Peptides as signals - Intercellular Communication in Plants - page 35

A homology search for Arabidopsis genes encoding the PSK sequence led to the
identification of at least four putative AtPSK s, named AtPSK1 , AtPSK2 , AtPSK3 ,
and AtPSK5 , whose designated numbers reflect their chromosomal location (Yang
et al. , 2001). These four genes encode 67-87-amino acid polypeptides. The deduced
Arabidopsis PSK precursors and the rice PSK precursor share little overall sequence
similarity except that they are identical in an 8-amino acid region (termed the PSK
domain), which includes the PSK sequence. In Arabidopsis cell cultures, both PSK-a
and PSK-b are secreted to the medium. The Arabidopsis PSK genes are expressed in
apical meristem, leaves, hypocotyls, and roots, again indicating that the PSK signals
function not only in culture cells but also in intact plants.
Specific high-affinity binding activities for PSK-a have been detected in the
outer surface of rice's plasma membrane. The specific binding was altered by the
pH condition and ionic interaction, suggesting that the ligand-receptor binding is
controlled by ionic interaction (Matsubayashi & Sakagami, 2000). The photoaffin-
ity cross-linking study revealed the existence of two putative PSK receptors in
rice. They are glycosylated proteins with molecular weights of 120 and 160 kDa
respectively. The carrot cell line NC contains a relatively high concentration of
high-affinity PSK-binding protein and therefore was used to purify the PSK recep-
tor. Photoaffinity labeling of membrane proteins from the carrot cell line with a
photoactivatable PSK analog revealed that a 120-kDa protein and a minor 150-kDa
protein, both of which are glycosylated proteins, bind to PSK. The two binding pro-
teins were purified and identified as an LRR-containing receptor kinase that shares
sequence similarity to those of the known hormone receptors such as BRI1 and
CLV1 (Matsubayashi et al. , 2002). The 150-kDa protein is likely a modified form
of the 120-kDa receptor.
The PSK receptor gene encodes a 1021-amino acid polypeptide with a predicted
cleavable signal peptide. It contains an extracellular N-terminus with 21 LRR re-
peats, a transmembrane domain, and a cytoplasmic serine/threonine kinase domain.
Suppression of the receptor gene by an antisense construct caused inhibition of PSK-
mediated cell proliferation, further demonstrating its function as the PSK receptor.
Apparently, multiple hormones including auxin, cytokinin, and PSK interact to
regulate differentiation and proliferation of cultured cells. It is intriguing to speculate
that many other auxin- and/or cytokinin-mediated biological pathways in plants
may involve peptide signals. Identification of downstream components in the PSK
signaling pathway should provide novel insights into molecular mechanisms that
control cell differentiation and proliferation.

2.2.5 CLAVATA 3 (CLV3) regulates stem cell homeostasis

Embryogenesis in animals generates a miniature version of an adult organism. In
contrast, a plant embryo has a much simpler structure that contains two stem-cell
populations: the shoot apical meristem and the root apical meristem (Clark, 2001).
The body patterns of an adult plant are largely established by postembryonic pat-
tern formation at these meristems. The shoot apical meristem (SAM) acts as a