Diseases container plants

While most plant viruses contain dsRNA, the geminiviruses,458 which cause a number of plant diseases, contain single-stranded DNA. The virus particle consists of a fused pair of incomplete icosahedra, evidently containing a single 2500-bp DNA strand. Replication may require coinfection with two virus particles of differing sequence. 

Enhanced resistance to disease in plants after an initial infection has fascinated observers for over 100 years. A review of the subject by Chester in 1933 contains 201 references (Jj. "Immunization", "acquired systemic resistance", or "induced resistance" of plants have been reviewed in recent years (2-11). We shall not exhaustively review the literature, but shall focus on general principles and phenomena of particular relevance to the use of "plant immunization" for the practical control of disease. This paper will stress examples from our own research program, but wi1i also include literature citations to provide the reader with an appreciation of important research contributions of others previously and presently active in the field. Most examples presented will deal with fungal, bacterial, or viral diseases of crop plants, but similar principles may apply to infestations by nematodes and, possibly, insects. 

Several picrotoxane-containing plants are used in traditional medicines. Cor-iaria species play an important role in Chinese traditional medicine, notably against mental diseases. Coriaria nepalensis is used against numbness, toothache, traumatic injury, and conjunctivitis (252). In the traditional medicine of Taiwan, gastrointestinal disturbances, rheumatism, and uterine cancer are treated with extracts of Coriaria intermedia (255). C. sinica is used for treatment of schizophrenia (29), and extracts of the parasitic plant on C. nepalensis, L. parasiticus, were used as shock therapy in schizophrenia (27). Okuda et al. point to corianin (21) as the main active component (27). 

Vitamin D metabolites have been found in plants. En-teque Seca, a disease of cattle that graze on the plant Solanum glaucophyllum (malacoxalon) in South America, is characterized by calcinosis and soft-tissue calcification. Aqueous extracts of the leaves of this and other plants that have been used to treat uremic bone disease contain glycosides of l,25-(OH)2D. 

The importation, handling and disposal of plants, seeds, soil, sediments and other materials that may contain plant diseases are often regulated nationally and care must be taken to avoid contravention of these controls. 

It can be assumed that seeds and dust from PA-containing plants are a major source for a human PA exposure [ 12,13]. In industrialized countries, grain cleaning methods reduce the PA contamination under a level where acute intoxications can occur, but the dust components are still remaining and there are indications that diseases such as cirrhosis, cancer, and pulmonary arterial hypertension are due to a long-time exposure of low doses of PAs [14]. 

The function of the essential oil in the plant is not fully understood. Microscopic examination of plant parts that contain the oil sacs readily shows their presence. The odors of flowers are said to act as attractants for insects involved in pollination and thus may aid in preservation and natural selection. Essential oils are almost always bacteriostats and often bacteriocides. Many components of essential oils are chemically active and thus could participate readily in metaboHc reactions. They are sources of plant metaboHc energy, although some chemists have referred to them as waste products of plant metaboHsm. Exudates, which contain essential oils, eg, balsams and resins, act as protective seals against disease or parasites, prevent loss of sap, and are formed readily when the tree tmnks are damaged. 

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