Avena coleoptiles

Various assay methods have been used to detect the presence of inhibitory substances. These include some of the classical tests used by investigators of growth-promoting substances—i.e., the various Avena coleoptile assays which utilize intact, decapitated, or isolated cylinders and the split pea stem test. Effects on seed germination and seedling shoot or root growth and development have also been measured in addition to other visible expressions of inhibition. Details of many of these tests have been compiled by Mitchell et al. (99). Tests have been carried out in Petri dishes, with various solution culture techniques, and by sand and soil culture. Effects so measured may or may not be similar to those obtained under field situations— i.e., the establishment of inhibition under controlled conditions pro-... 

Naringenin (5, 7, 4 -trihydroxyflavanone), isolated in pure form from dormant peach flower buds, strongly inhibited the growth of Avena coleoptiles at 4.6 X 10-4 M (71). Naringenin is the aglycone of the glycoside naringin. 

Coumarin, the lactone of o-hydroxycinnamic acid, and some of its derivatives have been isolated from many plant species 31). Thimann and Bonner 141) attributed the growth-inhibiting effects of coumarin to its action on enzyme sulfhydryl groups. Inhibitory effects of coumarin on Avena coleoptiles and pea stem sections could be overcome by 2,3-dimercaptopropanol (BAL). Coumarin has also been reported to disrupt mitosis 29,30). 

Pierce, W.S. Higinbothom, N. (1970). Compartments and fluxes of and Cl" in Avena coleoptile cells. Plant Physiology, 46, 666-73. 

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Nevertheless, the avena coleoptile exhibits a curvature to unilateral UV-illumina-tion with a satisfactory log-linear response/time relationship38) (the bending mode is similar to that observed for the second positive curvature which develops from the coleoptile base cf. 2.2). Fig. 5 338) shows that the double-peaked action spectrum does not match neither flavin (Fig. 5 5,16S)) nor carotenoid absorption (Fig. 5 4,183)), most likely excluding both as photoreceptors. The growth hormone auxin (cf. 2.4 and Scheme 1) has been discussed to be a possible photoreceptor. However, in this case, this is not supported by the action spectrum either. 

Table 1. Bluelight induced bending of decapitated avena coleoptiles with different fillings, with and without application of the growth-hormone auxin (cf. Sect. 2.4). Clearly the quantity of bending depends on the intrinsic light gradient (after v. Guttenberg75)).

The most photosensitive part of the coleoptile with respect to phototropism is the apical 50 qm-zone. The first millimeter of the tip is 160 times more sensitive than the second, and the second millimeter 1,800 times more sensitive than the third 108h In 1937, Bunning29 showed by microchemical method that there is a high concentration of carotenoid (lutein) below the tip of the avena coleoptile, from 250 jum to 2 mm below this point and just in the extreme tip no carotenoid is found. This calls in question whether the enormous local differences in sensitivity can solely be caused by corresponding different absorption gradients. This doubt is further substantiated by the fact, that carotenoid deficient mutants (containing 5 to 10% of the normal siblings) show normal phototropic sensitivity. 

Some thirty years ago Schrank and coworkers performed a series of electrophysiolog-ical experiments with avena coleoptiles, closely related to v.Guttenberg s experiment just mentioned. Schrank1S ) succeeded to induce bending of a coleoptile by a trans-... 

This latter explanation was verified by Webster and Schrank 187 who demonstrated that a transverse polarization of freshly decaptitated avena coleoptiles (containing no auxin ) could be electrically induced symmetrical application of exogenous auxin to the stump after electrical stimulation produced curvature A consistent behavior was exhibited if decapitated stumps, with auxin containing agar blocks on top, were electrically stimulated bending again occurred towards the positive side, similar to the case in which auxin was applied to only one side of freshly decapitated stumps without electrical stimulation (Fig. 12). 

Fig. 11. Effects of transverse application of 10 jaamperes for 10 min to the avena coleoptile on (A ) the transverse electrical polarity and (B) the bending of the same coleoptile. Bending occurs towards the positive pole, as sketched 157)...

Fig. 12. Curvature responses of decapitated avena coleoptiles as a function of auxin concentration. (A) curvature obtained in 90min, auxin applied asymmetrically (standard auxin test). (B) curvature produced in 60 min by symmetrical application of auxin upon succeeding transverse electrical stimulation. Bending occurs towards the positive pole, as sketched187 ...

Fig. 13. Electrical and curvature responses of avena coleoptiles to unilateral irradiation of two minutes. (A) intensity chosen to produce positive curvature (B) intensity chosen to produce negative curvature. Clearly the convex side of the coleoptile is electrically positive, regardless of the type of curvature. This indicates a strong correlation of bending and electrical potential gradient3)...

Fig. 15. Specific chemical inhibition of physiological bluelight responses 04) Swimming, inverse phobic response and direct photophobic response of Euglena as a function of iodide concentration118. (B) Phototropic and geotropic curvature of the avena coleoptile as function of azide concentration1S41...

Fig. 22. (A) Comparison of flavin triplet -> triplet absorption spectra (downwards drawn) with bluelight-induced (440 nm) phototropic curvature of aVena coleoptiles as inhibited by strong monochromatic light in the long wave visible region 154). (B) Comparison of the growth response of Phycomyces induced by strong laser light of wavelength longer than 590nm46, with the flavin phosphorescence spectrum los)...

Only for the (extrapolated) limit of zero bending the action spectrum reflects the absorption spectrum of the photoreceptor for phototropism (avena coleoptile). Just for this limit the UV-peak of the action spectrum disappears 162). 

The two-pigment-hypothesis explaining the two-peak action spectra (360 and 450 nm) is supported by blue fluorescing spots on the avena coleoptile, if it is excited in the UV162). An energy transfer from this pigment to a carotenoid is suggested. 

Hall, M. A., and L. Ordin. Subcellular location of phosphoglucomutase and UDP glucose pyrophosphorylase in avena coleoptiles. Physiol. Plant. 20 624-633, I%7. 

Ordin, L., M. J. Garber, and J. I. Kindinger. Effect of 2,4-dichlorophenoxyacetic acid on growth and on /1-glucan synthetases of peroxyacetyl nitrate pretreated Avena coleoptile sections. Physiol. Plant. 26 17-23, 1972. 

Ordin, L., and B. Propst. Effect of photochemically produced oxidants on growth of avena coleoptile sections. Plant Physiol. 36 326-330, 1961. 

Terminal complex consolidation has also been reported in vascular plants as loosely aligned files of rosettes associated with secondary wall formation (13,14,34,35). Similar rosette files were also observed during primary wall formation in rapidly elongating regions of Avena coleoptiles... 

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... 

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