Each filament is one cell thick and it is composed of many cells attached in series. Each cell is ~ 20 µm in width and ~ 200 µm in length. The cells at the end of these filaments exhibit tip growth and are classified as either chloronemata or caulonemata

Furthermore, scaling laws derived by Campàs and Mahadevan (Campàs and Mahadevan 2009), i.e.,

𝑅
𝑅
𝐴
∼
(
𝑎
𝑅
𝐴
)
2
/
3
(3)
predict that the geometry of the cell is related to the effective size of the secretion zone. Here
𝑅
is the radius of the cell shank,
𝑅
𝐴
is the radius of curvature at the cell tip, and
𝑎
is the effective size of the secretion zone (Campàs and Mahadevan 2009). This implies that by increasing the effective size of the secretion zone, the cell tip becomes more pointed relative to the cell width. These scaling laws are important because they relate the size of the secretion zone to the steady-state growth morphology.

Furthermore, scaling laws derived by Campàs and Mahadevan (Campàs and Mahadevan 2009), i.e.,

𝑅
𝑅
𝐴
∼
(
𝑎
𝑅
𝐴
)
2
/
3
(3)
predict that the geometry of the cell is related to the effective size of the secretion zone. Here
𝑅
is the radius of the cell shank,
𝑅
𝐴
is the radius of curvature at the cell tip, and
𝑎
is the effective size of the secretion zone (Campàs and Mahadevan 2009). This implies that by increasing the effective size of the secretion zone, the cell tip becomes more pointed relative to the cell width. These scaling laws are important because they relate the size of the secretion zone to the steady-state growth morphology

predict that the geometry of the cell is related to the effective size of the secretion zone. Here
𝑅
is the radius of the cell shank,
𝑅
𝐴
is the radius of curvature at the cell tip, and
𝑎
is the effective size of the secretion zone (Campàs and Mahadevan 2009). This implies that by increasing the effective size of the secretion zone, the cell tip becomes more pointed relative to the cell width. These scaling laws are important because they relate the size of the secretion zone to the steady-state growth morphology.

predict that the geometry of the cell is related to the effective size of the secretion zone. Here
𝑅
is the radius of the cell shank,
𝑅
𝐴
is the radius of curvature at the cell tip, and
𝑎
is the effective size of the secretion zone

is the radius of curvature at the cell tip, and
𝑎
is the effective size of the secretion zone

When growth is stopped by caffeine or low temperature, the Ca2+ gradient dissipates, and the Ca2+ influx is reduced

Similarly, when cytosolic Ca2+ concentration is artificially increased (Bibikova et al. 1997) or reduced with Ca2+ channel blockers Lanthanum (La3+) or Gadolinium (Gd3+), growth is arrested

In all these cases, growth reduction resulted in Ca2+ oscillations of longer periods, showing growth and Ca2+ are related

In chitin treated cells, Ca2+ oscillations are not restricted to the apex, but spread across the whole cell, and their periodicity is affected, resulting in lower frequency oscillations. Concurrently with the cytosolic Ca2+ alterations, upon chitin oligosaccharides treatment, the F-actin enrichment at the cell tip dissipates and cell growth stops

Consistently with the growth defect at the cell level, treatment with chitin oligosaccharides result in growth defect at the plant level (Galotto et al. 2020a, b). These results show that defects in Ca2+ oscillations, as a result of a simulated infection, are intimately related to defects in the F-actin cytoskeleton and tip growth.

Cytosolic Ca2+ gradients are thought to regulate the activity of the uniformly distributed actin-binding proteins, which include profilin, L1LIM1, and villins (Bascom et al. 2018a, b; Cheung and Wu 2008; Pollard 2016; Wang et al. 2008; Zhang et al. 2010). By modulating the activity of these proteins, Ca2+ can indirectly influence actin dynamics. This plasticity is probably important for the dynamic organization of the growing machinery at the cell tip.

Cytosolic Ca2+ has also been hypothesized to control vesicle trafficking by blocking myosin activity via its calmodulin domain (Cai and Cresti 2009) or by promoting exocytosis (Battey et al. 1999). Such a model is supported by in vitro evidence, which has shown that myosin XI motility is inactivated by calcium concentrations greater than 1 μM (Yokota et al. 1999). Ca2+-dependent control of myosin XI activity may play an essential role in vesicle clustering and delivery at the tip of protonemata.

In pollen tubes and root hairs, reactive oxygen species (ROS) can modulate intracellular Ca2+ dynamics (Boisson-Dernier et al. 2013; Foreman et al. 2003; Monshausen et al. 2007; Wu et al. 2010). We anticipate that ROS will also play a prominent role in the control of tip growth in protonemata. Increases in ROS and Ca2+ have been observed following fungal infection and chitin elicitation in moss tissue, respectively (Galotto et al. 2020a, b; Ponce De Leon et al. 2012), suggesting that feedback mechanisms involving ROS and Ca2+ could also be conserved in these tip growing cells.

predict that the geometry of the cell is related to the effective size of the secretion zone.

is the radius of the cell shank

This

mplies that by increasing the effective size of the secretion zone, the cell tip becomes more pointed relative to the cell width