In mosses, the spiral arrangement of leaves (phyllotaxis) is generated not by the hormone-feedback spacing mechanism of flowering plants but by the successive rotation of the division plane of a single tetrahedral apical stem cell. Kamamoto, Tano, Fujimoto and Shimamura (2021) showed in three moss species that the converged divergence angle between successive leaf-producing segments equals this division-plane rotation angle, making the rotation the major regulator of phyllotaxis diversity — while leaving its molecular mechanism unknown. Subsequent work has delivered live time-lapse verification in Physcomitrium patens (Cammarata et al. 2026) and 3D mechano-geometric models showing that geometric division rules (least-area/shortest-wall, maximal-tension) plus division-plane displacement can generate the rotation (Kamamoto and Fujimoto 2026), but both explicitly state that the cellular and molecular mechanism implementing the rotation remains elusive. The open problem is to identify the molecular machinery that sets the magnitude and direction of the rotation angle and thereby the species-specific phyllotaxis, and to establish whether it is conserved across bryophytes.

Background

Phyllotaxis is the arrangement of leaves or leaf-like organs around a plant stem; in spiral phyllotaxis, successive organs appear at a roughly constant divergence angle (the angle between consecutive organs seen from the shoot tip), in angiosperms typically near the golden angle of 137.5°. In flowering plants this pattern arises at a multicellular shoot apical meristem through a spacing mechanism: the plant hormone auxin is transported cell-to-cell by PIN proteins, so new primordia emerge where auxin accumulates, as far as possible from existing ones. Mosses (bryophytes) face the patterning problem with a meristem consisting of a single cell. The leafy moss shoot (gametophore) grows from one tetrahedral apical stem cell (AC) that repeatedly divides asymmetrically: each division cuts off one daughter, called a merophyte, that forms a future leaf plus a portion of the stem, while the other daughter remains the self-renewing AC. Successive division planes of the AC rotate around its axis, and this rotation, not an auxin spacing field, appears to place the leaves. Kamamoto et al. (2021) examined three mosses with different spiral phyllotaxis — Tetraphis pellucida (1/3), Physcomitrium patens (2/5) and Niphotrichum japonicum (3/8) — and measured the divergence angle between centroids of adjacent merophytes on cross-sections: it converged at the outer merophytes to about 120° in T. pellucida, 141° in P. patens and 136° in N. japonicum, with transient deviations near the AC. A mathematical model assuming scaling growth and a constant division-plane rotation angle reproduced the whole sequence, including the transient deviations caused by relocation of the AC centroid at each division, and showed that the converged divergence angle equals the rotation angle. The rotation angle is therefore the major regulator of moss phyllotaxis diversity — but the mechanism that sets it was left open. Two classes of candidate explanation exist. One is geometric/mechanical: plant cells often follow Errera's least-area rule (the new cell wall approximates the minimal-area surface, like a soap film; in cross-section the shortest wall), or Sachs's rules (new wall perpendicular to parental walls); Kamamoto and Fujimoto (2026) showed in 3D mechano-geometric models that the least-area rule combined with apical-cell surface curvature, or a maximal-tension rule, can drive the rotational divisions, and Cammarata et al. (2026) showed by live imaging and modeling that in P. patens the shortest-wall rule combined with displacement of the new wall away from the two oldest walls reproduces the spiral sequence. The other class is molecular: in flowering plants the division plane is commonly marked before mitosis by the preprophase band (PPB), a ring of cortical microtubules, but P. patens does not form PPBs in its asymmetric divisions, where the spindle is instead oriented by cytoplasmic microtubule organizing centers (MTOCs, protein complexes that nucleate microtubule arrays) (Kosetsu et al. 2017). In the Arabidopsis stomatal lineage, which shows spiral-like divisions, the polarity protein BASL (BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE) localizes to cell corners and displaces the nucleus and hence the division plane, with BRX (BREVIS RADIX) proteins co-localizing; P. patens has no BASL homolog but does encode two BRX-like proteins, which remain untested. Mosses do possess molecular machinery that overrides geometric division rules in other contexts — GRAS transcription factors (SHR/SCR/LATERAL SUPPRESSOR) force non-minimal-area divisions in P. patens leaf-vein precursors (Ishikawa et al. 2023) — so molecular control of division-plane choice is possible in this organism, yet nothing of the sort has been identified for the apical cell. Relatedly, the handedness (clockwise vs counter-clockwise) of the rotation is randomly chosen but faithfully maintained, and has been attributed to an unknown, universal direction-sensing mechanism (Zagórska-Marek et al. 2018).

Problem Statement

Identify the molecular mechanism that sets the rotation angle of the apical stem cell division plane in bryophytes, and through it the species-specific spiral phyllotaxis of mosses. Concretely: (i) which molecular components — polarity proteins (e.g., the untested BRX-like proteins of Physcomitrium patens), cytoskeletal structures such as the cytoplasmic microtubule organizing centers or actin/microtubule strands connecting the nucleus to the cortex, or wall-age and mechanical signals — implement the displacement of the division plane from the apical-cell centroid relative to particular daughter walls, such that each successive division rotates by a species-characteristic angle (approximately 120° to 141° across the mosses examined)? (ii) How is the direction (handedness) of the rotation chosen and then faithfully maintained through successive divisions? (iii) Is this mechanism conserved across bryophytes with different phyllotactic patterns (1/3, 2/5, 3/8 spirals) and, where applicable, in other land plants with apical-cell meristems, and how does it interact with the geometric division rules (least-area/shortest-wall and maximal-tension) that current models show can by themselves generate the rotation? Because current geometric and mechanical models reproduce the spiral pattern without identifying its cellular implementation, resolving the problem requires causally connecting molecular components to the measured rotation and divergence angles, rather than merely reproducing the pattern with such models.

References

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