Line Defense Mechanisms

Like in animals, the first line of defense of plants is the skin - the epidermis and the periderm.

If the first line of defense is penetrated, the second line of defense is the triggering of two different immune responses, which involves extensive genetic reprogramming.

The first being a chemical attack that would isolate the pathogen and prevent spread of infection. This immune response is a hypersensitive response called Pathogen-Associated Molecular Pattern (PAMP) triggered immunity. A hypersensitive response is a local cell and tissue death that would occur at and near the site of infection. It can restrict the spread of the pathogen, but also be a consequence of the overall defense response. It involves transcriptional activation of over 10% of the plant's genes that can encode enzymes that would hydrolyze components of cell walls of pathogens. Effector triggered immunity also stimulates the formation of lignin and cross linking of molecules within plant cell walls.

The second response is only triggered if PAMP is not sufficient. The second response is known as a systemic acquired resistance. This arises from plant-wide expression or defensive genes. Methylsalicylic acid is produced around the infection site and carried by the phloem throughout the plant, where it is converted to salicyclic acid in areas remote from sites of infection. The salicyclic acid activates signal transduction pathways that poises the defense system to respond rapidly to another infection. This response generally lasts for a while in order to ensure that infection is completely protected from.

Plants Spawning Invertebrates for Protection

Herbivores are organisms that solely rely on plants as their source of nutrients. Herbivores are a large threat to the plants as it reduces the size of the plants, hindering their ability to acquire resources to survive. Herbivores also restrict growth because many species of plants would focus their energy on developing an adaptation to defend themselves. Herbivores also opens up portals for infectious pathogens to infect the plants. Some plants would develop physical defensive mechanisms such as thorns or trichomes, and others would acquire chemical defenses such as a chemical that makes them taste bad or poisonous.

Some plants, such as corn are able to produce "green leaf volatile compounds" when it's leaves are chewed on. These compounds are a mixture of many chemicals, the most prevalent being terpenoid and phenolic (both of which are very attractive to parasitic wasps). The wasps would fly to the plant being eaten and the wasp would target the threat. The wasp would employ different defensive strategies depending on the species. Some species such as the digger wasps would pick up the host and relocate it elsewhere. Other wasps are more deadly, they would lay eggs in the threat. The eggs would hatch in a day or two and kill the host from within.

Plant Response to Various Stresses

Flooding
Too much water could cause the plant to suffocate because the soil would lack air spaces that provides oxygen for the roots to perform cellular respiration.

Oxygen deprivation generally will stimulate the production of ethylene and cause many cells in the root cortex to die. Destruction of the root cortex would create new air bubbles that would function as "snorkels", providing oxygen to the submerged roots.

Salt
Excess amounts of salt can lower the water potential of the soil solution, causing water deficiency in plants. When the water potential of soil solution become more negative, the water potential gradient from soil to roots is lowered, reducing the plant's water intake. NaCl and other ionic salts are toxic to plants when there is high concentrations of them.

Plants typically respond by produce solutes that are well tolerated at high concentrations, mainly organic compounds that can keep water potential of cells more negative then that of the soil solution while not admitting toxic quantities of ionic salts.

Heat
Heat could be detrimental to a plant's survivability as it could denature many enzymes, killing the plants.

Transpiration helps to keep the leaves cool by evaporative cooling. Hot and dry weather tends to dehydrate plants, closing off the stomate would help to conserve water loss but it would compromise the plant's ability for evaporative cooling, Many plants have a backup response, especially when the temperature exceeds 40 degrees Celsius - plants would synthesize heat-shock proteins that would help protect other proteins from the extreme heat.

Mechanical Stimuli Response

A plant's response to mechanical stimuli is called thigmomorphogenesis.

Plants are very sensitive when it comes to mechanical stress; for example, a ruler barely grazing a leaf could greatly alter its subsequent growth. Mechanical perturbation on a plant could result in many consequences.

Gravitropism/Geotropism

A plant's response to gravity is called gravitropism, or geotropism; this theory states that the roots are "positively geotropism/gravitropism" - which means that they generally will grow towards the direction of the pull of gravity. The shoot, however, will grow towards light (phototropism).

Plants are able to detect gravity by utilizing their settlements of statoliths, or dense cytoplasmic components that settle under the influence of gravity to the lower portions of the cell. In roots, the statoliths are located in certain cells in the root cap.

Light's Effect on Plants

A photoperiod is the amount of time that an organism experiences sunlight as well as night time. Photoperiodism is a physiological response to photoperiod, typically for the purpose of flowering. It also helps the plant be able to detect seasonal changes.

Photoperiodism influences flowering because it allows the plant to know when they should flower according to whether they are short-day plants or long-day plants. Short-day plants tend to flower in seasons such as winter due to their lack of need for sunlight. Long-day plants require long term exposure to the sun, so they tend to flower during seasons such as spring and summer.

