The absence of histone H2B monoubiquitination in the Arabidopsis hub1 (rdo4) mutant reveals a role for chromatin remodeling in seed dormancy

Seed dormancy is defined as the failure of a viable seed to germinate under favorable conditions. Besides playing an adaptive role in nature by optimizing germination to the most suitable time, a tight control of dormancy is important in crop plants. Extensive genetic and physiological studies have identified the involvement of several factors, but themolecularmechanisms underlying this process are still largely unknown.Wecloned theHISTONEMONOUBIQUITINATION1 (HUB1) gene, ofwhich the mutant (previously identified as reduced dormancy4) has reduced seed dormancy and several pleiotropic phenotypes. HUB1 encodes aC3HC4RINGfinger protein. TheArabidopsis thalianagenomecontains oneHUB1homolog,whichwenamedHUB2. The hub2 mutant also has reduced seed dormancy and is not redundant with hub1. Homologs of HUB1 and HUB2 in other species are required for histone H2Bmonoubiquitination. In agreement with this, the ubiquitinated form of histone H2B could not be detected in the hub1 and hub2mutants. In yeast and human cells, histone H2B monoubiquitination is associated with actively transcribed genes. The hub1 mutant showed altered expression levels for several dormancy-related genes. We propose a role for chromatin remodeling in seed dormancy by H2B monoubiquitination through HUB1 and HUB2