Lecture 22
Phase Changes
As the sporophyte develops it passes through
different phases. We have already discussed the embryonic phase which ended in
seed maturation and developmental arrest. Upon germination, the plant enters
the juvenile phase, followed by an adult phase and flowering. The most obvious
and dramatic post-germinative phase change is flowering but vegetative phases
can be quite distinct in some plants.
Vegetative Phases
Vegetative development is divided into juvenile and
adult phases. Juvenile plants are not competent to flower while adult phase
plants have gained the competence to flower. This has economic ramifications in
many crops, including fruit trees where trees cannot bear fruit until reaching
the adult phase. There are also morphological and physiological differences
between juvenile and adult which are not obvious in all species. Some of these
represent plant adaptations for survival. For example, some woody shrubs
protect their succulent juvenile tissues from grazing by accumulating bitter secondary
compounds.
Juvenile Phase represents the early phase of
vegetative development, therefore the juvenile organs are the first organs
produced. They will be at the bottom of the plant and will be the oldest organs
of the plant. Adult phase represents a later stage of development so adult
organs will be at the top of the plant and younger in age.
Examples of vegetative phase differences
Ivy—The juvenile grows as a prostrate vine with
alternate phyllotaxy while the adult grows as an upright shrub with spiral
phyllotaxy.
Eucalyptus—the juvenile has opposite phyllotaxy with
short broad leaves, the adult has spiral phyllotaxy with long slender leaves.
Maize—juvenile has short round leaves covered with
epicuticular wax and no epidermal hairs, adult has long narrow leaves with no
epicuticular wax but with epidermal hairs.
Regulation of vegetative phases
The hormone gibberellic acid (GA) promotes juvenility
in some plants. For example in ivy, if the apical bud is removed from an adult
branch, the subtending axillary buds will grow into typical adult branches.
However, if GA is applied to the axillary buds they will grow into a juvenile
branch. In maize, GA deficient mutants are delayed in their transition to adult
phase.
Dominant gain-of-function maize mutations, called
Teopod, prolong the juvenile phase but do not affect the onset of adult
characteristics. In other words, there are leaves that normally would display
adult characteristics which now display juvenile and adult simultaneously. This
indicates that the Teopod genes regulate phase change by promoting juvenility
and also that juvenile and adult phases are regulated independently. Genetic
mosaic experiments where sectors of wild type tissue were induced in Teopod
mutant plants show that the Teopod gene acts through some diffusible substance.
In leaves or stems that were half mutant and half wild type, the wild type
tissue showed the mutant characteristics.
Floral Induction
Upon reaching adult phase, plants are competent to
flower (enter reproductive phase). The process of floral determination is
called floral evocation. Some plants flower because of intrinsic signals while
others require environmental cues. The most common environmental cues are
temperature and photoperiod.
Vernalization
Many biennial plants and "winter crops"
such as winter wheat require a cold period to flower. An exposure to cold that
stimulates flowering is called vernalization. Different plants have
requirements for different vernalization temperatures and periods. In most plants
vernalization can be reversed by warm temperatures and so the cold period
must be relatively uninterrupted.
The site of cold perception is the SAM. A SAM grafted
from an unvernalized plant onto one that has received cold treatment will not
flower while a SAM grafted from a vernalized plant onto an unvernalized one
will flower.
Photoperiod
Plants are classified as short day, long day or day
neutral according to their photoperiod requirements. Short day plants flower
under regimes of short day, long night while day neutral plants have no
specific photoperiod requirements. In addition plants can be classified as
obligate or facultative in their photoperiod requirements. Obligate plants will
remain vegetative indefinitely without the inductive photoperiod while
facultative plants will eventually flower without the inductive photoperiod but
can be induced to flower sooner. (Plants can be obligate or facultative with
regard to vernalization requirements also).
