Male gametophyte development is defective in mms21-1 mutants. Anther development at stages 6, 7, 8, 11, 13 in the wild-type (A–E) and mms21-1 mutant (F–J). (A, F) Anther stage 6. PMCs have separated and are ready to undergo meiosis. (B, G) Anther stage 7. Uniform size tetrads (Tds) in wild-type, dyads and triads in mms21-1 mutant. (C, H) Anther stage 8. Note the dyads in mms21-1 mutant. (D, I) Anther stage 11. Pollen were uniform size in wild-type but variable size and shrunken in mms21-1 mutant. (E, J) Anther stage13. In mms21-1 mutant pollen grains were in variable size and shrunken.MC, meiotic cell; Tds, tetrads; MSp, microspores; PG, pollen grains; T, tapetum. Bars = 20 μm. (K-N) DAPI staining of wild-type anthers at different development stages. Pollen grains are of uniform size in the wild-type. (O-R) DAPI staining of mms21-1 anthers at stages equivalent to (K-N). Arrows in (O) indicate that there are dyads and triads at tetrad stage in mms21-1. Arrows in (P) indicate that visible variable size (red arrow) and some had two nuclei (white arrow) in uninucleus microspore stage in mms21-1. Arrows Q indicate that abnormal nuclear contents pollen Arrows in R indicate larger pollen (white arrow) and pollen with no DAPI staining (red arrow) in mms21-1. Bars = 20 μm. S, AtMMS21 expression was detected in PMCs by in situ hybridization. T, No hybridization signal was detected when a sense probe was used.