Mutations in the pilz group genes disrupt the microtubule cytoskeleton anduncouple cell cycle progression from cell division in Arabidopsis embryo and endosperm

Organised cell division and expansion play important roles in plant embryog enesis. To address their cellular basis, we have analysed Arabidopsis abnor mal-embryo mutants which were isolated for their characteristic phenotype: mutant embryos are small, mushroom-shaped ("pilz") and consist of only one or few large cells each containing one or more variably enlarged nuclei and often cell wall stubs. These 23 mutants represent four genes, PFIFFERLING, HALLIMASCH, CHAMPIGNON, and PORCINO, which map to different chromosomes. A ll four genes have very similar mutant phenotypes although porcino embryos often consisted of only one large cell, The endosperm did not cellularise a nd contained a variably reduced number of highly enlarged nuclei, By contra st, genetic evidence suggests that these genes are not required for gametop hyte development. Expression of cell cycle genes, Cdc2a, CyclinA2 and Cycli nB1, and the cytokinesis-specific KNOLLE gene was not altered in mutant emb ryos. However, KNOLLE syntaxin accumulated in patches but no KNOLLE-positiv e structure resembling a forming cell plate occurred in mitotic cells, A ge neral defect in microtubule assembly was observed in all mutants. Interphas e cells lacked cortical microtubules, and spindles were absent from mitotic nuclei although in rare cases, short stubs of microtubules were attached t o partially condensed chromosomes, Our results suggest that the cellular co mponents affected by the pilz group mutations are necessary for continuous microtubule organisation, mitotic division and cytokinesis but do not media te cell cycle progression.