Agriculture Reference
In-Depth Information
division of inner layers, suggesting that the ENOD40 peptide signals produced in the
epidermal cells may move to the inner cell layers to induce cell proliferation (Sousa
et al.
, 2001). Such a short-range movement can be achieved by diffusing through
plasmodesmata. Alternatively, the ENOD40 action may generate a secondary signal
that transmits the information to other cells to induce cell division. More work is
needed to understand the mode of action underlying the ENOD40-mediated nodu-
lation and cell proliferation processes.
2.2.4
PSK (phytosulfokine) is a mitogenic factor
Synthetic hormones such as auxin and cytokinin are usually added to cultured cells to
stimulate cell proliferation. In addition, dispersed culture cells must reach a required
initial cell density in order to proliferate. Low-density suspension cell cultures dis-
play strikingly low mitotic activity that could not be improved by supplementation
with known plant hormones. However, adding conditioned medium derived from
rapidly growing cell culture to a low-density culture can stimulate proliferation,
suggesting that a mitogenic factor is secreted into the medium.
A suspension culture system of mesophyll cells prepared from
Asparagus
officinalis
was used to purify the mitogenic factor (Matsubayashi & Sakagami,
1996). Two mitogenic factors were identified to be a sulfated pentapeptide (H-Tyr
(SO3H)-Ile-Tyr(SO3H)-Thr-Gln-OH) and a sulfated tetrapeptide (H-Tyr(SO3H)-
Ile-Tyr(SO3H)-Thr-OH). The two peptides are derived from a common precursor.
The peptides were named phytosulfokine-a and -b (PSK-a and -b ). Truncated
PSK which is missing the two C-terminal residues still retained up to 20% activity,
whereas truncation of any of the three N-terminal residues abolished its activity,
indicating that the N-terminal tripeptide fragment is the active core (Matsubayashi
et al.
, 1996). In addition, the sulfate group was found to be essential for its activity.
Incubation of low-density cells with either of the PSKs at nanomolar concentra-
tions in combination with other hormones stimulates cell proliferation. The failure
in proliferation of low-density cell culture is likely caused by the dilution of PSK
below its active concentration in initial cultures. It has been proposed that PSKs
are synthesized in culture cells in response to auxin and cytokinin and secreted into
medium to regulate cell dedifferentiation and activate the cell cycle (Matsubayashi
et al.
, 1999).
PSK-a was also identified in the conditioned medium of rice culture cells (Mat-
subayashi
et al.
, 1997). The cloned rice PSK gene encodes a 89-amino acid product,
preprophytosulfokine, which contains a 22-amino acid cleavable leader sequence
(Yang
et al.
, 1999). The PSK sequence is located near the C-terminus of the pre-
cursor. The PSK gene is expressed at a relatively high level in rice culture cells. It
is also expressed at a low level in rice seedlings, indicating that the hormone play
an important role in cell proliferation in culture
and
intact plants. Rice culture cells
overexpressing the rice PSK gene were found to divide approximately twofold faster
than wild-type cells. In contrast, antisense-mediated suppression of the rice gene
caused a decrease in cell division of the rice cultures.
