Chitosan Stimulates Root Hair Callose Deposition and Inhibits Root Hair Growth

The study demonstrates that root hairs (RHs) of Arabidopsis respond to low concentrations (0.001%) of chitosan by depositing callose and slowing their growth. The enzyme glucan synthase-like 5 (GSL5)/PMR4 is essential for this callose deposition but not for the inhibition of RH growth. At higher chitosan concentrations (0.01%), callose deposition does not occur, yet RH growth inhibition is more pronounced. The research also reveals that chitosan-induced callose deposition is a conserved response among soil-anchoring plant cells, including rhizoids and rhizophores from bryophytes and lycophytes, respectively, suggesting it is a common strategy to cope with mild biotic stress. High concentrations of chitosan disrupt intracellular dynamics and endomembrane compartments at the RH tip, which likely contributes to the inhibition of growth and prevents callose deposition.