The Arabidopsis thaliana histone H2A gene HTA1 is essential for efficient transformation of Arabidopsis roots by Agrobacterium tumefaciens. Disruption of this gene in the rat5 mutant results in decreased transformation. In Arabidopsis, histone H2A proteins are encoded by a 13-member gene family. RNA encoded by these genes accumulates to differing levels in roots and whole plants; HTA1 transcripts accumulate to levels up to 1000-fold lower than do transcripts of other HTA genes. We examined the extent to which other HTA genes or cDNAs could compensate for loss of HTA1 activity when overexpressed in rat5 mutant plants. Overexpression of all tested HTA cDNAs restored transformation competence to the rat5 mutant. However, only the HTA1 gene, but not other HTA genes, could phenotypically complement rat5 mutant plants when expressed from their native promoters. Expression analysis of HTA promoters indicated that they had distinct but somewhat overlapping patterns of expression in mature plants. However, only the HTA1 promoter was induced by wounding or by Agrobacterium infection of root segments. Our data suggest that, with respect to Agrobacterium-mediated transformation, all tested histone H2A proteins are functionally redundant. However, this functional redundancy is not normally evidenced because of the different expression patterns of the HTA genes.