Development and Morphogenesis (Environmental Complexity (if all plants…
Development and Morphogenesis
if all plants were extremely small and lived in completely uniform, non varying environments,
most would probably be simple and would experience little selective pressure for the evolution of complex shapes tissues, organs and metabolism
Gravity comes from only one direction.
the sun is either or overhead but never below
temperature are lower on the shady side of the plant
moisture depends on the depth below or height above the soil surface
most plants are so large that their bodies exist in several different micro environments
the need for inter communication and coordination also exists within a limited region of the body.
all levels of communication have in common a basic mechanism.
information about the environment or the metabolic status of the organ must be perceived
the plant must sense the environment cues such as change in temp, moisture or day length or the nucleus must receive chemical signals
next, information must be changed to form a that can be either acted upon or transported.
finally there must be be a response, the plant must enter dormancy, produce flowers, change type of leaf production and so on
if any these steps is missing, the plant cannot respond to the environment.
these principles apply to all organisms, and of course, higher animals have the most elaborate mechanisms.
their sophisticated sense organ for sight taste touch hearing and smell perceive external conditions
these senses organs transduced the perceived information to a transportable form such as nerve impulses or hormones that are secreted into blood stream
besides energy for photosynthesis, light also provides two important types of information about the environment
(1) the direction or more precisely, the gradient of light
this allows a plant to grow or orient its leaves toward a region of bright light ,
which increases the light available for photosynthesis
(2) the duration of light (Length of the day) provides information about the same time of year
air temp. may followed so quickly by severe cold that plants do not have enough time to become dormant, but day length is an infallible indicator of season
Pests and Damage
plants must detect when they are being attacked by pests or being damaged physically.
pest such as
attack plants chemically with toxins and digestive enzymes
plants suffer physical damage when larger animals trample them or chew into leaves and twigs
plants have two mechanisms that perceive such damage
(1) purely physical damage physically damage breaks a plants own cell wall molecules, producing fragments of cellulose and hemicellulose
(2) Pathogenic fungi and bacteria attack plants with digerctive enzymes such that the microbial cells can penetrate the cell walls
although plants do not move around like animals, their parts frequently grow against objects and respond to this contact
certain types of contact are detrimental
others types of contact are beneficial
in some cases, the contact is between two growing primordial and is normal developmental feature
many flowers that have fused petals or carpels start with separate primordial that grow together and fuse, acting as a single unit during development
in each case
the physical action of touching is similar
but each organ responds in a distinct way that is adaptive for the plant; the response of each would be inappropriate if it occurred
gravity itself is important because it causes weight stress
a vertical stem supports more weight than does a similar stem growing at random
whenever a plant is bent or tilted because of flooding or the slipping or a hillside, the plant must change its growth back to an upright direction
if it continues to grow at angle, it would need to produce many more fibers to support its weight
roots that grow downward are more likely to encounter water and mineral.
shoots that grow upward grow above other plants and encounter better conditions for photosynthesis, pollination and seed distribution
Normally shoots do this by growing toward the brightest light
the open sky, but shoots of seeds that germinate deep in the soil must determine which way is up while in the dark
direction of gravity is their only reliable guide
most bilaterally symmetrical flowers must be aligned with the body symmetry and flight pattern of their pollinator
such flowers must be bilaterally symmetrical vertically and project horizontally because insects and birds do not fly upside down or sideways
the flower must orient itself along the same environmental gradient that pollinator uses
Changing temperature can induce many specific types of plant development.
although most plants appear to be quiescent and virtually lifeless in winter
a considerable amount of critically important metabolism is occurring.
this metabolism usally does not proceed at tempurate about 1C to 7C
cold temp. are required for normal flowering of biennial and many perennial plants.
species of perennial trees that are adapted to habitats with cold winters
if the tree are grown in areas with warm winters, they form flowers that never open
in contrast, biennial plants spend their first year in a vegetative phase ad cannot be induced to form flowers
their first cold winter vernalizes them, it causes them to switch to a state in which they can since and respond to a stimulus that induces flower formation
Lower temp are required to induce deep dormancy in temperate trees and shrubs.
short days of fall induce plants to initiate preparations for winter and enter a mild state of dormancy , but the most resistnat stages are not entered until the plant actually experiences a week or two of cool temp.
in contrast, cool temp. are required to break the dormancy of many seeds, allowing them to germinate when their habitat becomes warm and moist
although water is an absoulte prereq. for life,
its presence probably does not act like a signal in the way other factors do
if enough water is avaible, plants grow; if not plants wilt and even die
roots often appear to grow towards water they actually grow in all directions and those which by chance,
grow towards water grow more rapidly because they are in a favorable environment
roots that grow away from water grow slowly, but only because they enter an environment too dry to permit growth.
