Agriculture Reference
In-Depth Information
glycolytic pathway and
LeT6
, which encodes a homeobox transcription factor. The
mutant shows more compound leaves than in wild-type plants. Grafting of wild-type
shoots onto stocks of the dominant
mouse ears
mutant caused the morphology of
leaves at the apex of the plant to appear similar to those of
mouse ears
mutants, and
this correlated with the presence of increased abundance of
mouse ears
mRNA in
the wild-type shoot (Kim
et al.
, 2001).
Positioning the floral stimulus within a regulatory hierarchy will allow it to be
approached using the molecular genetic tools available in
Arabidopsis
. Screening
for mutations that suppress the early-flowering phenotype caused by expressing
CO
or
FT
in the phloem, or screening randomly for further proteins and RNAs
that influence flowering-time when expressed in the phloem, is likely to identify
further components of the system. Similarly, use of full genome microarrays will
enable identification of all of the genes whose expression is induced by overexpres-
sion of
CO
in the phloem, and therefore provide leads as to the identity of mobile
components.
7.3
Intercellular communication during floral development
Dramatic changes in gene expression occur at the apex of the plant on the transition
to flowering. Recently, these changes were analyzed using microarrays containing
oligonucleotides derived from almost all
Arabidopsis
genes, and within 7 days of
a shift from short to long day lengths, the expression of 332 genes was induced or
repressed at least twofold in both the Landsberg
erecta
and Columbia accessions
(Schmid
et al.
, 2003). Perhaps surprisingly, of these 332 genes, the majority (231)
showed reduced expression during floral induction, whereas the remainder were
increased. At least one of the repressed genes, encoding an AP2-like transcription
factor, was shown to actively repress flowering when overexpressed, suggesting
that its downregulation during flowering may play an active role in floral induction
(Schmid
et al.
, 2003). Similarly, some of the earliest genes that show increased ex-
pression during floral induction appear to promote flowering. For example, the
SOC1
gene of
Arabidopsis
is induced at the meristem within a few hours of shifting plants
from short to long days (Borner
et al.
, 2000; Samach
et al.
, 2000), and the orthol-
ogous gene in
Sinapis alba
behaves similarly (Bonhomme
et al.
, 2000). Mutations
in
SOC1
delay flowering, but do not have an effect on floral development (Borner
et al.
, 2000; Lee
et al.
, 2000; Samach
et al.
, 2000). In contrast,
AP1
and
LFY
,two
genes that confer floral meristem identity on the developing floral primoridum, are
among the first genes with established roles in floral development whose transcripts
increase in abundance at the meristem (Mandel
et al.
, 1992; Weigel
et al.
, 1992).
Mutations in these genes cause shoots or flowers with shoot characters to develop
from primordia that would normally form flowers, and the proteins encoded by these
genes are involved in activation of downstream transcription factors that confer or-
gan identity on floral organs (Weigel & Meyerowitz, 1993). Recent work indicates
that
LFY
and orthologous genes in other species, as well as some of the transcription
