Nira Retig

Changes in peroxidase and polyphenoloxidase associated with natural and induced resistance of tomato to Fusarium wilt

Abstract Peroxidase and polyphenoloxidase activities, their electrophoretic patterns and the effect of Ethephon (2-chloroethylphosphonic acid) were tested in resistant and susceptible tomato plants, following inoculation with Fusarium oxysporum f. sp. lycopersici (Sacc.). Peroxidase and polyphenoloxidase activities increased in the roots of the resistant type shortly after inoculation, whereas in the susceptible type, a similar increase in peroxidase activity occurred 24 h later. No significant increase in polyphenoloxidase was found in the susceptible type. A marked increase in peroxidase activity in the stems of susceptible plants occurred when disease symptoms appeared. Ethephon treatment increased the resistance of susceptible plants to Fusarium and enhanced peroxidase and polyphenoloxidase activities. No changes in peroxidase and polyphenoloxidase isozyme patterns were found as a result of inoculation or Ethephon treatment.

Catechol-induced resistance of tomato plants to Fusarium wilt

Abstract Treatment of susceptible tomato plants with catechol prevented disease symptom expression after infection by Fusarium oxysporum f. sp. lycopersici . A marked accumulation of total phenols was observed in the catechol-treated plants. Though the treatment changed peroxidase and polyphenoloxidase activity, no changes appeared in their isozyme patterns. The pathogen was recovered from both inoculated-susceptible and catechol-treated tomato stem sections. It is suggested that the catechol treatment renders the susceptible plants symptomless carriers. The mechanism of this acquired resistance is discussed.

Changes in ribonucleases during differentiation (spherulation) of Physarum polycephalum.

Abstract Ribonuclease activity in Physarum polycephalum increased rapidly after the addition of mannitol for spherule induction. Most of the 12 ribonuclease isoenzymes detected by the gel isoelectric focusing technique increased, whereas one isoenzyme disappeared during spherulation. It is suggested that the rise in ribonuclease activity depends on protein synthesis de novo since it was prevented by cycloheximide.

Non-random segregation of gene I for Fusarium resistance in the tomato

Significant deviations from the ratios expected, according to the single dominant gene hypothesis for resistance to Fusarium wilt, were found in crosses involving several susceptible and resistant tomato lines. The susceptible class was the deficient one in F2 and F3 populations, as well as in backcrosses in which the heterozygotic resistant F1 served as the male parent. The reciprocal backcross, with the F1 as the female and the homozygous susceptible as the male, gave segregations better approximating or consistent with the single gene hypothesis. Reciprocal F1 and F2 generations did not give any evidence of cytoplasmic effects.The results were interpreted assuming preferential fertilization of ovules by pollen grains carrying the dominant I allele for resistance.The practical implications of the phenomenon of preferential fertilization in breeding for Fusarium resistance are discussed.

PENETRANCE OF GENE I FOR FUSARIUM RESISTANCE IN THE TOMATO

Tomato seedlings were inoculated, from one to ten days after emergence, with the tomato Fusarium wilt fungus race 1. The penetrance of gene I for Fusarium resistance in the homozygous resistant variety Homestead 24 was almost complete. In the F1 (Ii) between Homestead 24 and the susceptible Marmande penetrance was incomplete and ranged between 66.3% and 100% in different experiments. The age of seedlings at time of inoculation did not affect the final percentage of diseased plants while it influenced the nature and the time of appearance of disease symptoms. Possible consequences of incomplete penetrance for the resistance of F1 hybrids are discussed.

The mechanism of preferential fertilization in tomatoes carrying the I-Allele for fusarium resistance

SummaryPreferential fertilization of ovules by male gametes carrying the dominant I allele for Fusarium resistance occurred when heterozygous Ii plants served as male parent. Pollination with a mechanical l:l mixture of I and i pollen gave the normally expected Mendelian ratio.No significant differences were found in vivo in growth rate or in final length of pollen tubes of genotypes I and i. The experiments indicated that preferential fertilization was mainly caused by prepollination events leading to production of unequal amounts of fertile pollen grains of the two types.

Acquired tolerance of tomato fruits to sunscald

Abstract The effect of various conditions on tolerance of tomato fruits to sunscald injury was studied. Tolerance was acquired either during a developmental period in the field or during a short period of exposure to sub-damage temperature of sensitive detached fruits. Heat treatment, rather than light, was found to be the factor influencing the development of tolerance. Saturating the fruits with water increased their sensitivity to sunscald injury.

A Simplified method for determining the resistance of tomato seedlings toFusarium andVerticillium wilts

A simple test for determining the resistance of tomato lines toFusarium andVerticillium wilt diseases was developed. Roots of tomato seedlings at their first true leaf stage were dipped in a heavy suspension of the pathogen for 24 h. The seedlings were then transferred to small beakers containing various concentrations of Hoagland solution. The first and most severe disease symptoms appeared on susceptible cultivars grown in 25% Hoagland nutrient solution. Final results of resistance tests were obtained 10, 8 and 16 days after inoculation withFusarium race 1, race 2, andVerticillium, respectively. The test appears to be simple, results are obtained rapidly, and greenhouse space can be saved.

Changes in polygalacturonase, pectin lyase and cellulase in tomato plants, associated withFusarium inoculation

Activities of polygalacturonase, pectin lyase, and cellulase increased in susceptible, catechol-treated, and resistant tomato plants, after inoculation withFusarium oxysporum f.lycopersici (Sacc.) Snyder and Hansen race 2. The catechol-treated and the resistant plants remained symptomless, while susceptible plants developed symptoms of disease. It is therefore suggested that increased activity of cell-wall-degrading enzymes in inoculated plants does not necessarily cause the development of disease symptoms.