Monday, 23 December 2019

Expanding Role of Fungi

Role of Fungi

Following the observation by French farmers around the mid-1600s and, independently, by Connecticut farmers in the early 1700s that wheat rust was worse near barberry bushes, the farmers came to believe that barberry fathered the rust, which then moved to wheat. The request by farmers for legislation to force towns to eradicate barberries and in that way to protect the wheat plants from rust followed. At about the same time, spores of the rust fungus were observed with the compound microscope for the first time in England (Hooke, 1667). In Italy, Micheli 60 years later (1729) described many new genera of fungi, illustrated their reproductive structures, and noted that when he placed them on freshly cut slices of melon, these fungal structures generally reproduced the same kind of fungus that produced them. He proposed that fungi arose from their own spores rather than spontaneously, but nobody believed him. New information about plant pathogenic fungi continued to be developed, but most of it was not accepted by the scientists of the time for a long time.

As mentioned previously, in 1755, Tillet in France showed that wheat smut is a contagious plant disease, but even he believed that it was a poisonous substance contained in the smut dust, rather than a living microorganism, that caused the disease. In 1807, Prevost, also in France, repeated and expanded Tillet’s experiments and appeared to have demonstrated conclusively that wheat smut was caused by a fungus. His conclusions, however, were not accepted because the scientists were blinded by the belief that microorganisms and their spores were the results rather than the cause of disease.

These beliefs continued to be shared and expounded by scientists for at least another 50 years. The devastating epidemics of late blight of potato in northern Europe, particularly Ireland, in the 1840s not only dramatized the effect of plant diseases on human suffering and survival but also greatly stimulated interest in their causes and control. In 1861, DeBary finally established experimentally beyond criticism that a fungus (Ph. Infestans) was the cause of the plant disease known as late blight of potato, a disease that closely resembles the downy mildew.

It is, perhaps, worth noting here that it was during those years (1860–1863) that Louis Pasteur proposed, and finally provided irrefutable evidence, that microorganisms arise only from preexisting microorganisms and that fermentation is a biological phenomenon, not just a chemical one. Pasteur’s conclusions, however, were not generally accepted for many years afterward. Nevertheless, the proof for involvement of microorganisms (germs) in fermentation and disease signaled the beginning of the end of the theory of spontaneous generation and provided the basis for the germ theory of disease.

Although fungi had already been the object of study by many scientists, proof that they were causing disease in plants greatly increased interest in them. DeBary himself also carried out studies of the smut and rust fungi, of the fungi causing downy mildews, and of the fungus Sclerotinia, which induces rotting of vegetables. The German Kühn in the 1870s and later contributed significantly to the studies of infection and development of smut in wheat plants and promoted the development and application of control measures, particularly seed treatment for cereals.

During the years of Pasteur and Koch, several scientists also made significant contributions to plant pathology and to biology and medicine. After establishing beyond criticism in 1861 that the potato blight was caused by a fungus, DeBary went on to show conclusively that smut and rust fungi were also the causes and not the results of their respective plant diseases. Moreover, he showed that some rust diseases require two alternate host plants to complete their life cycle, e.g., the fungus causing the stem rust of wheat requires wheat and barberry. DeBary also showed (1886) that some fungi induce rotting of vegetables (Fig. 1) by secreting substances (enzymes) that diffuse into plant tissues in advance of the pathogen.

Plant Disease

FIGURE .1 Infection and advanced internal rotting of summer squash 
(A) by the fungus Choanephora, of peach fruit 
(B) by the fungus Rhizopus sp., and 
(C) of kiwi fruit by the fungus Botrytis cinerea. In all cases, fruit rot is a result of, primarily, pectinolytic enzymes secreted by the fungi and advancing ahead of the mycelium. A small amount of fungi can be seen on the surface of the fruits. 
(C) Courtesy of T. Michailides, University of California.

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