The composition of the root exudates of barley and wheat in the initial growth phases was investigated; amino acids, organic acids, sugars and certain aromatic compounds could be identified. A knowledge of the composition of root exudates is important from the standpoint of the interaction between the plants and the micro-organisms in the rhizosphere. Some aspects of the rhizosphere effect connected with the present work are discussed.

Root exudates of plants

CHAPTER 8 – The Rhizosphere of Mycorrhizae

SummaryQuantitative and qualitative differences are reported in the composition of the root exudates of six plants from four families, barley, wheat, cucumber, turnip cabbage, tomato, and red pepper, in the initial stages of growth and for the last two species, also at the fruiting stage. It is concluded that the less the plants are related phylogenetically, the greater is the difference in the composition of the exudates. These results are discussed in relation to certain aspects of the rhizosphere effect.

Root exudates of plants: II. Composition of root exudates of some vegetables

SUMMARY: Cultures of Azotobacter chroococcum strain A6 were grown for 14 days in a nitrogen-deficient mineral medium, the supernatant fluid and bacteria extracted and examined by paper partition chromatography with two solvent systems which separate authentic gibberellin (GA3) and indolyl-3-acetic acid (IAA). Gibberellin-like substances were not detected on the chromatograms examined under ultraviolet (u.v.) radiation, but were detected when chromatograms were cut into ten equal strips representing a sequence of RF
values and the eluates tested in dwarf pea and lettuce hypocotyl bioassays. Certain eluates applied to the roots of tomato seedlings also altered the later growth of stems, leaves and flowers. The Azotobacter cultures contained three gibberellin-like substances, of which probably the dominant was one with an RF
value similar to that of GA3; the other two were not identified. The average concentration of gibberellin/ml. culture was 0.03 μg. GA3 equivalent. The gibberellins in Azotobacter cultures probably cause the reported effects on plant development and yield when seeds or roots are inoculated with Azotobacter. Plant growth may also be affected by synthesis of further gibberellins in the root zone when the Azotobacter inoculum colonizes developing roots.

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Production of plant growth substances by Azotobacter chroococcum.

SUMMARY
1Although much is known about the effects of plant hormones and their role in the control of growth and differentiation, little is known about the way in which hormone production is itself controlled or about the cellular sites of hormone synthesis The literature on hormone production is discussed in this review in an attempt to shed some light on these problems
2The natural auxin of plants, indol-3yl-acetic acid (IAA) is produced by a wide variety of living organisms In animals, fungi and bacteria it is formed as a minor by-product of tryptophan degradation The pathways of its production involve either the transamination or the decarboxylation of tryptophan The transaminase route is the more important
3In higher plants auxin is also produced as a minor breakdown product of trypto phan, largely via transamination In some species decarboxylation may occur but is of minor importance Tryptophan can also be degraded by spontaneous reaction with oxidation products of certain phenols
4The unspecific nature of the enzymes involved in IAA production and the probable importance of spontaneous, nonenzymic reactions in the degradation of tryp to phan make it unlikely that auxin production from tryptophan can be regulated with any precision at the enzymic level The limiting factor for auxin production is the availability of tryptophan, which in most cells is present in insufficient quantities for its degradation to occur to a significant extent Tryptophan levels are, however, considerably elevated in cells in which net protein breakdown is taking place as a result of autolysis
5An indole compound, glucobrassicin, occurs in Brassica and a number of other genera It breaks down readily to form a variety of products including indole aceto-nitrile, which can give rise to IAA There is, however, no evidence to indicate that glucobrassicin is a precursor of auxin in vivo
6Conjugates of IAA, eg IAA-aspartic acid and IAA-glucose, are formed when IAA is supplied in unphysiologically high amounts to plant tissues These and other IAA conjugates occur naturally in developing seeds and fruits There is no persuasive evidence for the natural occurrence of IAA-protein complexes
7Tissues autolysing during prolonged extraction with ether produce IAA from tryptophan released by proteolysis IAA is produced in considerable quantities by autolysing tissues in vitro
8During the senescence of leaves proteolysis results in elevated levels of trypto phan Large amounts of auxin are produced by senescent leaves
9Coleoptile tips have a vicarious auxin economy which depends on a supply of IAA, IAA esters and other compounds closely related to IAA from the seed These move acropetally in the xylem and accumulate at the coleoptile tip The production of auxin in coleoptile tips involves the hydrolysis of IAA esters and the conversion of labile, as yet unidentified compounds, to IAA There is no evidence for the de novo synthesis of IAA in coleoptiles
10Practically all the other sites of auxin production are sites of both meristematic activity and cell death The production of auxin in developing anthers and fertilized ovaries takes place in the regressing nutritive tissues (tapetum, nucellus, endosperm) as the cells break down In shoot tips, developing leaves, secondarily thickening stems, roots and developing fruits auxin is produced as a consequence of vascular differ entiation; the differentiation of xylem cells and most fibres involves a complete auto-lysis of the cell contents; the differentiation of sieve tubes involves a partial autolysis There is no evidence that meristematic cells produce auxin
11The lysis and digestion of cells infected with fungi and bacteria results in elevated tryptophan levels and the production of auxin Viral infections reduce the levels of tryptophan and are asSociated with reduced levels of auxin
12Crown-gall tissues produce auxin It is suggested that the crown-gall disease may involve at any given time the death of a minority of the cells which produce auxin and other hormones as they autolyse; the other cells grow and divide in response to these hormones
13Auxin is produced in soils, particularly those rich in decaying organic matter, by micro-organisms This environmental auxin may be important for the growth of roots
14There is no convincing evidence that auxin is a hormone in non-vascular plants The induction of rhizoids in liverworts by low concentrations of auxin can be ex plained as a response to environmental auxin
15Abscisic acid is synthesized from mevalonic acid in living cells It is possible that under certain circumstances, abscisic acid or closely related compounds are formed by the oxidation of carotenoids
16The sites of gibberellin production are sites of cell death It is possible that precursors of gibberellins, such as kaurene, are oxidized to gibberellins when cells die
17Cytokinins are present in transfer-RNA (tRNA) of animals, fungi, bacteria and higher plants They are probably formed in plants by the hydrolysis of tRNA in autolysing cells There is evidence that they are also formed in living cells in root tips
18Ethylene is produced in senescent, dying or damaged cells by the breakdown of methionine
19It was shown many years ago that wounded and damaged cells produced sub stances which stimulate cell division It now seems likely that the production of wound hormones and the normal production of hormones as a consequence of cell death are two aspects of the same phenomenon Wounded cells can produce auxin, gibberellins, cytokinins and ethylene
20The control of hormone production in living cells is a biochemical problem which remains unsolved The control of production of hormones formed as a con sequence of cell death depends on the control of cell death itself Cell death is con trolled by hormones which are themselves produced as a consequence of cell death
21In spite of the fact that dying cells are present in all vascular plants, in all wounded and infected tissues, in certain differentiating tissues in animals, in cancerous tumours and in developing animal embryos, the biochemistry of cell death is a subject which has been almost completely ignored Dying cells are an important source of hormones in plants; some of the many substances released by dying cells may also be of physiological significance in animals

The production of hormones in higher plants

IT is known that Azotobacter cultures can affect the germination of seeds and growth of plants. It is assumed that this influence is due to the presence of physiologically active substances and vitamins in the cultures. Actually, indole-3-acetic acid has been detected in Azotobacter cultures1,2. In a previous paper2 the detection of another growth-active compound, in addition to indole-3-acetic acid, was described. More details about this compound, which appeared to be gibberellic acid, are presented here.

V Vancura

Papers

Detection of gibberellic acid in Azotobacter cultures.