Plants can also grow in response to light that they are exposed to. In the picture below, it shows the plants' growth being manipulated in accordance to the little variables present.

In the control, the plant leans towards the light, growing in the direction of the illuminated side. Since the tip of the plant is the main variable in the detection of light due to the high numbers of photoreceptors. As the tip of the plant is removed, the plant grows straight upwards. The same occurs when the tip is covered by an opaque cap. The plant with the tip covered with the opaque cap grows straight because phototropism only occurs if the tip is exposed to light, and light is being completely shielded from the tip. A plant with the tip covered in a transparent cap is the closest resemblance to the control as the transparent cap does not prevent the tip from being exposed to the light, thus promoting the plant to lean towards the light source. The base covered by the opaque shield does curve towards the source of light, but the base remains straighter than that of the control. Since the tip is exposed to light, it is supposed to lean, but the base is voided from light thus making it grow straight. The tip separated by a gelatin block does not do much in preventing the curvature of the plant towards the light. The gelatin block is similar to the transparent cap as it allows light to penetrate through and come into contact with the plant. The tip separated by mica is a unique case. Although the tip is separated by the impermeable mica suggested that the chemical signal was a growth stimulant as the phototropic response involves faster cell elongation on the shady side than on the illuminated side.

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Apoptosis Post-Reproduction

Post-reproduction, plants use apoptosis to senescence, or the programmed death of certain cells, organs, or the entire plant. Cells, organs, and plants are genetically programmed to die on a schedule. They do not simply, "shut down", and die. During apoptosis, newly formed enzymes would breakdown many chemical components that include: chlorophyll, DNA, RNA, proteins, and membrane lipids. The plant would salvage many of the products from the breakdown. A burst of a hormone, typically ethylene, is usually prevalent with apoptosis during senescence.

Fruit Ripening - Positive Feedback?!

Positive feedback is when the system responds to a foreign activity, or perturbation, by amplifying the perturbation stimulus.

When fruit ripens, it releases the hormone ethylene, which acts as a stimulus. Ethylene would promote the ripening of unripe fruits - thus forcing the fruits nearby to also ripen. Those fruits would release more ethylene as a response.

The hormone responsible for the ripening of fruit is ethylene.

Negative Feedback Mechanism of Plants

Negative feedback is a reaction that causes a decrease in a particular function as a response to a certain stimulus, it tends to stabilize the system.

Once a plant begins to experience water stress and starts wilting, abscisic acid (ABA) would start to accumulate in the leaves and force the stomata to close. The closing of the stomata would reduce transpiration and prevent further water loss. ABA would then affect second messengers, such as calcium ions, and cause potassium channels to open, causing a decrease in potassium ions. The accompanying osmotic loss of water would reduce guard cells turgor and lead to the closing of stomatal pores. ABA could also serve as an early warning system.

This is similar to human's blood glucose regulation because the blood glucose metabolism uses insulin which is used to increase glucose levels. ABA's effect in plants is similar to glucagon's effect in humans. When there is an excess of glucose within the blood, glucagon would be triggered to perform glucogenesis in order to lower the levels of glucose.

Plant Hormones

Auxin (IAA)
Auxin stimulates stem elongation in low concentrations only. It also promotes the formation of lateral and adventitious roots. Auxin plays a main role in regulating the development of fruits, enhancing apical dominance, functions in phototropism and gravitropism, promotes vascular differentiation, as well as retard leaf abscission.

 Cytokinins (CK)
Cytokinins regulates cell division in shoots as well as roots. They help modify apical dominance and promotes lateral bud growth. Cytokinins promotes movement of nutrients into sink tissues, stimulates seed germination, and delay leaf senescence.

Gibberellins (GA)

Gibberellins stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed development and germination. This hormone also regulates sex determination and helps with the transition from the plant's juvenile phase to its adult phase. 

Abscisic Acid (ABA)

Abscisic acid inhibits growth, yet promotes the stomata's closure during drought stress. ABA helps with seed dormancy and inhibiting early germination. The hormone is a primary promotor of leaf senescence and desiccation tolerance.

Ethylene

Ethylene is the promotor of the ripening of fruit, leaf abscission, the triple response in seedlings, lateral expansion, horizontal growth, root and root hair formation, and flowering in pineapples. Ethylene inhibits stem elongation; it enhances rate of senescence.

Brassinosteroids (BR)

Brassinosteroids promote cell expansion and cell division in shoots, root growth at low concentrations, xylem differentiation, seed germination, and pollen tube elongation. BR inhibits traits such as root growth at high concentrations, and phloem differentiation.

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Angiosperm Cycle of Reproduction

Angiosperm's reproductive cycle is more complex in comparison to many of the other species of land plants. Angiosperms undergo two main functions: meiosis and fertilization.

