callose

Callose is a plant polysaccharide composed primarily of β-1,3-linked glucose units, with occasional β-1,6 branches. It is synthesized by enzymes known as callose synthases (glucan synthase-like enzymes) and deposited in the cell wall at specific sites where rapid remodeling is needed. Major sites include plasmodesmata, sieve plate pores in phloem tissue, developing pollen walls, and wound or infection sites.

In development and defense, callose plays several key roles. During pollen development, callose forms a transient wall around pollen mother cells and around developing microspores; later, callase enzymes degrade this wall to release mature pollen. At plasmodesmata, callose deposition reduces cell-to-cell transport by narrowing or sealing the channels, thereby regulating intercellular communication during development or in response to stress. In the phloem, callose can be deposited around sieve plate pores after wounding or pathogen attack, helping to contain damage and regulate translocation, though excessive deposition can impede transport.

Regulation and turnover are dynamic. Callose synthesis is triggered by calcium signaling and reactive oxygen species, and is modulated by plant hormones such as salicylic acid and jasmonic acid in defense responses. Degradation is carried out by β-1,3-glucanases (callases), which remove callose when normal communication or wall remodeling is needed again. Callose is distinct from cellulose and represents a rapid, reversible barrier that supports development and defense, making it a useful marker in studies of plasmodesmatal function and phloem physiology.