Etiolation: An Example of Hormone-mediated Response What happens to a potato if you wrap it up in a brown paper bag and then forget about it?

Etiolation is the term applied to the morphological adaptations exhibited by a plant growing in the dark:

• long, spindly stems (why is this adaptive?)
• lack of chlorophyll (why is this adaptive?)
• stunted leaves (why is this adaptive?)

Upon reaching light, de-etiolation ("greening") takes place:

• stem elongation slows
• chlorophyll is manufactured
• leaves expand to normal proportions

...but how does the plant sense the light?
The receptor in this case is a pigment known as PHYTOCHROME.

Phytochromes are proteinaceous molecules with a sulfur-linked (covalently bonded) pigment active group (the chromophore) consisting of a linear tetrapyrrole.

The pigment absorbs primarily in the red and far-red region of the spectrum, and so appears blue-green to our eyes.

The chromophore portion of the molecule can exist in one of two forms, P_R or P_FR:

The functional phytochrome consists of two identical proteins, each with a chromophore. One part of the protein acts as the photoreceptor, and the other as a kinase, which triggers cellular responses.

Let's take a tour of the signal transduction pathway for de-etiolation, triggered by the isomerization of phytochrome:

It's all about gene expression. Ultimately, the signal transduction pathway results in the activation (or, in some cases, the suppression) of genes involved in the production of a specific, environmentally-induced phenotype.

• stimulating transcription of the mRNA coding for that enzyme (transcriptional control)
  OR
  
• activating existing enzymes (post-translational control)

Note:

A single phytochrome isomerization can trigger the production of hundreds of secondary messenger molecules, thus amplifying the response to what might have been a very weak initial environmental stimulus (i.e., it can take very little light to trigger de-etiolation.)

(Note: These genes were discovered in Arabadopsis thaliana (commonly known as "Thale Cress"; Family Brassicaceae). This little guy is the plant equivalent of Caenorhabditis: it is one of the most highly utilized plant models in studies of botanical molecular and cellular processes.)

Other Responses to Light: The Hard-working Phytochromes and Blue Light Receptors Light is one of the most critical environmental factors for plant survival and growth. Evolution has provided them with at least two different classes of photoreceptors:

• phytochromes (whom we've met already)
• blue light photoreceptors

Blue Light Photoreceptors
Blue light stimulates

The blue light receptor was so difficult to find that for a long time researchers referred to it as "cryptochrome!" But in the last decade, three blue-absorbing receptors have been found:

• cryptochromes (inhibit hypocotyl elongation)
• phototropin (its name implies its function)
• zeaxanthin (induces stomatal opening)

Phytochromes and Germination
Germination is the critical starting point of a plant's life. Especially if you're a small seed (with little endosperm), it you germinate too early, when you're buried too deeply in the soil, or when you're in a spot that's too shady, you're OUT OF THE GENE POOL.

In the 1930s, USDA researchers used lettuce seeds to determine the effects of light on germination rate. Seeds were soaked, and then:

Seeds last exposed to red light had the highest germination--and the effect was reversible (as indicated by the last protocol). Red light stimulated germination, and far red light inhibited it.

(What's the adaptive significance of this?)

Remember:

Given this information, which phytochrome (P_R or P_FR) might you expect to promote germination?

Which might inhibit germination?

Tropisms A tropism is a permanent, directed movement (growth) in response to an external stimulus. Tropisms may be positive (going towards a stimulus) or negative (going away from a stimulus).

Plant tropisms are changes in growth pattern in response to stimuli.

...and you can think of more tropisms, no doubt (halotropism, chemotropism, etc.)

Gravitotropism
Lay a plant on its side, and--given enough time--it will reorient both its shoots and roots in the proper position.

But how does the plant know which way is up, and which way is down? It starts with germination (remember the little song). As you might guess, auxin is a key player in gravitotropism.

Amyloplasts
Starch-filled plastids called amyloplasts (a.k.a. statoliths) tumble downwards in response to gravity in specific cells of the shoots and the roots.

• In shoots (and in coleoptiles), the amyloplast-rich cells form a starch sheath around the vascular tissues.
• In roots, the amyloplast-rich cells are located in the root cap, and especially in its central axis (the columella).

When a root or shoot is placed on its side, the amyloplasts slide to the formerly vertical cell wall and rest there. A few hours later, the shoot or root begins to bend/grow vertically (up or down, depending on whether it's a shoot or a root). When the horizontal wall becomes more vertical again, the amyloplasts slide back to their position on the "bottom" wall of the cell.

Why does the displacement of the amyloplasts result in differential growth? Several hypotheses have been proposed...

