Analysis of the histone H3 gene family in Arabidopsis and identification of the male-gamete-specific variant AtMGH3

Histones are major components of chromatin, the protein-DNA complex involved in DNA packaging and transcriptional regulation. Histone genes have been extensively investigated at the genome level in animal systems and have been classified as replication dependent, replication independent or tissue specific. However, no such study is available in a plant system. In this paper we report that there are 15 histone H3 genes in the Arabidopsis genome, including five H3.1 genes, three H3.3 genes and five H3.3-like genes. A gene structure analysis revealed that gene duplication causes redundancy of the histone H3 genes. The expression of one of the H3 genes, termed AtMGH3/At1g19890, is cell-specific, being restricted to the generative and sperm cells of Arabidopsis pollen as shown by in situ hybridisation and reporter gene analysis. Thus, we conclude that in Arabidopsis, AtMGH3 is a male-gamete-specific histone H3 gene. A T-DNA insertion line for AtMGH3 revealed decreased expression and ectopic RNA splicing. The T-DNA insertion lines for AtMGH3/At1g19890 and other H3 genes revealed a normal growth phenotype and reproductive fertility. These findings suggest that other H3 genes are likely to compensate for the T-DNA-insertion-induced loss of a single H3 gene because of the high redundancy of these genes in the Arabidopsis genome. These T-DNA mutant lines should be useful for accumulating different H3 gene mutations in a single plant and for studying replication-dependent and replication-independent H3 genes and the specific role of AtMGH3 in chromatin remodelling and transcriptional regulation during development of male gametes.