Tom K. Scott

Auxin transport in roots : II. Polar flux of IAA in Zea roots.

SummaryThe movement of IAA has been investigated in roots of dark-grown seedlings of Zea mays using IAA-I-14C.With 6-mm segments excised 1 mm below the apex of the root it has been shown that: (a) There is a strictly acropetal flux of IAA through the tissues, the amount of IAA found in an apical receiving block increasing almost linearly with increasing transport period up to about 6–7 hours, but thereafter declining for at least a further 18 hours. The onset of this decline appears to be dependent upon the concentration of IAA in the donor block. (b) The amount of IAA recovered in the apical receiving block increases with increasing concentration of IAA in the donor block over the range from 0.1–10 μM, with transport periods of both 4 and 9 hours. (c) The radioactivity in the receiving block is confined to the IAA molecule. (d) The orientation of the segment with respect to gravity did not significantly affect the acropetal polar flux of IAA in the tissue.With non-decapitated 7-mm root apices it has been found that the presence of the apex has no effect on the strictly acropetal flux of IAA in the tissues, but that it entirely prevented the emergence of IAA into an apical receiving block.The movement of IAA has been investigated in roots of dark-grown seedlings of Zea mays using IAA-I-(14)C.With 6-mm segments excised 1 mm below the apex of the root it has been shown that: (a) There is a strictly acropetal flux of IAA through the tissues, the amount of IAA found in an apical receiving block increasing almost linearly with increasing transport period up to about 6-7 hours, but thereafter declining for at least a further 18 hours. The onset of this decline appears to be dependent upon the concentration of IAA in the donor block. (b) The amount of IAA recovered in the apical receiving block increases with increasing concentration of IAA in the donor block over the range from 0.1-10 μM, with transport periods of both 4 and 9 hours. (c) The radioactivity in the receiving block is confined to the IAA molecule. (d) The orientation of the segment with respect to gravity did not significantly affect the acropetal polar flux of IAA in the tissue.With non-decapitated 7-mm root apices it has been found that the presence of the apex has no effect on the strictly acropetal flux of IAA in the tissues, but that it entirely prevented the emergence of IAA into an apical receiving block.

Cellulosic Microfibrils: Nascent Stages of Synthesis in a Higher Plant Cell

Freeze-fracturing of untreated plasma membrane and inner wall surfaces of stelar tissue in corn roots demonstrated the association of globular complexes with the ends of nascent microfibrils. It is proposed that the granule complexes associated with the outer leaflet of the plasma membrane coordinate the assembly of the cellulosic microfibrils.

Rapid Growth Responses of Corn Root Segments: Effect of pH on Elongation

The effect of indole-3-acetic acid (IAA) on the elongation rates of 2 mm corn (Zea mays L.) root segments induced by citrate-phosphate buffer (or unbuffered) solutions of pH 4.0 and 7.0 was studied. At pH 7.0, auxin initially reduced the elongation rate in both buffered and unbuffered solutions. Only in buffer at pH 7.0 was auxin at a concentration of 0.1 μM found to promote the elongation rate though briefly. THis promoted rate represented only ca. 20% of the rate achieved with only buffer at pH 4.0. Auxin in pH 4.0 buffered and unbuffered solutions only served to reduce the elongation rates of root segments. Some comparative experiments were done using 2 mm corn coleoptile segments. Auxin (pH 6.8) promoted the elongation rate of coleoptile segments to a level equal or greater than the maximal H ion-induced rate. The two responses of root segments to auxin are compared to auxin action in coleoptile growth.

Hydrotropism and its interaction with gravitropism in maize roots

We have studied hydrotropism and its interaction with gravitropism in agravitropic roots of a pea mutant and normal roots of peas (Pisum sativum L.) and maize (Zea mays L.). The interaction between hydrotropism and gravitropism in normal roots of peas or maize were also examined by nullifying the gravitropic response on a clinostat and by changing the stimulus-angle for gravistimulation. Depending on the intensity of both hydrostimulation and gravistimulation, hydrotropism and gravitropism of seedling roots strongly interact with one another. When the gravitropic response was reduced, either genetically or physiologically, the hydrotropic response of roots became more unequivocal. Also, roots more sensitive to gravity appear to require a greater moisture gradient for the induction of hydrotropism. Positive hydrotropism of roots occurred due to a differential growth in the elongation zone; the elongation was much more inhibited on the moistened side than on the dry side of the roots. It was suggested that the site of sensory perception for hydrotropism resides in the root cap, as does the sensory site for gravitropism. Furthermore, an auxin inhibitor, 2,3,5-triiodobenzoic acid (TIBA), and a calcium chelator, ethyleneglycol-bis-(β-aminoethylether)-N,N,N′,N′- tetraacetic acid (EGTA), inhibited both hydrotropism and gravitropism in roots. These results suggest that the two tropisms share a common mechanism in the signal transduction step.

Auxin transport in roots : IV. Effects of light on IAA movement and geotropic responsiveness in Zea roots.

