BARREN INFLORESCENCE2 Interaction with ZmPIN1a Suggests a Role in Auxin Transport During Maize Inflorescence Development

Polar auxin transport, mediated by the PIN-FORMED (PIN) class of auxin efflux carriers, controls organ initiation in plants. In maize, BARREN INFLORESCENCE2 (BIF2) encodes a serine/threonine protein kinase co-orthologous to PINOID (PID), which regulates the subcellular localization of AtPIN1in Arabidopsis. We show that BIF2 phosphorylates ZmPIN1a, a maize homolog of AtPIN1, in vitro and regulates ZmPIN1a subcellular localization in vivo, similar to the role of PID in Arabidopsis. In addition, bif2 mutant inflorescences have lower auxin levels later in development. We propose that BIF2 regulates auxin transport through direct regulation of ZmPIN1a during maize inflorescence development.