Primers designed from sequences of the gene encoding the elongation factor Tu (tuf gene) of several culturable mollicutes amplified most of the tuf gene from phytoplasmas of the aster yellows, stolbur and Xdisease groups. About 85% of the tuf gene from two aster yellows strains and a tomato stolbur phytoplasma was sequenced. The nucleotide sequence similarity between these related phytoplasmas was between 87.8 and 97G0/o, whereas the homology with other mollicutes was 663-7207 %. The similarity of the deduced amino acid sequence was significantly higher, ranging from 960 to 994% among the phytoplasmas and 785% to 833% between phytoplasmas and the culturable mollicutes examined. From the nucleotide sequences of the phytoplasma strains, two pairs of primers were designed; one amplified the phytoplasmas of most phylogenetic groups that were established, the other was specific for the aster yellows and stolbur groups. The phytoplasmas of the various groups that were amplified could be distinguished by RFLP analysis using Sau3A1, AIul and Hpall. The aster yellows group could be divided into five Sau3Al RFLP groups. These results showed that the tuf gene has the potential to be used to differentiate and classify phytoplasmas. Southern blot analysis revealed that the tuf gene is present as a single copy.1 Keywords : phytoplasmas, differentiation, classification, elongation factor, PCR INTRODUCTIONPhytoplasmas, formerly called mycoplasma-like organisms (MLOs), are plant-pathogenic prokaryotes of the class Mollicutes that cannot be cultured under axenic conditions. The inability to culture phytoplasmas has made it difficult to characterize these pathogens. Only recently, by the introduction of molecular methods into plant mycoplasmology, has it become possible to determine the phylogenetic and taxonomic relationships of the phytoplasmas to each other and to other prokaryotes. Currently, classification is based on sequence analysis of the 16s rRNA gene (Lim & Sears, 1989 ;Kuske & Kirkpatrick, 1992 ;Namba et al., 1993 ;Gundersen et al., 1994; Schneider et al., 1995b). This gene is present in all prokaryotes and the conserved and variable regions make it suitable for phylogenetic classifications. Other phytoplasma genes or DNA regions that have been used for classificationThe GenBank accession numbers for the sequences reported in this paper are L46368 ( M Y ) , L46369 (KV) and L46370 (STOLF).are the ribosomal protein genes rpl2.2 and rps3 (Lim & Sears, 1992;Gundersen et al., 1994; Toth et al., 1994) and the 16S/23S rRNA spacer region Schneider et al., 1995b). The latter are considerably more variable than the 16s rRNA gene but phylogenetic analysis of all three sequence categories has resulted in a similar classification of the phytoplasmas in relation to each other and to other mollicutes (Lim & Sears, 1989Kuske & Kirkpatrick, 1992;Namba et al., 1993;Gundersen et al., 1994; Toth et al., 1994).Although the relationship between the phylogenetic classification of the phytoplasmas and pathological and other biological tra...

The chromosome sequence of “Candidatus Phytoplasma australiense” (subgroup tuf-Australia I; rp-A), associated with dieback in papaya, Australian grapevine yellows in grapevine, and several other important plant diseases, was determined. The circular chromosome is represented by 879,324 nucleotides, a GC content of 27%, and 839 protein-coding genes. Five hundred two of these protein-coding genes were functionally assigned, while 337 genes were hypothetical proteins with unknown function. Potential mobile units (PMUs) containing clusters of DNA repeats comprised 12.1% of the genome. These PMUs encoded genes involved in DNA replication, repair, and recombination; nucleotide transport and metabolism; translation; and ribosomal structure. Elements with similarities to phage integrases found in these mobile units were difficult to classify, as they were similar to both insertion sequences and bacteriophages. Comparative analysis of “Ca. Phytoplasma australiense” with “Ca. Phytoplasma asteris” strains OY-M and AY-WB showed that the gene order was more conserved between the closely related “Ca. Phytoplasma asteris” strains than to “Ca. Phytoplasma australiense.” Differences observed between “Ca. Phytoplasma australiense” and “Ca. Phytoplasma asteris” strains included the chromosome size (18,693 bp larger than OY-M), a larger number of genes with assigned function, and hypothetical proteins with unknown function.

Hypolithic microbes, primarily cyanobacteria, inhabit the highly specialized microhabitats under translucent rocks in extreme environments. Here we report findings from hypolithic cyanobacteria found under three types of translucent rocks (quartz, prehnite, agate) in a semiarid region of tropical Australia. We investigated the photosynthetic responses of the cyanobacterial communities to light, temperature and moisture in the laboratory, and we measured the microclimatic variables of temperature and soil moisture under rocks in the field over an annual cycle. We also used molecular techniques to explore the diversity of hypolithic cyanobacteria in this community and their phylogenetic relationships within the context of hypolithic cyanobacteria from other continents. Based on the laboratory experiments, photosynthetic activity required a minimum soil moisture of 15% (by mass). Peak photosynthetic activity occurred between approximately 8 degrees C and 42 degrees C, though some photosynthesis occurred between -1 degrees C and 51 degrees C. Maximum photosynthesis rates also occurred at light levels of approximately 150-550 micromol m(-2) s(-1). We used the field microclimatic data in conjunction with these measurements of photosynthetic efficiency to estimate the amount of time the hypolithic cyanobacteria could be photosynthetically active in the field. Based on these data, we estimated that conditions were appropriate for photosynthetic activity for approximately 942 h (approximately 75 days) during the year. The hypolithic cyanobacteria community under quartz, prehnite and agate rocks was quite diverse both within and between rock types. We identified 115 operational taxonomic units (OTUs), with each rock hosting 8-24 OTUs. A third of the cyanobacteria OTUs from northern Australia grouped with Chroococcidiopsis, a genus that has been identified from hypolithic and endolithic communities from the Gobi, Mojave, Atacama and Antarctic deserts. Several OTUs identified from northern Australia have not been reported to be associated with hypolithic communities previously.

The influence of green ants, Oecophylla smaragdina (Fabricius), on insect pests of cashew, Anacardium occidentale Linnaeus, in tropical northern Australia was investigated using field surveys and field observations. Oecophylla smaragdina was abundant in the native vegetation of the area, and it was a dominant predator when found in cashew plantations. Oecophylla smaragdina significantly reduced the numbers of the four most important species of insect pests (Helopeltis pernicialis Stonedahl, Malipatil & Houston, Penicillaria jocosatrix (Guenee), Amblypelta lutescens (Distant) and Anigraea ochrobasis Hampson) on cashew trees, and trees with higher numbers of O. smaragdina produced higher quality nuts than trees with fewer numbers of O. smaragdina. Other ant species also reduced pest numbers, but not as much as O. smaragdina. The possibility of using O. smaragdina to control insect pests of cashew in the future is discussed.

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