLABELLED: SUMMARY Taxonomy: Bacteria; Proteobacteria, Gammaproteobacteria; Xanthomonadales; Xanthomonadaceae, Xanthomonas. Microbiological properties: Gram-negative, obligately aerobic, straight rods, motile by a single polar flagellum, yellow pigment. Related species: X. campestris, X. axonopodis, X. oryzae, X. albilineans. HOST RANGE: Affects Rutaceous plants, primarily Citrus spp., Fortunella spp., and Poncirus spp., world-wide. Quarantined pathogen in many countries. Economically important hosts are cultivated orange, grapefruit, lime, lemon, pomelo and citrus rootstock. Disease symptoms: On leaves, first appearance is as oily looking, 2-10 mm, similarly sized, circular spots, usually on the abaxial surface. On leaves, stems, thorns and fruit, circular lesions become raised and blister-like, growing into white or yellow spongy pustules. These pustules then darken and thicken into a light tan to brown corky canker, which is rough to the touch. On stems, pustules may coalesce to split the epidermis along the stem length, and occasionally girdling of young stems may occur. Older lesions on leaves and fruit tend to have more elevated margins and are at times surrounded by a yellow chlorotic halo (that may disappear) and a sunken centre. Sunken craters are especially noticeable on fruit, but the lesions do not penetrate far into the rind. Defoliation and premature abscission of affected fruit occurs on heavily infected trees. USEFUL WEBSITES: ;