Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Sensory Systems in Plants

I.  Overview

A.  General - both plants and animals require mechanism to respond to environmental conditions.  This involves the response scheme mentioned earlier (signal →  receptor →  transducing mechanism →  response).  The transducing mechanism typically involves hormones (each of which has its own receptor).

II.  Lettuce Seed Germination 

A.  General
    Recall from our Plant Way of Life notes that one problem a non-motile organism like plants have is getting started in the right spot. Obviously a motile organism can move to a favorable location, but a plant is stuck in one spot once the seed germinates. For most plants getting started in the right place is a matter of luck.  They produce lots of offspring with little parental care of offspring and very few survive - think oak tree and acorns.  However, there are a few "tricks" that seeds use to help increase the odds that they germinate in a favorable environment.  One of my favorites involves light.

    But first, consider your garden.  What happens in the spring after you dig up the soil?  Right - you get a huge crop of weeds!  Why?  The reason is that some seeds, like certain varieties of lettuce, require light for germination. This is a mechanism to insure that they germinate on the soil surface.  Thus, tilling your garden brings a new crop of seeds that were buried in the soil to the surface where they are exposed to light.  Light-sensitive seeds are usually small and without much stored food.  Thus, it is important that they begin to photosynthesize soon after germination.

    Action spectra for this response show that red light (ca. 660 nm) triggers seed germination and that treatment with far-red light (ca. 730 nm) inhibit/prevent germination. A typical experiment would yield the following results:

    Note that the seeds are responding to the last "flavor" of light to which they are exposed. Essentially the response is "reversible" much like using a switch to turn on a light bulb.

B.  Phytochrome
    Phytochrome is the receptor for this response and other photo-reversible responses in plants. Phytochrome is a blue-green and has a pigment molecule that is associated with a protein.  Phytochrome exists in two form: Red form (Pr; absorbs red light at about 660 nm) and Far Red form (Pfr; absorbs far-red light at about 730 nm). Pr is converted to Pfr when exposed to red light. Pfr is converted back to Pr when exposed to far-red light. Since the plant only makes phytochrome in the red form, initially all phytochrome is in the Pr form until the plant is exposed to light. The reversibility is one of the characteristics of a phytochrome effect. The last wavelength determines the germination response.

    Pfr is the "active" form of phytochrome that will either induce or inhibit a response.

C.  Mechanism of Action
    So, how does phytochrome work?  Evidence suggests that phytochrome induces germination by presumably activating a number of processes in the dormant seed including amylase production.  Recall that amylase is the enzyme for digesting starch.  Amylase production is activated when the seed is exposed to light (or when non-light requiring seeds are soaked in water).  Thus:

GA is produced by the cotyledon of the embryo → stimulates the production of amylase by the aleurone layer → amylase hydrolyzes starch to simple sugars → absorbed by cotyledons and translocated to embryo for growth.

    The production of amylase occurs de novo. That is, gibberellin stimulates transcription. In short: GA → binds to membrane receptor → interacts with a protein complex (heterotrimeric G protein) that → activates a GA signaling intermediate → turns off a repressor → transcription of GA-MYB mRNA → translated in cytosol to make GA-MYB protein → returns to nucleus to bind to alpha-amylase gene promoter region → activates transcription of alpha-amylase mRNA → translated in ribosomes on RER → transported to golgi → secretory vesicles release alpha-amylase. This last step is apparently regulated by a calcium dependent mechanism that was also activated by the heterotrimeric G protein complex.  Phew!

D.  Commercial Applications
    Brewers take advantage of GA's ability to stimulate germination and enzymes which are important in the brewing process.  Here's a quick overview of the beer-making process:

Step 1.  Malting - the function of this step is to induce hydrolytic (digestive) enzymes that breakdown the starches in the grains to fermentable (simple) sugars like glucose.

• Steps in the malting process are:  barley → cleaned → soak → germinate (rotating drums) → dry in kiln (raise temp to about 180 F) → grind → screen → malt

• Malt - gets its name from the maltose (one of products of enzyme breakdown of starch)

• Preparation of malt will determine flavor of beer.  For example, stouts use malt that is caramelized

• barley may be treated with gibberellic acid to insure uniform germination

Step 2.  Mashing - the purpose of this step is to convert starch to simple (fermentable) sugar

• Malt + water + adjuncts (other materials, i.e., rice & corn added to US beers) → enzymes continue to convert starch into fermentable sugar → spent grain removed feed to animals; liquid → wort

Step 3.  Brewing - the function of this step is to convert sugars into alcohol (and other flavor components)

• wort + hops (related to marijuana; use the female flowers; provides flavor, disguises sweetness from sugars in the beer; stops enzyme action, precipitates proteins, antimicrobial action) → boil (stops enzyme action, extracts flavor, sterilizes) → spent hops removed (used for fertilizer) → add yeast to liquid → brew 5-12 days

• Saccharomyces cerevisae - ales top fermented (yeast floats), higher alcohol content, higher hops, paler color; lagers bottom fermented (yeast sinks). 

