FHA2 is a plant-specific ISWI subunit responsible for stamen development and plant fertility.

Imitation Switch (ISWI) chromatin remodelers are known to function in diverse multi-subunit complexes in yeast and animals. However, the constitution and function of ISWI complexes inArabidopsis thalianaremain unclear. In this study, we identified forkhead-associated domain 2 (FHA2) as a plant-specific subunit of an ISWI chromatin-remodeling complex inArabidopsis. Byin vivoandin vitroanalyses, we demonstrated that FHA2 directly binds to RLT1 and RLT2, two redundant subunits of the ISWI complex inArabidopsis. The stamen filament is shorter in thefha2andrlt1/2mutants than in the wild type, whereas their pistil lengths are comparable. The shorter filament, which is due to reduced cell size, results in insufficient pollination and reduced fertility. Therlt1/2mutant shows an early-flowering phenotype, whereas the phenotype is not shared by thefha2mutant. Consistent with the functional specificity of FHA2, our RNA-seq analysis indicated that thefha2mutant affects a subset of RLT1/2-regulated genes that does not include genes involved in the regulation of flowering time. This study demonstrates that FHA2 functions as a previously uncharacterized subunit of theArabidopsisISWI complex and is exclusively involved in regulating stamen development and plant fertility.