Long day plants have a minimum required light period
for photoinduction; anything above that will result in flowering and there is
no dark period requirement. Short day plants have a maximum photoperiod over
which they will not flower; they require a dark period. These are more
accurately described as long night plants because it is the dark period that is
inductive. The darkness must be continuous. Interrupting the dark period with a
flash of light prevents photoinduction.
Different plants have different requirements for the
number of photoinductive cycles. Some require only one cycle, others require
several weeks. Once a plant is induced, placing it in noninductive cycles will
usually not reverse the photoinduction.
Photoperiod is perceived through the photoreceptor phytochrome. Phytochrome is
a protein with a covalently attached chromophore. Phytochrome exists in 2
interconvertable forms. Pr absorbs light in the red part of the spectrum (~660
nm). Pfr absorbs far red light (~730 nm). Absorption of red light converts Pr
to Pfr and absorption of far red light converts Pfr to Pr. Pfr also undergoes
spontaneous "dark reversion" to Pr. Daylight has a relatively high
proportion of red light and thus most phytochrome is in the Pfr form. In the
dark, the equilibrium slowly shifts to Pr. This cycling of phytochrome somehow
interacts with the circadian clock to determine the proper seasonal time for
flowering.
The site of photoperiod perception is the leaves.
Exposure of the SAM to photoinductive cycles has no effect. In cocklebur (a
short day plant), exposure of 8 cm2 of leaf surface to short days while the
rest of the leaves receive long days induces flowering. Not all leaves are
equally receptive. Juvenile leaves cannot perceive or transmit the
photoinductive signal. The first adult leaves may require more photoinductive
cycles to induce flowering than later adult leaves. Grafting experiments show
this is a property of the state of the leaf, not it’s position relative to the
SAM. Early adult and late adult leaves grafted onto identical positions of
recipient plants will still require different numbers of photoinductive cycles.
The induced state is stable in the leaf. A Perilla leaf that has been
photoinduced can be grafted to an uninduced plant and cause flowering. The same
leaf can then be removed and regrafted several times to other plants under
noninductive conditions and continue to induce flowering.
SAMs from different phases differ in their competence
to respond to floral induction. Grafting juvenile meristems of tobacco onto
florally induced plants did not produce flowering until after a period of
growth (presumably until after adult phase transition) while grafting adult
meristems onto induced plants resulted in rapid flowering. The veg gene
in pea is required to confer floral competence to SAMs. veg mutant SAMs
grafted onto wild type plants under inductive photoperiods will not flower,
however wild type SAMs grafted onto veg mutant plants will.
Florigen: Because of the hormonal nature of the
signal sent from the leaves to the SAM, it was believed that a floral inducing
hormone existed. This was called florigen. It has never been isolated and
current opinion favors a more complex explanation than just a single substance.
Increased hormone sensitivity is correlated with floral induction
Several lines of evidence, including culturing
meristems with various levels GA or GA inhibitors suggest that an increased
sensitivity of the SAM to GA is associated with floral induction. Recently, a
gene called FPF1 (floral promoting factor) was identified in Arabidopsis. FPF1
mRNA is not present in the vegetative SAM but is induced in the SAM at the
onset of floral induction. Constitutive expression of FPF1 leads to an early
flowering phenotype. In addition, plants show additional phenotypic aspects
like elongated internodes that are reminiscent of GA responses. The FPF1
overexpressors do not show early flowering or internode elongation if combined
with GA deficient mutants or treatment with paclobutrazol (a GA inhibitor).
Therefore these effects of FPF1 are GA dependent indicating that expression of
FPF1 increases sensitivity to GA and that this increased sensitivity of the SAM
leads to floral induction. Consistent with the role for GA in floral induction
is a report that the LEAFY (a gene involved in arabidopsis flower development)
promoter is GA responsive.
