Biology is full of examples of plants that rotate as they grow, but one particular fungus starts off rotating counterclockwise as it grows, then later switches to clockwise. Previous explanations have been incomplete, but theoretical work in the 1 April Physical Review Letters unravels the full mystery. In the model, both directions of rotation occur simultaneously in different regions as a result of mechanical stresses, but each direction dominates at different stages of growth. The work suggests some general principles that may apply to other examples of spiral growth.
Despite being single-celled, the fungus Phycomyces blakesleeanus manages to erect a giant stalk that rises as much as 10 centimeters above the ground, with a spherical shell containing spores at the top. A lot of scientific work has focused on this stalk’s sensitivity to light and other stimuli, but its unique rotation has also garnered much attention. At the start of its final growth stage, the 100-micron-wide cylindrical stalk rotates counterclockwise as it grows upward, motion that’s equivalent to the sense of a right-handed screw, but then after one hour it spontaneously changes direction. It remains left-handed for 2 days–although it sometimes switches again–before completing its growth.
Microscopic studies of spiral or helical patterns in seashells, pinecones, and other life forms often find some preferred orientation, or anisotropy, at the cellular level, that seems to account for the overall spiral direction. However, the anisotropy inside Phycomyces’s stalk wall only seems to account for the left-handed rotation.
The stalk is reinforced by microscopic fibers that wind in a right-handed fashion around the top portion of the stalk, like the stripes on a candy cane. As the outer surface stretches upward during the growth process, the helical fibers also stretch, causing a clockwise rotation–as you would expect from stretching out a coiled spring, so that it begins to “unwind.” This clockwise rotation during growth is in the left-handed sense.
The initial right-handed rotation has been harder to explain. Earlier attempts postulated a separate fiber motion that had little observational basis . Alain Goriely of the University of Oxford and Michael Tabor of the University of Arizona in Tucson have now devised a mathematical model that encompasses all of Phycomyces’s twists and turns. They assume that new fiber material is continuously added inside the so-called growth zone, which is a small region at the top of the stalk. As a fiber grows like a vine coiling around the stalk, it rotates the stalk in the direction of the coil, which is counterclockwise, or right-handed.
This rotation runs in opposition to the stretching and unwinding effect–mentioned above–that occurs simultaneously. Which of these two effects dominates varies along the stalk and depends on the pitch (winding angle) of the helical fiber at each point. Where the pitch is small (nearly horizontal), right-handed expansion is dominant, because this orientation maximizes the growing fiber’s contribution to rotation, while left-handed unwinding becomes more important for a steep (nearly vertical) pitch.
The authors constructed a growth scenario for Phycomyces that accounts for this pitch dependence. They imagined that the stalk fibers begin in a tight (small pitch) helix, which initially rotates the stalk in a right-handed sense. But as the fibers get longer, their lower portions become increasingly more vertical. The top and bottom of the stalk will therefore be turning in opposite directions, which agrees with observations. Eventually, the longer section of left-handed motion at the bottom overwhelms the right-handed motion at the top. Experiments could confirm the pitch dependence derived in the model, says Goriely.
“I think the model is very elegant, as it is able to produce the twisting in different directions without having to recur to complicated biological regulation mechanisms,” says biologist Anja Geitmann from the University of Montreal. She thinks the results could have wider applications, since the fungus’s rotation is representative of other twisting motions in plants. “Modeling this process is valuable, as it will allow us to link biochemical structure with biological form and function,” she says.
- J. Ortega and R. Gamow, “The Problem of Handedness Reversal During the Spiral Growth of Phycomyces,” J. Theor. Biol. 47, 317 (1974).