Lessons from leaf epidermal patterning in plants - Intercellular Communication in Plants - page 236

9.4.2 Cell signalling and the control of asymmetric cell divisions during
stomata development
The genetic, morphological and molecular characterization of two genes, TOO
MANY MOUTHS (TMM) and STOMATAL DENSITY AND DISTRIBUTION1
(SDD1) , has revealed some insights into the underlying mechanism of stomata
patterning.
In tmm mutants, stomata are found in clusters that can contain more than 20
individual stomata (Yang & Sack, 1995). The main defects in tmm mutants are
the randomization of the orientation and the number of asymmetric divisions in
cells adjacent to pre-existing stomata or meristemoids (Fig. 9.4B) (Geisler et al. ,
2000). The molecular cloning of the TMM gene indicated that TMM serves to
receive and relay signals coming from the neighbouring cells (Nadeau & Sack,
2002a). TMM encodes a protein with sequence similarity to leucine-rich-repeat-
containing receptor-like proteins (LRR-RLP). The presence of a signal peptide and
a transmembrane domain and in vivo localization studies with a TMM:GFP fusion
protein suggest that TMM protein localizes to the plasma membrane. By analogy to
other known LRR-RLP proteins, a likely molecular function of TMM is to perceive
extracellular signals via its extracellular domain. Upon binding of a specific ligand
the signal would be relayed into the cell. As TMM has no cytoplasmic kinase
domain, it is likely that TMM interacts with additional factors for intracellular signal
transduction. Consistent with its proposed function in controlling asymmetric cell
divisions in meristemoids and neighbouring cells, TMM is expressed in exactly
these cells.
The sdd1 mutants show increased stomatal density and a very much higher fre-
quency of adjacent stomata than wild-type plants. However, individual clusters con-
tain far fewer stomata than tmm mutants. Also, the proportion of cells that enter
the stomatal pathway is increased by about twofold compared to wild type. It has
been found that all the stomata in these clusters are derived from satellite meriste-
moids (Fig. 9.4B). SDD1 encodes a subtilisin-like serine protease and it is there-
fore likely that SDD1 acts to cleave or modify other proteins (Berger & Altmann,
2000; Von Groll et al. , 2002). The expression of SDD1 was found specifically in
meristemoids and not in neighbouring cells. Although SDD1 has neither a predicted
transmembrane domain nor post-translational membrane-association motifs, in vivo
localization experiments using a SDD1:GFP fusion showed that the fusion protein
is localized to the plasma membrane. It is therefore possible that SDD1 is involved
in the production of the ligand that relays the signal to TMM (Fig. 9.4C). This view
is supported by genetic experiments that show that SDD1 and TMM act in the same
pathway. Over-expression of SDD1 causes a drastic reduction of stomata. This dom-
inant effect of SDD1 over-expression is rescued when TMM is absent, indicating
that SDD1 function is mediated by TMM (Von Groll et al. , 2002).
A likely scenario of how asymmetric cell divisions are controlled is that SDD1 is
expressed in meristemoids or guard mother cells where it produces an extracellular
signal. This is received and relayed into meristemoid as well as the sister cells by