The reproductive cycle of the angiosperm begins with the anthers. Anthers contain microsporangiums, or pollen sacs. On the anther, each pollen sacs contains microsporocytes (2n) that would undergo meiosis for division, creating microspores (n). After mitosis it would become a pollen grain (n). The pollen grain contains a generative cell and a tube cell (with a tube nucleus). The generative cell would divide, forming two sperm cells. The tube cell would create the pollen tube. After the pollen grains connect to the stigma, the sperm and the tube nucleus would travel down the pollen tube, this is the process of pollination. After pollination, the sperm cells would eventually be discharged into each ovule. The ovule would already have divided it's megasporocyte in the megasporoangium by the process of meiosis. The product would be four megaspores. Only one would survive to become the female gametophyte that receives the sperm.

Next, there would be double fertilization. One sperm would fertilize the egg, forming a zygote; the other sperm would fertilize the central cell, which would turn into an endosperm (3n) which would act as a food supply. The zygote would then develop into an embryo, which is then packaged along with the endosperm, forming a seed. The seed would eventually germinate and the embryo would develop into a mature sporophyte.

http://www.mun.ca/biology/scarr/139450_Angiospermae.jpg

Seductive Reproduction

Angiosperms are the most diverse group of land plants but they all have similar structures to help with reproduction. 

For example, all angiosperms have petals which act as eye candy for organisms. The petals would be the first thing that other organisms see and the vibrant colors would cause organism to approach the flower. Then the angiosperms would use the pollen that is generated by the anther to stick pollen to the organism, allowing pollen to be spread to other plants. Pollen can also be transferred by means of forces of nature, for example, wind or rain. The carpel of the angiosperm contains the ovaries, so the stigma would receive pollen from other organisms or the wind and begin reproduction.

https://sixlegsonecorolla.files.wordpress.com/2012/07/generalised_flower_diagram.jpg

    

Soybean plant and Rhizobium bacteria's "special relationship"

Soybean plants' roots would emit a chemical signal that is attractive to Rhizobium bacteria. The bacteria would respond with signals that causes the root hairs to elongate; allowing the bacteria to infect the plant by penetrating and causing the invagination of the plasma membrane.

The Rhizobium bacteria would then penetrate the root cortex within the "infection thread". Root cortex cells and pericycle cells would begin to divide. Growth would continue in the affected regions of the cortex and pericycle until the two dividing cells fuse, which forms the root nodule. The nodule would continue to grow while vascular tissues connect the nodule to the xylem and phloem. The vascular tissue would supply nutrients to the nodule as well as carry nitrogenous compounds out from the nodule to be distributed to the rest of the plant.

Soybean plants and Rhizobium bacteria have a type of symbiosis relationship. They exhibit a mutualistic relationship, or cooperation, due to the fact that the Rhizobium bacteria supplies the soybean plant with fixed nitrogen while the soybean plant provides the Rhizobium bacteria with carbohydrates and other organic compounds that helps the bacteria to grow.

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Ye Ole Gas Exchange

In order for there to be photosynthesis, plants must be able to exchange gases in their leaves. This picture shows the process of gas exchange within a cell of the leaf. During transpiration, diffusion occurs and the water vapor from the moist air spaces would transfer to the drier air outside by means of the stomata. The water vapor that is lost is replaced by evaporation that occurs from the water film that coats the mesophyll cells. The evaporate would cause the air-water interface to delve deeper into the cell wall and cause a greater curvature. This curve would increase surface tension as well as the rate of transpiration. Then, it would pull water from the xylem into surrounding cells and air spaces in order to compensate for the water loss from transpiration.

Guard cells play a crucial role in gas exchange; they are the regulators of the entire process. The stoma must open to allow gas exchange and they must close to regulate water loss. Guard cells are the structures that regulate the opening and closing of the stomata.

Sucrose Relocation into Phloem and Bulk Flow in a Sieve Tube

Sucrose is manufactured in mesophyll cells and can travel by symplast to sieve-tube elements. In some plants, sucrose would diverge from symplast near sieve tubes and travels through apoplast. Sucrose is then actively transported from apoplast into companion cells and sieve-tube elements. The proton pumps generate a hydrogen ion gradient that allows the buildup of sucrose with the help of cotransport proteins that couples sucrose transport to the diffusion of hydrogen ions back into the cell.

This is the bulk flow by positive pressure (pressure flow) in a sieve tube. Sugar is loaded into the sieve tubes at the source and that causes a reduction in the water potential inside the sieve tube elements. This causes the tube to undergo osmosis to take up water. The uptake of water then generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water at the sink. In leaf-to-root translocation, the xylem would recycle water from sink to source. 

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Tree of Water Potential

The diagram indicates that water moves from higher to lower water potential areas.

