Gravitropism in Phycomyces: violation of the so‐called resultant law – evidence for two response components

Abstract

We investigated gravitropic bending of sporangiophores of the zygomycete fungus Phycomyces blakesleeanus in response to centrifugal accelerations to test the so-called resultant law of gravitropism ('Resultantengesetz'; Jahrbuch der wissenschaftlichen Botanik, 71, 325, 1929; Der Geotropismus der Pflanzen, Gustav Fischer, Jena, Germany, 1932), which predicts that gravitropic organs orient in a centrifuge rotor parallel to the stimulus vector resulting from the centrifugal acceleration and gravity. Sporangiophores of wild-type strain C171 carAcarR and of several gravitropism mutants were subjected for 7 h to centrifugal accelerations in a dynamic range between 0.01 and 3 × g. The stimulus-response curves that were obtained for C171 carA carR, C2 carA geo and C148 carA geo madC were complex and displayed two response components: a low-acceleration component between 0.01 and 0.5 × g and a high-acceleration component above 0.5 × g. The low acceleration component is characterised by bending angles exceeding those predicted by the resultant law and kinetics faster than that of the second component; in contrast, the high-acceleration component is characterised by bending slightly below the predicted level and kinetics slower than that of the first component. Sporangiophores of the wild-type C171 centrifuged horizontally displayed the opposite behaviour, i.e. low accelerations diminished and high accelerations slightly enhanced bending. Further proof for the existence of the two response components was provided by the phenotype of gravitropism mutants that either lacked the first response component or which caused its overexpression. The tropism mutant C148 with defective madC gene, which codes for a RasGap protein (Fungal Genetics Reports, 60 (Suppl.), Abstract # 211, 2013), displayed hypergravitropism and concomitant deviations from the resultant law that were twice as high as in the wild-type C171. Gravitropism mutants with defects in the genes madF, madG and madJ lacked the low-response component below 0.5 × g. Our data are at variance with the so-called resultant law and imply that gravitropic orientation cannot depend exclusively on the classical sine stimulus (i.e. acting perpendicularly on the side walls); it rather must also be controlled by the cosine stimulus acting parallel to the longitudinal axis of the gravisensing organ. Our studies indicate that the threshold for the cosine response is the same as that of the sine response, and thus close to 0.01 × g.

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

“…The resultant law claims that in such situations the gravitropic organ orients itself parallel to the acceleration vector resulting from the centrifugal and gravitational acceleration. A rigorous quantitative examination of this ‘law’ showed, however, that it does not provide a precise description of the bending of centrifuged sporangiophores of Phycomyces (Göttig & Galland ; this volume). Centrifuge experiments done with sporangiophores of Pilobolus crystallinus also showed a substantial deviation from the resultant law (Horie et al .…”

“…When plant material is tested over an extended dynamic range it becomes apparent that the ‘resultant law’ does not correctly describe the bending pattern of centrifuged hypocotyls, and in particular roots of Arabidopsis seedlings (M. Dümmer & P. Galland, in preparation). The ‘resultant law’ is a corollary of the sine law, and any possible deviation from it represents a strong indication that basic assumptions of the classical statolith–receptor paradigm fail to fully explain the complex spectrum of graviresponses (Göttig & Galland , this volume).…”

“…Removal of Ca 2+ ions elicited hyperpolarisation in the wild type but depolarisation in a madC mutant, from which we now know that it encodes a Ras GTPase activating protein (Polaino Orts et al . ) that confers a hypergravitropic phenotype (Göttig & Galland ; this volume). The tips of submerged sporangiophores display strong outwardly directed ion currents at pH 7.…”

A Ras GTPase associated protein is involved in the phototropic and circadian photobiology responses in fungi

“…The P. blakesleeanus madC mutants have no defects in growth, development (e.g. mating or asexual sporulation) or other tropisms (gravity, avoidance response, photoinduction of sporangiophores or photoinduction of carotene synthesis) although a recent report suggests that one strain carrying a madC mutation may be hypersensitive to gravity40. The madC gene may have specialized its function after gene duplication to be specific to phototropism allowing the other Ras GAP proteins to perform their basic cellular roles.…”

Gravity sensing, a largely misunderstood trigger of plant orientated growth

“…In other words this law predicts that the amplitude of gravitropism depends of the sinus of inclination angle. Even if this sine law has been confirmed in several species both in stems and roots, it is valid only in a limited range of inclination angles, ranging from 0 to 90° (see introduction in Göttig and Galland, 2014) and therefore, it does not characterize gravitropic movements of the whole organ over-time (reviewed in Moulia and Fournier, 2009). Later on, a curvature angle which is the change in tip inclination angle over time was proposed (Galland, 2002; Perbal et al, 2002; Hoshino et al, 2007).…”