Sandrine Eveillard

Tomato Flower Abnormalities Induced by Stolbur Phytoplasma Infection Are Associated with Changes of Expression of Floral Development Genes

Tomato (Lycopersicon esculentum cv. Micro-Tom) plants infected by the stolbur phytoplasma (isolate PO) display floral abnormalities, including sepal hypertrophy, virescence, phyllody, and aborted reproductive organs, which are reminiscent of those observed in Arabidopsis thaliana mutants affected in flower development genes. Semiquantitative reverse transcription-polymerase chain reaction and in situ RNA hybridization were used to compare expressions of meristem and flower development genes in healthy and stolbur phytoplasma-infected tomatoes. In infected plants, FALSIFLORA (FA), controlling the identity of the inflorescence meristem, was up-regulated, whereas LeWUSCHEL (LeWUS) and LeCLAVATA1 (LeCLV1), regulating the meristem development, and LeDEFICIENS (LeDEF), responsible for the organ (petals and stamens) identity within the flower, were down-regulated regardless of the development stage of the flower bud. In contrast, expression of TAG1, which regulates stamen and carpel identities and negatively controls LeWUS, was up-regulated at the early stages and down-regulated at the late stages. In situ RNA hybridization analyses revealed that TAG1 transcripts were restricted to the same floral meristem territories in healthy and infected tomatoes, indicating that tissue-specific expression of TAG1 was not affected by the stolbur phytoplasma infection. Taken together, these data indicate that flower malformations of stolbur phytoplasma-infected tomatoes are associated with early changes in the expression of key flower development genes. The possible mechanisms by which the multiplication of stolbur phytoplasma in tomato sieve tubes deregulates floral development are discussed.

Partial Chromosome Sequence of Spiroplasma citri Reveals Extensive Viral Invasion and Important Gene Decay

ABSTRACT The assembly of 20,000 sequencing reads obtained from shotgun and chromosome-specific libraries of the Spiroplasma citri genome yielded 77 chromosomal contigs totaling 1,674 kbp (92%) of the 1,820-kbp chromosome. The largest chromosomal contigs were positioned on the physical and genetic maps constructed from pulsed-field gel electrophoresis and Southern blot hybridizations. Thirty-eight contigs were annotated, resulting in 1,908 predicted coding sequences (CDS) representing an overall coding density of only 74%. Cellular processes, cell metabolism, and structural-element CDS account for 29% of the coding capacity, CDS of external origin such as viruses and mobile elements account for 24% of the coding capacity, and CDS of unknown function account for 47% of the coding capacity. Among these, 21% of the CDS group into 63 paralog families. The organization of these paralogs into conserved blocks suggests that they represent potential mobile units. Phage-related sequences were particularly abundant and include plectrovirus SpV1 and SVGII3 and lambda-like SpV2 sequences. Sixty-nine copies of transposases belonging to four insertion sequence (IS) families (IS30, IS481, IS3, and ISNCY) were detected. Similarity analyses showed that 21% of chromosomal CDS were truncated compared to their bacterial orthologs. Transmembrane domains, including signal peptides, were predicted for 599 CDS, of which 58 were putative lipoproteins. S. citri has a Sec-dependent protein export pathway. Eighty-four CDS were assigned to transport, such as phosphoenolpyruvate phosphotransferase systems (PTS), the ATP binding cassette (ABC), and other transporters. Besides glycolytic and ATP synthesis pathways, it is noteworthy that S. citri possesses a nearly complete pathway for the biosynthesis of a terpenoid.

Molecular study of the effect of exogenous phytohormones application in “stolbur” phytoplasma infected tomatoes on disease development

‘Candidatus Phytoplasma solani’ alters developmental processes in tomato and cause malformations of both vegetative and reproductive organs. The study was performed to determine the effect of application of phytohormones on “stolbur” strain PO infected tomato and to assess whether application of BTH, an analogue of salicylic acid (SA), jasmonic acid (JA) and ethylen (Et) could protect tomato against the “stolbur” disease development. Expression of SA- and JA-dependent marker genes was also studied in tomato by qRT-PCR. Results indicated that the SA-mediated defence response delay the “stolbur” phytoplasma strain PO multiplication in contrast to the JA and Et-dependent defence pathways.

Contrasting Susceptibilities to Flavescence Dorée in Vitis vinifera, Rootstocks and Wild Vitis Species

Flavescence dorée (FD) is a quarantine disease of grapevine, involving interactions between the plants, leafhopper vectors, and FD phytoplasma. Characterizing the susceptibility of vine varieties could limit disease propagation. After extensive surveys in vineyards, we showed that Cabernet Sauvignon (CS) is highly susceptible, with a high proportion of symptomatic branches and phytoplasma titers, in contrast to Merlot (M). Localized insect transmissions and grafting showed that phytoplasma circulate in the whole plant in the CS cultivar, but in M they are restricted to the transmission point. Insect-mediated transmission under high confinement mimicking natural conditions confirmed these phenotypes and allowed the classification of 28 Vitis accessions into three distinct categories, according to the percentage of infected plants and their phytoplasma titers. Reduced symptoms, low phytoplasma titers, and low percentages of infected plants were found to be associated in the Vitis vinifera cultivars tested. Interestingly, the low susceptibility of M was observed for one of its parents, i.e., Magdeleine Noire des Charentes. Rootstocks and their Vitis parents, although having high percentages of infected plants and intermediate to high phytoplasma titers, shared a symptomless response. This is troubling, because rootstocks can constitute a silent reservoir of contamination in mother plants or when they grow wild nearby vineyards. Altogether, data suggest distribution of genetic traits within the Vitis genus involved in insect-mediated phytoplasma transmission, multiplication, circulation, and symptom development.