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Agrobacterium rhizogenes (updated scientific name: Rhizobium rhizogenes) is a Gram negative soil bacterium that produces hairy root disease in dicotyledonous plants. A. rhizogenesinduces the formation of proliferative multi-branched adventitious roots at the site of infection; so called 'hairy roots'  In the rhizosphere, plants may suffer from wounds by soil pathogens or other sources. This leads to the secretion of phenolic compounds like acetosyringone which have chemotactic effects that attract the bacteria. Under such conditions, certain bacterial genes are turned on leading to the transfer of its T-DNA from its root inducing plasmid (Ri plasmid) into the plant through the wound. After integration and expression, in vitro or under natural conditions, the hairy rootphenotype is observed, which typically includes overdevelopment of a root system that is not completely geotropic, and altered (wrinkled) leaf morphology, if leaves are present. Bacterial genes may be retained within the plant.
The hairy roots are grown in vitro in bioreactors to study their soil interaction with other pathogens like fungi and nematodes. This technique has also led to the commercial production of certain metabolic compounds that the plant is known to secrete, especially in regard to the medicinal plants that are difficult to cultivate in sufficient quantities by other means. The root cultures are also used for genetic engineering. -------------------------------------------------
Blackleg of potato
De Boer, S. H. 2004. Blackleg of potato. The Plant Health Instructor. DOI:10.1094/PHI-I-2004-0712-01 DISEASE: Blackleg of potato
PATHOGEN: Erwinia carotovora subsp. atroseptica (Synonym: Pectobacterium atrosepticum) HOSTS: potato (Solanum tuberosum)
Solke H. De Boer,
Centre for Animal and Plant Health, Charlottetown, PE, Canada
The blackened stem and wilted leaves are typical
of the potato blackleg disease. (Courtesy S.H. De Boer)
Potatoes are grown world-wide and the crop is usually considered to be the fourth most important staple food source after wheat, rice, and corn. It is one of the few staple food crops that are vegetatively propagated. Vegetative propagation means that the potato crop is not grown from true seed but rather from asexually produced propagules or "seed potatoes." Potatoes are underground storage organs known as tubers and are attached to the mother plant by stolons. Potato tubers are not only harvested as a food source for fresh market and processed products, but are also used for planting a new crop. Seed potatoes only differ from eating and processing potatoes in that they are produced as a highly regulated crop to keep them free of potential pathogens and pests. True botanical seed tends to exclude many disease-causing microorganisms even if they are present in the parent plant. Vegetative propagules such as tubers, on the other hand, are often infected or contaminated by the pathogens associated with the parent plant. The bacterium that causes the blackleg disease of potato is one of the pathogens that is tuber-borne. The blackleg disease can cause severe economic losses to the potato crop. However, the occurrence of blackleg depends very much on the growing conditions, particularly temperature and rainfall after planting. Symptoms and signs
Blackleg disease sometimes develops early in the growing season soon after the plants emerge. This is referred to as early blackleg and is characterized by stunted, yellowish foliage that has a stiff, upright habit (Figure 1). The lower part of the belowground stem of such plants is dark brown to black in color and extensively decayed (Figure 2). The pith region of the stem is particularly susceptible to decay and in blackleg-infected plants the decay may extend upward in the stem far beyond...