SummaryLight promotes the net acropetal movement of 14C through 6-mm subapical segments of dark-grown roots of Zea mays supplied at their basal ends with 1 μM IAA-1-14C in agar blocks. This promotion occurs only when the segments are irradiated during the transport period, and both red and blue light appear to be as effective as white light at the radiant flux densities used in this investigation. The promotion is not found if the segments are pretreated with light and then returned to darkness before the trasport of IAA-1-14C is determined. The very slight basipetal movement of 14C through the segments supplied with an apical source of IAA-1-14C is unaffected by light.Only one radioactive substance is found in the apical receiver blocks. This substance has an Rf virtually identical to those of the stock solution of IAA incorporated into the donor block and of unlabelled IAA. The movement of radioactivity into the receiver blocks through, the illuminated segments therefore appears to reflect the movement of IAA. Light thus increases the acropetal movement of IAA through the Zea root segment.The primary roots of Zea mays var. Giant Horse Tooth seedlings grown in total darkness do not exhibit a positive geotropic response. When the seed is orientated with the embryo uppermost the radicle grows out horizontally. On exposure to light, however, the roots bend down. This reaction appears about 3–9 hours after the onset of illumination, and white, red and blue light appear to be equally effective at the flux densities employed in this study. Green light in the spectral band between 510–530 nm did not appear to induce this positive geotropic responsiveness.

Auxin transport in roots : III. Dependence of the polar flux of IAA in Zea roots upon metabolism.

SummaryThe dependence upon metabolism of the acropetal polar flux of IAA through 6-mm Zea root segments has been investigated using IAA-I-14C.The results are as follows: 1. At 0°C the acropetal movement of IAA is reduced to the level of basipetal movement and the polar flux is thus abolished, even after transport periods of 24 hours. 2. In dead tissue the acropetal polar flux is abolished and replaced by a basipetal polar flux. 3. On exposing segments to anaerobic conditions, the acropetal polar flux of IAA is severely reduced during the first 4 hours, but reappears during the subsequent 4 hours. The tissue thus appears to have the ability of adapting to anaerobic conditions and of resuming the acropetal transport of IAA. 4. The acropetal flux of IAA found in segments deprived of oxygen for 8 hours is dependent upon anaerobic metabolism since it is abolished and replaced by a net basipetal flux in the presence of sodium fluoride.The acropetal polar flux of IAA through the Zea root segments is thus entirely dependent upon metabolic energy.

Gravity-regulated formation of the peg in developing cucumber seedlings

It has been proposed that peg formation in the vascular transition region (TR zone) between the hypocotyl and the root in Cucurbitaceae seedlings is a gravimorphogenetic phenomenon. Initiation of the peg became visible 36 h after imbibition when cucumber (Cucumis sativus L. cv. Burpee Hybrid II) seeds were germinated in a horizontal position at 24°C in the dark. Simultaneously, sedimented amyloplasts (putative statoliths) were apparent in the sheath cells surrounding the vascular strands, and in the cortical cells immediately adjacent to them, in the TR zone. In contrast, the other cortical cells, some of which were destined to develop into the peg, contained amyloplasts which were not sedimented. These results suggest that the graviperception mechanism for peg formation may be like that of statoliths in shoot gravitropism. By 48 h following imbibition, the cells of the TR zone still had sedimented amyloplasts but had lost their sensitivity to gravity, possibly because of their maturation.

Rapid-growth responses of corn root segments: Effect of citrate-phosphate buffer on elongation

SummaryThe effects of citrate-phosphate buffer on the elongation rate of 2 mm root segments of Zea mays L. was investigated under O2 and N2 conditions by comparing with the effects in non-buffered solution. The buffer at pH 7.0 has an inhibiting effect on the elongation rate of root segments. At pH 4.0 the buffer has a promoting effect on the elongation rate twice that of the H-ion concentration. Nitrogen experiments indicate that growth in neutral buffer is O2 dependent. On the other hand the acid-growth response of root segments in citrate-phosphate buffer appears to have two components, 1) a H-ion component which is partially inhibited by N2 and fluoride and 2) a citrate component which is inhibited in part by N2 but not by fluoride.

Phytochrome-mediated swelling of etiolated leaf protoplasts and its possible biological significance.

SummaryRed light, mediated by the photoreceptor phytochrome, induces maize leaf unrolling as well as leaf expansion. Protoplasts prepared from maize leaves still in the rolled condition swell in a red-far red photoreversible manner indicating that phytochrome mediates this phenomenon. To determine if protoplast swelling is related to leaf unrolling, leaf expansion, or both, we compared red-light induced swelling of protoplasts from rolled maize leaves to protoplasts prepared from tissues that are known to grow in response to light but do not unroll. We also compared the swelling response of protoplasts from rolled vs. unrolled leaves. In all cases, we found that swelling correlated with the unrolling response and not leaf expansion.

Agricultural Management Strategies, Initiatives, and Goals for Survival

During the course of the Advanced Study Institute, there was a great deal of discussion concerning many and various topics related to plant growth regulation and world agriculture. Those discussions were helpful and illuminating to the participants. However, it was agreed at the outset that only points of particular strategical significance for the future of agriculture, horticulture, and associated research imperatives be given special note in this volume. There follows, therefore, a compilation of points, arguments, comments, and philosophies which grew out of these discussions and which we have brought together under an umbrella title, AGRICULTURAL MANAGEMENT STRATEGIES, INITIATIVES, AND GOALS FOR SURVIVAL.