Abstract
One of the longest standing theories and, therein-based, regulation-model of plant root development, posits the inhibitory action of auxin (IAA, indolylacetic acid) on elongation growth of root cells. This effect, as induced by exogenously supplied IAA, served as the foundation stone for root growth regulation. For decades, auxin ruled the day and only allowed hormonal side players to be somehow involved, or in some way affected. However, this copiously reiterated, apparent cardinal role of auxin only applies in roots immersed in solutions; it vanishes as soon as IAA-supplied roots are not surrounded by liquid. When roots grow in humid air, exogenous IAA has no inhibitory effect on elongation growth of maize roots, regardless of whether it is applied basipetally from the top of the root or to the entire residual seedling immersed in IAA solution. Nevertheless, such treatment leads to pronounced root-borne ethylene emission and lateral rooting, illustrating and confirming thereby induced auxin presence and its effect on the root — yet, not on root cell elongation. Based on these findings, a new root growth regulatory model is proposed. In this model, it is not IAA, but IAA-triggered ethylene which plays the cardinal regulatory role — taking effect, or not — depending on the external circumstances. In this model, in water- or solution-incubated roots, IAA-dependent ethylene acts due to its accumulation within the root proper by inhibited/restrained diffusion into the liquid phase. In roots exposed to moist air or gas, there is no effect on cell elongation, since IAA-triggered ethylene diffuses out of the root without an impact on growth.
Highlights
Based on the early experiments with coleoptiles of Avena sativa carried out by Paâl (1919) and Went (1926), the existence of “growth accelerating substances” had been postulated early last century (Went 1926; Went and Thimann 1937)
Other plant hormones with diverse competences have been and are still being discovered (Kleine-Vehn and Sauer 2017); in addition to what has been thought for many years, namely that plant development is regulated by a total of five hormone groups, namely auxins, gibberellins, cytokinins, abscisic acid and ethylene
The inhibiting effect on root elongation is at higher IAA concentrations, i.e. 1 0−5 M, and 1 0−6 M, macroscopically characterised by root swelling (Fig. 2). This effect is accompanied by inhibited cell elongation and an increase in the cell circumference of the cells within the elongation zone of the root (Fig. 3), which is reminiscent of an effect that has been shown many times for the effect of ethylene
Summary
Based on the early experiments with coleoptiles of Avena sativa carried out by Paâl (1919) and Went (1926), the existence of “growth accelerating substances” had been postulated early last century (Went 1926; Went and Thimann 1937). Handling Editor: Peter Nick (IAA, indolyl-3-acetic acid) was increasingly recognised as a chemical messenger in plants (“growth substance”), relevant for a multitude of diverse physiological processes (Davies 1995; Lv et al 2019). Other plant hormones with diverse competences have been and are still being discovered (Kleine-Vehn and Sauer 2017); in addition to what has been thought for many years, namely that plant development is regulated by a total of five hormone groups, namely auxins, gibberellins, cytokinins, abscisic acid and ethylene.
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