| Summary: | A review of the literature indicated that the purine cytokinin 6-(benzylamino)purine
(SA) may be converted to a wide range of metabolites. Although the functional
significance of these metabolites remains obscure, cytokinin physiologists have
essentially classed them as either active or inactive. Inactivation of cytokinins is
considered to proceed via catabolic oxidation (side-chain cleavage), or N-conjugation
with glucose or alanine moieties.
The literature survey was hampered by the confusing array of synonyms which
have been coined for cytokinin metabolites. Accordingly, a working system of
(semi-systematic) abbreviations was devised which accommodated all groups and
classes of purine cytokinins.
Prior to commencing metabolic interactive studies, it was necessary to resolve the
contentious issue associated with the successful extraction of cytokinin
nucleotides. Five-week-old soybean callus was fed [8[-14]C]BA and subsequently
extracted using four widely used cytokin in extraction techniques. Techniques
compared were a modified Bieleski method, 80% ethanol with tissue
homogenisation, 80% ethanol without homogenisation, and boiling ethanol. All
four procedures produced similar results, showing that all metabolites of SA,
including the nucleotide, were adequately extracted. It was concluded that the
extraction of nucleotides with Bieleski solvents did not warrant the inconvenience.
Auxins have been shown to interact with cytokinins in the regulation of many
physiological processes, although little is known of the mechanisms of interaction
which proceed at the metabolic level. Previous investigators have shown that auxin
promoted cytokinin degradation through catabolic oxidation, Shoot-apex and seed derived
cell suspensions of Dianthus zevheri subsp. natalensis were incubated with
[8[-14]
C]BA for between 30 minutes and 48 hours in the presence of both low (2 mg
l-1) and high (4 mg 1¯¹) levels of exogenously supplied 2,4-dichlorophenoxyacetic
acid (2,4-D), In both systems, the auxin 2,4-D was shown to promote SA inactivation through 7-glucosylation (N-conjugation). This observation represents
the first report of auxin-promoted cytokinin N-conjugate formation. The auxin
effect on metabolism was transient in the case of shoot-apex, but not in seed-derived
systems over a 48 hour period. Formation of the 7-glucoside of SA was
dose-dependent in apex-derived cultures. Further studies were undertaken with
indole-3-acetic acid (lAA) and α-naphthaleneacetic acid (NAA). It was found that
auxin-promoted 7-glucosylation of SA was only minimally effected by these two
auxins.
In comparable studies with soybean suspension cultures (Glycine max cv. Acme),
2,4-D-promoted catabolic oxidation was observed between 18 and 48 hours,
following application of phytohormones. The main catabolite was tentatively
identified as adenosine-5'-monophosphate (AMP), based on chromatographic
characteristics.
Carrot (Daucus carota) cell suspensions similarly supplied with 2,4-D and SA
maintained a large active cytokinin pool. Neither substantial oxidative nor Nconjugative
processes were observed. Instead, there was a transient effect by 2,4-
D on the relative formation of the riboside and the 7- and 9-glucosides of SA.
The effect of auxin on the metabolism of SA thus varied with the species and
system investigated. Generally, auxin promoted (rather than inhibited), the
formation of inactivated metabolites and catabolites of SA, possibly by the
induction of relevant enzyme systems.
Transient auxin effects on the metabolism of SA are discussed in relation to the
role of the auxin/cytokinin balance in the induction of developmental processes. === Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1993.
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