6.3.2 Asymmetric Hormone Distribution Leads to Directed
Growth
Cholodny and Went independently discovered a connection between phytohormones and tropisms in higher plants. Cholodny observed that a gravitational stimulus disturbs the even distribution of phytohormones in the root of higher plants.
Went also observed that the phytohormone auxin specifically enriches on the
bending side. Later experiments on the influence of light on hormone distribution
led to similar results (Went and Thimann 1937). Based on these observations the
Cholodny-Went hypothesis was postulated that claims that the bending growth
typical for tropisms is a result of polar auxin distribution (Cholodny 1929).
Later, the Cholodny-Went hypothesis was refined by the observation that besides
auxin, the phytohormone jasmonic acid (JA) also shows a polar distribution. Inhibition of the JA gradient led to a delay of gravitropic responses. JA-deficient Oryza
sativa mutants show delayed but observable gravitropic responses (Gutjahr et al.
2005). This suggests a modulating function of JA in gravitropism.
A central element of the gravitropic growth response is, therefore, the generation
of hormone gradients within the growing organ. For auxin this is achieved via
carrier-mediated asymmetric transport.
6.4 Gravitropic Growth
6.4.1 Polar Hormone Distribution
While the importance of auxin for tropic growth was known since the CholodnyWent hypothesis, the mechanism of auxin transport was only identified later. The
most important member of the group of auxins, free Indole-3-acetic acid can be
protonated (IAAH) or deprotonated (IAA
À ) depending on the pH. Only IAAH can
freely diffuse through the plasma membrane. At pH 7 IAA
À prevails and requires
active transport from cell to cell (Friml and Palme 2002).
Experiments with auxin transport inhibitors led to different results in Boston
Ivy (Parthenocissus tricuspidata). Triiodobenzoic acid (TIBA) inhibits auxin efflux
from the cytoplasm. Auxin accumulation in the cytoplasm was still observed besides
TIBA application. 2,4-Dichlorphenoxyacetic acid inhibits auxin influx. Based on
these observations, the chemiosmotic theory was postulated. The interplay of passive auxin transport with the activity of auxin efflux and influx carriers allows polar
auxin transport in plants (Rubery and Sheldrake 1974).
Modern molecular biology methods allowed for the identification of the efflux
and influx carriers. Mutant analyses identified AUXIN RESISTANT 1 (AUX1) as the
gene coding for the influx carrier (Bennett et al. 1996). Similar mutant experiments
identified PIN-FORMED 1 (PIN1) as a gene coding for an efflux carrier, with polar
localization in the plasma membrane and thereby allowing for polar auxin transport
6.4 Gravitropic Growth
85
Growth
Cholodny and Went independently discovered a connection between phytohormones and tropisms in higher plants. Cholodny observed that a gravitational stimulus disturbs the even distribution of phytohormones in the root of higher plants.
Went also observed that the phytohormone auxin specifically enriches on the
bending side. Later experiments on the influence of light on hormone distribution
led to similar results (Went and Thimann 1937). Based on these observations the
Cholodny-Went hypothesis was postulated that claims that the bending growth
typical for tropisms is a result of polar auxin distribution (Cholodny 1929).
Later, the Cholodny-Went hypothesis was refined by the observation that besides
auxin, the phytohormone jasmonic acid (JA) also shows a polar distribution. Inhibition of the JA gradient led to a delay of gravitropic responses. JA-deficient Oryza
sativa mutants show delayed but observable gravitropic responses (Gutjahr et al.
2005). This suggests a modulating function of JA in gravitropism.
A central element of the gravitropic growth response is, therefore, the generation
of hormone gradients within the growing organ. For auxin this is achieved via
carrier-mediated asymmetric transport.
6.4 Gravitropic Growth
6.4.1 Polar Hormone Distribution
While the importance of auxin for tropic growth was known since the CholodnyWent hypothesis, the mechanism of auxin transport was only identified later. The
most important member of the group of auxins, free Indole-3-acetic acid can be
protonated (IAAH) or deprotonated (IAA
À ) depending on the pH. Only IAAH can
freely diffuse through the plasma membrane. At pH 7 IAA
À prevails and requires
active transport from cell to cell (Friml and Palme 2002).
Experiments with auxin transport inhibitors led to different results in Boston
Ivy (Parthenocissus tricuspidata). Triiodobenzoic acid (TIBA) inhibits auxin efflux
from the cytoplasm. Auxin accumulation in the cytoplasm was still observed besides
TIBA application. 2,4-Dichlorphenoxyacetic acid inhibits auxin influx. Based on
these observations, the chemiosmotic theory was postulated. The interplay of passive auxin transport with the activity of auxin efflux and influx carriers allows polar
auxin transport in plants (Rubery and Sheldrake 1974).
Modern molecular biology methods allowed for the identification of the efflux
and influx carriers. Mutant analyses identified AUXIN RESISTANT 1 (AUX1) as the
gene coding for the influx carrier (Bennett et al. 1996). Similar mutant experiments
identified PIN-FORMED 1 (PIN1) as a gene coding for an efflux carrier, with polar
localization in the plasma membrane and thereby allowing for polar auxin transport
6.4 Gravitropic Growth
85