Plants acquire their nutrients by the root hairs absorbing soil solute, which includes water molecules and dissolved mineral ions. The soil solution is then drawn into the hydrophilic walls of epidermal cells and travels along the cell walls and the extracellular spaces into the root cortex. The flow amplifies the exposure of the cortex cells to the solution which provides a greater membrane surface area for absorption.

Then, the water and nutrients obtained from the soil solution travels through the endodermis, which is the last layer of cells in the root cortex, and continue onto the xylem by the use of root pressure. The transportation of the xylem sap, or the water and minerals in the xylem, requires the extensive loss of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant. Now, root pressure causes more water to enter the leaves than the amount of water that is transpired, so that creates guttation or the exudation of water droplets that can be seen in the morning.

There is also the cohesion-tension hypothesis. This theory suggests that transpiration is the force that pulls the xylem sap upwards and cohesion is responsible for water molecules continuous structure through the use of hydrogen bonds. The transpirational pull works as the negative pressure potential causes water to travel through the xylem develops at the surface of mesophyll cell walls in the leaf. The cell wall functions similarly as a very thin capillary network. Water would stick to the cellulose microfibrils and other hydrophilic components of the cell wall. Water would evaporate from the water film that encases the cell wall. Due to the high surface tension of water, there is tension, or negative pressure potential, in the water. So when water evaporates from the cell wall, the air-water interface would increase, causing the pressure of the water to become more negative. Water molecules that are found at the more hydrated parts of the leaf are attracted to the negative area, reducing the amount of tension.

The ever-important role of the negative pressure potential in transpiration is consistent with the water potential equation since tension, or the negative pressure potential, are key factors in lowering water potential. This is significant as osmosis is the movement of water from areas with high water potential to the areas with lower water potential - the more negative pressure potential present at the air-water interface would allow water in the xylem cells to be pulled into the mesophyll cells and diffuse the water out of the leaf by means of stomata.

Root hairs are significantly similar to microvilli in the human gut. Microvilli vastly increase the surface area available for absorption of microscopic or molecular sized particles compared to the same linear inches of surface area if they were not present. The same function is performed by the tiny root hairs from the cells of the roots of plants. The value of the structure is to get a huge increase in the surface area without having to add to the length.

Alternation of Generations (of plants)

Alternation of generations is a process that allows each generation to give rise to the other generation. Alternation of generation typically occurs in algae that are closely related to the land plants.

Alternation of generation is characterized by a cycle that includes both multicellular haploid and diploid organisms. The haploid organism is known as gametophytes, or gamete producing plant. Gametophytes are produced through the mitosis of eggs and sperm (haploid gametes). The gametes would undergo fertilization, and form diploid zygotes (2n). Through mitotic division of the zygote, a diploid known as sporophytes, or spore producing plants, is formed. Through meiosis, the sporophyte would product spores, which act as the sporophytes' reproductive cells that would develop into a new haploid organism. Through mitotic division of one of the spores, a gametophyte is formed and the cycle would resume.

Mitosis and meiosis are not used for the same thing. Meiosis in plants are used to produce spores to initiate the gametophyte generation while mitosis is to turn the gametes into sporophytes.

http://kvhs.nbed.nb.ca/gallant/biology/alternation_generations.jpg

Land plants vs. Charophycean algae

Land plants and algae are strikingly similar, however, they are significantly different in many aspects.

Algae can be unicellular, colonial, or multicellular while plants are only multicellular.
Even when algae is multicellular, they only possess holdfasts, stapes, and blades. Multicellular land plants exhibit characteristics that include roots, stems, leaves, flowers, fruits, seeds and cones.

The more advanced plants have vascular tissues, which means that they are able to absorb and transport water and nutrients throughout their body. Each algae cell must be able to fend for themselves by taking in nutrients from the surrounding water.

Algae are water plants, which means that they grow completely submerged in water. Land plants, as their name suggests, typically live in land. Both require water for survival, but if land plants were submerged in water they would suffocate.

Reproduction of the two differ the greatest. Algae can reproduce in many ways that include tiny spores, replication, and regeneration of broken pieces. Plants have a very complex reproductive system as they could require the assistance of other organisms as well as forces of nature to spread pollen.

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Plant Phylogeny?!?

Like animals, plants have a phylogeny tree to indicate new adaptations and advancements in structure as well as function. The ancestors of all plants is the "green plants" which are closely related to the bryophytes, or green algae. The first land plants include nonvascular plants which means that the plants do not have vascular tissue; thus, cannot retain water and deliver it to the rest of the plant. Next is the introduction of the vascular plants. The first vascular plants are the seedless vascular plants. These plants do have vascular tissues, which means that water is vital for their survival. Seedless vascular plants, or pteridophytes, typically favor moist environments and they reproduce by using spores. Seed plants are the most advanced plant species and there are two kinds - gymnosperms (conifers) and angiosperms (flowering). 

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