Download Monocots vs

Nick Lorenz
IB 423 Final
2.
The plant kingdom has not always been divided up into two clear cut families
such as monocots and dicots. In fact, today the distinction is not prefect as well. Yet
today we use the distinction of these two groups to facilitate describing plant species that
are studied because there are a large number of differences between the groups and they
are fairly consistent across taxa. From the acknowledgement of their differences, we can
then ask about the evolution of these two groups. Did they both branch off from a
common ancestor or is one derived from the other? The relationship seems to be the later
case. Current hypotheses about the origin and diversification of the flowering plants
suggest that the dicots are the older group, from which the monocots evolved. In fact,
some dicots, called paleoherbs, are now believed to be closer relatives of the monocots
than of the other dicots. In other words, the dicots include a basal paraphyletic group
from which the monocots evolved. It is now believed that some of the dicots are more
closely related to monocots than to the other dicots, and that the angiosperms do not all
fit neatly into two clades. Counting species, there are at least three times more dicots than
monocots (60,000 species). With the assistance of evidence from DNA sequences, now
scientists with confidence place monocots as a branch within the dicotyledons, further
destroying the old model that depicts two branches at the base.
So how did they actual decide monocots were more advanced than dicots, and
was this possible even before DNA sequencing? It seems that the two groups have
different strategies and both make sense. In general dicots have more specialization in
tissue development and are also more organized to a specific plant structure. This strategy
argues for the best consistency among species, and if the structure is successful then you
want to make it consistent. On the other hand monocots have a simpler structure. There is
less tissue specialization because some of the same tissues in dicots take on other tasks
which negate the necessity of a specialized tissue (the lack of periderm will depict this).
There is also a reduction in plant parts for monocots. While this may seem primitive at
first, it also may tell of the increased efficiency in monocots. The randomness of some
parts of monocots may actually point towards this hypothesis as well.
Starting off with the most apparent difference is the number of cotyledons.
Monocots have one less cotyledon in their embryo than dicots obviously, and this tells us
about the differences is quality. The single cotyledon has everything the plant needs and
it is able to have primary development from one source. The dicot plant may be using up
more resources in creating that second cotyledon thus making it less efficient than the
monocot.
In the leaf venation the monocot again seems simpler than the dicot leaf. Dicots
have palmate major vein formation and reticulate minor veins that are immersed in the
mesophyll and make sure every cell is at least in the vicinity of a nutrient supplying vein.
The dicot venation pattern looks very complex. The parallel venation of monocot leaves
looks less appealing but if you think about its structure it seems like it would provide the
plant with a better system for supplying nutrients to leaves. Since the pattern does not
branch the nutrient flow is very direct and fast, straight from the main body of the plant.
Dicots do not do this because the branching allows for the minor veins to reach every
cell. However the monocot does this as well with commissural veins that connect parallel
major veins. In fact, this system seems better because it is less random and all cells seem
to be more equal in their nutrient intake. The reticulate pattern of dicot may put some
cells at a greater distance from the nutrient source.
Monocots have an atactostele also are more random in their vascular bundles than
dicots and this may have the same strategy as their leaf venation. Dicots instead have a
ring of vascular bundles in their subepidermal tissues. Both of these systems obviously
work very well, but the monocot random development shows that vascular bundles
develop where they are most needed. This feature is not unique to monocots, but is found
also in the paleoherbs and certain magnolia-like dicots. Vascular tissue like xylem not
only provides nutrients but also provides support and when the xylem develops randomly
it develops where it is necessary. Using the word random seems to imply there is no
purpose, but that would be wrong. It seems just the opposite, that vascular bundles
develop in plants were they are sensed to be needed the most. This may insure that all
cells receive a more equal amount of water from xylem and nutrients from phloem. The
transfer of water and nutrients within dicots is probably much slower for cells that are far
from the subepidermal ring.
Since monocots do not have a continuous vascular cambium they do not have
secondary growth, which initially sounds like a disadvantage. However, monocots
develop with adequate vascular systems so they seem more efficient. In fact, they
probably waste less primary tissue that is frequent in dicots after their secondary
development.
Like having no vascular cambium, monocots have no periderm, but it seems
unnecessary to have separate tissues to achieve the periderm function. In monocots they
inside have the outer epidermis become thickened, suberized and/or sclerified. This outer
protective layer is even more specialized than a dicot periderm. Within the monocot outer
cortex, parenchyma cells become meristematic and through periclinal divisions they
produce short radial files (similar to phellem). These are non-living and heavily
suberized. Some non-suberized cells become trapped between suberized zones but they
receive the same protection with the need for more suberin production. This process
seems much more efficient than the periderm in dicots (Beck 246).
