Phototropism is the growth of organisms in response to a light stimulus. It is
most often observed in plants, but can also occur in other organisms such as
fungi. The cells on the plant that are farthest from the light have a chemical
called auxin that reacts when phototropism occurs. This causes the
plant to have elongated cells on the farthest side from the light.
Phototropism is one of the many plant tropisms or movements which respond to
external stimuli. Growth towards a light source is called positive phototropism,
while growth away from light is called negative phototropism. Most plant shoots
exhibit positive phototropism, and rearrange their chloroplasts in the
leaves to maximize photosynthetic energy and promote growth. Roots usually
exhibit negative phototropism, although gravitropism may play a larger role in
root behavior and growth. Some vine shoot tips exhibit negative
phototropism, which allows them to grow towards dark, solid objects and climb
them. The combination of phototropism and gravitropism allow plants to grow in
the correct direction. Mechanism
There are several signaling molecules that help the plant determine where the
light source is, and this activates several genes, which change the hormone
gradients allowing the plant to grow towards the light. The very tip of the
plant known as the coleoptile is necessary in light sensing. The middle
portion of the coleoptile is the area where the shoot curvature occurs. The
Cholodny-Went hypothesis, developed in the early 20th century, predicts that in
the presence of asymmetric light, auxin will move towards the shaded side and
promote elongation of the cells on that side to cause the plant to curve towards
the light source. Auxins activate proton pumps, decreasing the pH in the cells on
the dark side of the plant. This acidification of the cell wall region
activates enzymes known as expansins which break bonds in the cell wall
structure, making the cell walls less rigid. In addition, the acidic
environment causes disruption of hydrogen bonds in the cellulose that
makes up the cell wall. The decrease in cell wall strength causes cells to
swell, exerting the mechanical pressure that drives phototropic movement.
A second group of genes, PIN genes, have been found to play a major role in
phototropism. They are auxin transporters, so it is thought that they
are responsible for the polarization of auxin. Specifically PIN3 has been
identified as the primary auxin carrier. It is possible that phototropins receive
light and inhibit the activity of PINOID kinase, which then promotes the activity
of PIN3. This activation of PIN3 leads to asymmetric distribution of auxin,
which then leads to asymmetric elongation of cells in the stem. pin3
mutants had shorter hypocotyls and roots than the wild-type, and the same
phenotype was seen in plants grown with auxin efflux inhibitors. Using anti-PIN3
immunogold labeling, movement of the PIN3 protein was observed. PIN3 is
normally localized to the surface of hypocotyl and stem, but is also
internalized in the presence of Brefeldin A, an exocytosis inhibitor.
This mechanism allows PIN3 to be repositioned in response to an
environmental stimulus. PIN3 and PIN7 proteins were thought to play a role in
pulse-induced phototropism. The curvature responses in the "pin3" mutant
were reduced significantly, but only slightly reduced in "pin7" mutants.
There is some redundancy among "PIN1", "PIN3", and "PIN7", but it is thought
that PIN3 plays a greater role in pulse-induced phototropism.
There are phototropins that are highly expressed in the upper region of
coleoptiles. The two main phototropins are phot1 and phot2. phot2 single
mutants have phototropic responses like that of the wild-type, but phot1 phot2
double mutants do not show any phototropic responses. The amounts of
PHOT1 and PHOT2 present are different depending on the age of the plant and
the intensity of the light. There is a high amount of PHOT2 present in mature
Arabidopsis leaves and this was also seen in rice orthologs. The expression
of PHOT1 and PHOT2 changes depending on the presence of blue or red light. There
was a downregulation of PHOT1 mRNA in the presence of light, but upregulation
of PHOT2 transcript. The levels of mRNA and protein present in the plant were
dependent upon the age of the plant. This suggests that the phototropin
expression levels change with the maturation of the leaves. Mature leaves
contain chloroplasts that are essential in photosynthesis. Chloroplast
rearrangement occurs in different light environments to maximize photosynthesis.
There are several genes involved in plant phototropism including the NPH1
and NPL1 gene. They are both involved in chloroplast rearrangement. The nph1 and
npl1 double mutants were found to have reduced phototropic responses. In fact,
the two genes are both redundant in determining the curvature of the stem.
= Five models of auxin distribution in phototropism =
In 2012, Sakai and Haga outlined how different auxin concentrations could be
arising on shaded and lighted side of the stem, giving birth to phototropic
response. Five models in respect to stem phototropism have been proposed, using
Arabidopsis thaliana as the study plant. First model
In the first model incoming light deactivates auxin on the light side of
the plant allowing the shaded part to continue growing and eventually bend the
plant over towards the light. Second model
In the second model light inhibits auxin biosynthesis on the light side of the
plant, thus decreasing the concentration of auxin relative to the unaffected
side. Third model
In the third model there is a horizontal flow of auxin from both the light and
dark side of the plant. Incoming light causes more auxin to flow from the
exposed side to the shaded side, increasing the concentration of auxin on
the shaded side and thus more growth occurring.
Fourth model In the fourth model it shows the plant
receiving light to inhibit auxin basipetal down to the exposed side,
causing the auxin to only flow down the shaded side.
Fifth model Model five encompasses elements of both
model 3 and 4. The main auxin flow in this model comes from the top of the
plant vertically down towards the base of the plant with some of the auxin
travelling horizontally from the main auxin flow to both sides of the plant.
Receiving light inhibits the horizontal auxin flow from the main vertical auxin
flow to the irradiated exposed side. And according to the study by Sakai and
Haga, the observed asymmetric auxin distribution and subsequent phototropic
response in hypocotyls seems most consistent with this fifth scenario.
Effects of wavelength Phototropism in plants such as
Arabidopsis thaliana is directed by blue light receptors called phototropins.
Other photosensitive receptors in plants include phytochromes that sense red
light and cryptochromes that sense blue light. Different organs of the plant may
exhibit different phototropic reactions to different wavelengths of light. Stem
tips exhibit positive phototropic reactions to blue light, while root tips
exhibit negative phototropic reactions to blue light. Both root tips and most
stem tips exhibit positive phototropism to red light. Cryptochromes are
photoreceptors that absorb blue/ UV-A light, and they help control the
circadian rhythm in plants and timing of flowering. Phytochromes are
photoreceptors that sense red/far-red light, but they also absorb blue light.
The combination of responses from phytochromes and cryptochromes allow the
plant to respond to various kinds of light. Together phytochromes and
cryptochromes inhibit gravitropism in hypocotyls and contribute to
phototropism. See also
Scotobiology Cholodny–Went model
References External links
Media related to Phototropism at Wikimedia Commons
Time lapse films, Plants-In-Motion