Structural, biochemical and physiological aspects of a systemic virus host interaction

摘要

A Susceptible tobacco variety infected with a mosaic-inducing strainudof tobacco mosaic Virus displayed certain alterations in normal growthudpatterns. This work confirmed that such plants, as a result of infection,udwere reduced in plant height, internode length and leaf size. Infectedudplants also had reduced root systems. The rate of leaf growth., that isudthe time taken for leaves to reach full expansion size, was similar forudinfected and virus-free plants although the number of leaves formed overuda 6 week period following inoculation was slightly greater for virus udinfected plants. Infection of young plants delayed both flower initiationudand the appearance of inflorescences. A reduced rate of cell divisionudin sub-apical regions appeared to be the major factor in reducing plantudsize.udThe survival of plant supporting virus synthesis appeared to be relatedudto certain metabolic changes. Specific enzymes associated with photorespirationudand "dark" respiration had lower activities in Virus infectedudplants.udEnzymes indirectly associated with chloroplasts and photosyntheticudpigments were also reduced in virus-containing tissues. To some extent,udthe reduced potential of carbon fixation by photosynthesis was offset byuda reduced cellular demand from carbon utilized in photorespiration andud"dark" respiration, this enabling cells to support a level of virus synthesis.udNormal activity of an enzyme associated with the Embden-Mbyerhofparnasudpathway of carbohydrate metabolism higher activity of an enzymeudassociated with the pentose phosphate pathway and reduced activity ofudan enzyme associated with "dark" respiration suggested that most carbohydratesudmoving through the major pathways of metabolism are channelledudtowards virus synthesis through conversion to amino acids. The high activityudof hydrolytic enzymes in virus-containing tissues suggested a highudrate of metabolism of carbohydrates and nucleic acids that would be expectedudof 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 leafudmosaics. Biochemical and ultrastructural studies revealed that over-matureudleaves on virus-infected plants were metabolically active and composedudof cells containing intact membrane systems. Most enzymes studiedudin similarly aged leaves from uninoculated plants had greatly reducedudactivities and cells in these tissues contained membrane components thatudshowed signs of deterioration and disorganization. Virus-free areas ofudmosaic diseased leaves, referred to as dark green island tissues, wereudMetabolically more active than comparably aged tissues from uninfectedudplants. Enzymes associated with photosynthesis and carbohydrate metabolismudand photosynthetic pigments were at greatest levels in these tissues.udOf the three tissue types compared, virus -containing and virus-free tissuesudfrom infected plants and leaf tissues from uninfected plants, darkudgreen island tissues had the greatest potential for photosynthesis andudCarbohydrate metabolism.udGrowth patterns of virus-infected plants, delayed flower initiationudand onset of leaf senescence, the altered activities of enzymes associatedudwith chloroplasts and carbohydrate metabolism and the stimulated activitiesudof enzymes in dark green island tissues suggested that basic responsesudof plants to infection occurred through shifts in endogenousudgrowth regulator levels. Bioassays of the major growth regulators revealec that although a gibberellin A3-like compound was unaffected by infection,udcytokinin-Iike compounds and an indole acetic acid-like compoundudwere at higher levels in infected tissues and an abscisic acid-like compoundudwas at a reduced level. The altered balance of the major growthudstimulating and growth retarding hormones is sufficient to account forudmost of the measured and observed changes in diseased plants.