Agriculture Reference
In-Depth Information
needs to be faster. In addition, it is necessary that the activator can activate its own
synthesis because this positive feedback loop allows the rapid amplification of small
fluctuations. In such a scenario, small fluctuations create a local enhancement of the
activator and also of the inhibitor, which in turn leads to the suppression of the ac-
tivator around this local peak. At a distance from already existing peaks, new local
enhancements can occur. This model predicts that the activator and inhibitor show
maximal levels at the same position, which intuitively would seem to be paradoxical.
Currently it is speculated that trichome patterning is in principle based on this
model. The positive patterning genes
GL1
,
TTG
and
GL3
are assumed to locally ac-
tivate their own expression and that of
TRY
and
CPC
. The inhibitors can counteract
their activity by a competition mechanism as described above. Cell-cell interactions
are likely to be mediated by the movement of TRY and CPC through the plasmodes-
mata (Fig. 9.2B) (see also Chapter 5, this volume). This is supported by the finding
that in the root system CPC can move from the cells in which it is expressed into
neighbouring cells (Wada
et al.
, 2002; see also Chapter 8, this volume).
While this model is consistent with the current data, many aspects of the model
still remain to be proven.
9.3.6 Long-range control of trichome initiation by hormones
Among the plant hormones, only gibberellins have been implicated in the control
of trichome initiation. Plants deficient for gibberellin develop no trichomes and
the application of exogenous gibberellin rescues the trichome phenotype (Chien &
Sussex, 1996). Gibberellin appears to act upstream of the trichome patterning genes
as the over-expression of
GL1
and the maize homologue of
GL3
(the R gene) rescues
the trichome phenotype in gibberellin-deficient plants (Perazza
et al.
, 1998). This
regulation of trichome initiation by gibberellins appears to be important to regulate
trichome density on organs and to change the density accompanying the switch from
vegetative to reproductive growth (Telfer
et al.
, 1997).
9.4
Stomatal development and patterning
Stomata are small openings in the plant epidermis that control gas exchange between
the plant and its environment to balance the water household and the levels of carbon
dioxide in relation to photosynthetic activity. The actual pore consists of two guard
cells that can regulate the size of the opening by their turgor (Nadeau & Sack, 2002b).
In dicots the distribution of stomata is not regular, suggesting a random distribution.
However, some patterning mechanism must exist, as stomata are normally not found
directly next to each other. Since new stomata can develop between already existing
stomata, Bunning proposed that stomata create an inhibitory field that prevents new
stomata from developing in their neighbourhood (Bunning & Sagromsky, 1948;
Bunning, 1956). The finding that stomata development involved a stereotypic series
of cell divisions that placed new stomata away from already existing ones suggested
