Cloning and characterization of Tag 1, a tobacco anther β-1,3-glucanase expressed during tetrad dissolution

A critical stage in pollen development is the dissolution of the four products of meiosis, the tetrads, into free microspores. The tetrads are surrounded by a thick callose wall composed of β-1,3-glucan. At the completion of meiosis, the tetrads are released into the anther locule after hydrolysis of the callose by a β-1,3-glucanase. Using the polymerase chain reaction, we have amplified and subsequently cloned a cDNA corresponding to a β-1,3-glucanase, tobacco (Nicotiana tabacum cv. Samsun) anther glucanase (Tag 1), which is expressed exclusively in anthers from meiosis to the free microspore stage of pollen development. The identity of the clone was determined by DNA and deduced protein sequence similarity to other known β-1,3-glucanases. Several regions strictly conserved among four classes of glucanases are also conserved in the Tag 1 protein. Tag 1 represents a novel class of β-1,3-glucanase based on phylogenetic analysis and RNA expression pattern. Tag 1 RNA was detected in situ only in the tapetum, with maximal expression just prior to tetrad dissolution. Due to its expression pattern and sequence similarity to other β-1,3-glucanases, we believe Tag 1 may be involved in tetrad dissolution.