roots do not turn and grow towards water in the way they turn and grow towards gravity
Responding to Environmental stimuli
is a response in which a cell swims toward (positive taxis) or away from (negative Taxis) a stimulus
even in plants like moses, ferns, cycads and maidenhair tree,
sperm cells swim to egg cells by following a chemical gradient (chemotaxis)
in algae, chemotaxis is similary important for reproduction,
and in many species phototaxis allows them to swim toward light for photosytheisi or away from light that is too intense
is growth response oriented with regard to the stimulus
is growth at an angle
most tap root are positively gravitropic
growing downward in response to gravity whereas shoots are negatively gravitropic, grwoing upward in response to gravity
branches and secondary roots grow horizontally or at an angle, plagiogravitropically
when touch is
growing along the style to the ovary by following a gradient of chemical release from the ovule, probably from the synergids.
however this is still uncertain: the chemical responsible is not known , and pollen tubes in its concentration
often involves a change in direction of growth
one side of the tendril must grow more than the other
(1) growth on the contact side slowing
(2) growth on the opposite side accelerating
is a stereotyped nongrowth response that is not oriented with regard to the stimulus
"Positive and negative" are not used because the response is not oriented with regard to the stimulus
many pollinators are active only at night or during the day, and the flowers they pollinate are open only at the right time
only active during the day
only active at night
presence of the pollinator is the critical factor for pollination
the cue that stimulates flower opening is the presence or absence of light
the opening and closing always happen in the same manner, even if the light is given artificially from the west, north, or south, above or below. the response is
Nastic movement are based on changes of turgor pressure rather than growth; thus movement can be repeated.
blade elevated in the day, lowered at night
such leaves often have an enlarged area, a pulvinus at their petiole base, and it contains motor cells that cycle between being turgid and flaccid
causes a change in the quality of the plant that is a fundamental change occurs in the metabolism of a tissue or even the entire plant.
because day length is such an excellent indicator of season , a photomorphogenic responses are numerous
the induction to form flowers
the induction of dormant seeds germinate
and the induction of buds to become dormant
is the formation of fibrous wood when a stem or branch is tilted and becomes stressed by gravity
include formation of extra bark where branches rub against an object and formation of a suture when petal or carpel primordia grow against each other
Plants response to the diverse types of information presents in the environment can be grouped into four simple classes
Communication within the plant
Converts some plants from little to big condition, converts other species from big to to little conditions, involving in flowering; releases some seeds and buds from dormancy, stem elongation, stimulates pollen tube growth
Intiation of dormancy; resistance to stress conditions; stimulation of growth at very low doses; stomatal closure, probably not involved in abscission
Bud activation, cell division, fruit and embryo development, mimics the effects of phytovhrome and red light in several cases; preventing leaf senescence
formation of aernchyma in submerged roots and stems; fruits ripening and abscission; initation of root hairs; latex production
Abscission suppression, apical dominance; cell elongation,; formation of roots in cutting; fruits maturation tropisms; xylem differentiation
Perception and transduction
many, possibly most, responses occur in tissues or organs different from those that sense the stimuli
the site of perception is not the site of responses, so a form of communication must exist
in plants most sites of perception and responses are not specialized for those functions but seem to be rather ordinary cells
in root caps certain cells are called
do have large starch granules,
, that sink in response to gravity;
day length is probably perceived by all living leaf cells
no specialized region of cells has been discovered,
low temperatures for vernalization appear to be detected by buds
which do not contain a particular group of cells specialized just for temperature perception
the site of perception is tentatively assumed to be the site of transduction, where the stimulus is converted into form that can be transmitted and can trigger a reaction at a response site
transduction is still a complete mystery in almost all plants responses
we do not know how changes in temp, light, weight, or humidity are converted into chemical signals
two factors are important in perception and transduction
Presentation and timing
is the length of time the stimulus must be present for the perceptive cells to react and complete transduction
for roots gravitropism is easy to understand
a root must lie on its side long enough for the statoliths to sink to the new bottom of the cell
if the root returned to vertical before they can settle, no perception occurs.
in many tropic responses, only a brief touch or unilateral lighting is sufficient to cause curvature, presentation times are often only a few seconds
refers to the level of stimulus that must be present during the P.T. to cause perception and transduction.
in phototropism, plants are extremely sensitive to very dim unilateral light if they are in an otherwise dark environment
the threshold is higher and curvature
All or nothing response
after the threshold and P.T. requirements are met, the stimulus is no longer important, the responses is now completely internal.
Dosage -Dependent responses
the amount or duration of the stimulus affects the amount or duration of the responses
Activation and Inhibition of shoots by Auxin
the second site of response to apically produced auxin is the buds located in leaf axils;
their response is not cell elongation but rather inhibition of growth.
Apically produced auxin induces dormancy in these axillary buds, the results being that each shoot tip has only on active apical meristem
a phenomenon called
this is a threshold response: as the terminal shoot apical meristem grows away, the concentration of auxin around an axillary bud gradually decreases until at some point it drips below the threshold
Differentiation of Vascular Tissues
response to auxin produced in shoot tips is vascular cambium;
the response is cell division and morphogenesis
IN SPRINGTIME, AS AIR TEMPERATURE RISE AND BUDS BECOME ACTIVE ,
their auxin moves basipetally, activating the dormant V.C..
