No abstract

In addition to shoots and roots, the gravity (g)-vector orients the growth of specialized cells such as the apical cell of dark-grown moss protonemata. Each apical cell of the moss Ceratodon purpureus senses the g-vector and adjusts polar growth accordingly producing entire cultures of upright protonemata (negative gravitropism). The effect of withdrawing a constant gravity stimulus on moss growth was studied on two NASA Space Shuttle (STS) missions as well as during clinostat rotation on earth. Cultures grown in microgravity (spaceflight) on the STS-87 mission exhibited two successive phases of non-random growth and patterning, a radial outgrowth followed by the formation of net clockwise spiral growth. Also, cultures pre-aligned by unilateral light developed clockwise hooks during the subsequent dark period. The second spaceflight experiment flew on STS-107 which disintegrated during its descent on 1 February 2003. However, most of the moss experimental hardware was recovered on the ground, and most cultures, which had been chemically fixed during spaceflight, were retrieved. Almost all intact STS-107 cultures displayed strong spiral growth. Non-random culture growth including clockwise spiral growth was also observed after clinostat rotation. Together these data demonstrate the existence of default non-random growth patterns that develop at a population level in microgravity, a response that must normally be overridden and masked by a constant g-vector on earth.

Abstract. Gravitropic bending of fruiting bodies of Flammulina velutipes (Curtis) Karst. is based on the dierential growth of the transition zone between stem and cap. Reorientation becomes visible as early as 2 h after displacing the fruiting body from the vertical to the horizontal position. It is preceded by a preferential accumulation of microvesicles within the hyphae on the lower side of the transition zone and related to an increase in the vacuolar compartment required for hyphal extension. A model made of a bundle of interconnected balloons is used to demonstrate that a dierential volume increase at one¯ank is sucient to bend the entire structure in the opposite direction. Gravitropic raising of intact stems or segments derived from the transition zone requires positional information which can be accomplished by three major, coordinated events: (i) gravisensing by the individual hyphae within the transition zone, (ii) unidirectional signalling by means of a soluble growth factor creating a vertical concentration gradient, and (iii) translation of the concentration signal into elongation growth.

No abstract