Cell cycle events in developing neem (Azadirachta indica) seeds: are they related to intermediate storage behaviour?

Abstract

AbstractSeeds of neem (Azadirachta indicaA. Juss.), a multipurpose tropical tree, have limited desiccation tolerance. Whether their intermediate storage behaviour might be caused by an elevated percentage of 4C nuclei in the embryo at maturity was investigated. Morphological development of neem seeds was monitored on selected trees in Ouagadougou, Burkina Faso. Developing seeds were tested for germinability, and cell cycle events were determined by using flow cytometry and analysing the level of β-tubulin. Germination could occur after 8 weeks of seed development, but normal seedlings resulted only after 10 weeks or more. The change in fruit colour from green to yellow began after approximately 12 weeks of seed development. Immature, 4-week-old embryos about 2 mm in length had 15% of cells in the G2phase and 60% in the G1phase of the cell cycle, as indicated by 4C and 2C nuclear DNA levels, respectively. During maturation, the proportion of G2cells declined to 3% at full maturity and slightly further after drying, and the proportion of G1cells increased to approximately 90%. A strong β-tubulin signal was observed in tips of young embryonic radicles and cotyledons, but a weak or non-detectable signal was found in 9-week-old ones and in those from green-mature and yellow fruits. Because DNA replication and β-tubulin level were almost negligible at seed maturity, as in orthodox tomato seeds, it is suggested that these factors are not involved in the intermediate storage behaviour of neem seeds.