Diffusible auxin

Fig. 10. Diffusion of auxin into agar blocks in three hours from corn coleoptile tips. The numbers given are curvatures obtained from a standard auxin test, i,.e. a measure for the auxin diffused into the agar. (A) dark control bisecting does not significantly reduce the total amount of auxin. Unilateral bluelight (B) the total amount of flavin is not significantly reduced by irradiation (C) partial bisecting indicates strong lateral-basipetal auxin transport23). (D) Radio-actively labeled auxin, symmetrically applied to the coleoptile tip, proves the conclusion drawn from experiment (C)13 G...

Oat coleoptie last Arena test a biotest for the quantitative determination of auxins, which is carried out as follows. The tip of the coleoptile is cut off, and the cotyledon within it is removed by pulling on its base. An agar block with the test substance is set on one side of the coleoptile stump. Auxin diffuses into the side of the coleoptile covered by the agar and, due to the one-sided stimulation of growth, causes it to bend. The angle of the bend is a function of the auxin concentration. 

Bioassay of the auxin diffusing out of buds and leaves of Vida faba showed that the apical bud is by far the largest source of diffusible auxin ... 

The present survey will summarize these negative data and demonstrate that, instead, phototropic curvature results from a lateral gradient of growth-inhibiting substances that cause differential flank growth by differential inhibition of cell elongation with unchanged auxin distribution. This also applies to the auxin diffusion from unilaterally illuminated coleoptile tips. 

Fig. 7.1 A-C. Amount of endogenous auxin diffused to agar gels from the cam-bial regions of pine stem sections during 6 min. A Apical a) or basal h) efflux of auxin from sections of different lengths. B and C Basal efflux from successive sections 5 or 6 mm long. Solid lines no pretreatment broken lines pretreatment by apical application of exogenous lAA for 100 min prior to sectioning and positioning for 6-min basal efflux test. (A and B adapted from Zaj czkowski and Wod-ziCKi 1978 a C adapted from Wodzicki et al. 1979)...

The original proposal concerning the mode of action of auxin stated that auxin made cell walls more plastic, thus inducing a water-diffusion-pressure deficit and causing the cells to expand (7). Much evidence has since been gathered in support of this hypothesis. For example, in Avena coleoptiles it has been established that the auxin-induced increase in plasticity precedes, or coincides with, the... 

On reflection, it is interesting to review the earlier (pre-gibberellin) studies on the physiology of genetic dwarfism in maize. For example, in 1938 van Overbeek [19] reported lower levels of diffusible auxin from dl, d2, dS, and d5 coleoptiles. In 1951, Harris [11] confirmed the lower levels from dl coleoptiles based on ether extractable auxin he also found that addition of lAA to decapitated coleoptiles... 

It has not previously been possible to determine the relationship between the rate of growth and the amount of the growth hormone, lA A. Early experiments involved exodiffusion of coleoptile tips, followed by a bioassay of the amount of lAA in the agar blocks that were used to collect the diffusing growth substance. Such experiments measured the capacity of the tips to supply lA A over a sustained time period and undoubtedly involved conversion of a seed auxin precursor [2, 23]. We now believe that the seed auxin precursor is indole-3-acetyl-wj< -inositol (lAInos), which upon hydrolysis yields free lAA [8,9,15,22]. Also, it is now possible to extract the tissue rapidly with aqueous acetone and to measure the free lAA by GC-MS. 

Application of an auxin concentrate, or later of lAA, at a concentration somewhat higher than that identified as coming directly from the bud, duplicated the inhibition caused by the apex. Auxin at the same level as that determined by bioassay of diffusate from the apex caused a significant growth inhibition but it was incomplete. 

NPA and other synthetic auxin transport inhibitors can block PAT when they are incorporated into the receiver blocks in a classical donor-to-receiver polar transport assay. Figure 4 shows that quercetin in receiver blocks can inhibit lAA arrival, but only for the first 2 h of the transport test. Thereafter, the radioactivity delivered to the receiver blocks is increased compared to controls. In some experiments lAA (or NAA) arrival in quercetin-containing receiver blocks is increased at earlier times. A crossover is never observed when NPA is used. Quercetin and other flavonoids bind strongly to cellulose [31 ] and, unlike NPA, do not diffuse away... 

Table 3 demonstrates the reproducibility of the Went experiment, the agar blocks from the shaded side showing, on an average, 2.6 times as much auxin activity as those from the illuminated side. However, the interpretation that this would reflect differences in auxin content is apparently wrong, the lAA-diffusion... 

Tables. Distribution of auxin activity (10-fold determinations) and of lAA (duplicate determinations) in agar blocks after 2 h diffusion from the lighted and shaded sides of tips of unilaterally illuminated Avena coleoptiles ...

Sembdnee and co-workers have extracted the crude extract with ethyl acetate and then with n-butanol, thereby obtaining the more polar gibberellins from the flowers of Phaseolus [113] and shoots of Nicotiana [115]. Jones and PHmLiPS [40] have detected diffusible gibberellins in agar after 18 hours diffusion (see diffusible auxins above). The compounds were extracted with methanol at room-temperature the agar had been frozen at —15° C. 

Under this conceptual framework, hormone systems may be viewed in terms of three primary functional components (1) a site of synthesis or source of the hormone, (2) movement from the source to the site of action (this may be passive as in the diffusion of ethylene or active as in polar transport of auxin) and (3) action by the hormone upon the target site. Through such a system, hormones may initiate a new path of development at the target site or simply control an established pathway. 

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