A conceptual framework for investigating plant growth movements, with special reference to root gravitropism,...

Microgravity (micro-g) has become established as an important environment in which to conduct experiments that aid in the understanding of gravity-responsive processes in plant growth and development. The gravitropic movement of roots is one example of a gravity-driven differential growth process that can be so investigated, autonomous nutational movements may be another. Although comparison of root movements of agravitropic mutants and their isogenic graviresponsive wild-types in l-g can answer many questions about their gravity-driven processes, they often relate only to gravity perception. Micro-g, however, can help define other constitutive processes which are also influenced by gravity. Examples of the latter are the so-called "tonic" effects, where organ or protoplast weight influences root growth and cellular polarity. A flow-chart is presented which orders the processes of differential growth in roots for analysis using both agravitropic mutants insensitive to gravity and mutant and wild-type roots grown in the micro-g environment. This approach addresses the question whether gravitropism and nutation of root tips are independent or co-existent processes, and also whether gravity determines any features of the nutation, e.g. its amplitude and period. While agravitropic mutants at 1-g can partially answer these questions, micro-g can validate the conclusions and indicate requirements for postulating additional influences such as tonic effects on signal transmission. Gravitropism of roots normally depends on the presence of the root cap, but embryonic tissue and its connection with the seed or grain can also influence gravitropic growth. A hypothetical scheme, based on experimental observations, is proposed for this grain-root-cap interaction. Testing the scheme could be achieved by physically dissociating the compartments presumed to interact in bringing about the tropism. The use of micro-g and agravitropic mutants to compare results with those obtained in 1-g would be of great value in further defining this system.