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Fig. l. Auxin production by pea leaf homogenate incubated ~t 37 ~ C, [ ]-~ ; Auxin production by autolysing yeast incubated at 37 ~ C, o-o ; Auxin production by rat liver homogenate incubated at 37 ~ C, A-A
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Autolysing plant tissues are known to produce auxin when extracted with ether. It has been shown that autolysing plant, yeast and rat liver tissues produce auxin in vitro; this suggests that relatively unspecific mechanisms are involved. Furthermore, sterile plant and animal tissues which have been killed by freezing and thawing induce nodules of d...
Citations
... The auxin activity decreased in the epidermis above primordia and increased in AR primordia after the induction of AR growth, and both cell death and AR emergence were reduced by inhibition of auxin efflux suggesting an antagonistic regulation [50]. The localized PCD of cells thereby providing a burst of auxin to control developmental processes in neighbouring tissues is certainly plausible in the light of the dying cell hypothesis of auxin production, first proposed over 50 years ago [51] and recently revisited [52]. This thought-provoking idea postulates that in a plant much of the auxin is produced from the elevated levels of amino acid tryptophan, the auxin precursor, released by hydrolysis of proteins in dying cells [52]. ...
Both auxin signalling and programmed cell death (PCD) are essential components of a normally functioning plant. Auxin underpins plant growth and development, as well as regulating plant defences against environmental stresses. PCD, a genetically controlled pathway for selective elimination of redundant, damaged or infected cells, is also a key element of many developmental processes and stress response mechanisms in plants. An increasing body of evidence suggests that auxin signalling and PCD regulation are often connected. While generally auxin appears to suppress cell death, it has also been shown to promote PCD events, most likely via stimulation of ethylene biosynthesis. Intriguingly, certain cells undergoing PCD have also been suggested to control the distribution of auxin in plant tissues, by either releasing a burst of auxin or creating an anatomical barrier to auxin transport and distribution. These recent findings indicate novel roles of localized PCD events in the context of plant development such as control of root architecture, or tissue regeneration following injury, and suggest exciting possibilities for incorporation of this knowledge into crop improvement strategies.
... (vi) The non-specific nature of auxin production is shown by the fact that IAA is produced by autolysing yeast and rat liver, as well as by autolysing plant tissues (Sheldrake and Northcote, 1968a). In humans, IAA is a non-specific breakdown product of tryptophan. ...
In this review, I discuss the possibility that dying cells produce much of the auxin in vascular plants. The natural auxin, indole-3-acetic acid (IAA), is derived from tryptophan by a two-step pathway via indole pyruvic acid. The first enzymes in the pathway, tryptophan aminotransferases, have a low affinity for tryptophan and break it down only when tryptophan levels rise far above normal intracellular concentrations. Such increases occur when tryptophan is released from proteins by hydrolytic enzymes as cells autolyse and die. Many sites of auxin production are in and around dying cells: in differentiating tracheary elements; in root cap cells; in nutritive tissues that break down in developing flowers and seeds; in senescent leaves; and in wounds. Living cells also produce auxin, such as those transformed genetically by the crown gall pathogen. IAA may first have served as an exogenous indicator of the presence of nutrient-rich decomposing organic matter, stimulating the production of rhizoids in bryophytes. As cell death was internalized in bryophytes and in vascular plants, IAA may have taken on a new role as an endogenous hormone.
... Synchronous and periodic PCD in root cap cells might release auxin bursts into the oscillation zone that contributes to the regular spacing of LRs. Interestingly, dying plant tissue was reported to produce auxin in vitro, probably due to the release and subsequent degradation of free tryptophan during protein hydrolysis [45]. This suggests that de novo production of auxin by dying root cap cells might also contribute to the auxin release into the oscillation zone. ...
... This suggests that de novo production of auxin by dying root cap cells might also contribute to the auxin release into the oscillation zone. Sheldrake and Northcote suggested, already in 1968, that auxin production by controlled cell death stimulates cell division during plant development [45]. Therefore, the recently revealed PCD in the root cap might represent a more general mechanism in plant development in which PCD-derived auxin is capable in affecting local accumulation patterns to influence cellular differentiation processes. ...
