
Plants are remarkably sensitive to their environment, responding by appropriate growth and development to a wide range of environmental stimuli. In the case of gravity, the appropriate response is for stems to grow upwards (βaway from the source of gravityβ; negative geotropism),Β and for roots to grow downwards (βtowards the source of gravityβ; positive geotropism β for examples, see reviews by Elison Blancaflor and Patrick Masson Β and by Miyo Morita). Although the details of the full pathway involved are still the subject of intense research efforts, a role for gravity-stimulated repositioning of cell-located statoliths (starch-bearing amyloplasts) in the gravity-detection side of things has long been proposed. However, the dynamic β rather than settled β nature of such amyloplasts has cast doubt on their effectiveness to act in this way. Now, elegant work by Masatsugu Toyota et al. has demonstrated that amyloplast displacement is necessary for gravisensing (in arabidopsis shoots). Using a custom-built centrifuge microscope they show that βsedimentary movements of amyloplasts under hypergravity conditions are linearly correlated with gravitropic curvature in wild-type stemsβ. Furthermore, and using a range of gravitropic mutants that do not exhibit a normal response under the Earthβs usual 1Β g gravity field, Β they demonstrate that their βhypergravity-induced amyloplast sedimentation and gravitropic curvatureβ¦ was identical to that of wild-type plantsβ. Such work supports the view that arabidopsis shoots do have a gravisensing mechanism that converts the number of gravity-settling amyloplasts into gravitropic signals. And restoration of the gravitropic response by hypergravity in the gravitropic mutants examined indicates that those plants probably also have a functional gravisensing mechanism, albeit one that is not triggered at 1Β g. Nice work. But in view of recent upsets (see previous βOuch! That must hurtβ¦β post), I wonder if it also applies to non-arabidopsis plantsβ¦? Still, it is good to have the odd positive story about arabidopsis (I suppose…!).
[For more on the tangled web that is plant gravity-sensing and involvement of the actin cytoskeleton, check out the recent review by Elison Blancaflor Β β Ed.]
1 comment