Biology Reference
In-Depth Information
& Smyth, 1999). Further,
CRABS CLAW
functions independently of the ABC
floral identity and its ectopic expression is not sufficient for nectary develop-
ment (Baum et al., 2001).
Recent studies indicate that
CRABS CLAW
is regulated by a number of
positive and negative regulators expressed in the nectary Anlagen (Lee et al.,
2005a). A phylogenetic footprinting of the
CRABS CLAW
promoter identi-
fied a number of regulatory elements in the promoter that included putative
binding sites for
LEAFY
and MADS-box transcription factors. The authors
propose that B-class and C-class genes act redundantly with each other and
in combination with the
SEPALLATA
genes to activate
CRABS CLAW
in the
nectary Anlagen. The transcription factors
SHATTERPROOF1/2
may also
participate in the transcriptional regulation of
CRABS CLAW
in appropriate
backgrounds (Lee et al., 2005a). Additional studies demonstrate that both
Rosids and Asterids (Brassicaceae, Solanaceae, and Malvaceae) utilize
CRABS
CLAW
as a general regulator of nectary development (Lee et al., 2005b).
Thus,
CRABS CLAW
is widely expressed among the angiosperms and
appears to function early in nectary gland formation. Further studies will be
required to define the biochemical mechanisms and downstream events me-
diated by
CRABS CLAW
expression that (along with other transcription
factors) result in nectary formation.
3.2
Conversion of chloroplasts into chromoplasts
Perhaps the most dramatic change in the nectary of ornamental tobacco, as it
develops, is its change in colour. At early stages of development, the nectary
is lime green due to the differentiation of chloroplasts in the nectary cells.
About the middle of the developmental profile, the nectary begins to take on
a pale yellow hue and with further development, the nectary becomes bright
orange. This bright orange pigment, which accumulates to tremendous levels
in the nectaries, has been isolated and characterized. It matches both the re-
tention time and the spectral properties of pure β-carotene (Horner et al., 2007).
The reason that the nectary would accumulate such a high concentration
of β-carotene is not entirely clear. However, it must be noted that the nectary
undergoes a huge oxidative stress as a result of the nectar redox cycle (see
Nicolson & Thornburg 2007, Chapter 5 in this volume) and the β-carotene
very likely serves as an intracellular antioxidant to inhibit damage to the nec-
tary caused by the highly oxidative environment in the nectar.
Because β-carotene is synthesized and accumulates in plastids, we began
by examining the nature of nectary plastids during development. The plastids
that accumulate in the nectary can be morphologically divided into at least
