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Q23 - Statoliths and Root Geotropism in Microgravity

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Tropisms & Nastic Movements

Statocytes are cells in the central root cap thought to be crucial in geotropism of the plant root. These cells possess starch-filled granules, statoliths, which have a higher density than the statocyte cytoplasm, so their position in the statocyte depends on the root’s orientation relative to the direction of the gravitational force it experiences.

In order to determine the role of statoliths in geotropism and their mechanism of action, Brassica napus seeds were taken to and grown on the International Space Station. Seedlings were grown either in a centrifuge that exerts 1g force (fig A) or in microgravity (negligibly small gravity, fig B). Root tissues were fixed after 40 hours of germination and statolith position determined, represented in a unified coordinate system whose origin (0;0) is the centre of the statocyte.

Two scatter-plot panels (A: 1g, B: microgravity), each with a main scatter (x-axis = statocyte transverse position µm, y-axis = statocyte longitudinal position µm) plus marginal histograms (Axis 3 = count). Panel A (1g): points tightly clustered around a mean near (0, -6.9), with a narrow, unimodal transverse histogram. Panel B (microgravity): points much more widely and evenly scattered, mean near (0, -3.7), with a flatter, more spread-out transverse histogram. Fig.1. Axis 1 = Statocyte transverse position (µm). Axis 2 = Statocyte longitudinal position (µm). Axis 3 = Count.

Each tile below represents the final growth direction of a root tip relative to the direction of gravity (arrow). Determine which of the diagrams depict the distribution of growing root tips in microgravity and in 1g gravitational force - there are two correct answers for each condition.

Seven labeled circular diagrams (A-G), each showing small tick-mark “growth” segments radiating from a circle in different patterns: A = a single dense, tightly-focused directional bundle (straight up); B = segments scattered fairly evenly all the way around the circle; C = two opposite dense bundles (left and right); D = segments scattered around the circle, somewhat sparser than B; E = a dense directional bundle (down) plus some scattered segments around the rest of the circle; F = a dense directional bundle (up) plus a few scattered segments; G = a strongly dominant single directional bundle (down) with almost no scatter elsewhere. Fig.2.

Assume statocytes are cylindrical. Statoliths can move freely across the diameter of the statocyte, but can only move 40% of the statocyte’s length. Estimate the approximate diameter and length of a statocyte, in µm.

Q23.1. Brassica napus seedlings were grown on the ISS either in a centrifuge exerting 1g force, or in microgravity, to test how statolith position guides root geotropism. Diagrams A-G show different distributions of final root-tip growth direction relative to gravity (arrow). Two diagrams correctly depict the 1g condition, which two?
Q23.2. Which two diagrams correctly depict the microgravity condition?
Q23.3. Assume statocytes are cylindrical, and statoliths can move freely across the full diameter of the statocyte but only 40% of its length. Using the transverse (diameter-axis) spread of statolith positions from Fig.1, estimate the statocyte's diameter, in µm.
Q23.4. Using the longitudinal spread of statolith positions and the fact that statoliths only cover 40% of the statocyte's length, estimate the statocyte's length, in µm.
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key