Structural, biochemical and physiological aspects of a systemic virus host interaction
Creators: name: Boucher, Wayne Desmond
Publisher: name: University Of Tasmania
Year: 2023

description: A Susceptible tobacco variety infected with a mosaic-inducing strain of tobacco mosaic Virus displayed certain alterations in normal growth patterns. This work confirmed that such plants, as a result of infection, were reduced in plant height, internode length and leaf size. Infected plants also had reduced root systems. The rate of leaf growth., that is the time taken for leaves to reach full expansion size, was similar for infected and virus-free plants although the number of leaves formed over a 6 week period following inoculation was slightly greater for virus infected plants. Infection of young plants delayed both flower initiation and the appearance of inflorescences. A reduced rate of cell division in sub-apical regions appeared to be the major factor in reducing plant size. The survival of plant supporting virus synthesis appeared to be related to certain metabolic changes. Specific enzymes associated with photorespiration and \dark\" respiration had lower activities in Virus infected plants. Enzymes indirectly associated with chloroplasts and photosynthetic pigments were also reduced in virus-containing tissues. To some extent the reduced potential of carbon fixation by photosynthesis was offset by a reduced cellular demand from carbon utilized in photorespiration and \"dark\" respiration this enabling cells to support a level of virus synthesis. Normal activity of an enzyme associated with the Embden-Mbyerhofparnas pathway of carbohydrate metabolism higher activity of an enzyme associated with the pentose phosphate pathway and reduced activity of an enzyme associated with \"dark\" respiration suggested that most carbohydrates moving through the major pathways of metabolism are channelled towards virus synthesis through conversion to amino acids. The high activity of hydrolytic enzymes in virus-containing tissues suggested a high rate of metabolism of carbohydrates and nucleic acids that would be expected of cells supporting both virus synthesis and cellular metabolism. Two features of virus infected plants ensure their survival: delayed onset of leaf senescence and the presence of virus-free tissues with leaf mosaics. Biochemical and ultrastructural studies revealed that over-mature leaves on virus-infected plants were metabolically active and composed of cells containing intact membrane systems. Most enzymes studied in similarly aged leaves from uninoculated plants had greatly reduced activities and cells in these tissues contained membrane components that showed signs of deterioration and disorganization. Virus-free areas of mosaic diseased leaves referred to as dark green island tissues were Metabolically more active than comparably aged tissues from uninfected plants. Enzymes associated with photosynthesis and carbohydrate metabolism and photosynthetic pigments were at greatest levels in these tissues. Of the three tissue types compared virus -containing and virus-free tissues from infected plants and leaf tissues from uninfected plants dark green island tissues had the greatest potential for photosynthesis and Carbohydrate metabolism. Growth patterns of virus-infected plants delayed flower initiation and onset of leaf senescence the altered activities of enzymes associated with chloroplasts and carbohydrate metabolism and the stimulated activities of enzymes in dark green island tissues suggested that basic responses of plants to infection occurred through shifts in endogenous growth regulator levels. Bioassays of the major growth regulators revealec that although a gibberellin A3-like compound was unaffected by infection cytokinin-Iike compounds and an indole acetic acid-like compound were at higher levels in infected tissues and an abscisic acid-like compound was at a reduced level. The altered balance of the major growth stimulating and growth retarding hormones is sufficient to account for most of the measured and observed changes in diseased plants."