These filaments consist of two types: a slow-growing, photosynthetically active type and a rapidly expanding type with underdeveloped chloroplasts, called chloronemata and caulonemata, respectively. Both types expand exclusively by highly polarized tip growth to effectively explore the plant’s immediate environmen

(1) Two types of filamentous protonemata (chloronemata + caulonemata) both grow by polarized, unidimensional cell expansion at their apex.

(1) Two types of filamentous protonemata (chloronemata + caulonemata) both grow by polarized, unidimensional cell expansion at their apex. The former produces division planes (red line) perpendicular to the growth axis, while the latter exhibits tilting of the division apparatus (phragmoplast), leading to slanted division planes.

Cell-intrinsic factors are required for internal symmetry breaking and often function via cortically located polarity protein complexes.

Initially, primary filaments have a chloronemal identity, which, after several division rounds of the tip cell, can transition to a caulonemal identity. Notably, the division planes in chloronemata are perpendicular to the growth axis, while those in caulonemata are consistently slanted (Figure 2(B1)). The physiological or developmental relevance of the slanted cross walls for the organism has not yet been established. The chloronema-to-caulonema identity transition is controlled by the plant hormone auxin and a set of conserved transcription factors [9,10]. Interestingly, auxin signaling is important for division plane positioning in other plant systems [11,12], hinting that similar roles may be encountered in moss.

(2) A secondary growth axis (indicated by arrows) within protonemal tissue can be established by branching of subapical cells. This involves cell polarization and control over nuclear position and division plane orientation.

Cell-intrinsic and extrinsic factors are not independent and can operate synergistically or antagonistically in providing positional information

Branching normally occurs on the apex-directed side of a mother cell and is oriented according to environmental inputs like gravity and ligh

Four P. patens tissues/life stages where various aspects of cell division plane orientation and the establishment of new organismal axes can be studied: (1) Two types of filamentous protonemata (chloronemata + caulonemata) both grow by polarized, unidimensional cell expansion at their apex. The former produces division planes (red line) perpendicular to the growth axis, while the latter exhibits tilting of the division apparatus (phragmoplast), leading to slanted division planes. (2) A secondary growth axis (indicated by arrows) within protonemal tissue can be established by branching of subapical cells. This involves cell polarization and control over nuclear position and division plane orientation.

Branching is initiated by a subapical cell and is under the control of hormonal and carbon-related signaling [10,14,15], although it also shows probabilistic elements, with a variable frequency of branching occurring in a typical filament

Branching is initiated by a subapical cell and is under the control of hormonal and carbon-related signaling [10,14,15], although it also shows probabilistic elements, with a variable frequency of branching occurring in a typical filament. Branching normally occurs on the apex-directed side of a mother cell and is oriented according to environmental inputs like gravity and light [16,17]. Prior to visible outgrowth of a new branch, the mother cell undergoes intracellular reorganization (cell polarization) to bring its nucleus and cell division machinery towards the designated branching site.

The formed outgrowth will be separated from the subapical mother cell at the moment of cell division and continues to grow at its tip as a secondary protonemal apical cell.

. Contrary to branching, though, after an initial transition division, the bulge will instead swell in a diffuse manner and then divide in an oblique manner. This oblique division will generate an apical–basal and medial–lateral axi

Thus, a series of asymmetric divisions accomplishes the transition to the 3D body patterning of the more mature gametophore tissues from a precursor tissue with a 2D growth mode. Despite the similarities between branch formation and bud initiation on a protonemal parental cell, the morphology of the outgrowth and the angle of the division plane distinguish the two

However, the precise moment the competency of a subapical caulonemal cell to produce buds is determined is unclear, but fate determination seems at least to be initiated in the parental cell before the division leading to this transition takes place

RIC proteins are characterized by containing the ROP-interactive CRIB (Cdc42-and Rac-Interactive Binding) motif, which is able to physically interact with GTP-bound ROPs. Different functions have been assigned to few members of the Arabidopsis RIC family that involve cytoskeleton reorganization (either actin or microtubule filaments) [27,28]. Based on sequence homology, there is only one putative RIC protein in P. patens

P. patens, with only four almost identical ROP protein family members, has been proposed as a model to study the role of ROPs and their effectors. Recently, it has been suggested that accumulation of tagged PpROP4 not only predicts the sites of filamentous outgrowth (tips and branches), but also the position of new division planes in protonema filaments during cell divisions

SOSEKIs represent an outstanding and intriguing class of polar proteins because their accumulation appears to be independent of the conventional cellular trafficking pathways involved in polar protein delivery

Functionally dissecting the constituent protein domains revealed that SOSEKIs associate with the plasma membrane at specific cell edges via a centrally located domain, where they oligomerize via their N-terminal domain. Collectively, these properties lead to their highly polarized accumulation at cell corners

The function of the phragmoplast is to assemble smaller building blocks supplied by the secretory system into a straight, disc-shaped precursor of the dividing wall

The second is a microtubular ring in the form of the preprophase band (PPB) that forecasts the division plane prior to the start of cell division proper. While the PPB microtubule structure is transient, its position and orientation coincide with a ring-shaped domain at the plasma membrane with a specialized molecular makeup that persists throughout mitosis, called the cortical division zone (CDZ; see glossary).

CDZ fulfils the role of a “molecular memory” to guide expansion of the phragmoplast such that, ultimately, the nascent wall connects to the parental wall at the CDZ-defined position. For recent comprehensive reviews on the CDZ’s molecular makeup and function, see

On the one hand, the net orientation of microtubules at the cellular cortex which is preserved by the PPB correlates well with decisions about where to position the division plane

Consistently, important CDZ markers like POK1 (Phragmoplast Orienting Kinesin 1) localize correctly in absence of PPB microtubules, albeit less efficiently

One such pre-mitotic structure with clear links to division plane determination is a cytoplasmic cloud of microtubules typically associated with one (but occasionally more) side of the nucleus

For example, recent findings show that the microtubule-associated protein TPX2 is essential for maintenance of a central spindle position along the apical–basal axis in buds [62]. This defect could surprisingly be compensated by actin cytoskeleton disruption, which, under normal conditions, does not significantly interfere with spindle/phragmoplast positioning [21,62]. These findings open new avenues for study on the mechanisms controlling “tugging” of the division apparatus during mitosis and its implications for division plane positioning. Another principle that positions the division apparatus involves its communication with the CDZ. In protonemal moss cells, a physical link between the two, mediated by actin and associated myosin Class VIII motor proteins, is established, which assists in division plane guidance