Identification of a constitutive 45 kDa soluble protein associated with resistance to downy mildew in muskmelon (Cucumis melo L.), line PI 124111 F

Abstract Analysis by sodium dodecylsulphate-polyacrylamide gel electrophoresis of the soluble proteins extracted from muskmelon ( Cucumis melo L.), inbred line PI 124111F (PI) with resistance to Pseudoperonospora cubensis (Berk, et Curt.) Rost, and two susceptible cvs, Hemed and AnanasYokneam revealed the presence of a unique 45 kDa protein (P45) in the resistant but not in the susceptible plants. Subcellular fractionation indicated that P45 is a cytoplasmic soluble protein. It was detected in leaves and cotyledons but not in stems or roots. F 1 hybrid plants (Hemed × PI) that displayed only a partial resistance against P. cubensis expressed P45 at an intermediate level, whereas plants of the inbred generations F 5 , F 7 and F 10 that displayed a high degree of resistance, expressed P45 at a level similar to the parental PI. Back-cross progeny plants [(Hemed × PI) × Hemed] segregate 1:2:1 partially resistant: susceptible: highly susceptible to the disease. The partially resistant plants showed an intermediate level of P45 (similar to F 1 ) whereas the highly susceptible plants had no P45, thus indicating cosegregation of P45 with resistance. The resistance of PI to P. cubensis was found to decrease at a colonization temperature of 12 °C. 35 S-methionine in vivo protein labelling revealed a reduction in the intensity of the P45 band in plants incubated for 11 h at 12 °C. The application of P45 in breeding programs of muskmelon for downy mildew resistance and its possible involvement in resistance to P. cubensis are discussed.