Studies on the early development of zygotic and synergid embryo and endosperm in polyembryonic rice ApIII

Abstract

A morphological, cytological and embryological investigation was conducted on the early development of embryo and endosperm in polyembryonic rice ( Oryza sativa L.) ApIII. We found that the percentage of single-, twin- and triple-seedlings from mature caryopses was 82.4–85.6%, 11.3–14.6% and 2.0–3.8%, respectively. From twin-seedlings one of the plants and in the triple-seedlings usually two seedlings were very weak having a mortality of about 50–60% and over 95%, respectively, and most of the dead seedlings were haploid. In addition, among 674 young caryopses the percentage of single-, double- and triple-embryo sacs was 77.60%, 18.10% and 4.30%, respectively. The development of both embryo and endosperm was observed in the embryo sacs of 653 young caryopses, while embryo development occurred in absence of endosperm in the embryo sacs of 21 ones. Furthermore, after pollen tube penetration, frequently one, rarely two, additional embryos arise via apogamy from the typical specialized synergid of normal egg apparatus with a collapsed cap-neck region that had a damaged filiform apparatus at the micropylar end. Finally, the formation of a strikingly degenerated endosperm cell region is a specialized functional structure that serves for the active transport of metabolites from endosperm to the developing embryo. Taken together, these results suggest that synergid apogamy seems to be the unique reason of polyembryony, poor growth and high mortality of many seedlings may be due to haploidy, and high genetic stability of polyembryony occurs in rice ApIII.