Biology Reference
In-Depth Information
the nectary, located at the base of the gynoecium, is a distinct organ from the
remainder of the gynoecium. It is small and does not extend out from the
gynoecium. However, by stage 6, the nectary has enlarged significantly, and
by stage 12, the nectary is considerably larger still.
It is also apparent that the colour of the nectary of ornamental tobacco
changes significantly during the process of development. At stage 2, the
colour is indistinguishable from the remainder of the gynoecium. By stage 6,
it is a pale yellow colour and by stage 12, a deep pumpkin-orange. There-
fore, it is clear that the development of the ornamental tobacco nectary is a
complex process that results in both obvious and non-obvious changes in the
organ. Defining these changes in the floral nectary as it develops will lead to
an increased understanding of this unusual floral organ, and may permit
long-term manipulation of nectary function to achieve improved pollinator
attraction and increased yields among a variety of angiosperms.
3.1
Origin of the floral nectary
Very little is known about the origins of the floral nectary in ornamental
tobacco. To investigate this, one must turn to other species.
Arabidopsis tha-
liana
, in particular, has proven instrumental in understanding the origins of
the floral nectary.
In attempts to understand the origin of the carpel, a unique gene termed
CRABS CLAW
was identified that affected not only the carpel but also the
floral nectaries. To date, this is the only known gene that uniquely affects
nectary function and development.
CRABS CLAW
belongs to the small
YABBY family of transcription factors. The
CRABS CLAW
protein contains
a zinc finger and a helix-loop-helix domain that are thought to mediate DNA
binding (Bowman & Smyth, 1999). Mutants of
CRABS CLAW
cause the gy-
noecium to develop into a wide, short structure in which the apical ends of
the two carpels remain unfused. In addition, the
CRABS CLAW
mutants also
lack floral nectaries (Bowman & Smyth, 1999).
Most of the subsequent work in this arena has been aimed at characteriz-
ing the other YABBY family members (
FILAMENTOUS FLOWER/YABBY3,
INNER NO OUTER
,
YABBY2,
YABBY5
) and the YABBY interaction part-
ners that function in plants to promote an adaxial/abaxial asymmetry in the
lateral organs (Siegfried et al., 1999; Eshed et al., 1999; Golz et al., 2002).
However,
CRABS CLAW
has been shown to suppress the radial growth of
the developing gynoecium and to promote its longitudinal growth (Alvarez