Ans.: Adjoining cells can transport ions (e.g., K+) into the xylem that would cause the pectic matrix in the pit membranes of xylem cells to shrink, decreasing the resistance of the "membrane" (pectic wall) to water flow.

Ans.: Low [CO2] in the air spaces of leaves would induce stomates to open, allowing a replenishment of the [CO2], thus increasing the rate of carbon fixation. Light induces the stomates to open. The rate of photosynthesis, and ultimately the rate of CO2 utilization, increases in the light, and open stomates allow the more rapid replenishment of the CO2 in the leaf air spaces.

Ans.: The same blue light that induces stomates to open also causes protoplasts from guard cells to take up salt (K+) from the medium, which in turn causes them to take up water and visibly swell, sometimes to the point of bursting. In vivo guard cells swell (become turgid) when they are induced to open.

Ans.: You would expect to find high concentrations of stachyose and raffinose in the intermediary cells (companion cells) of plants that show symplastic phloem loading. These compounds are polymers produced enzymatically from sugar precursors.

Ans.: In the companion cells of plants that show apoplastic phloem loading. The cooperative functioning of these two proteins would result in sugar loading into the companion cells from the apoplasm. The proton pumping ATPases create the pH gradient (lower pH outside) that is used to drive the symport of sugar through the sucrose transporter.

Ans.: Factors injected into phloem cells by the worm induce these cells to form syncytia by dissolving the membranes and wall that separate cells. The sugar is then concentrated in these syncytia by apoplastic loading through sucrose transporters.

Ans.: Growth changes are induced in plans by regulatory chemicals, so a gravitropically induced asymmetrical distribution of these chemicals would induce the asymmetric (curved) growth of gravitropism. Auxin asymmetry precedes gravitropic growth in roots and shoots and induce (depending on concentration) either enhanced growth or reduced growth on the side with the higher [auxin]. Calcium accumulates asymmetrically in the walls of graviresponding shoots and roots, and inhibits growth on the side with the higher [Ca2+].

Ans.: Rice roots growing in a medium of equal density to the density of the protoplasm show severely reduced gravitropism. In this situation the protoplasm floats, but the amyloplasts in the root cells sink, indicating that the more important cellular structure that has to fall to cue roots as to which way is up is the protoplasm not the amyloplasts. In these cells the primary function of the amyloplasts would be to add mass or weight to the protoplasm thus helping it to create more tension and compression forces to trigger the gravitropic response.

Ans.: The beads were coated with peptides containing the RGD sequence. This allowed the beads to grab on to extracellular binding sites on integrins and "pull" on them as the beads rolled across the cell surface. Because on the inside of cells integrins are connected to the whole cytoskeleton, which forms a continuum all the way into the nucleus where it connects with chromosomes, pulling on the integrins ultimately makes the chromosomes move.

Ans.: When Chara cells are in the vertical orientation all the tension and compression generated by the protoplasm is on the horizontal (top and bottom) walls, and none is on the side. Thus using RGD peptides to disconnect integrins from the protoplasm along the sides of the cells would not affect the tension and compression forces, but using it to disconnect integrins along the top of the cell would.

Ans.: In the process of RNA interference, double stranded RNA that enters the cytoplasm is recognized by dicer molecules that cut the dsRNA into small pieces,, which then serve as templates to recognize and bind to other RNA in the cell with sequences identical to either of the two strands. The binding of the small RNAs generated by dicer to other cellular RNAs allows an enzyme complex called RNA Interference Silencing Complex (RISC) to digest the cellular RNA into more small RNAs (21-25 nucleotides long) that can then go and bind to other cellular RNAs and subject them to RISC attack. The ultimate result is that all RNA that looks like either strand of the original dsRNA is destroyed and rendered non-functional. This method is particularly attractive to use in Ceratopteris spores because these cells take up the dsRNA directly from the medium, unlike other cells that have to be microinjected with the dsRNA.

Ans.: Good genes to choose would be ones that could affect the calcium current, which is directed by gravity and is required for gravity to direct the cell’s polarity. These genes would include those that encode calcium channels or pumps. Identification of the molecular machinery that creates the current would allow a dissection of how gravity alters the activity of this machinery.

Ans.: Blue light would not affect hypocotyl growth in a cry mutant, but would inhibit hypocotl growth in a nph1 mutant. Blue light would not affect phototropism in a cry mutant but would in a nph1 mutant.

Ans.: Because the photoreceptors that induce this light response have peak absorbances in the blue region (CRY1) and red region (PHY) of the spectrum.

Ans.: CRY1 physically binds to COP1, which is known to be a nuclear protein.

Ans.: R is a major component of unfiltered sunlight and promotes germination; FR is highly enriched in light filtered through green leaves and inhibits germination. Seeds receiving primarily FR in nature are most likely surrounded by mature plants and are at a competitive disadvantage, so there is an adaptive advantage for these seeds not to germinate. Because in nature wind can temporarily move leaves allowing unfiltered sunlight to penetrate, seeds surrounded by other plants would occasionally receive unfiltered sunlight. The stimulatory effects of this light would have to be reversible by FR when the wind dies down and the leaves of the surrounding plants once again shade the seeds, otherwise occasional sunflecks could induce the seeds to germinate even though they are still at a competitive disadavantage.

HW (high-intensity white light) represents unfiltered sunlight, and HWFR represents HW with FR (or light filtered through green leaves) mixed in. One question is whether changing the quality of light changes the growth rate of plant stems, and a second question is, If light affects growth rates, how fast does it do so? The answer to the first question is Yes: HW inhibits the growth rate, and HWFR accelerates it. The answer to the 2^nd question is, In less than 10 min. If we assume that phy is the photoreceptor responding to the HW (which converts phy to the active Pfr form), then the rapidity of the HW effects suggests that the initial effects of Pfr on growth inhibition can occur before phy enters the nucleus and modifies gene expression.

Ans.: Red light also promotes photosynthesis through chlorophyll. For R to induce anthocyanin synthesis in apple skin (make it turn red) both photosystems are needed: Photosynthesis for sugar synthesis (energy production) and phy to induce the enzymes needed to synthesize anthocyanin.

Ans.: COP1 is a nuclear protein that suppresses the expression of transcription factors, such as CIP4, needed to turn on light-regulated genes. COP1 suppresses CIP4 levels in the nucleus; in the absence of COP1, CIP4 levels are constitutively high.

Ans.: The activation of CRY1 by blue light leads to inhibition of hypocotyl growth in wild-type plants, and this requires the presence of CIP4 a transcription factor that turns on genes needed for the inhibition of hypocotyl elongation. This can be demonstrated by using antisense constructs to suppress CIP4 production. Under these conditions, blue light cannot inhibit hypocotyl growth.

Ans.: COP1 is a nuclear protein and Phy is not in the nucleus in darkness. (Phy moves to the nucleus only after it is activated by light).

Ans.: For heterotrimeric G-proteins to interact with target receptors, the receptors have to be activated by their physical or chemical agonist. The alpha subunit of the G-protein is what binds to the activated receptor.

Ans.: One can microinject activated G-proteins into mutants that cannot make phy and induce these mutants to turn on light-regulated responses (such as gene expression).

Ans.: PLC can hydrolyze PIP2 into IP3 +DAG. The IP3 can bind to and open calcium channel receptors in the ER and/or vacuolar membranes, releasing calcium into the cytoplasm. A rise in cytosolic calcium activates calmodulin, which then can turn on the activity of calmodulin-binding proteins.