Witch's Broom Disease of Lime (Candidatus Phytoplasma aurantifolia): Identifying High-Risk Areas by Climatic Mapping

Protein interaction mapping reveals widespread targeting of development-related host transcription factors by phytoplasma effectors

Phytoplasmas are pathogenic bacteria that reprogram plant host development for their own benefit. Previous studies have characterized a few different phytoplasma effector proteins that destabilize specific plant transcription factors. However, these are only a small fraction of the potential effectors used by phytoplasmas; therefore, the molecular mechanisms through which phytoplasmas modulate their hosts require further investigation. To obtain further insights into the phytoplasma infection mechanisms, we generated a protein-protein interaction network between a broad set of phytoplasma effectors and a large, unbiased collection of Arabidopsis thaliana transcription factors and transcriptional regulators. We found widespread, but specific, interactions between phytoplasma effectors and host transcription factors, especially those related to host developmental processes. In particular, many unrelated effectors target specific sets of TCP transcription factors, which regulate plant development and immunity. Comparison with other host-pathogen protein interaction networks shows that phytoplasma effectors have unusual targets, indicating that phytoplasmas have evolved a unique and unusual infection strategy. This study contributes a rich and solid data source that guides further investigations of the functions of individual effectors, as demonstrated for some herein. Moreover, the dataset provides insights into the underlying molecular mechanisms of phytoplasma infection.

Assessing the Impact of Climate Change on the Distribution of Lime (16srii-B) and Alfalfa (16srii-D) Phytoplasma Disease Using MaxEnt

Witches' broom disease has led to major losses in lime and alfalfa production in Oman. This paper identifies bioclimatic variables that contribute to the prediction of distribution of witches' broom disease in current and future climatic scenarios. It also explores the expansion, reduction, or shift in the climatic niche of the distribution of the disease across the different geographical areas of the entire country (309,501 km²). The maximum entropy model (MaxEnt) and geographical information system were used to investigate the potential suitability of habitats for the phytoplasma disease. This study used current (1970-2000) and future projected climatic scenarios (2021-2040, 2041-2060, 2061-2080, and 2081-2100) to model the distribution of phytoplasma for lime trees and alfalfa in Oman. Bioclimatic variables were downloaded from WorldClim with ± 60 occurrence points for lime trees and alfalfa. The area under the curve (AUC) was used to evaluate the model's performance. Quantitatively, the results showed that the mean of the AUC values for lime (16SrII-B) and alfalfa (16SrII-D) future distribution for the periods of 2021-2040, 2041-2060, 2061-2080, and 2081-2100 were rated as "excellent", with the values for the specified time periods being 0.859, 0.900, 0.931, and 0.913 for 16SrII-B; and 0.826, 0.837, 08.58, and 0.894 for 16SrII-D respectively. In addition, this study identified the hotspots and proportions of the areas that are vulnerable under the projected climate-change scenarios. The area of current (2021-2040) highly suitable distribution within the entire country for 16SrII-D was 19474.2 km² (7.1%), while for 16SrII-B, an area of 8835 km² (3.2%) was also highly suitable for the disease distribution. The proportions of these suitable areas are very significant from the available arable land standpoint. Therefore, the results from this study will be of immense benefit and will also bring significant contributions in mapping the areas of witches' broom diseases in Oman. The results will equally aid the development of new strategies and the formulation of agricultural policies and practices in controlling the spread of the disease across Oman.

Association of the 16SrII-D Phytoplasma with African Marigold (Tagetes erecta) Phyllody in Oman

The African marigold (Tagetes erecta L.) is an ornamental, herbaceous plant commonly found in Oman. In 2019, African marigold plants showing phyllody and virescence symptoms, which are typical symptoms of phytoplasmas disease, were found in at Sultan Qaboos University in Oman. Transmission electron microscopy of marigold leaf midrib from phyllody disease plants showed the presence of numerous phytoplasma bodies in the sieve tube of all of the symptomatic samples. DNA was extracted from asymptomatic and symptomatic marigold plant samples, followed by PCR of the 16S ribosomal RNA (rRNA) and imp genes. The PCR assays showed that the symptomatic plants are positive for phytoplasma. The DNA sequence analysis and phylogenetic trees showed that the 16S rDNA and imp gene sequences from all marigold phyllody strains shared 100% sequence identity to 16SrII-D subgroup sequences in the GenBank. This is the first report of a phytoplasma of the 16SrII-D subgroup associated with the African marigold (T. erecta) worldwide.

Bacterial Vector-Borne Plant Diseases: Unanswered Questions and Future Directions

Vector-borne plant diseases have significant ecological and economic impacts, affecting farm profitability and forest composition throughout the world. Bacterial vector-borne pathogens have evolved sophisticated strategies to interact with their hemipteran insect vectors and plant hosts. These pathogens reside in plant vascular tissue, and their study represents an excellent opportunity to uncover novel biological mechanisms regulating intracellular pathogenesis and to contribute to the control of some of the world's most invasive emerging diseases. In this perspective, we highlight recent advances and major unanswered questions in the realm of bacterial vector-borne disease, focusing on liberibacters, phytoplasmas, spiroplasmas, and Xylella fastidiosa.