XVI International Botanical Congess

Moss development affords unrivalled opportunities for studying cell polarity. A polar axis forms during protoplast regeneration while tropic responses of protonemal apical cells modify existing polarity. Protoplasts of both species regenerate by polar outgrowth. A directional light input orients the regeneration axis but since high light levels are needed for regeneration in Physcomitrella but not in Ceratodon, a wider range of conditions can be investigated in the latter where it has been established (Cove et al Development 122,371) that axis alignment is fixed first and commitment to grow towards or away from the light is fixed later. Studies of apical cell tropisms also show that axis alignment and polarity are separable. The wrong way response phenotype of wwr mutants shows this most spectacularly (Wagner et al Planta 202,149). wwr mutants still align to gravity but grow down rather than up.