<i>HANABA TARANU (HAN)</i> Bridges Meristem and Organ Primordia Boundaries through <i>PINHEAD</i>, <i>JAGGED</i>, <i>BLADE-ON-PETIOLE2</i> and <i>CYTOKININ OXIDASE 3</i> during Flower Development in <i>Arabidopsis</i>

<div><p>Shoot organ primordia are initiated from the shoot apical meristem and develop into leaves during the vegetative stage, and into flowers during the reproductive phase. Between the meristem and the newly formed organ primordia, a boundary with specialized cells is formed that separates meristematic activity from determinate organ growth. Despite interactions that have been found between boundary regulators with genes controlling meristem maintenance or primordial development, most boundary studies were performed during embryogenesis or vegetative growth, hence little is known about whether and how boundaries communicate with meristem and organ primordia during the reproductive stage. We combined genetic, molecular and biochemical tools to explore interactions between the boundary gene <i>HANABA TARANU</i> (<i>HAN</i>) and two meristem regulators <i>BREVIPEDICELLUS (BP)</i> and <i>PINHEAD</i> (<i>PNH</i>), and three primordia-specific genes <i>PETAL LOSS (PTL)</i>, <i>JAGGED</i> (<i>JAG</i>) and <i>BLADE-ON-PETIOLE (BOP)</i> during flower development. We demonstrated the key role of <i>HAN</i> in determining petal number, as part of a set of complex genetic interactions. <i>HAN</i> and <i>PNH</i> transcriptionally promote each other, and biochemically interact to regulate meristem organization. HAN physically interacts with JAG, and directly stimulates the expression of <i>JAG</i> and <i>BOP2</i> to regulate floral organ development. Further, <i>HAN</i> directly binds to the promoter and intron of <i>CYTOKININ OXIDASE 3</i> (<i>CKX3</i>) to modulate cytokinin homeostasis in the boundary. Our data suggest that boundary-expressing HAN communicates with the meristem through the PNH, regulates floral organ development via JAG and BOP2, and maintains boundary morphology through CKX3 during flower development in <i>Arabidopsis</i>.</p>