In dicot root systems, lateral roots are in general regularly spaced along the longitudinal axis of the primary root to facilitate water and nutrient uptake. Recently, recurrent programmed cell death in the root cap of the growing root has been implicated in lateral root spacing. The root cap contains an auxin source that modulates lateral root patterning. Periodic release of auxin by dying root cap cells seems to trigger lateral root specification at regular intervals. However, it is currently unclear through which molecular mechanisms auxin restricts lateral root specification to specific cells along the longitudinal and radial axes of the root, or how environmental signals impact this process.
... The ability of E. strenuana to induce development of nutritive callus tissue by rupturing the vascular connections during feeding, and to place frass between disconnected vascular strands, is critical in host plant control activity. The 1st activity, in conjunction with the induced metabolism of senescence, has profound implications in the abnormal synthesis of plant growth regulators at these sites (Sheldrake andNorthcote 1968, Wilson andWilson 1991) and in destabilizing the normal physiology of the host plant. The 2nd activity provokes the production of polyphenolics and resists the development of reparatory vascular bridges. ...
The North American Epiblema strenuana (Walker) may prove to be an important agent for the control of the weed Parthenium hysterophorus L. in Australia. In this article we report time-related changes in the galled shoots of P. hysterophorus consequent to attack by E. strenuana. In addition to many structural adjustments involving tissue regeneration, the host plant shows a number of metabolic alterations as a result of its response to galling by the moth larva. Through histochemical localization we demonstrate how the plant synthesizes some of the key metabolites facilitating the nutritional requirements of the larva and displays compensatory behavior to neutralize the stress induced by larval feeding. As the larva prepares for pupation, host-plant metabolism changes abruptly, by accumulating polyphenolic materials in the translocatory cells and ultimately blocking them. This event coincides with a modest decline in gall mass that was previously increasing with larval mass. The ability of the moth to damage the growth point, phloem, and the associated parenchyma, making them nonfunctional, and induce the plant to lose vigor, indicates that it is a potential biocontrol agent.
... All plant tissues produce callus (if the usually autonomously produced cytokinin is not limiting) with water, nutrients, and IAA (Wilson and Wilson 1991). IAA is produced by the catalysis of tryptophan (Kutacek 1991) from the autolysis of dead and dying cells (Sheldrake and Northcote 1968) and accumulates when basipetal polar transport in vascular tissue is interrupted. Thus severing vascular tissue in a nutrient sink (thistle receptacle and knapweed rosette root) should provide both the necessary IAA and nutrients for callus production. ...
Gall inducers are favoured as biocontrol agents of weeds because they tend to have a narrow host range. Six insect and one nematode gall inducer used in Canada are described in terms of their biology, gall morphology, gall physiology, and effectiveness in weed control. The species differ in plant organ attacked, requirement for moisture, whether the galls are induced by secretions or by severing xylem, and effectiveness, which in part relates to the ability of the gall to import nutrients. The most powerful galls divert assimilates from other sinks via a gall’s vascular system joined to that of their host. One of our examples also has mechanisms to compensate for reduction of turgor during drought. Two of the gall inducers enhance their nutrient supply by severing xylem in a plant nutrient sink. One, in the short-term sink of a thistle capitulum, obtains about a quarter of its assimilates at the expense of other capitula. The other, in the long-term sink of a rosette root, approximately halves seed production. Hypotheses are presented to explain various aspects of gall development and function.
... However, when the amount of free IAA in mung bean hypocotyls was estimated using a bioassay, a higher level (about 1542 pmol/g fresh wt) was reported (14). The high level of " IAA" detected by bioassay was neither the result of synthesis, during extraction (19) n or spontaneous conversion of indole-3-pyruvic acid (2). Amide-conjugated IAA or other active substances may have co-chromatographed with IAA, adding to the activity detected by the oat mesocotyl bioassay. ...
