Volume 69, Issue 3
March 1969, pages 95-184
pp 95-103 March 1969
The impact of physical sciences, particularly biochemistry, has played a significant role in understanding etiology and syndrome in pathogenesis. Critical tissue respiration and enzyme changes, deranged carbohydrate and nitrogen metabolism, transpiratory disturbances and ionic imbalance produced by fungal toxins, exaggerated auxin relationships, formation of abnormal metabolite (s) (phytoalexins) have all contributed to a better understanding of the ‘sick’ plant.
Plant virologists have made phenomenal progress in the applied field of the biochemistry of the infected plant and some of the recent researches on the nature of viruses and control measures adopted are worth emulating in other fields of plant pathology.
A new field is developing round environment and disease proneness. This has reference to the rice blast disease where low nyctotemperatures for a long enough period makes for alterations in the nitrogen metabolism of the host and is governed by the balance between primary nitrogen metabolism and secondary metabolic events leading to synthesis of structural metabolites.
pp 104-114 March 1969
Little information is available on the sequence of physiological changes from virus inoculation to full development of disease symptoms. In this paper, we discuss (1) activity of chlorophyllase, (2) ferrous and ferric iron changes, (3) inorganic and organic phosphorus, and (4) respiration in pigeon pea sterility mosaic infected pigeon pea plants and cassava mosaic infected cassava plants.
In both healthy and diseased plants, chlorophylla andb increased with age. However, these were significantly lower in diseased plants than in healthy plants, from early stages. Chlorophyllase activity increased with age, such increase being much steeper in diseased plants. There appeared to be a progressive conversion of ferrous Fe to ferric Fe in diseased leaves. Diseased leaves at all ages had higher levels of total P. There was a greater conversion of inorganic P to the organic form as the disease progressed. Respiration increased with progress of disease up to the production of full symptoms, and dropped thereafter reaching levels lower than those in healthy leaves of corresponding age. In PSMV-affected pigeon pea leaves activity of catalase, peroxidase, ascorbic acid oxidase and cytochrome oxidase increased while polyphenol oxidase activity decreased. Mitochondrial nitrogen was much higher in diseased leaves than in healthy leaves.
pp 115-119 March 1969
Trace metals play a very important role in the health and disease of plants. Iron, zinc, manganese, copper and molybdenum are essential to all fungi. Boron is not shown to be essential for fungi. The phenomenon of ion antagonism in fungi, although conclusively demonstrated, has been little understood. Further work is necessary to elucidate the interaction between pH of the medium, ion antagonism and trace metal requirements by fungi.In vitro experiments revealed the influence of trace metals on the production of enzymes and/or toxins by plant pathogenic fungi and these results point to the possibility of control of vascular wilt diseases by altering the metal concentrations available to plants by soil amendments.
pp 120-132 March 1969
Resistance to fungal and bacterial diseases of sugarcane is a hypersensitive reaction (HR). Meristematic tissues do not respond with HR; therefore, though present in differentiated cells, HR is not useful against root rot which destroys the root tips. Resistance against this disease is an exclusion phenomenon depending on the actinomycete and fungal flora of the rhizosphere.
HR in red rot is related to the speed of activation of polyphenol oxidase (PPO) and this in turn is affected by temperature.
On juvenile leaves, following germination of spores of an avirulent strain of the red rot pathogen, diffusates contain a factor (a phytoalexin) inhibitory to the germination of other spores. Dormant infections in bud scales and leaf-scar tissue is associated with partial resistance. Accumulation of phenolic aglycones in sublethal concentrations in host cells in response to infection induces formation of dormant structures of the pathogen. Higher temperatures (Ca 32° C.) favour development of avirulent types, whereas at lower temperatures virulent types persistin vivo. Rapid degeneration of the pathogen and greater host resistance at higher temperatures explain the infrequency of red rot in tropical areas; the reverse favours the frequent epiphytotics in sub-tropical areas.
