Redifferentiation of moss protonemata: an experimental and immunofluorescence study of brood cell formation

The formation of ovoid or spherical brood cells from chloronemal filaments by swelling and symmetrical subapical divisions preceded by phragmosomes, but not preprophase bands, is described in the mosses Physcomitrella patens, Dicranoweisia cirrata, Bryum tenuisetum, Bryum bicolor, and Rhytidiadelphus loreus. This is a widespread phenomenon that occurs in protonemal colonies cultured for long periods of time or allowed to dry out. It involves the redifferentiation of highly polarized chloronemal cells with a net axial array of microtubules into nonpolar cells containing microtubules with no preferred orientation. Brood cells in some species become thick walled and retain their viability for long periods even in a desiccated state. When transferred to new medium they immediately regenerate protonemata from new filaments of no fixed position. Experiments using activated charcoal, which prevents brood cell formation, and transferring protonemata or spores onto old medium in which brood cells had been produced previously, indicate that the redifferentiation process is triggered by substances released into the medium by protonemata. Addition of abscisic acid, an endogenous growth regulator in mosses, causes precocious brood cell formation in young cultures and is very likely the natural compound triggering their development and inducing their tolerance to desiccation. Key words: abscisic acid, activated charcoal, redifferentiation, desiccation tolerance, microtubules, polarity, protonema, regeneration.