Hormones in zygotic and microspore embryos of Brassica napus

Abstract

A number of studies have used microspore-derived embryos (MDEs) as amodel for examining a range of processes, including hormonal regulation ofembryo development. We examined the hormonal physiology of MDEs with theprimaryobjective of testing the validity of using the MDE system as a model forhormonally-regulated development in zygotic embryos, through late stages. To dothis we identified and quantified endogenous levels of abscisic acid (ABA),indole-3-acetic acid (IAA) and a number of gibberellins (GAs), includingGA19, GA20, GA1 and GA8 in bothMDEsand zygotic embryos. The presence of GA19, together with itsC19 metabolites indicates that the early-13 hydroxylation pathway isoperative in both embryo systems. Gibberellins A4 and GA9were also identified, thereby confirming the presence of the earlynon-hydroxylation pathway in B. napus MDEs and zygoticembryos. In general, the pattern of change of hormone (ABA, IAA, GA1and GA20) content per embryo through embryogenesis was similar forMDEs and zygotic embryos. Indole-3-acetic acid and GA1 increased toamaximum at day 30 after culture (DAC) before decreasing. Abscisicacid levels increased to a maximum at ∼day 35, and declined in zygoticembryos but not in MDEs. GA20 increased to the final harvest atmaturity, or day 40. The absolute content (g/embryo) of each hormone, howeverwas appreciably lower (5- to 15- fold) in the MDEs. This was not the result ofdilution into surrounding medium for ABA or IAA; GA1, however, didaccumulate in the medium. Although there were absolute quantitative differencesin the levels of IAA and ABA found in the two embryo systems, the similaritiesin the pattern of hormone changes suggests that the MDE system can serve as auseful model for examining the physiological roles of hormones duringembryogenesis.