1. Calcium/Auxin Redistribution Hypothesis
The aggregation of amyloplasts at the lowest points in the cells triggers a redistribution of Ca⁺ ions, and this, in turn triggers the (active) lateral transport of auxin within the root. But--counterintuitively to what you might expect--the auxin and calcium accumulate on the BOTTOM side of the displaced root.

Recall: At HIGH concentrations, auxin actually inhibits cell elongation. Hence, growth on the lower side of the root slows down, and proceeds as normal on the upper side. Once the root is growing straight down (or shoot straight up), the statoliths take up their usual position on the "bottom" cell wall, and growth direction stabilizes.

2. Protoplast Pressure Hypothesis
Mutant Arabidopsis and tobacco planst lacking statoliths are still capable of gravitotropism, albeit more slowly than wild type plants. So there must be more to this phenomenon than just the starch granules.

Some investigators have suggested that the entire protoplast is involved in sensing a change in gravity: cytoskeletal components may "tug" on the proteins that bind the protoplast to the cell wall, causing

This could explain how plants "sense" gravity, but not how the cell responds on a molecular level.

3. Tensegrity Model
Tension integrity, or "tensegrity", (a term used in architecture to describe structural integrity generated by the tension among interconnected parts of a building or other structure) has been suggested as another mechanism by which plants sense gravity.

Proponents of this model suggest that actin filaments stretched by the plant's orientation stimulate an increase in local [Ca⁺²]. It has been noted that

As we've seen, Ca⁺² ion concentration changes are part of many signal transduction pathways. But the specific way in which this one works is not yet known.

Hormonal Involvement in Gravitotropism

Thus, two types of altered cell growth may contribute to the end result: the root bending downwards towards gravity.

Response to Mechanical Stimuli An organism rooted to the spot has an adaptive advantage if it can alter its growth in response to mechanical touch, if it's unable to physically move its existing cells and tissues.

Thigmomorphogenesis
Recall our earlier discussion of ecotypes.

The Jeffrey Pine (Pinus jeffreyi) is one of the most striking examples of showing two distinct ecotypes, one growing on flat lowland areas, and the other on high cliffs (Yosemite National Park) where winds are strong and constant.

The morphological differences can be explained by a phenomenon known as thigmomorphogenesis.

As we've already hinted, plants are extremely sensitive to mechanical stimuli.

• measuring a leaf with a ruler will affect its subsequent growth
• rubbing the stems of a young plant daily will stunt its growth (relative to controls)

Mechanical stimulation of plants in this way results in an increase in Ca⁺² ions in the cytosol. As we saw previously, this can trigger/mediate gene expression. In the case of thigmomorphogenesis, the genes activated encode enzymes that affect the physical properties and construction of the cell walls themselves, increasing lignification and reducing cell elongation.

Thigmotropism
Certain plants (especially vining plants) are very sensitive to touch, and will grow in response to mechanical touch.

Vines (and their tendrils) will usually grow straight until they are touched. At that point, they will curve and coil in the direction of the touch (differential growth on either side of the stem) until they are able to attach to whatever solid thing attracted them.

Fast Responses to Mechanical Stimuli Turgor movements are relatively quick plant movements that result from changes in internal water pressure.
Examples include stomatal opening and closure, as well as the sudden movements of the sensitive plant's leaves or the Venus flytrap's insect-capturing leaves. (They are not tropisms.)

Turgor movements are also known as thigmonastic movements because they usually occur in response to touch, shaking, or even electrical stimuli.

In Mimosa pudica, specialized cells within the pulvinus at the base of each leaflet and the main petiole rapidly lose water in response to a change in potassium ion potential in the apoplast. This, in turn, appears to be triggered by the loading of sugar into the apoplast from the phloem, all in response to a small environmental stimulus!

Definition: A pulvinus is a sort of plant motor organ. It is an an enlarged area at the base of a movable plant structure (petiole, leaflet attachment to rachis, node, etc.) that can produce movement by a change in turgor pressure in some of its cells.

Long a mystery, the means by which the Venus Flytrap closes its specialized leaves in response to touch may be close to discovery. It took an international team of

Lakshminarayanan Mahadevan (Professor of Applied Mathematics, Harvard University) explains...

• the open leaf trap is somewhat akin, physically, to a tennis ball cut in half and pushed inside out
• the leaves are held in place by regions of cells with high turgor pressure that apply enough tension to the open leaves to keep them in place
• mechanical stimulus of tiny hairs inside the trap cause a change in ion potential, and water shifts out of the turgid cells
• the slight change in curvature releases the tension holding the leaves open, allowing them to snap shut.

The Venus Flytrap discriminates:

Circadian Rhythms Circadian Rhythms are adaptations in response to environmental changes over the course of 24 hours.

