Cytokinins: Metabolism and Biological Activity of N-(Delta-Isopentenyl)adenosine and N-(Delta-Isopentenyl)adenine in Tobacco Cells and Callus

The cytokinin, N⁶-(Δ²-isopentenyl)adenine, is found to be at least 3.3 times as active as N⁶-(Δ²-isopentenyl)adenosine in promoting the growth of cytokinin-requiring tobacco (Nicotiana tabacum) callus. Absorption rates of N⁶-(Δ²-isopentenyl)adenine and N⁶-(Δ²-isopentenyl)adenosine by tobacco cells in liquid suspension do not differ significantly. In these cells, N⁶-(Δ²-isopentenyl)adenosine-5′-monophosphate, di-, and triphosphate are synthesized in both cases, but 7-glucosylation occurs significantly only with N⁶-(Δ²-isopentenyl)adenine, protecting thereby its N⁶-isopentenyl side chain from cleavage. Degradation by N⁶-side chain removal appears to be intense, leading to the formation of adenine, adenosine, and adenylic nucleotides. Thus, it is suggested that N⁶-(Δ²-isopentenyl)adenine-7-glucoside is a protected or storage form of the cytokinin which could account for the higher biological activity of N⁶-(Δ²-isopentenyl)adenine than of N⁶-(Δ²-isopentenyl)adenosine.     

Effects of Indoleacetic Acid on the Quantity of Mitochondria, Microbodies, and Plastids in the Apical and Expanding Cells of Dark-grown Oat Coleoptiles

We determined the number of mitochondria, microbodies, and plastids in dark-grown oat (Avena sativa) coleoptiles following incubation in indoleacetic acid (IAA) for a period of 60 minutes at 6-minute intervals. In the apical outer epidermis of coleoptiles, the mitochondria increased from 31.4 to 35 per cell section with a 6-minute incubation in IAA, and this trend persisted over the 60-minute incubation. Neither the microbodies, plastids, nor the dicytosomes (Gawlik and Miller 1974 Plant Physiol 54:217-221) responded to the hormone. The apical parenchyma showed no change in quantity of any of the organelles including the dictyosomes during IAA incubation. The quick response of mitochondria in the coleoptile tip could be interpreted as an association of this organelle with hormone transport, growth, or perhaps with gravity perception. In the subapical expansion region, IAA caused significant reductions of mitochondria, microbodies, and dictyosomes in the outer epidermis compared to the control, the timing of which preceded the IAA-induced elongation and of geotropism. The fast response of organelles in the various cells is probably a change in organelle volume rather than number. That microbodies show a response to the plant hormone in the permanently achlorophyllous epidermis indicates that these organelles, in addition to their peroxisomal functions in green leaves, also may have a growth regulation function. IAA treatment was without effect on the quantity of the various types of plastids (including the amyloplasts) in the different oat coleoptile cells.     

Water Flow in Beta vulgaris Storage Tissue

The relative magnitudes of the hydraulic resistances, water capacities, and water potential equilibration time constants for the single cell, for the apoplast, and for the symplast in higher plant tissue are assessed. Swelling of beetroot (Beta vulgaris, var. `Detroit Red') storage tissue sections in pure water is measured using a displacement transducer. This method of measurement avoids the difficulty of solute diffusion in the apoplast. Theoretical analysis of the experimental results shows that the main path of water flow into the tissue is the apoplast rather than the symplast, that the main resistance to water flow into the cells is usually the cell membrane rather than the apoplast, but that in some cases the apoplast resistance and water capacity can contribute significantly to the water potential equilibration time constant of the tissue.     

Specific inhibition of phototropism in corn seedlings

Geotropism was used as a control for the specificity of potential inhibitors of phototropism by the coleoptiles of corn (Zea mays) seedlings. The compounds tested fall into three categories showing: (a) no inhibition of either phototropism or geotropism (KCl); (b) nonspecific inhibition of both phototropism and geotropism (KCN); and (c) specific inhibition of phototropism (KI, NaN₃, and phenylacetic acid). Simultaneous irradiation of coleoptiles with phototropically inert light in addition to the phototropically active blue light also results in an inhibition of phototropism. Since azide, iodide, and phenylacetic acid are known to interact with flavins while a simultaneous irradiation with a phototropically inert light may depopulate the first triplet state of flavins, these data support the hypothesis that the photoreceptor pigment for phototropism in corn is a flavin.     

Photosynthetic Pod Wall of Pea (Pisum sativum L.): Distribution of Carbon Dioxide-fixing Enzymes in Relation to Pod Structure

The pod wall of pea (Pisum sativum L.) was shown to contain two distinct photosynthetic layers. The outer, comprising chlorenchyma of the mesocarp, captured CO₂ from the outside atmosphere; the inner, a chloroplast-containing epidermis lining the pod gas cavity, was involved in photoassimilation of the CO₂ released from respiring seeds.     

Effect of ethylene on the endogenous cytokinin and gibberellin levels in tuberizing potatoes

High concentrations of 2-chloroethylphosphonic acid inhibited tuberization on aged potato tubers (Solanum tuberosum) that had been predisposed to the little tuber disorder. As a result of this treatment sprouts developed which contained relatively high levels of endogenous gibberellins and which elongated normally. The endogenous cytokinin levels in the different treatments did not change appreciably. It is suggested that tuberization is prevented by ethylene either as a direct inhibition of cell division or that it prevents the endogenous cytokinins from functioning. Irrespective of the mode of action of ethylene, cell division apparently is the primary process affected, the result being that storage tissue required for the accumulation of starch is not formed.     

Quantitative Assessment of Ultrastructural Changes in Primary Roots of Corn (Zea mays L.) after Geotropic Stimulation: II. Curving and Noncurving Zones of the Root Proper

In outer cortical cells of corn (Zea mays L.) roots we made ultrastructural comparisons between the geotropically responding (661 nm irradiated) and nonresponding (dark control) roots in both the curving and noncurving regions of the root. In the control treatment, Golgi apparatus (dictyosomes) and mitochondria exhibited centrifugal distribution (taking the stele as the center) in both regions of the roots (the organelles localized in the top of the cells in the upper tissues, and in the bottom of the cells in the lower tissues). In the geotropically responding roots, the distribution patterns were the same as those of the controls. However, in the zone of curvature the dictyosomes (but not the mitochondria) were randomly distributed in the cells of the upper tissues. This change in pattern of dictyosome distribution could be related to the change in cell elongation of upper cells.     

Kinetic determination of the genome size of the pea

Renaturation of pea (Pisum sativum) DNA has been used to estimate the size of the pea genome and the fraction of pea DNA containing repeated DNA sequences. Pea DNA renaturation and single copy tracer renaturation indicate that the size of the pea genome is 0.5 picograms. More than 70% of pea DNA sequences are repeated from 100 to 5,000 times.