TRANSDUCTION AND RESPONSE:
Much of the research concerning the gravitropic growth response has been conducted in the context of elucidating the mechanism of the transduction signal. Therefore, information which relates growth response to the presence of various growth regulators is detailed in Transduction of the signal. More general information surrounding the nature of the growth response is found here.
For details on the perception of the signal, click here.

MOLECULAR MECHANISMS OF THE GROWTH RESPONSE:
A number of mutants in various species of plants exhibit atypical graviresponsiveness, indicating a genetic component to the growth response. What this component may be and how specific it is to each species is still unclear, but the identification of gravitropic mutants has yielded varied results. In Arabidopsis thaliana, mutants with altered auxin transport mechanisms (rgr1) or with auxin resistance (rcn1) inhibt gravitropic response (Simmons, et al, 1995); so do some mutants (sgr1, sgr2, sgr3) with normal auxin transport and distribution (Fukaki, et al, 1996). For the latter class of mutants, it has been suggested that the "gene products are acting on the gravity perception mechanism or the signal transduction mechanism such as that which regulates the auxin distribution within gravistimulated shoots"(Fukaki, et al, 1996). In both cases, it appears that stem, hypocotyl, and root gravitropism are genetically separable.

TEMPERATURE AND GRAVIRESPONSIVENESS:
The genetic component of the gravitropic growth response may be regulated by environmental factors such as temperature. General trends reveal that as temperature increases, the initial shoot angle decreases and the vertical shoot growth increases. The intitial root angles were least vertical at 20°C (Matthews and Zobel, 1995).

DEVELOPMENT AND GRAVITROPIC SIGN REVERSAL:
There is evidence that "the directionality of the gravitropic response of some organs is not consistent and it can change during development" (Myers, et al, 1994). This gravitropic sign reversal is easily seen in hanging plants: as time passes, nodes which are originally negatively gravitropic become positively gravitropic. This change is influenced by light, perhaps as "the result of a direct effect of light on the gravitropic perception or response mechanism" (Myers, et al, 1994). Myers, et al propose two models to explain gravitropic sign reversal, one in which "different cells act as gravity sensing elements as the organ develops" and one in which "the polarity of the sensing cells changes during development" (Myers, et al, 1994).

GRAVITROPISM AND GRAVIMORPHOGENESIS:
The coenocyte Caulerpa prolifera ultimately responds to gravity by changing the site of rhizoid formation . This gravimorogenetic response is preceded by the gravitropic response, in which the "inverted rhizome tip shows a negative gravitropism that restores it to its normally upturned position" (Jacobs, 1993). In turn, that gravitropic response is preceded by the sedimentation of amyloplasts, although there is not a causative relationship between the two.

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