Host-associated selection of a P3 mutant of zucchini yellow mosaic virus affects viral infectivity in watermelon

In this study, we found that the infectivity of zucchini yellow mosaic virus (ZYMV) in watermelon lines H1 and K6 changed from partial to complete after propagation in the susceptible watermelon line ZXG637. When using cucumber infected with strain ZYMV-CH87 as an inoculum (named ZYMV-CH87C), the mean incidences of infection in lines H1 and K6 were 6% and 11%, respectively. However, when these lines were inoculated with ZXG637 infected with ZYMV-CH87C (named ZYMV-637), 100% of the plants became infected. Sequencing of ZYMV from these different inoculums revealed two nucleotide changes in the P3 cistron in ZYMV-637, which resulted in changes in the amino acids at positions 768 and 857 of the P3 protein, compared with the original strain ZYMV-CH87. We named this variant the M₇₆₈I₈₅₇-variant. The M₇₆₈I₈₅₇-variant was detected at low levels (3.9%) in ZYMV-CH87C. When ZYMV-CH87C was passaged with ZXG637, the M₇₆₈I₈₅₇-variant was selected by the host, and the original sequence was replaced entirely after two passages. These results may be explained by host-associated selection due to an unknown host-encoded factor. Using the M₇₆₈I₈₅₇-variant as an inoculum, 100% of the H1 and K6 plants showed systemic symptoms. These results suggest that (1) changing the individual amino acids at the end of the P3 N-terminus induces resistance-breaking, and (2) the P3 N-terminus may be involved in host recognition.