Phytoplasmal infection derails genetically preprogrammed meristem fate and alters plant architecture.

In the life cycle of higher plants, it is the fate of meristem cells that determines the pattern of growth and development, and therefore plant morphotype and fertility. Floral transition, the turning point from vegetative growth to reproductive development, is achieved via genetically programmed sequential changes in meristem fate from vegetative to inflorescence, and to floral, leading to flower formation and eventual seed production. The transition is rarely reversible once initiated. In this communication, we report that a bacterial infection can derail the genetically programmed fate of meristem cells, thereby drastically altering the growth pattern of the host plant. We identified four characteristic symptoms in tomato plants infected with a cell wall-less bacterium, phytoplasma. The symptoms are a manifestation of the pathogen-induced alterations of growth pattern, whereas each symptom corresponds to a distinct phase in the derailment of shoot apical meristem fate. The phases include premature floral meristem termination, suppressed floral meristem initiation, delayed conversion of vegetative meristem to inflorescence meristem, and repetitive initiation and outgrowth of lateral vegetative meristems. We further found that the pathogen-induced alterations of growth pattern were correlated with transcriptional reprogramming of key meristem switching genes. Our findings open an avenue toward understanding pathological alterations in patterns of plant growth and development, thus aiding identification of molecular targets for disease control and symptom alleviation. The findings also provide insights for understanding stem cell pluripotency and raise a tantalizing possibility for using phytoplasma as a tool to dissect the course of normal plant development and to modify plant morphogenesis by manipulating meristem fate.

Evaluation of the DNA-dependent RNA polymerase ß-subunit gene (rpoB) for phytoplasma classification and phylogeny.

Phytoplasmas are classified into 16Sr groups and subgroups and 'Candidatus Phytoplasma' species, largely or entirely based on analysis of 16S rRNA gene sequences. Yet, distinctions among closely related 'Ca. Phytoplasma' species and strains based on 16S rRNA genes alone have limitations imposed by the high degree of rRNA nucleotide sequence conservation across diverse phytoplasma lineages and by the presence in a phytoplasma genome of two, sometimes sequence-heterogeneous, copies of the 16S rRNA gene. Since the DNA-dependent RNA polymerase (DpRp) ß-subunit gene (rpoB) exists as a single copy in the phytoplasma genome, we explored the use of rpoB for phytoplasma classification and phylogenetic analysis. We sequenced a clover phyllody (CPh) phytoplasma genetic locus containing ribosomal protein genes, a complete rpoB gene and a partial rpoC gene encoding the ß'-subunit of DpRp. Primers and reaction conditions were designed for PCR-mediated amplification of rpoB gene fragments from diverse phytoplasmas. The rpoB gene sequences from phytoplasmas classified in groups 16SrI, 16SrII, 16SrIII, 16SrX and 16SrXII were subjected to sequence similarity and phylogenetic analyses. The rpoB gene sequences were more variable than 16S rRNA gene sequences, more clearly distinguishing among phytoplasma lineages. Phylogenetic trees based on 16S rRNA and rpoB gene sequences had similar topologies, and branch lengths in the rpoB tree facilitated distinctions among closely related phytoplasmas. Virtual RFLP analysis of rpoB gene sequences also improved distinctions among closely related lineages. The results indicate that the rpoB gene provides a useful additional marker for phytoplasma classification that should facilitate studies of disease aetiology and epidemiology.

'Candidatus Phytoplasma solani', a novel taxon associated with stolbur- and bois noir-related diseases of plants.

Phytoplasmas classified in group 16SrXII infect a wide range of plants and are transmitted by polyphagous planthoppers of the family Cixiidae. Based on 16S rRNA gene sequence identity and biological properties, group 16SrXII encompasses several species, including 'Candidatus Phytoplasma australiense', 'Candidatus Phytoplasma japonicum' and 'Candidatus Phytoplasma fragariae'. Other group 16SrXII phytoplasma strains are associated with stolbur disease in wild and cultivated herbaceous and woody plants and with bois noir disease in grapevines (Vitis vinifera L.). Such latter strains have been informally proposed to represent a separate species, 'Candidatus Phytoplasma solani', but a formal description of this taxon has not previously been published. In the present work, stolbur disease strain STOL11 (STOL) was distinguished from reference strains of previously described species of the 'Candidatus Phytoplasma' genus based on 16S rRNA gene sequence similarity and a unique signature sequence in the 16S rRNA gene. Other stolbur- and bois noir-associated ('Ca. Phytoplasma solani') strains shared >99 % 16S rRNA gene sequence similarity with strain STOL11 and contained the signature sequence. 'Ca. Phytoplasma solani' is the only phytoplasma known to be transmitted by Hyalesthes obsoletus. Insect vectorship and molecular characteristics are consistent with the concept that diverse 'Ca. Phytoplasma solani' strains share common properties and represent an ecologically distinct gene pool. Phylogenetic analyses of 16S rRNA, tuf, secY and rplV-rpsC gene sequences supported this view and yielded congruent trees in which 'Ca. Phytoplasma solani' strains formed, within the group 16SrXII clade, a monophyletic subclade that was most closely related to, but distinct from, that of 'Ca. Phytoplasma australiense'-related strains. Based on distinct molecular and biological properties, stolbur- and bois noir-associated strains are proposed to represent a novel species level taxon, 'Ca. Phytoplasma solani'; STOL11 is designated the reference strain.