The process of microtubule nucleation in plant cells is still a major question in plant cell biology. {gamma}-Tubulin is known as one of the key molecular players for microtubule nucleation in animal and fungal cells. Here, we provide genetic evidence that in Arabidopsis thaliana, {gamma}-tubulin is required for the formation of spindle, phragmoplast, and cortical microtubule arrays. We used a reverse genetics approach to investigate the role of the two Arabidopsis {gamma}-tubulin genes in plant development and in the formation of microtubule arrays. Isolation of mutants in each gene and analysis of two combinations of {gamma}-tubulin double mutants showed that the two genes have redundant functions. The first combination is lethal at the gametophytic stage. Disruption of both {gamma}-tubulin genes causes aberrant spindle and phragmoplast structures and alters nuclear division in gametophytes. The second combination of {gamma}-tubulin alleles affects late seedling development, ultimately leading to lethality 3 weeks after germination. This partially viable mutant combination enabled us to follow dynamically the effects of {gamma}-tubulin depletion on microtubule arrays in dividing cells using a green fluorescent protein marker. These results establish the central role of {gamma}-tubulin in the formation and organization of microtubule arrays in Arabidopsis.