Auxin and Cytokinin Activities in Plant Growth and Development
Auxins and cytokinins may trigger responses in plants either directly through stimulation or inhibition of the expression of sets of certain genes, or by means independent of gene expression. One pathway leading to the changes of gene expression involves the reception of auxin by TIR1 protein which has been demonstrated to be an auxin receptor.
On the cellular level, auxin/cytokinin activities are essential for cell proliferation, affecting both cell division and cellular expansion. Depending on the specific tissue, and auxin/cytokinin ratio, activities may promote axial elongation (as in shoots), lateral expansion (as in root swelling), or isodiametric expansion (as in fruit growth) and delay plant senescence. In some cases, auxin-promoted cellular expansion occurs in the absence of cell division. In other cases, auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue (root initiation, fruit growth). In a living plant, auxins and other plant hormones nearly always appear to interact to determine patterns of plant development.
Expansion/elongation and division of plant cells lead to a magnification of plant bio-mass, and specific tissue growth contributes to the development of specific plant organs. To cause growth in the required domains, auxins must of necessity be active preferentially in them. Auxins are not synthesized in all cells (even if cells retain the potential ability to do so, only under specific conditions will auxin synthesis be activated in them). For that purpose, auxins have to be not only translocated toward those sites where they are needed, but also they must have an established mechanism to detect those sites. For that purpose, auxins have to be translocated toward those sites where they are needed. Translocation is driven throughout the plant body, primarily from peaks of shoots to peaks of roots (from up to down).
As auxins contribute to organ shaping, they are also fundamentally required for proper development of the plant itself. Auxin employment begins in the embryo of the plant, where directional distribution of auxin ushers in subsequent growth and development of primary growth poles, then forms buds of future organs. 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 overal architecture of the plant. Throughout the plant's life, auxin helps the plant maintain the polarity of growth, and actually "recognize" where it has its branches (or any organ) connected.Finally, the sum of auxin arriving from stems to roots influences the degree of root growth. If shoot tips are removed, the plant does not react just by outgrowth of lateral buds — which are supposed to replace to original lead. It also follows that smaller amount of auxin arriving to the roots results in slower growth of roots and the nutrients are subsequently in higher degree invested in the upper part of the plant, which hence starts grow faster.
Auxin/cytokinin activities stimulate cell elongation by inducing wall-loosening factors, such as elastins, to loosen cell walls. The effect is stronger if gibberellins are also present. Auxin also stimulates cell division if cytokinins are present. When auxin and cytokinin are applied to callus, rooting can be generated if the auxin concentration is higher than cytokinin concentration. Xylem tissues can be generated when the auxin concentration is equal to the cytokinins. Auxin also plays a role in wound setting and induces sugar and mineral accumulation at the site of application.
Rhizosphere stimulation and expansion
Auxins promote root initiation. Auxin induces both growth of pre-existing roots and adventitious root formation, i.e., branching of the roots. As more native auxin is transported down the stem to the roots, the overall development of the roots is stimulated. If the source of auxin is removed, for example the tips of stems are trimmed, the roots are less stimulated accordingly, and growth of stem is supported instead. In horticulture, auxins, especially NAA and IBA, are commonly applied to stimulate root initiation when rooting cuttings of plants. However, high concentrations of auxin inhibit root elongation and instead enhance adventitious root formation. Removal of the root tip can lead to inhibition of secondary root formation.
Apical dominance
Auxin induces shoot apical dominance; the axillary buds are inhibited by auxin, as a high concentration of auxin directly stimulates ethylene synthesis in lateral buds, causing inhibition of their growth and potentiation of apical dominance. When the apex of the plant is removed, the inhibitory effect is removed and the growth of lateral buds is enhanced.
Fruiting and flower development
Auxin and cytokinin are required for fruit growth and development and the delay of fruit senescence. For example when seeds are removed from strawberries, fruit growth is stopped; exogenous auxin stimulates the growth in fruits with seeds removed. For fruit with unfertilized seeds, exogenous auxin results in parthenocarpy ("virgin-fruit" growth). Fruits form abnormal morphologies when auxin/cytokinin transport is disturbed. In Arabidopsis fruits, auxin controls the release of seeds from the fruit (pod). The valve margins are a specialised tissue in pods that regulates when pod will open (dehiscence). Auxin must be removed from the valve margin cells to allow the valve margins to form.
Axial stem expansion and elongation
In exogenous exposure or 'supplimentation' of auxin/cytokinin ratios, activities may promote axial elongation (as in shoots), lateral expansion in early vegetative development. In some cases, auxin-promoted cellular expansion occurs in the absence of cell division. In other cases, auxin-promoted cell division and cell expansion may be closely sequenced within the same tissue. Expansion/elongation and cytokinesis (division of plant cells) lead to a magnification of plant bio-mass, and specific tissue growth which contributes to the development of specific plant organs.
Here are several examples of auxin/cytokinin induced stimulation in young vegetative plants. These three Blueberry (Royal Queen) phenos were part of an experiment to determine the effects of a high auxin-cytokinin ratio on the acceleration of growth dynamics and shoot explansion/elongation. Plant subjects were grown under cold conditions with a maximum day temperature of 12 degrees celcius and low humidity (<30%) in organic media. Varying aqueous auxin-cytokinin supernates (2:1 / 1:1 / 1:2) were prepared by freeze/thaw methodology at 30% concentration (300,000ppm). The high auxin-cytokinin supernatant (2:1) was administered to Ax-1 at 14 days, the equi-molar supernatant (1:1) applied to Eq-1 and the low auxin-cytokinin supernate applied to Cy-1 via a series of foliar applications with surfactant or 'wetting agent' (cationic surfactant 0.1%) over a three day period. Plants were re-potted into larger 2ltr containers @ day 32.
Ax-1 (left) Eq-1 (back,center) Cy-1 (right) @ 14 days
Following a five week growth phase the growth profiles and characteristics of the these plants were examined; at day 50 both the Ax-1 and Eq-1 phenotypes had gained a significant difference in vertical height of appox.10- 12'' with Ax-1 exhibiting the greatest advancement in vertical height of 12''. Multiple and profuse branching and thickening of vascular tissues and meristem are evident upon examination of these plants. Note that changes in stem vigor/expansion and inter-node spacing in these subjects is evident only from the third node.
Ax-1 @ day 50
Eq-1 @ day 50
In comparison with Ax-1 and Eq-1, the Cy-1 which received a low auxin-cytokinin treatment give only a very minimal change in vertical height of 3'' with minor stem expansion and swelling of vascular tissues. Interestingly Cy-1 was induced to flower following administration of the low auxin-cytokinin supernate (1:2) despite no obvious enhancement of vegetative bio-mass in contrast to both the Ax-1 and Eq-1 subjects that remain in the vegetative phase.
Cy-1 @ day 50
Cy-1 @ day 50