In most organisms, these rhythms are timed by internal mechanisms that collectively function as a biological clock that synchronizes the organism's metabolism in response to environmental changes. This allows the organism to optimize its functions over a 24-hour day, or over the course of changing seasons. Circadian flowering rhythms can be vital to the evolutionary fitness of a given individual.

Plants...

• close their leaves and flowers daily in a sleep cycle
• go dormant (in temperate latitudes) over the winter
• flower at different times of the year (photoperiodism)

The exact timing of the cycle is variable (about 21-27 hours, depending on the particular Circadian response), but these will synchronize with the environmental day/night cycle in an outdoor plant (or animal) and shift with the seasons and other stimuli (think: jet lag).

Photoperiodism: How do Plants Know When to Flower? Some plant species bloom in the summer (when days are long), whereas others bloom in the winter (when days are short). Others bloom at times in between. Plants that bloom only when there is a specific day to night ratio are said to be photoperiodic.

How does each individual "know" how to flower exactly when its conspecifics are flowering, thus increasing its likelihood of pollination? What might be the evolutionary significance of plant species that have developed these differences in the timing of their reproduction?

What factors might induce plants to flower?

Which of these is the most reliable gauge of the season in temperate latitudes?

Although temperature, water, and other environmental conditions may vary from year to year, the duration of daylight is predictable.

The Secret of Flowering

In 1920, Garner and Allard reported two species of plants that would not flower unless their daylight hours were of a certain, critical shortness. Even a flash of light during the critical dark period would inhibit flowering. They called this phenomenon photoperiodism.

There are three "flavors" of plants with respect to photoperiodicity:

• short day plants flower in early spring or fall; they need daylight shorter than a particular, critical length, or they will not flower.

• long day plants flower in summer; they need daylight longer than a particular, critical length, or they will not flower.

• day neutral plants flower without respect to day length (flowering mechanisms are various, and some are not well understood)

Particular species tend to be one type, but

• ecotypes of differing flowering stimulus periods are known in species that have a broad north/south distribution
• the number of days of critical period depends on species. (Some species need only one day of the critical period, whereas others need several weeks of a critical period before they will flower.)

How does it work? It's our Old Pal phytochrome.

Recall the Two Phytochromes

The P_R form:

• has a λ_max (maximum absorbance) at 666nm (red)
• is synthesized in dark-grown seedlings
• is converted to P_FR in the presence of red light
• is the more stable of the two forms

• has a λ_max of 730nm (far red)
• is biologically active, eliciting a physiological response in the plant
• is converted to P_R in the presence of far red light
• is the less stable form, and will spontaneously revert to P_R in darkness (dark reversion)
• can be destroyed by proteases in the cell
• Because P_FR absorbs some red light, there is usually a proportion of about 85% P_FR to 15% P_R in the cell when the cell is exposed to red light.
• However, because P_R is not very sensitive to far red wavelengths, in far red or darkness, the cell has about 97% P_R to 3% P_FR.

In all photoperiodic plants,

• In the winter when days are short, [P_R] > [P_FR]

• In the summer when days are long, [P_R] < [P_FR]

The critical difference is...

• In short-day (i.e., winter flowering) plant species, P_FR inhibits flowering.
• In long-day (i.e., summer-flowering) plant species, P_FR induces flowering.

Therefore,

• In WINTER, P_FR when concentration is low, the flowering inhibition it exerts in winter-flowering plants is "lifted." They bloom!

• In SUMMER, P_FR when concentration is high, the flowering induction it exerts in summer-flowering plants starts working. They bloom!

Hormonal Control of Flowering
The putative receptor stimulated by the phytochrome isomerization is known as florigen. It may be a hormone--or even a team of hormones--but it has not yet been identified.

In any case, the physical result of photoperiodism is the expression of genes in meristem cells that cause differentiation of the cells into the tissues and various parts of the flower we now know so well. Signal transduction pathways that trigger these gene expressions are still under study. Stay tuned!

Plant Responses to Environmental Stress Flooding, drought, extreme temperatures (abiotic), pathogens, herbivores and competition (biotic) are all selective factors that have resulted in a marvelous array of plant responses.

Drought
Plants need water to survive, of course. But they do have mechanisms by which they can survive short periods of drought.

• loss of water from cells reduces turgor, closing stomatal guard cells
• in some species, leaves roll into a water-loss reducing shape (e.g., grasses)
• loss of turgor will inhibit further plant growth (photosynthesis slows!)
• drought inhibits the formation of surface roots, promotes deep root formation

Flooding
Overwatering kills plants because of oxygen deprivation (drowning). But plants have some short-term survival strategies that can allow them to cope with temporary inundation.