Changes in the levels of putative free and conjugated indole-3-acetic acid (IAA) were examined by high performance liquid chromatography (HPLC) during the first 96 hr of adventitious root formation in mung bean [ Vigna radiata (L.) R. Wilcz. ‘Berken’] stem cuttings. The putative IAA was characterized as biologically and chemically similar to IAA; ester- and amide-conjugated IAA also were found. Amide-conjugated IAA was an order of magnitude more abundant than either free or ester-conjugated IAA, both of which were present at low levels. In duplicate experiments, the relative levels of free and conjugated IAA in the rooting region fluctuated similarly during root formation, although some differences in timing and magnitude were observed.
... This may also be true of the control of polyploid cell formation. Auxins are synthesized by leaves (Jacobs, 1952 ) and cytokinins are produced by roots (Kende, 1965) and both of these hormones may be formed during the controlled autolytic processes of xylem and phloem formation (Wooding & Northcote, 1964; Northcote & Wooding, 1966; Sheldrake & Northcote, 1968). The ratio of these hormones at a particular point within a plant would depend on the physical dimensions of the plant as well as on other factors and thus such a ratio may differ from plant to plant. ...
Comparisons have been made between a sycamore callus (S4) isolated in 1970 and one first isolated in 1958 (S2). Incubation of S4 on media containing different concentrations of auxin and kinetin showed that the ratio of the hormones was the factor which controlled the growth pattern within the tissue. High ratios of auxin: cytokinin favoured a white friable tissue, whereas a compact callus with a much harder texture was formed at low ratios. Roots were differentiated at intermediate values.
Callus (S2) has not been induced to undergo cellular differentiation. No soluble factors which were capable of stimulating differentiation in S2 were produced during the differentiation of S4, and S2 did not affect the ability of S4 to form roots.
Divisions seen in suspension cultures of S4 callus were mainly diploid; the cells of S2 were predominantly tetraploid. When S2 suspensions were grown in the cytokinin-free medium used for S4 a considerable number of diploid cells were seen. A number of S2 clones have been isolated and all these developed chloroplasts on exposure to continuous light and, when grown on the medium used to induce differentiation in S4, five of them showed a growth pattern in which the cells were more closely packed than is usually the case in S2 callus.
A greater degree of cell contact has also been induced by including polyethylene glycol in the medium and so restricting the availability of water to the callus. Under these conditions the cells must have a greater capacity for interaction with one another. The effect of tissue texture on the extent of cell contact and adhesion between the cells is discussed in relation to the composition of the plant cell wall and ideas on the totipotency of plant tissue cultures.
... The idea of strand maintenance may be reinforced, in the case of xylem, by the proposition that material from an autolysing cell is transferred to the contiguous cells in the strand. This material then acts to stimulate or initiate the next steps in the developmental sequence (Table 1. 1, Sheldrake 1973, Sheldrake andNorthcote 1968a). ...
Don Northcote became eminent in the field of plant biochemistry following his identification of the processes involved in the synthesis and deposition of polysaccharides that constitute the cell wall of plants. His researches spanned lower and higher plant species and he showed by the application of a variety of experimental techniques, including radioautography, electrophoresis, freeze etching and the novel use of electron microscopy, that much of the material of the cell wall is synthesized in cytoplasmic organelles before being transported to the developing wall in vesicles assembled from the membranes of the Golgi body. His findings inspired many colleagues to build on the foundation he laid for understanding the biochemistry of cell morphogenesis. Nearly his entire career was spent in fundamental research and teaching in the Department of Biochemistry, University of Cambridge, from 1948 until his retirement in 1992. In addition, he was a fellow of St John's College, Cambridge, from 1960 to 1976, and he served Sidney Sussex College, Cambridge, as master from 1976 to 1992.
Indole or indoleglycerol phosphate were incubated with crude or dialyzed enzyme extracts (pH 5.4-8.0) of pea seedlings. Neither with nor without addition of serine-like cosubstrates, any formation of auxin or an auxin precursor could be detected by means of the Avena curvature test or thin-layer chromatography. In presence of indole, the auxin content of crude homogenates (pH 6.8) was slightly enhanced; that was due to an indole-induced inhibition of the breakdown of IAA produced by bacteria during the incubation period. Indoleglycerol was not altered by wheat tissues and was physiologically inactive in the Triticum section test.
