Gravitropic responses of wild-type and mutant strains of the moss Physcomitrella patens

The gravitropic responses of dark-grown caulonemata and gametophores of wild-type and mutant strains of the moss Physcomitrella patens have been investigated. In the wild-type both caulonemata and gametophores show negative orthogravitropism. No gravitropic response is observed when plants are rotated slowly on a clinostat and the inductive effect of gravity can be replaced by centrifugal force. The gravitropic response of caulonemanta is biphasic, consisting of an initial phase producing a bend of about 20 degrees within 12 h of 90 degrees reorientation and a subsequent slower phase leading to completion of the 90 degrees curvature. No obvious sedimentation of statoliths accompanies this response. Several mutants have been isolated that are either partially or completely impaired in caulonemal gravitropism and one mutant shows a positive gravitropic response. Complementation analysis using somatic hybrids obtained following protoplast fusion indicates that at least three genes can mutate to give an altered gravitropic phenotype. None of these mutants is altered in gametophore gravitropism, suggesting that the gravitropic response of caulonemal filaments may require at least some gene products that are not required for the response of the multicellular gametophores. One class of mutant with impaired caulonemal gravitropism shows a pleiotropic alteration in leaf shape.