• The purpose of the yeast is to convert sugars to carbon dioxide and alcohol and to impart flavor from other metabolic products. 

Step 4.  Lagering - this is the aging and maturing step.  

• Beer is aged from 14 days to 3+ months.  

• The yeast is removed → transferred to lagering tanks where the proteins settle out and flavor improves → carbonated → pasteurized → bottled

• Carbonation can be done by: (a) adding beechwood chips and green beer to stimulate a final yeast growth (krausening); (b) introduce CO2 under pressure.

III.  Gravitropism

A. Stimulus - gravity, hence the name gravitropism. It used to be called geotropism, but gravitropism is more accurate since the response isn�t toward the earth ("geo").  Note that the response varies - roots and shoots grow parallel to the direction of the stimulus whereas rhizomes grow perpendicular to the stimulus and lateral roots develop at an angle relative to the gravity vector.

B. Receptor

1. Location in root?
       Root cap. Decapitated roots don�t respond to gravity. Interestingly, gravity seems to be necessary for the regeneration of the root cap in Zea mays - when decapped in microgravity (on the Space Shuttle) maize roots don�t regenerate a cap. This observation suggests that a part of the root other than the cap also perceives gravity.
    
2. Location in shoot?
       Not known precisely, though it seems to be associated with a starch sheath in stems, such as sunflower hypocotyls and the dandelion flowering stalk.
    
3. Nature of the receptor - statoliths
       These are small bodies that have a high specific gravity and presumably settle to the bottom of cells (statocytes) to "tell" the plant which way is down.  Statoliths are probably amyloplasts (or starch grains).  Specifically, which cells in the root cap are the statocytes? The root cap has three layers of cells: (a) calyptrogen (which are meristematic producing new root cap cells; (b) columella cells (named for their shape; filled with amyloplasts); and (c) peripheral cells (secrete mucilage).  Thus, the columella cells are the likely statoliths.  There are suggestions that the receptor may work via other mechanisms - for example stretch receptors.

C. Physiological response
    Proton efflux on upper side of a horizontal root (and lower side of the shoot) loosens the cell walls of the upper root cells causing them to elongate more than the lower ones.  Thus, roots bend down and shoots bend up. This is rather similar to the way a vehicle with tracks, like a tank, turns - one tread remains stationary while the other continues along (Mulkey et al. - data from acid efflux experiments with maize roots).

D. Some observations

1. Cholodny/Went hypothesis - they suggested that auxin accumulates on the lower side of gravistimulated tissues. The increased auxin content of the stem stimulates elongation of cells on the lower side but inhibits roots; 
2. Auxin is required for graviresponsiveness 
3. IAA is laterally transported downward in some tissues like maize coleoptiles there are conflicting reports about the presence of an IAA gradient across shoots and roots. 

D. The transducing mechanism
    Plant on side → amyloplast (or protoplast) settles → associates with ER/microfilaments/proteins or wall or stretch receptors → stimulates redistribution of auxin → auxin accumulates on the lower side → stimulates/inhibits proton pump → loosens cell wall → unidirectional growth.

IV. Phototropism
    Growth response to unidirectional light or a gradient (more on one side than the other) of light.

V. Thigmomorphogenesis
    Plant growth response to a mechanical stimulation such as rubbing, wind, raindrops, etc. The termed was first coined by M. Jaffee. Seismomorphogenesis is specifically the response to shaking.

    Compared to unstimulated plants, mechanically-stimulated plants: (a) grow more slowly; (b) increase more in diameter. In essence, they are shorter and fatter. This response makes "sense" to minimize the risk of breaking which is especially true for plants in the mountains. As an example, compare plants grown in indoors (houseplants, greenhouse) with those grown outdoors.

    This phenomenon is due to ethylene (the triple response) for the following reasons: (a) ethylene concentrations increase in response to mechanical stimulation; and (b) ethylene treatment mimics these effects, i.e., inhibits shoot elongation and induce stem swelling.

V.  Apical Dominance 

VI.  Photoperiodism