• low oxygen concentration in tissues stimulates ethylene production
• ethylene triggers apoptosis in some root cortex cells
• this creates air spaces and even air tubes continuous with the non-submerged plant parts (snorkels!)

• some plants can produce organic solutes (non-toxic, even in high concentrations) that help equalize water potential
• a halophyte is a plant evolutionarily adapted for life in soils with relatively high salt content. These have other mechanisms (e.g., salt-excretion glands) for reducing water potential between root and soil.

Heat Stress
Excessive heat can denature enzymes and cause other metabolic damage.

• In times of extreme heat, increased transpiration can effect evaporative cooling
• special "chaperone" proteins known as heat shock enzymes which may link to other enzymes and structural proteins, preventing them from denaturing in the heat--at least temporarily.

Cold Stress
Cold temperatures increase cytosol viscosity (slowing things down). I don't need to tell you that freezing ruptures cells and causes biological havoc.

• lipid composition in plasma membranes changes in response to cold: unsaturated lipids (which have a lower freezing point) increase, and saturated lipids decrease
• this change can take hours or even days
• hence, plants are often more sensitive to cold than they are to other environmental stresses: there's sometimes not enough time to respond
• freezing temperatures can be mitigated by solutes in the cytosol which lower its freezing point (sometimes simply removing water from the cell is the method!)
• in this case, resistance to dehdration is the key to survival

Plant Defenses Against Herbivores Physical defenses against herbivores can be obvious: thorns, pubescent leaves, sticky resins, etc. Chemical defenses can be more subtle.

Canavanine is an unusual amino acid that closely resembles arginine. If an insect eats enough tissue from a plant containing canavanine, the compounds replaces arginine in many of the insect's newly forming proteins, rendering them inactive. Death ensues!

Calling in the Cavalry
When a corn plant is munched by a caterpillar, the physical damage and certain chemicals in the caterpillar's saliva combine to elicit release of volatile compounds from the corn leaf. This attracts parasitoid wasps that lay their eggs in the hapless caterpillars' bodies.

Warning the Neighbors
A damaged plant can even send warning to its nearby conspecifics.
Lima beans suffering attack by spider mites release volatile compounds that elicit the expression of genes that encode enzymes that make the plant

• less susceptibel to spider mites
• attractive to another species of mites that preys on spider mites

Plant Defenses Against Pathogens Pathogens can be

• virulent - causing disease in a plant because the plant has little or no genetic defense
• avirulent - causing mild harm, but not death (a result of a coevolutionary "Cold War")

Gene-for-gene Resistance

This form of protection, utilized by many plants, is the production of protein by specific plant disease resistance genes (collectively called R genes) that can recognize molecules produced by pathogens (often encoded by the pathogen's Avr, or "avirulence" genes).

Different R genes respond to different pathogens, but if a plant does not have an R product that matches the Avr of a pathogen, then that pathogen may be virulent in that plant species.

Pathogen-produced molecules known as elicitors can induce a less specific type of defense response.

Cell wall damage can cause fragments of cellulose to convert to oligosaccharins can act as elicitors, and can stimulate the production of antimicrobial phytoalexins.

Other microbe infections can elicit the cell to produce compounds that

• attack bacterial cell walls
• increase lignification of plant cell walls

  An avirulent pathogen (i.e., one that has an R-Avr match in that plant species) will elicit a particularly strong response, and the plant will actually wall off the localized infection. This Hypersensitive Response (HR) will produced small lesions where the infection has been corralled. The leaf will be ugly, but it will survive.

A Hypersensitive Response will also set up signal transduction pathways that result in SAR - Systemic Acquired Resistance.

Recall that the activation of the SAR response requires accumulation of endogenous salicylic acid, which is triggered by the presence of a pathogen.

In Arabidopsis, a high concentration of salicylic acid activates a molecular signal transduction pathway that is identified by a gene called nim1 (also known as npr1 or sai1). The pathway results in heightened immunity to all pathogens in uninfected parts of the plant, sometimes for many days after the attack.

Because the SAR elicits production of various phytoalexins and PR proteins, it tends to be non-specific, and can afford protection against a variety of pathogens.

An overview of plant defense against an avirulent pathogen...

Key:

• 1 - Plant R protein (receptor) and pathogen Avr protein (ligand) join
• 2 - signal transduction pathway is initiated
• 3 - HR response: cells seal off pathogens and then undergo apoptosis
• 4 - dying cells release a chemical signal (salicylate)
• 5 - salicylate is transported to the other parts of the plant
• 6 - signal transduction pathways are initiated in distant parts of the plant
• 7 - SAR is achieved when phytoalexins and PR proteins are produced in those remote tissues.