Lecture 7 Outline
1.
2.
3.
4.
5.
Chimeras II
History of “Sports”
The Chimera Concept
Sectorial Chimeras
Definitions
Green-white-green (GWG)
periclinal chimeras
History and Application of
Grafting
1.
2.
3.
4.
5.
Plant Collectors
Darwin’s travels
Winkler’s Chimera Concept
Types of Grafts
Modern Applications
Grafting occurs naturally
Chloroplast genomes can be transferred from one
tree to the other across the graft junction
Figure 1A from Stegemann et al., 2012, PNAS 109:2434
1.
History
Explorers, like Charles Darwin, traveled
to new and exotic places and collected
‘treasures’in the form of novel species
of plants and animals
2. New varieties of plants were highly
prized and breeders were therefore
motivated to generate new and improved
stocks
a) by making interspecific crosses
b) by vegetative propagation (or the
generation of a “sport”)
History
1. A Sport was defined as, “A spontaneous
change or mutation in a part of a plant
that created a feature not previously
known within the species, their sexual
hybrids and descendants”.
2. Sports were notoriously difficult to
maintain and would often be unstable
3. The first reproducible and stable
variant was described in 1674 and was
called the ‘Bizzarria orange’
Pietro Nati (1625-1685), who was the director of the Pisa
Botanical Gardens, published one of the first
descriptions of a graft chimera known as “La Bizzarria”
http://www.homecitrusgrowers.co.uk/citrusvarieties/bizzarria.html
Resulted from grafting a sour orange scion to a citron
stock. A bud arose at the graft junction with a unique
mix of parental characteristics.
Historical paradox: What was the
nature of graft hybrids?
Were the resulting hybrid branches the
result of sexual hybridization or a union
of vegetative tissue (somatic
segregation)?
The argument against sexual hybridization
was the you could not produce such
plants by conventional breeding methods.
Winkler 1907
1.
Did experiments to test the nature of graft
hybrids and, using tomato and nightshade
plants, reproduced results akin to those seen
previously
2. Got shoots that were all tomato, all
nightshade or 1/2 nightshade and 1/2 tomato
3. Winkler was responsible for coining the term
“chimera” (a fire breathing monster from
Greek mythology that had the foreparts of a
lion, torso of a goat and the hindquarters of a
dragon)
Convention
L2
L1
L3
SAM
Periclinal chimeras are described
according to the relative
contribution of graft partners,
starting with the L1
1. If plant A is grafted to plant B
and a periclinal chimera is
produced, the layer
contributions are listed
beginning with the L1, then the
L2 and finally the L3
2. ABA: L1 is A, L2 is B, L3 is A
3. BBA: L1 is B, L2 is B, L3 is A
Vegetative Propagation
1.
Plants can be maintained by taking cuttings
and rooting them
2. By grafting a cutting onto a stock
3. Maintaining them in tissue culture
4. Grafting is very important to
a) The horticultural industry (roses)
b) For maintaining grape varieties for wine
production (root stocks resistant to
pathogens)
c) The fruit industry (apples)
Two types of chimeras
Periclinal and Sectorial
At the SAM
Within Organs
Sectorial Chimeras
1. Plants with sectorial meristems show
sectors that run vertically down the
shoot
2. Meristem is divisible into
circumferential lineages and therefore
there must be a small number of initial
cells at the top of the meristem (subset
of central zone)
3. Sectors are not usually stable (do not
persist for the lifetime of the plant)
Sectorial Chimeras
1. Position of an initial
does not restrict the
fate of the initial’s
descendants
2. The position of a
marked band is
independent of leaf
initiation and may or
may not encompass a
whole leaf or even part
of a leaf
Sectorial Chimeras
Within organs reveals division patterns and
clonal relationships
Daughter cells arising
from one marked cell
Variegation
Spontaneous sectors can arise from the
disruption of pigment biosynthetic genes
caused by transposon insertion and excision.
Definitions
Genotype: The DNA constitution
a) the genetic constitution of a cell, an
organism, or an individual (i.e. the specific
allele makeup of the individual) (from
Wikipedia)
b) the actual alleles present in an individual
Phenotype: Visible consequences
a) an observable characteristic
b) any observable characteristic or trait of
an organism (from Wikipedia)
Gene expression patterns
In theory, every nucleus in a multicellular organism has
the same genetic information but expression
patterns are different. Genes are turned on and off
by multiple factors (temporal/spatial).
A direct relationship: gene is on, function is on
or gene is off and function is off
Factors affecting gene expression
Location, location, location
Developmental Regulation
A gene may be off if it resides in a
cell in the L1 but not in the L2 (where
you live matters!)
Activation due to a change in fate
Through cell division, an L1
daughter cell may end up in an L2
neighbourhood and hence different
genes become activated and
inactivated (turned on/off)
Green-white-green chimeras
1.
L1=green, L2=albino, L3=green
Conventionally written as GWG
What does that mean?
The L1 cells are genetically green but visibly
(phenotypically) white because chloroplasts do
not mature (suppressed)
2. The L2 cells are genetically albino or white
(genes do not function: they have mutations)
and phenotypically white
3. The L3 cells are genetically and phenotypically
green (green genes are functional)
Green-white-green chimeras
The L2 meristematic layer is albino
From Leyser and Day, Mechanisms in Plant Development. 2003 Blackwell Publishing,Chapter 3.
Genotype of L1, L2 and L3
L1=green, L2=albino (white), L3=green
Pigments are cell autonomous markers because
they are contained within a plastid
Leaf in cross-section
Phenotype of L1, L2 and L3
L1=white (gene expression suppressed)
L2=albino (genes have mutations)
L3=green (genes are expressed)
Layer displacements
L1 cells that divide periclinally (rare) will give rise to L2
daughter cells that can make chloroplast pigment while
L2 cells cannot, even if displaced into the L3. L2 cells
invading the L3 occurs more commonly.
L1
L2
L3 layer
GWG Chimeras Demonstrate that….
1.
2.
3.
4.
L1 layer is very stable and rarely invades the
subjacent L2 layer
L2 cells typically remain in the subepidermal
layer in most of the leaf but near leaf margins
are more likely to become displaced into the L3
If L2 cells invade, there is a compensatory
reduction in cell divisions in the L3 so that the
number of cell layers in the leaf remains
constant
Hence one sees this fairly common pattern of
variegation in garden and house plants (white
margins and green centre)
Sectorial Chimeras are Rare
1.
GWG to WWG, GWW or GGG sectors can
occur but they tend not to be stable
2. GWW sector would give rise to a white
sector on the stem and would run down
the vertical axis of the shoot (leaf and
stem tissues would be white)
3. GGG sector would produce a solid green
band with no variegation
4. Seeds produced in any of these sectors
would have the genotype of the L2 only
What have we learned so far?
L2
1.
L1
L3
SAM
Meristem layers are
relatively stable (L1>L2>L3).
2. Proteins and RNAs can
move between layers and
there seems to be
directionality.
3. Directionality is determined
on a case-by-case basis.
4. There are tissue-specific
constraints.
Some additional questions
L2
1.
L1
L3
SAM
Who is in charge? For
example, is the
coordination of meristem
activity due to the
response of L2/L3 tissues
to signals from the L1?
2. Is there lateral movement
of signals within a layer?
3. Are there communication
rules and if so, what are
the rules?