ISIS Press Release 09/03/05
View from MADS House
New opportunities for manipulating flowering in plants set the stage
for extensive alteration of crop geography Prof. Joe Cummins
A fully referenced version
of this article is posted on ISIS members’ website.
Details here
MADS-box genes are a large family of genes coding for protein
transcription-factors that recognize short stretches of DNA - the MADS-box
– to which they bind directly to regulate transcription. MADS-boxes and
transcription factors are present in all multi-cellular eukaryotes from fungi
to plants and humans where they regulate developmental pathways. The MADS-box
transcription factors bend DNA at the site of transcription initiation to
juxtapose transcription factors on adjacent sites (boxes).
In plants, the MADS-box gene families are conserved among gymnosperms,
angiosperms, ferns and mosses. They serve a wide range of functions from floral
development to root formation, but the range of effects is not yet fully
explored. The controls of floral development pathways and time of flowering
have a common evolutionary origin. Flowering plants prefer transcription from
chromosomes of maternal rather than paternal origin and even that epigenetic
effect is mediated through MADS-box controls.
The discovery of the MADS-box gene families has had an instant impact
on agriculture and forestry. A wide array of inventions to control flowering,
seed production as well as other growth modifications has not yet reached
commercial farms and forests, but many modifications have been reported and
others have been patented. The main commercial transgenic crops now available
are modified for herbicide resistance or insect resistance, the MADS-box
constructions are modified with flowering controls or flowering timing, even
alterations in yield are contemplated. Agronomy, horticulture and forestry will
all be greatly affected by the genetic modifications involving the
MADS-box.
Much of the initial work on the plant MADS-box transcription factors
was done using the tiny mustard plant Arabidopsis. Recently a floral
transcription factor was found to control the agronomic traits of seed yield
and seed mass. Of course, such trait in Arabidopsis would mainly please
a few voles, but the trait can easily be manipulated in grain crops such as
maize, rice and wheat. In rice, for example, MADS-box genes have been
identified which control the timing of flowering. As flowering time determines
regional adaptability of rice varieties, manipulating that timing will allow
greater use of regional varieties. Genes determining rice floral morphology
have been identified allowing rice spikelet development to be manipulated.
Vernalization is the long winter cold treatment required for flowering
in grain and some oil crops. Usually, the crops requiring vernalization have
spring planted cultivars that do not require the cold treatment so they can be
planted in spring rather than autumn. Nevertheless, the winter requiring
varieties have desirable traits that are not present in the spring varieties.
In wheat, vernalization is controlled by the MADS-box gene WAP1. Unlocking
vernalization should allow quality wheat to be produced in warm climates.
Bolting is another aspect of cold temperature-induced flowering. Exposing the
germinating seeds or plantlets to a range of low temperatures accelerates
flowering causing cabbage or lettuce to lose commercial value. An anti-bolting
MADS-box gene has been identified in Chinese cabbage. The stage is set for
extensive alterations in crop geography.
The MADS-box genes expressed during tomato seed and fruit development
have been identified. Such findings may lead to commercial applications.
An anther-specific MADS-box was identified in peas and is expressed
also in a number of other plant species. The anther-specific transcription
regulator can be manipulated to produce male-sterile varieties used to produce
high value hybrid seeds.
A root nodule-specific MADS-box gene was identified in alfalfa root
nodules. Transferring nitrogen-fixing ability to non-legumes has been discussed
for decades, and this discovery may spur developments in that area.
The MADS-box gene DAL1 was identified as a mediator of juvenile to
adult transition in Norway spruce. Hastening floral development in forest trees
can accelerate breeding programs.
The first of many patents on MADS-box related functions have begun to
appear, all of them broad patents covering reproductive development in plants
in general. United States Patent 6 828 478 provides the surprising finding that
ectopic expression of certain MADS-box-containing gene products, such as SEP1,
SEP2, SEP3 or AGL24, combined with the ectopic expression of AP1, CAL or LFY
gene products, result in modulated reproductive development. Thus, this
invention provides plants comprising such ectopically expressible gene products
as well as methods of modulating the timing of reproductive development in
plants.
US patent 6 693 228 deals with the flowering locus (FLC) to delay or
advance flowering, and US patent 6 713 663, with FT protein that modulates
flowering in plants and dominant negative mutations of that protein (dominant
negative mutations usually disrupt the function of a wild type gene by
producing peptides that inactivate the wild type product). A Canadian patent
application deals with floral homeotic genes for manipulation of flowering in
poplar, and involves ablation of reproductive cells using toxins activated by
promoters for the flower specific transcription factors, or by inhibiting the
transcription factors with genetic anti-sense or dominant negative mutants. In
the patents described above, the genes involved all originate from plants, but
selectable markers and synthetic genes are also used to modulate the plant
genes.
The discovery of MADS-box transcription regulators has open doors to
modulating a wide array of agronomical properties of which the control of
flowering and seed production is only the first. Other properties include
nitrogen fixation, plant growth and disease resistance. The potential benefits
from such manipulation must be evaluated alongside the safety considerations,
including the genetic modifications themselves, which may involve synthetic
genes and ablation toxins that pose a threat to animals. Indeed, the homology
between plant and animal MADS-box genes should receive special attention.
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