Phototropin-dependent weak and strong light responses in the determination of branch position in the moss Physcomitrella patens.

Branch position in the moss Physcomitrella patens is regulated by blue light. In this study, fluence rate dependency of branch position determination was investigated by partial cell irradiation with a microbeam. With a 30 Wm(-2) or lower fluence rate, branches formed at the microbeam area, but formed outside the microbeam when the fluence rate was raised to > or = 200 Wm(-2). Thus, both weak and strong light responses influence the determination of branch position. Further, light sensitivity of both responses was reduced in phototropin knock-out lines, revealing an involvement of phototropin as the blue light receptor.