Physiology

AbstractIn 2010, cabbages (Brassica oleracea L.) showing symptoms of proliferated axillary buds, crinkled leaves and plant stunting with shortened internodes typical to phytoplasma infection were found in a breeding facility in Beijing, China. Three symptomatic plants and one symptomless plant were collected, and total DNA was extracted from the midrib tissue and the flowers. With phytoplasma universal primers R16F2n/R16R2, a special fragment of 1247 bp (16S rDNA) was obtained from all three symptomatic cabbage plants, but not from the one symptomless cabbage plant. The 16S rDNA sequence showed 99% similarity with the homologous genes of the aster yellows group phytoplasma (16SrI group), and the phytoplasma was designed as CWBp‐BJ. Phylogenetic and computer‐simulated restriction fragment length polymorphism (RFLP) analysis of the 16S rDNA gene revealed that CWBp‐BJ belongs to subgroup 16SrI‐B. This is the first report of a phytoplasma associated with cabbage witches’‐broom in China.

AbstractWe report the detection of phytoplasmas in Picea abies, Picea glauca and Picea pungens trees with witches’ brooms and other growth abnormalities and also in symptomless trees. Phytoplasmas were detected in c. 25% of the tested plants by polymerase chain reaction using phytoplasma universal P1/P7 followed by R16F2n/R16R2 primer pairs. The phytoplasmas were classified as members of the phylogenetic groups: 16SrI, ‘Ca. Phytoplasma asteris’; 16SrIII, X‐disease phytoplasma group; and 16SrXXI, ‘Ca. Phytoplasma pini’.

AbstractThe bacterium Candidatus Liberibacter asiaticus (Las) has been strongly associated with huanglongbing, or citrus greening, which is one of the most devastating citrus diseases worldwide. Las is transmitted by the Asian citrus psyllid (ACP) Diaphorina citri (Hemiptera: Psyllidae) in a persistent manner, but its interactions with the psyllid vector, particularly at the organ and cellular levels, are poorly understood. We have tested several fluorescence in situ hybridization (FISH) protocols and three nucleic acid probes for the localization of Las in haemolymph smears and dissected organs of ACP adults that fed on Las‐infected plants in the field or laboratory and in sections from Las‐infected citrus leaves. Las was detected by FISH and confocal laser scanning microscopy in the filter chamber, midgut, Malpighian tubules, haemolymph, salivary glands, ovaries and in muscle and fat tissues of ACP that acquired Las from infected plants, as well as in the phloem of infected citrus leaves. Las appeared as pleiomorphic bodies or short thin rods that were much more dispersed and individually distinct in citrus leaf phloem and in ACP haemolymph, but more densely aggregated in cells of the salivary glands and other ACP organs and tissues. Our results provide the first in situ demonstration of Las infection in various psyllid organs and tissues and show the near‐systemic infection of ACP by Las.

Citrus Huanglongbing (HLB) has been threatening citrus production worldwide. In this study, a comparative proteomic approach was applied to understand the pathogenic process of HLB in affected sweet orange leaves. Using the isobaric tags for relative and absolute quantification (iTRAQ) technique, we identified 686 unique proteins in the mature leaves of both mock‐inoculated and diseased ‘Madam Vinous' sweet orange plants. Of the identified proteins, 20 and 10 were differentially expressed in leaves with and without symptoms of HLB (fold change > 2.5), respectively, compared with mock‐inoculated controls. Most significantly, upregulated proteins were involved in stress/defense response, such as four miraculin‐like proteins, chitinase, Cu/Zn superoxide dismutase and lipoxygenase. Microarray analysis also showed that stress‐related genes were significantly upregulated at the transcriptional level. For example, remarkable upregulations of miraculin‐like proteins and Cu/Zn superoxide dismutase transcripts were observed. Moreover, the transcriptional patterns of miraculin‐like protein 1 and Cu/Zn superoxide dismutase were examined at different stages of HLB disease development. Combined with the transcriptomic data, the proteomic data can provide an enhanced understanding of citrus stress/defense responses to HLB.