Poinsettia is a colorful blooming potted plant, in the Euphorbia family. It is a popular holiday plant especially used for decorations during the winter holidays in Europe and American countries. Poinsettia is more attractive because of its colorful bracts (leaves). Different colored poinsettias in the market are from different varieties. The Poinsettia flower is made up of bracts that look like a petal, but in the center, there are tiny yellow flowers called cyathia. The potted poinsettia has more branches and it grows like a small bush and it is an important feature in the market. They are usually produced through cutting propagation of the axial bud or terminal bud of vegetative stock plants. It is necessary to treat the plant with chemicals 6-7 times. Today all the commercial poinsettia varieties are planted with free branching phenotypes. The secret of this free branching is a result of the infection by phytopathogens known as Phytoplasma.
Phytoplasmas are a group of prokaryotes which are very small bacteria without a cell wall but covered by a membranous envelope. Phytopathogenic studies of various genes of phytoplasma suggest that they are descended from walled-bacteria in the Bacillus/Clostridium group. They are less than 1µm in diameter with a reduced genome greatly in size. Therefore, they lack enzymes needed for biosynthetic pathways. Thus, what should they do to obtain the necessary compounds? All known phytoplasmas infect plants and transmit through insect vectors and obtain necessary compounds from plants and vectors. Phytoplasmas are phloem-limited pathogens. In plants phytoplasma mainly lives in the sieve cells of phloem tissues. Therefore, phytoplasma transmission has been confirmed only through phloem-feeding insects. Phloem-feeding insects such as leafhoppers and planthoppers of the order Hemiptera are identified as vectors. Vectors acquire phytoplasmas passively during feeding and have a long time period of acquisition during feeding time to acquire sufficient phytoplasmas to establish in the vector body. It can take up to nearly three weeks. Once phytoplasmas are established, they will be found in most organs of the infected vector. Phytoplasma replicates in the salivary glands of vector insects. In addition, phytoplasma can be transmitted through vegetative propagation.
Phytoplasma diseases spread mainly in tropical and sub-tropical countries. The presence of phytoplasma is associated with a wide range of symptoms. Phyllody is the development of leaf-like growth in place of normal flowers. Some plants develop green color in place of normal flower color (green flowers) called virescence and witches’ broom is an abnormal excessive proliferation of axillary shoots resulting in a broom-like growth. The other characteristic symptom is little leaves, the development of abnormally small leaves.
The proliferation of axillary shoots is useful in the production of poinsettia. Witches’ broom symptoms are shown at the infection of poinsettia. The phytoplasma associated with poinsettia branching belonging to group 16SrIII-H produces more auxiliary shoots that have more than one coloured shoot with flowers. Therefore, some symptoms of phytoplasma are commercially important except for their negative impacts.
References
Pondrella, M.; Caprara, L.; Bellardi, M.G.; Bertaccini, A., Roll of Different Phytoplasma in Inducing Poinsettia Branching. Proc.10th IS Virus Disease Ornamentals, 2002, pp 170-176.
Assunta, B., Plants and Phytoplasma: When bacteria modify Plants. MDPI 2022, 11
Hogenhout, S.A., Plant Pathogen – Minor (Phytoplasma). Elsevier 2009
Kumari, S.; Nagendran, K.; Bahadur Rai, A.; Singh, B.; Rao, G.P.; Bertaccini, A., Global Status of Phytoplasma Disease in Vegetable crop. Frontiers in Microbiology 2019, 10
Bertaccini, A.; Oshima, K.; Maejima, K.’; Namba, S., Phytoplasma: Plant Pathogenic Bacteria-III, Chapter 2
Image courtesy
Featured image – https://www.ambius.com/blog/wp-content/uploads/2016/12/Poinsettia-1024×679.jpg
