Linker histones play a role in male meiosis and the development of pollen grains in tobacco

To examine the function of linker histone variants, we produced transgenic tobacco plants in which major somatic histone variants H1A and H1B were pre sent at similar to 25% of their usual amounts in tobacco chromatin. The dec rease in these major variants was accompanied by a compensatory increase in the four minor variants, namely, H1C to H1F. These minor variants are smal ler and less highly charged than the major variants. This change offered a unique opportunity to examine the consequences to a plant of major remodeli ng of its chromatin set of linker histones. Plants with markedly altered pr oportions of H1 variants retained normal nucleosome spacing, but their chro mosomes were less tightly packed than those of control plants. The transgen ic plants grew normally but showed characteristic aberrations in flower dev elopment and were almost completely male sterile. These features correlated with changes in the temporal but not the spatial pattern of expression of developmental genes that could be linked to the abnormal flower phenotypes. Preceding these changes in flower morphology were strong aberrations in ma le gametogenesis. The earliest symptoms may have resulted from disturbances in correct pairing or segregation of homologous chromosomes during meiosis . No aberrations were observed during mitosis. We conclude that in plants, the physiological stoichiometry and distribution of linker histone variants are crucial for directing male meiosis and the subsequent development of f unctional pollen grains.