Cabbage root hairs provide a useful system for studying responses to infection byPlasmodiophora brassicaeat the cellular level. Roots of cabbage seedlings grown in liquid culture were inoculated with zoospores ofP. brassicae, and the effects of infection on host nucleolar development and root hair cell extension were studied. Within 18ensp;h after the root hairs emerged from the trichoblasts the nucleoli of noninfected cells declined in diameter from 5ensp;mu; to less than 1ensp;mu;. Root hairs which became infected, whether 6ensp;h old or 24ensp;hold, had a nucleolar diameter ranging between 1.5 and 3ensp;mu;. The diameter of the nucleolus did not increase above 1.5ndash;3ensp;mu; regardless of the age and number of Plasmodia within the cell. In addition to nucleolar enlargement, parasitized cells became stunted and often enlarged at their tips. Growth of Plasmodia within hair cells occurred with synchronous division of their nuclei. After penetration by the parasite there was an 18ensp;h lag before the first nuclear division within the Plasmodia after which synchronous nuclear division occurred at about 4-h intervals.
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