Gravitropism in Physcomitrella Patens: A Microtubule Dependent Process

Abstract

The plant cytoskeleton plays an important role in the early stages of gravisignaling (Kiss, 2000). Although in vascular plants, actin filaments are used predominantly to sense changes in the gravity vector, microtubules have been shown to play an important role in moss gravitropism (Schwuchow et al., 1990). The moss Physcomitrella patens is a model organism and was used here to investigate the role of microtubules with respect to the gravitropic response. Dark grown caulonemal filaments of P. patens are negatively gravitropic and the readily imaged tip growing apical cell is a 'single-cell system' which both senses and responds to changes in the gravity vector. MTs were imaged before and after gravistimulation with and without MT depolymerizing agents. Six-day-old filaments were embedded in low melting agarose under dim green light, allowed to recover overnight in darkness and gravistimulated for 15, 30, 60 and 120 min. Using indirect immunofluorecence and high resolution imaging, MTs were seen to accumulate in the lower flank of the gravistimulated tip cell starting 30 min post turning and peaking 60 min after gravistimulation of the cells. The microtubule depolymerizing drug, oryzalin (0.1 μM for 5 min), caused MTs to disintegrate and delayed MT redistribution by 3hrs 30min. Growth of the oryzalin treated filaments was analyzed and a delay in growth was observed for both gravi and non-gravistimulated filaments. Tip cells bulged and sometimes branched after 75 min. This study demonstrates that microtubules are important for growth in P. patens and MT depolymerization leads to a delayed growth and graviresponse.

(Supported by NASA grant NAGW 4984)