Another feature common to all monocots is the structure of certain plastid
inclusions. All plant cells contain plastids, some of which become photosynthetic, but
those which are found in the phloem tissue do not do so. When examined in properly
prepared material, the plastids in the phloem are shown to contain crystalloid proteins,
the shape of which is distinctive for different major groups of plants. With an overall
abundance of chloroplasts, the monocot group is able to perform photosynthesis more
often if necessary, which is great for peak photosynthetic periods.
While most monocots are herbs, and lack vascular cambium and true wood, they
do have the ability to create similar forms. At least four times in their history, monocots
have evolved tree-like growth forms. A lack of cambium might be expected to make the
evolution of tree-like growth forms difficult, but it obviously has not been an
insurmountable obstacle. In monocots species like palms, grass-trees, bamboo, pandanes,
and yuccas all simulate woody growth.
DNA analysis has even been used to show that monocots are evolved from dicots.
The most useful multigene analysis so far has been based on nucleotide sequences of
three genes, rbcL and atpB from chloroplasts and 18s for ribosomal DNA sequences from
the nucleus. By analyzing the patterns of evolution for all three genes at the same time, a
computer produced an evolutionary tree having a very high degree of support. From this
analysis, one conclusion was inescapable: the monocots evolved from dicotyledonous
stock, not from the base of the tree, and monocots originated after several groups of
dicotyledons had already evolved. Computer analyses tell us that we can be essentially
100% certain that ALL monocots arose as a single evolutionary lineage (monophyletic).
Theoretically this means that all 2800 monocot genera on earth today can trace their
origins back to a single ancestral population. Thus probably ends another age-old debate,
whether monocots evolved more than one time.
3.
After an ovule is fertilized it begins to expand and develops into a seed The ovary
eventually comes to form, along with other parts of the flower, a structure surrounding
the seed called the fruit. Fruit development continues until the seeds have matured. We
see a wide variety of fruits and this is a result of the differences in the number of ovules
that are fertilized. Fruits are so varied in form and development, that it is difficult to
devise a classification scheme that includes all known fruits. There are three different
types of fruits: simple, aggregate, and multiple. What type of fruit develops is thought to
be controlled by the phase of rapid cell division. The number of fertilized ovules in a fruit
is correlated with both the initial cell division rate and the final size of the fruit.
Simple fruits can be either dry or fleshy and consists of a single carpel, or several
fused carpels, without any attached floral parts. Dry fruits may be either dehiscent or
indehiscent. This is the most abundant groups with many different groups each with a
multitude of species. There dry fruits like achenes (buttercup), caryopsis (wheat),
fiberous drupe (coconut, walnut), legume (pea, bean, peanut) and many more. Fleshy
simple fruits are berry (tomato, avocado), drupe (plum, cherry, peach) and others. These
simple fruits develop from hypogynous flowers in which the ovaries are superior.
An aggregate fruit is one that consists of several to many carpels of a single
flower. An example is the raspberry, whose simple fruits are termed drupelets because
each is like a small drupe attached to the receptacle. In a bramble fruit, like blackberry
the receptacle is elongate and part of the ripe fruit, making the blackberry an aggregateaccessory fruit. The strawberry is also an aggregate-accessory fruit, but the seeds are
contained in achenes. In all these examples, the fruit develops from a single flower with
numerous carpels (Beck 370).
Multiple fruits consists of the fused ovaries of several flowers (called an
inflorescence). Each flower produces a fruit, but these mature into a single mass. First an
inflorescence of flowers called a head is produced. After fertilization, each flower
develops into a drupe, and as the drupes expand, they merge into a multiple fleshy fruit
called a syncarp. Examples are the pineapple, edible fig, mulberry, osage-orange, and
breadfruit. If these fruits are composed of floral parts other than the carpels they are
termed accessory fruits.
6.
Plastids can have many functions and are responsible for photosynthesis, storage
of products like starch and for the synthesis of many classes of molecules such as fatty
acids, which are needed as cellular building blocks and/or for the function of the plant.
Depending on their morphology and function, plastids are commonly classified as
chloroplasts, leucoplasts, amyloplasts or chromoplasts. However, these different forms
are not fixed, and plastids have the ability to differentiate, or redifferentiate, between
these forms. All plastids are derived from proplastids, which are present in the
meristematic regions of the plant. Proplastids and young chloroplasts commonly divide,
but more mature chloroplasts also have this capacity.