Auxin not only stimulates cambial cells to begin mitois and cytokinesis but also causes new daughter cells to differentiate into xylem cells.
if the apical mristem is destroyed, by insects or late frost
the basipetal flow of auxin stops, Vascular Differentiation is interrupted inter node elongation ceases and apical dominance is broken
some axillary buds, now free of apical dominance active and re-establish a flow of auxin that maintains the V.C. and any other cells that depend in it
three separate tissues give three distinct responses .
not because there are three separate chemical messengers
there is only auxin- but because part of their previous differentiation was preparation to respond to auxin in a particular way
in cells of young inter nodes just below the apical meristem, auxin triggers cell elongation
when IAA contacts these responsive cells, which are prepared for growth, it binds to receptors thought to be small proteins called
the cells begin to transport protons actively out across the plasma membrane
this has the effect of acidifying the cell wall
the protons beak some of the chemical bonds that hold one cellulose microfibrils to another and activate enzymes that weaken other bonds so that the wall becomes weaker
if protoplast is turgid and pressing against the wall
it exerts enoough pressure to stretch the weakened wall and growth results.
Immature cells neither excrete protons nor grow if auxin is lacking at lower inter nodes , fully grown, mature cells apparently lack proper auxin receptors because auxin does not cause them the extrude protons to grow
Auxin is often described as a growth hormone whereas ABA is considered an inhibitor
Unfortunately, such characterizations are confusing.
Hormones simply carry information about the status of a particularity region, nothing more
whether the elicited response is inhibition or the complexity is provided by shoot tips
as shoot apical meristems grow and initiate the new cells of shoots and leaf primordial,
they also produce the auxin, IAA.
young leaves are also a rich source of this hormone
no external signal must be perceived to initiate auxin production; instead
this is a means of integrating the plant during the ordinary growth.
large quantities of auxin indicate to cells that shoots are elongating and producing new leaves
neither signal perception nor transduction occurs, transport takes place
in stem auxin undergoes basipetal, polar transport at a speed of 5 to 20 mm per hour, perhaps by means of molecular pumps in plasma membrane
the downward flow of auxin surrounds all stem, cells and at least three cell types are set to respond to it
Hormones as signals of Environmental factors
hormones communicate to various parts of a plant the information that a particular part has encountered an environmental Change
Export of ABA by wilted leaves has been mentioned, and another example involves abscission of leaves and fruits.
a young leaf produces large amounts of auxin but production falls to a low but steady level in mature leaf
as long as auxin flows out through the petiole, activity in the abscission zone is inhibited
if the leaf is damaged by animals feeding or water stress, auxin production drops to such low levels hat its flow through the petiole does not keep abscission zone quiescent
Perception and transduction in this case may be simply that insect or wilted damage makes it impossible for the impaired cells to produce enough auxin o inhibit the abscission
old age of the leaf may also result in the lack of sufficient production of ethylene
which the suppresses auxin production and transport in the time abscission before winter
fruits are prevented from abscising prematurely by the presence and export of sufficient amounts of auxin through the pedicel.
ripening is under control of both auxin and the ethylene ,at least in edible fleshy fruits
Initial transformation of the ovary wall into a fruit is a response to auxin synthesized in developing embryos and transported to the enlargement and differentiation;
there is usually surprisingly little cell division during the formation of fruits, even large ones
Ripeness to flower
almost all plants must reach a certain age before they can be induced to flowers
Annual plants need to be only several weeks old before they become competent to respond to a floral stimulus
but many perennials must be 5 or 10 years old
before this time, conditions that should induce flowering have no effect. Virtually nothing is known about the metabolic difference between the
when plants are incapable of being induced to flower, and the
, when they are sensitive to floral stimuli.
the conversion from Juvenile to Adult is called
Cold temp. are the stimulus responsible for the phase change in biennial plants, and the proceed is vernalization
the site of perception is the shoot apex itself; if it is cooled while while the rest of the plant remains warm, vernalizaton occurs
however if the rest of the plant is cooled while a small heater keeps the apex warm no Vernalization occurs
presentation time is a short as 1 day in some plants
the transduction process is known to require oxygen carbohydrates as an energy source, and an optimal temperature just above freezing between 1C to 7C
the conversion of an adult plant from the vegetative to the flowering condition may be most complex of all morphongenic processes
this is not one process- different mechanisms exist in different species.
in certain annual species, size appears to be the only important factor;
which acts as a season indicator
bloom when days are long
bloom when days are short
day neutral plants
do not respond to the day length
detects the presence or absence of light
Critical Night length
if a short night plant receives night shorter than the critical length it flowers , whereas a long night plant must receive nights longer than its own critical night length