A. Indrianto et al., Tracking individual wheat microspores in vitro: identification of embryogenic microspores and body axis formation in the embryo, PLANTA, 212(2), 2001, pp. 163-174
The development of isolated, defined wheat microspores undergoing in vitro
embryogenesis has been followed by cell tracking. Isolated wheat (triticum
aestivum L.). microspores were immobilized in Sea Plaque agarose supported
by a polypropylene mesh at a low cell density and cultured in a hormone-fre
e, maltose-containing medium in the presence of ovaries serving as a condit
ioning factor. Embryogenesis was followed in microspores isolated from imma
ture anthers of freshly cut tillers or from heat- and starvation-treated, e
xcised anthers. Three types of microspore were identified on the basis of t
heir cytological features at the start of culture. Type-1 microspores had a
big central vacuole and a nucleus close to the microspore wall, usually op
posite to the germ pore. This type was identical to the late microspore sta
ge in anthers developing in vivo. Microspores with a fragmented vacuole and
a peripheral cytoplasmic pocket containing the nucleus were defined as typ
e 2. In type-3 microspores the nucleus was positioned in a cytoplasmic pock
et in the centre of the microspore. Tracking revealed that, irrespective of
origin, type-1 microspores first developed into type 2 and then into type-
3 microspores. After a few more days, type-3 microspores absorbed their vac
uoles and differentiated into cytoplasm-rich and starch-accumulating cells,
which then divided to form multicellular structures. Apparently the three
types of microspore represent stages in a continuous process and not, as pr
eviously assumed, distinct classes of responding and non-responding microsp
ores. The first cell division of the embryogenic microspores was always sym
metric. Cell tracking also revealed that the original microspore wall opene
d opposite to a region in the multicellular microspore which consisted of c
ells containing starch grains while the remaining cells were starch grain-f
ree. The starch-containing cells were located close to the germ pore of the
microspore. In more advanced embryos the broken microspore wall was detect
ed at the root pole of the embryo.