Undifferentiated plastids (proplastids) will differentiate into several forms,
depending upon which function they need to play in the cell. The most commonly known
plastid is chloroplasts which contain chlorophyll along with photosystems II and I for
photosynthesis. Photosynthesis results in primary assimilate starch. Closely related to
chloroplasts would be amyloplasts because they contain thylakoids and are vital to starch
accumulation also. However, amyloplasts occur in non-green cells like roots and seeds
and are more important to long-term or secondary starch storage. Both plastids
accumulate starch in the stroma. Similar to amyloplasts (but lacking starch) are
eliaoplasts. They store different materials instead like oils and fatty acids for the plant
which could be later secreted or consumed. Leucoplasts have a little in common with the
two previous plastids, but the starch accumulation is much less common and they also
lack pigmentation and ribosomes in mature form. They act like continual proplastids and
keep replicating in new dividing cells so that they could develop into other plastid forms.
Chromoplasts are important to pigment synthesis and storage. They have very large
plastoglobuli which store the carotenoids. Since they lack a thylakoid system and starch
grains, chromoplasts see to be less related to other plastids. Etioplasts are like
chloroplasts which have not been exposed to light. Instead of a large thylakoid system
they have a lattice of tubules called prolamellar bodies. Their growth is very different to
light sufficient cells as their vertical growth is accelerated but their leaves are usually
white or yellow. What makes this occur is their inhibition of chlorophyll production by
protochlorophyllide. When introduced back to the sun, the light will degrade
protochlorophyllide and chlorophyll accumulates, causing the thylakoid system to
develop. An etioplast can morph into a chloroplast within minutes.
The development and specialization of plastids are enough to hint that they have
had a rich evolution in order to have such complexity and diversity. However it was not
always that plants developed their plastids from their own DNA (and it still really is not
the case). A process better understood now is prokaryotic genome assimilation into the
plant nucleus. This arose, way back in prehistoric time, when plant ancestors assimilated
photosynthetic prokaryotes. These unicellular organisms, like bacteria, lacked a
membrane bound nucleus unliked eukaryotes. For the creation of functional plastids in
plants there needed to be migration of huge numbers of genes from the prokaryote
genome to the plant nucleus. Scientists have a relatively good idea about the identity and
gene content of the organisms that were involved when the original eukaryotic plant
ancestor took in the prokaryotic forerunner of the plastid. Currents research is trying to
figure out the evolutionary process of gene transfer from the plastid to the nucleus and
also ask is transfer of plastid DNA into the nucleus of higher plants still occurring?
How did scientists make this discovery? After analyzing plastid DNA and
completing the sequence for the genome, they were able to deduce that the genome has
clear remanents of eubacterial genes. By comparing gene sequence, organization, and
expression of plastids with similar data on varying eubacteria, researchers were able to
pinpoint the evolutionary origin of plastids (along with mitochondria) and identify extant
eubacterial lineages to which they are closely related (Gray 21). While they were not able
to define a specific lineage, scientists decided that plastids orginated from within
Cyanobacteria.
A discovering this origin of plastids some scientists then went on to finding out
how many times did plastids orgininate in evolution? Also is their any non-eubacterial
proteins in plastids and where did they come from and when and how? The current
plastid genome helped them answer some of these questions. Plastid genomes are pretty
large (100-200kb) compared to say a mitochondrial genome. They also have a lot of
genes but they are much less diverse in structure and they tend to be conservative in their
retention of ancestral eubacterial features. Scientists have looked at this conservation and
used it as evidence to support the hypothesis of a monophyletic evolution of plastids.
There are some who still argue for a polyphyletic evolution, but there is no strong
evidence to support this alternate hypothesis.
The initial assimilation of plastids is thought to have occurred by primary
endosymbiosis in which cyanobacteria was taken up by a phagotrophic eukaryotic host.
Plastids that derived from this are called primary plastids and are in three eukaryotic
lineages: red algae, green algae, and glaucophytes. However, it became very difficult to
unite a host component for these three types of algae in phylogeny. This could point to a
very early acquisition of a primary plastid and a following diversification and plastid loss
in some eukaryotic lineages (which are now aplastidic). It has also been found that some
algae have obtained plastids via secondary endosymbiosis. In this process they took up a
another photosynthetic alga already containing a plastid. This addition of plastids via
horizontal transfer is important to the abundence and advancement of plastids. However
they have decided that secondary plastids must have derived independantly at least twice
because we have green and red algae.