Floral Determination
Day-neutral tobacco consistently forms about 40 nodes
and then a terminal flower. Undetermined vegetative SAMs can be cut off and
rerooted indefinitely. Removal of leaves from plants has no effect on the
number of nodes to flowering, but induction of adventitious roots by piling up
soil as the stem grows increases the number of nodes produced. This suggests
that the distance between the SAM and roots is important for floral
determination. SAMs become florally determined prior to actual floral
development. Determined SAMs that are cut off and rerooted will produce 4 more
nodes, then a flower (the same as if they had remained on the plant).
Subtending axillary buds also form floral branches in
tobacco. The axillary buds form over a period of 5 days following SAM floral
determination. The earliest the axillary buds are large enough to remove and
root is 9 days after. At this time most but not all axillary buds are florally
determined. Thus floral determination in lower axillary buds occurs later than
in the terminal meristem.
The florally determined state is not restricted to
tissues that will actually form flowers. Internode segments will produce shoots
when cultured on hormone free medium. Vegetative state segments will produce
vegetative shoots while segments from the upper nodes of flowering plants will
form floral shoots.
Sunflower and maize represent extremes in the timing
of floral determination. Rooting experiments showed that the sunflower SAM is determined
to flower during the seedling stage, approximately 14 nodes prior to
morphological differentiation. The apical bud was removed and rooted at
different times. Whether the apex was removed from the seedling or after
producing 9 nodes, the total number of nodes produced before flowering remained
about 16.
In maize, floral determination does not occur until
after vegetative development is complete and it appears to be a stepwise
process. Cultured SAMs form vegetative shoots until floral transition. Early
tassel meristems develop vegetative shoots from their branch primordia. Later
tassel primordia develop sterile flowers with leaf like organs. Only flowers
that are initiating floral organs at the time of culture will go on to complete
normal floral development.
Arabidopsis is a facultative long day plant with a
facultative response to vernalization. Genes that regulate flowering have been
identified in genetic screens for mutants that are either early flowering or
late flowering relative to wild types under the same set of conditions. Early
flowering mutants identify genes that inhibit flowering, late flowering mutants
define genes that promote flowering (or inhibit inhibitors). At least 2
independent pathways function. The photoperiod dependent pathway is defined by
mutants that flower late only under inductive photoperiod but flower at the
same time as wild type under non-inductive photoperiod. Mutants with altered
flowering times under non-inductive conditions are involved in the “autonomous”
pathway.
Genes of the autonomous pathway include LUMINODEPENDENS
(LD), FRIGIDA (FRI) and FLOWERING
LOCUS C (FLC). FLC encodes a Mads box transcription factor. FLC
expression is positively regulated by FRI and negatively regulated by LD and by
vernalization.
Genes of the photoperiod sensitive pathway include CONSTANS
which encodes a zinc finger transcription factor.
Measuring developmental time
Because of the nature of plant development, spatial
relationships change over time. For example the SAM becomes increasingly
separated from the root system over time. This makes it difficult to tell
whether the mechanisms regulating the duration of developmental phases are
based on temporal or spatial considerations. Thus there is debate over whether
plant homeotic mutants are really heterochronic. Homeotic is when structures
form in an inappropriate place. Heterochronic is when something occurs at an
inappropriate time. This dilemma is particularly evident in the control of
phase changes. Phase change could be based on spatial relationships (distance
between the SAM and the root system for example) or on temporal considerations
or counting mechanisms. The phenotype of one mutant in arabidopsis called
paused has been interpreted to suggest there might be a "developmental
clock." paused plants initiate leaves more slowly than normal. The first
seedling leaf after germination is delayed by several days relative to wild
type. The first leaf that forms in the mutant resembles the leaf that the
normal plant would be making at the same time although it is not in the same
position. Furthermore, this mutant flowers at the same time as normal even
though it possesses fewer total leaves. Thus it appears to enter phase
transitions at the proper time, regardless of position. There are other
possible explanations and the majority of the evidence (from rooting
experiments etc.) suggests that spatial relationships are more important for
determining phase.