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INTRODUCTION |
This month's The Hot and
the Classic pays tribute to the scientific contributions of Dr. Folke
Skoog who passed away this February in his 92nd year. Chief among his
many awards and honors were his memberships in the National Academy of
Sciences and the American Academy of Arts and Sciences and his receipt
in 1991 in a White House ceremony of a National Medal of Science. Dr. Skoog's name will forever be linked with the discovery of cytokinins (CKs) and the perfecting of techniques that allowed for the
regeneration of whole plants from tissue culture, but these were just a
few of his many contributions to plant physiology. We hope that the sampling of articles discussed below will remind the readers of Plant
Physiology of the breadth and importance of Dr. Skoog's numerous and
varied contributions to science.
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1933: Auxin Promotes Apical Dominance |
Struck by the fact that apical buds usually have a higher auxin
concentration than any other bud on the shoot, Thimann and Skoog
(1933)
, performed a simple experiment that demonstrated that auxin was
the previously demonstrated but as-yet-unidentified inhibitor of
lateral bud growth. They removed the apical buds from a number of
etiolated fava bean (Vicia faba) seedlings and noted the
characteristic enhanced growth of the previously repressed lateral
buds. They found, however, that the application of auxin in lanolin
paste to the cut end of such decapitated seedlings mimicked the action
of the terminal bud and prevented the release of lateral bud growth.
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1937: An Improved Auxin Bioassay |
Early bioassays of auxin were based on the ability of added
auxin to restore growth to decapitated oat (Avena sativa)
coleoptiles. However, it was noted that although decapitation led to an
abrupt cessation of growth, after a short time a limited amount of
growth resumed. However, if the top few millimeters of the stump were removed, this renewed growth ceased. This phenomenon of physiological regeneration of the tip was a confounding and annoying aspect of these
early auxin bioassays. Skoog (1937) discovered that this physiological
regeneration could be prevented by removal of the "seed" of the oat
before the initial decapitation. He proposed that some precursor of
auxin (he guessed tryptophan) originates in the "seed" and passes
up to the tip where it is converted into auxin.
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1940: Reduced Auxin Levels in Zn-Deficient
Plants |
Internode elongation is so severely inhibited in Zn-deficient
plants that such plants often assume a rosette-like growth habit. Skoog
(1940) determined that the levels of extractable auxin were dramatically reduced in tomato (Lycopersicon esculentum) and
sunflower (Helianthus annuus) plants grown in Zn-deficient
culture solutions. The decrease in auxin preceded the appearance of
visible symptoms of Zn deficiency. Although Skoog proposed that Zn
deficiency leads to the enhanced destruction of auxin, later
researchers concluded that Zn deficiency impairs auxin biosynthesis.
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1948: Adenine Promotes Bud Development in Tobacco
Explants |
While searching for factors that influence the development of
buds in tobacco callus, Skoog and Tsui (1948) discovered that adenine
together with phosphate not only counteracted the inhibitory effects of
auxin on bud growth, but promoted the formation of buds and increased
the growth of the callus tissue. This seminal finding redirected the
research focus of Skoog's laboratory and led ultimately to the
discovery of CKs and their recognition as plant hormones.
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1954: Vascular Tissue Promotes Cell Division in Pith
Explants |
The treatment of pith parenchyma with indole-3-acetic acid (IAA)
was shown to lead to enormous cell enlargement without cell division.
Jablonski and Skoog (1954) report that the placement of a piece of
vascular tissue atop an explant of tobacco pith induced the pith cells
to divide. These results lent credence to the idea that there are
chemical factors, later to be identified as CKs, that induce cell
division in plant tissues maintained in vitro and that these factors in
planta may be localized chiefly in the vascular tissue of stems.
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1955: Kinetin Isolated and Described |
Based on the results of tobacco bioassays, a small amount
of highly active CK concentrate was obtained from yeast
(Saccharomyces cerevisiae) which, although not identified,
exhibited the properties of a purine. Various nucleic acid preparations
were then tested, and it was discovered that the degraded DNA from
herring sperm and autoclaved DNA were the two richest sources of CK
activity to date. The chemical structure of the isolated material (i.e. kinetin or 6-furfurylaminopurine) was deduced from its elementary composition (C10H9N5O) and
degradation products (adenine and levulenic acid) (Miller et al.,
1955). Although kinetin was later shown to be an artifact that arises
spontaneously in DNA preparations from deoxyadenosine, its
effectiveness as a CK at very low concentrations was undeniable, and it
continues to be widely used as a synthetic CK to this day.
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1956: Triiodobenzoic Acid (TIBA) Inhibits Polar
Transport of Auxin |
Although previous researchers had established that TIBA produces
aberrations in plant development, including loss of apical dominance,
the common belief was that TIBA acts by preventing the synthesis of IAA
or by acting as its competitive inhibitor. Niedergang-Kamien and Skoog
(1956) report that TIBA abolished the normal tip-to-base IAA gradient
in shoots and prevents the polar transport of IAA through 5-mm sections
of sunflower stems.
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1962: Tissue Culture Medium Perfected |
With over 17,000 citations, Murashige and Skoog's (1962) report
of a new plant tissue culture medium may well be the most cited plant
physiology paper of all time. Although originally designed for the
purpose of testing organic growth factors for their effects on cell
expansion and division in tobacco, the Murashige and Skoog medium
proved to have wide applicability and soon became a standard for plant
cell and tissue culture.
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1966: Cytokinins and tRNA |
In the early 1960s, reports began to emerge that certain
species of tRNA from a wide variety of organisms had peculiar CK-like bases associated with their anticodon loops. Expanding immensely upon
these initial findings, Skoog et al. (1966) determined that tRNAs from
yeast, liver, and Escherichia coli had CK activity in the tobacco
callus bioassay, whereas ribosomal RNA from yeast was inactive. The
idea that the activity or synthesis of CKs in plants was somehow
related to their presence in certain species of tRNA became a major
focus of Skoog's research toward the end of his active career, but
this notion has few adherents today.
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1967: Structure/Activity of Cytokinins |
The discovery of kinetin led researchers to ask, "What are the
structural requirements for CK-like biological activity?" Skoog et
al. (1967) tested 69 compounds, mostly purine derivatives and closely
related substances, for their abilities to promote growth and regulate
organ formation in tobacco tissue bioassays. Virtually every structural
aspect of these purine derivatives was systematically altered to
determine which moieties were necessary for optimal CK activity. The
highest CK activity was achieved with N6-monosubstituted
adenine compounds, but the structure, size, shape, composition,
saturation, and charge of the substituent groups were found to also
strongly influence activity.
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INTRODUCTION
1933: Auxin Promotes Apical...