When looking at the evolution of plastids into land plants, they have noticed that a
lot of rRNA is conserved (81%) from land plants to chlorophyte algae. This conservation
of genes allow scientists to divide the plastid genome into two repeat and two single-copy
regions. Most of the conservation is within certain regions and in clusters, reinforcing the
hypothesis of extreme conservation of the genome amongst lineages (Gray 29). Another
finding as of late is the cases of plastid gene loss following divergence of red and green
algae. By trying to map out these losses with phylogenetic trees, this can really help our
ability to describe evolutionary relationships among compared plastids genomes. This
will reveal the nature and timing of losses and then will showcase multiple independent
losses of the same gene in different lineages. We will be able to discover the purpose to
why plastids have loss certain genes when compared to more derived and newer genes.
7.
Plants are a culmination of complex processes that often require specific
substances. A lack of certain substances would hinder many functions of the plant and
also be detrimental to the structure of the plant.
Resin- A lack of this substance would put the plant at risk to invasion. At times when a
plant is injured, they secrete resin to block up the injury. Since resin is normally under
pressure, a rupture causes resin to flow out due to the reduced pressure and its viscosity.
Without the flow of resin, the plant will be able to be infected by harmful fungi or
insects. (Beck 189)
Suberin- This substance frequently coats plant tissue near the vascular tissue and
periderm. Without suberin there would be many instances of unregulated movement
throughout the plant. Unregulated substances could be ions or water. In the periderm, the
layers of phellem are coated with suberin to make it impermeable to water. If water were
able to penetrate the periderm it would most likely lead to the erosion of dead mature
cells that have the purpose of protecting the inside of the plant or supporting it. The weak
structure of the plant could easily be fatal. Suberin is also characteristic of the Casparian
strip in the endodermis. Without this Casparian strip the apoplastic movement of water
and ions would not be regulated and could cause imbalances in water pressure and ion
concentration. Besides the endodermis, the inner layers of many primary walls are
suberized (called the suberin lamella). Transfer between cells in this instance would also
be unregulated (Beck 284).
Lignin- As lignin polymerizes with primary and secondary cells walls; it provides
support to cells by reducing elasticity and increasing hardness and tensile compression.
Lignin, like suberin, also makes the cell impermeable to water. Without these features,
vertical growth of plants seems almost impossible. A lack of Lignin will make it
impossible for the plant to counteract gravity and withstand movement from weather or
animals. Lignin is vital to xylem and the uptake of water to cells is also important for
growth. Xylem vessels would simply collapse without lignin, and tissues would not
receive water. In general other tissues like phloem rely on the support of other lignified
tissues or else they would collapse as well from stress.
Hemicellulose- This matrix polysacride is important to cellulose microfibril deposition in
cell walls. Fibrous hemicellulose xyloglucan coats the cellulose microfibrils and is then
bonded to another hemicellulose via hydrogen bonding. Without it cellulose would not be
deposited in the manner that gives primary cell walls their greatest potential of strength.
As a result, the cell walls would be extremely weakened which could collapse the cell
and even the plants overall structure. Hemicellulose also forms a cross link between
pectin molecules (Beck 59).
Pectin- Like hemicellose, pectin is vital to primary cell wall composition. In fact, the
middle lamella is composed of only pectin. The pectic cell plate initiates the middle
lamella during telophase, and the space between neighboring cells is created. Without the
middle lamella, neighboring cells would not be stabilized together would be disastrous
for cell to cell transfer of materials. The network of pectin integrates with the separate
network of hemicellolose-cellulose.
Starch- These grains are the immediate result of photosynthesis that develop within
amyloplasts. Starch is largely for storage for the plant and is consumed for the plants own
metabolic processes. Starch is also a large reason for herbivory and responsible for
energy for herbivores as well. If a plant was not able to store energy, it would be have to
be doing photosynthesis continuously and this is impossible during nighttime when light
excitation is impossible. Plants would not be able to survive during times when rates of
photosynthesis are not high, and plants encounter these situations quite frequently.
Callose- The carbohydrate β-1,3-glucan is involved in a few places within a plant. Upon
wounding callose encloses the sieve plates of sieve elements. It encases the plasmalemma
and endoplasmic reticulum because these transverse pores. As a sieve element
approaches the end of its functional life, callose accumulates and this is called Definitive
Callose. Callose seems to be a barrier for plant cells so that current cell contents are not
lost upon wounding and foreign substances are prevented from entering the cell and
infiltrating the whole plant. Callose also develops in pollen tube as a way to protect the
exine until sporopollenin completely coats the exine. The gametes of the plant might be
degraded if not for the temporary protection of callose.
