Suspensor development in the nun orchid, Phaius tankervilliae

Abstract

The suspensor of the Nun orchid (Phuiur trtnkenilliae) consists of a single cell, which originates as follows: after fertilization, the zygote divides once, unequally, giving rise to a smaller terminal cell and a larger basal cell. At the two-celled stage, a prominent cortical array of microtubules is present in the basal cell. The actin filaments are more centrally located within the cell having a perinuclear arrangement, and some actin material is also present at the micropylar end of the basal cell. After the basal cell divides one more time, the cell near the micropyle enlarges and differentiates into the suspensor cell. The enlargement of the suspensor results primarily from vacuolation. Characteristic organization of the cytoskeletal elements can be found at the time of cell enlargement. The actin filaments are localized in the cortical region of the cell, while the microtubules become internalized, forming a perinuclear array with extensions toward the poles of the cell. The final structural event in the development of the suspensor is the extension of the suspensor cell through the inner integument into the outer integument. At this stage, a cortical array of microtubules once again reappears. The cortical microtlibules of the suspensor near the embryo proper are arranged transversely while those near the tip portion of the suspensor are aligned parallel to the long axis of the cell. The actin filaments remain unchanged and appear as a netlike framework located in the cell cortex. As the embryo matures, the cytoskeletal elements become disorganized and defined structures cannot be discerned.