170 Y.-R.J.Lee·B.Liu
rical restriction defined by the rigid cell wall. For example, at metaphase
Tradescantia generative cells have kinetochore fibers arranged along the
cell axis (Liu and Palevitz 1992). Nevertheless, plant spindle, no matter
how “distorted”, still functions perfectly to faithfully segregate genetic ma-
terial. During mitosis and meiosis, specific activities like the organiza-
tion of spindle poles, attachment of kinetochores to the microtubule plus
end, chromotid movement towards the poles, and microtubule polymeriza-
tion/depolymerization would be dependent on activities of motor proteins
acting on spindle microtubules. Compared to microtubules, during mitosis
microfilaments exhibit a cortical array encaging the two half spindles (Liu and
Palevitz 1992). Thus, microfilament-based motor proteins unlikely contribute
to mitosis and meiosis directly in flowering plants.
The phragmoplast is organized with a framework of an anti-parallel mi-
crotubule array in which microtubule plus ends are juxtaposed in the middle.
Microfilaments as well as membranous structures like the endoplasmic retic-
ulum, vesicles, and other organelles are arranged in close proximity to micro-
tubules (Staehelin and Hepler 1996). Microtubules are essential for assembly
of the cell plate as vesicles destined to the division site are largely transported
along phragmoplast microtubules. In addition to being motors for vesicle
transport in the phragmoplast, microtubule-based motors also contribute to
organization of phragmoplast microtubules. Microfilaments are arranged in
a similar but distinct pattern as that of microtubules in the phragmoplast
as they do not occupy the equatorial region (Zhang et al. 1993). Microfila-
ments are required for proper orientation of the cell plate although they are
not essential for cell plate formation (Palevitz 1980).
The presence of both microtubule-based and microfilament-based motor
proteins in plant cells was first confirmed by biochemical means around the
late 1980s and early 1990s (Asada et al. 1991; Ma and Yen 1989). The splendid
pictureofthediversityofthesemotorproteinswasnotrevealeduntilrecently
when the genomes of model plants were completely sequenced. Compared to
animal species, plants have a more complicated family of microtubule motor
kinesins and a simpler family of microfilament motor myosins.
1.1
Kinesin Superfamily Proteins in Plants
Directional movement along microtubules is powered by microtubule-based
motors. Members of the kinesin superfamily are one of the two types of such
motor proteins (Kreis and Vale 1999). The other type is dynein. However, to
date completed genomes of flowering plants do not contain genes encoding
the heavy chain of dynein (Lawrence et al. 2001). Thus, microtubule-based
motility in plant cells is likely driven by kinesins only.
Members of the kinesin superfamily contain a conserved kinesin motor
domain of approximately 350 amino acids with an ATP-binding motif and