Sporopollenin- This hydrophobic biopolymer coats the exine and makes it safe from
degradation. The occurrence of degradation is frequent in cells and important substances
have to be marked in order to be saved sometimes. With the protective sporopollenin, the
exine would degrade and the reproduction of the plant would be extremely fragile.
8.
Higher plants have a complex life that involves two phases: a diploid sporophyte
phase and a haploid gametophyte stage. This revolving life is called alternation of
generations. The large plant bodies that we usually see are sporophytes, which develop
from fertilized zygotes. That zygote results from the fusion of two gametes, which are
very small plant bodies.
The development of fruit begins with meiosis occurring in both the anther and
pistil. For anthers, the process is called microgametogenesis. It starts with
microsporocytes that are contained in microsporangia, and microspores are created.
These microspores are what develop into pollen grains, and upon germination they will
turn into microgametophytes, the male part of fertilization. Each pollen grain has two
cells, a generative cell and a tube nucleus. When germinated, the generative cell nucleus
divides into two sperms. Both of these sperms have a nucleus and cytoplasm. Before
fertilization, the two sperms are in contact with each other and the leading sperm is also
in contact with the vegetative tube nucleus. It has more recently been found that the two
sperms differ in their size and cellular contents in some taxa, and these differences can
decide which sperm fuses with the egg cell.
For megagametogenesis meiosis also occurs in megasporocytes, and one is
contained in each of the developing ovules. Meiosis creates a linear tetrad of megaspores.
Three of these spores degenerate and the remaining spore develop into the
megagametophyte or embryo sac. Eight nuclei results from three mitotic divisions and as
these divisions happen the original cell expands and elongates (Beck 360). Four of the
nuclei migrate to each end of the developing embryo sac. At the micropylar end, one
nucleus will become at least partially enclosed by a cell wall, and functions as an egg
cell. The other two nuclei differentiate into synergids. These cells stay in contact with the
egg cell and possess a filiform apparatus. The purpose of this apparatus is thought to be a
transfer structure. At the opposite end of the embryo sac, the chalazal end, the three
nuclei differentiate as antipodal cells. The remaining fourth nucleus at each end migrates
to the center of the embryo sac. The central cell is then comprised of the two polar nuclei,
the remaining cytoplasm, the synergids and the egg cell. The central cell along with the
three antipodal cells makes up the mature female gametophyte. Not every instance of
megagametogenesis processes this way, but this is the most frequent type in angiosperms
called monosporic.
After pollination and germination of the pollen grain on the stigma, the pollen
tube proceeds through the style and into the locule of the carpel. It is impressive to see
the distance for the pollen to grow in order to reach the embryo sac. It grows through the
micropyle and as is does so, one of the synergids deteriorates to create an entrance for the
pollen tube. When inside, the two sperms are released into the synergid. A recent study
has shown that the surfaces of the sperms contain myosin and these motor complexes
interact with microfibrils of F-actin in order to transport down the pollen tube (Beck
361). When inside the embryo sac, the leading sperm will fuse with the polar nuclei and
forms the triploid endosperm nucleus. The following sperm fuses with the egg cell and
forms a diploid zygote, and once this occurs double fertilization is complete. At this stage
the ovule has an embryo sac, encased in a nucellus, and one or two integuments.
This process occurs in gymnosperms with a few differences. One major difference
is the reduction of structures in angiosperm reproduction as compared to gymnosperms.
Ovules and microsporangia of gymnosperms are produced in large female and male
cones respectively. In conifers, ovules are attached to the adaxial surface of ovuliferous
scales, and the micropylar end of the ovule is facing the middle or cone axis.
Microsporangia are borne on the abaxial surface of microsporophylls. The division of the
developing megagametophyte occurs for much longer in gymnosperms (several months),
and this creates a larger gametophyte.
Following microsporogenesis (which is mostly similar to angiosperms), the
conifer pollen grain is dispersed by wind and it lands in a pollination droplet, which will
then retract and draw the pollen grain through the micropyle and contacts the
megasporangium. Gymnosperms can also fertilize several eggs and several embryos will
develop (called polyembryony). After this only one of the embryos will actually continue
development and the others abort (Beck 354).
Sources
Beck, Charles. Plant Structure and Development. Cambridge University Press.
Cambridge. 2005.
Grey, Michael W. “The Evolutionary Origins of Plant Organelles”. Molecular Biology
and Biotechnology of Plant Organelles. Springer. Dordrecht. 2004.
No author. “Monocots versus Dicots: The Two Classes of Flowering Plants”. Internet.
5/05/06. http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html