Auxin

For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds.

And as native auxin, its equilibrium is controlled in many ways in plants, from synthesis, through possible conjugation to degradation of its molecules, always according to the requirements of the situation.

Auxin can act in a heat sensitive manner in many situations, which will in turn effect a plant's phenotypic development.

In 1881, Charles Darwin and his son Francis performed experiments on coleoptiles, the sheaths enclosing young leaves in germinating grass seedlings.

[11] In 1910, Danish scientist Peter Boysen Jensen demonstrated that the phototropic stimulus in the oat coleoptile could propagate through an incision.

[13][14] He found that the tip could be cut off and put back on, and that a subsequent one-sided illumination was still able to produce a positive phototropic curvature in the basal part of the coleoptile.

He demonstrated that the transmission could take place through a thin layer of gelatin separating the unilaterally illuminated tip from the shaded stump.

Thus, the longitudinal half of the coleoptile that exhibits the greater rate of elongation during the phototropic curvature, was the tissue to receive the growth stimulus.

[15][16] In 1911, Boysen Jensen concluded from his experimental results that the transmission of the phototropic stimulus was not a physical effect (for example due to a change in pressure) but serait dû à une migration de substance ou d’ions (was caused by the transport of a substance or of ions).

In 1928, the Dutch botanist Frits Warmolt Went showed that a chemical messenger diffuses from coleoptile tips.

If the chemical was distributed unevenly, the coleoptile curved away from the side with the cube, as if growing towards the light, even though it was grown in the dark.

Went later proposed that the messenger substance is a growth-promoting hormone, which he named auxin, that becomes asymmetrically distributed in the bending region.

Went concluded that auxin is at a higher concentration on the shaded side, promoting cell elongation, which results in coleoptiles bending towards the light.

The precise mechanisms by which this occurs are still an area of active research, but there is now a general consensus on at least two auxin signalling pathways.

When at these promoters, Aux/IAA repress the expression of these genes through recruiting other factors to make modifications to the DNA structure.

The large number of Aux/IAA and ARF binding pairs possible, and their different distributions between cell types and across developmental age are thought to account for the astonishingly diverse responses that auxin produces.

This has led some scientists to suggest that there is an as yet unidentified TIR1-dependent auxin-signalling pathway that differs from the well-known transcriptional response.

In some cases (coleoptile growth), auxin-promoted cellular expansion occurs in the absence of cell division.

So, precise control of auxin distribution between different cells has paramount importance to the resulting form of plant growth and organization.

PIN proteins can be phosphorylated by PINOID, which determines their apicobasal polarity and thereby the directionality of auxin fluxes.

[28] Auxin has a significant effect on spatial and temporal gene expressions during the growth of apical meristems.

STM (SHOOT MERISTEMLESS), which helps maintain undifferentiated cells, is down-regulated in the presence of auxin.

The CUC (CUP-SHAPED COTYLEDON) genes set the boundaries for growing tissues and promote growth.

[26] Experiments making use of GFP (GREEN FLUORESCENCE PROTEIN) visualization in Arabidopsis have supported these claims.

Next, it helps to coordinate proper development of the arising organs, such as roots, cotyledons, and leaves and mediates long-distance signals between them, contributing so to the overall architecture of the plant.

If shoot tips are removed, the plant does not react just by the outgrowth of lateral buds — which are supposed to replace to original lead.

The uneven distribution of auxin, due to environmental cues, such as unidirectional light or gravity force, results in uneven plant tissue growth, and generally, auxin governs the form and shape of the plant body, direction and strength of growth of all organs, and their mutual interaction.

[28] The evolutionary transition from diploid to triploid endosperms - and the production of antipodal cells - may have occurred due to a shift in gametophyte development which produced a new interaction with an auxin-dependent mechanism originating in the earliest angiosperms.

Excess ethylene can inhibit elongation growth, cause leaves to fall (abscission), and even kill the plant.

This hindrance to the plant causes a response that increases carbohydrate production, leading to larger fruit.

The auxin signal cascade: In the absence of auxin, Aux/IAA bind to and suppress the transcriptional activity of ARFs. When auxin is present it forms a 'molecular glue' between TIR1 and Aux/IAAs, which leads to the degradation of these repressors. ARFs are then free to bind to DNA and to cause changes in transcription.
Auxin diffuses along the shaded side of the plant, and causes cellulose in the cell wall to break, allowing turgor (water pressure) to expand the cell.
A healthy Arabidopsis thaliana plant (left) next to an auxin signal-transduction mutant with a repressed response to auxin.
Crown galls are caused by Agrobacterium tumefaciens bacteria ; they produce and secrete auxin and cytokinin , which interfere with normal cell division and cause tumors .