pp 133-141 March 1969
The events leading to the development of a pathogen-induced disease in a plant are included in the term pathogenesis. The early steps in this interaction is confined to the phase when the pathogen gains entry into the host tissues, into the cells or between the cells by dissolving the cell-walls. At this stage the ability of the pathogen to degrade the cell-wall constituents such as cellulose, pectin, hemi-celluloses and protein by secreting the appropriate hydrolytic enzymes seems important. Although evidence for such enzymatic breakdown of cell-walls have been obtained in some cases such as soft rot, damping-off of seedlings, foot-rot of cereals, the role of these extracellular enzymes of the pathogen in other diseases is less clear.
The other facet of this problem, namely, the reaction of host tissues to the action of these pathogen factors is also important in the development of the disease. Several enzymes are either stimulated or inhibited in and around the infection loci. For example, oxidases such as polyphenolase and peroxidase show increased activity in several host-pathogen interactions studied. Increased activity of these enzymes could have been owing to their activation consequent upon release from “inactive” combinations with other cellular components or it could mean synthesis of these enzymesde novo in response to the activity of the pathogen. Be that as it may, the excessive activity of these enzymes would bring about death of cells and tissues if the stimulation is violent enough as encountered in “hypersensitive” reaction. Perhaps, plants can afford to loose the diseased portions in this battle without any serious repercussion on the rest of the portions.
pp 142-145 March 1969
Respiration is stimulated in leaves of groundnut plants following infection byCercospora personata. The degree of stimulation varies with the variety of the host plant. A decreased sensitivity to malonate and an accumulation of sugars are noted in the infected tissues. The actual increase in oxygen uptake induced by 2, 4-dinitrophenol (DNP) is almost the same in healthy and infected tissues although when expressed as a percentage of the control rate (i.e.,—DNP) the stimulation is less in infected tissues. The culture filtrate ofCercospora personata induces marked stimulation in respiratory rate of groundnut leaf tissue.
pp 146-151 March 1969
It is when a bacterial lesion is first visible that the bacterium would be multiplying most rapidly. Further expansion of lesion size suggests continuing interaction between the pathogen and host. This interaction between host and pathogen involves production of some compounds and/or utilisation of soluble organic compounds of the host cells by the pathogen, and a host reaction by producing compounds antagonistic to bacterial compounds and/or increased metabolic activity. Pathogen’s multiplication necessarily involves increased synthesis of some compounds in the host cells, and less of others.
Decreased photosynthesis, increased host catabolic activity, proteolysis, decreased carbohydrate content, and imbalanced enzymic activity are noted in the diseased plant. Specific identification of the crucial changes in the affected host cell, like activity of some lipases, transamination and polymerisation of amino-acids or inhibition of some oxidases is suggested.
pp 152-172 March 1969
The technique of growing plant tissues and cells on nutrient media under conditions of controlled environment has greatly enlarged the scope of experimental investigations. In phytopathology, application of this technique has given an insight into the nature of abnormal growth, factors affecting penetration, infection and multiplication of pathogens, the weapons of attack of the pathogen, and the morphogenetic potential of the diseased cell. The paper reviews recent applications of the technique in studies on parasitism and points some future possibilities.
pp 173-180 March 1969
Isotope techniques have been valuable in understanding many fundamental aspects of plant disease, particularly these caused by obligate parasites like rusts and mildews. Autoradiography, microautoradiography and other tracer techniques have thrown considerable light on the mobilization of materials to the infection court, shifts in metabolic pathways, RNA and protein synthesis in the host-parasite complex as well as on the metabolic machinery of uredospores. The present article summarizes current knowledge on obligate parasitism gained through tracer techniques.
pp 181-184 March 1969
Using immunoserology, certain strains of phytopathogenic fungi have been compared. Recent work has demonstrated the presence of common antigens between a pathogen and its susceptible host and the importance of such antigens in host-parasite interaction.