The cell polarity landscape in Arabidopsis male and female gametophytes

Plant cell polarity underlies the capacity to directionally transport auxin and other small molecules, but also precedes asymmetric cell division in the stomatal lineage. Except for specific contexts, mechanisms underlying cell polarization are poorly understood. Most insights on plant cell polarity have come from studies on flowering plant sporophytes, while no polar protein localization has yet been reported in flowering plant gametophytes, despite clear anisotropic growth and asymmetric cell divisions in this generation. Inspired by the identification of deeply conserved SOSEKI proteins and their associated partners that mark polar domains in bryophyte gametophytes and Arabidopsis sporophytes, we here investigate cell polarity in Arabidopsis gametophytes. We discover a rich landscape of polar domains throughout male and female gametophyte development, marked by one or more polar proteins. We find that successive steps in male gametophyte polarization can be mechanistically uncoupled, and the first polarization step precedes asymmetric cell division. Using ectopic expression, we find that there is a near-universal capacity for polar protein localization in gametophytes through a single polarity system. Our work offers the first insight into gametophyte cell polarity in Arabidopsis, identifies novel polar domains, and presents a starting point for future dissection of mechanisms underlying plant cell polarity.