In vitro response of Indica rice microspores subjected to cold stress: a cytological and histological perspective

This study investigated developmental changes in cold stressed microspores of Indica rice variety At 303. After 3 d at 10 degrees C, approximately 76% of microspores were in the late uni-nucleate stage. Even after 5 d, 49% of viable cells remained in the late uni-nucleate stage without advancing to the bi-nucleate stage. In comparison, microspores undergoing normal gametogenesis in planta progressed rapidly from uni-nucleate to bi-nucleate stage eventually forming tri-nucleate pollen during this period. Thus, cold stress prevented normal microspore development and retained cells in the late uni-nucleate stage which is the most favorable stage for in vitro induction of sporophytic development in rice. Uni-nucleate microspores subjected to cold stress showed a characteristic pattern consisting of several minute vacuoles surrounding a centrally positioned nucleus, which can be interpreted as an early indicator of sporophytic determination in Indica rice microspores. During in vitro culture phase, freshly plated yellow anthers became brown. After 4 wk in culture, 51% of the anthers had discolored. Significantly, all yellow anthers contained only non-viable cells whereas 10% of the brown anthers had few viable cells. Some microspores in brown anthers underwent division on callus induction medium. The first division was symmetrical and occurred after 2 wk. The second division occurred after 4 wk and resulted in four-celled structures. Anther-derived callus was either compact or friable. Histo-differentiation occurred mostly from compact callus. Cell clusters, each delimited by a protoderm, were observed in histological sections of callus grown for 2 to 4 wk on regeneration medium. Within a cellular unit, two heterogeneous cell populations were arranged in concentric rings with larger cells in the center and smaller cells towards the periphery. However, an apical-basal polarity that is present in embryo-like structures was not observed. Therefore, it may be surmised that in Indica rice, regeneration from anther-derived callus takes place not by the formation of somatic embryos but by direct organogenesis.