Studies on the action of abscisic acid on IAA-induced rapid growth of Avena coleoptile segments

Effect of auxin and abscisic acid on cell wall extensibility in maize coleoptiles

Plastic and elastic in-vitro extensibilities (E pland E el ) of cell walls from growing maize (Zea mays L.) coleoptile segments were measured by stretching frozen-thawed tissue, pre-extended to its in-vivo length, at constant force (creep test) in a custom-buildt extensiometer, equipped with a linear-displacement transducer. The indole-3-acetic acid (IAA)-induced change of E pl (ΔE pl ) is strictly correlated with the growth rate for a period of 3-4 h. Subsequently, ΔE plremains constant while the growth rate is slowing down. Since this discrepancy can be accounted for by a growth-dependent reduction of osmotic pressure, it is concluded that ΔE plrepresents quantitatively the relative increase of in-vivo extensibility (cell wall loosening) involved in IAA-mediated cell growth over a much longer time. On the other side it is argued that the growth rate may not be strictly correlated with wall extensibility during long-term growth. Abscisic acid (ABA) inhibits segment growth induced by auxin, fusicoccin, or exogenous acid, and this effect can be quantitatively attributed to an ABA-mediated reduction of cell wall extensibility as determined by the ΔE plmeasurement. Both, IAA and ABA have no effect on total protein synthesis, RNA synthesis, and amount of osmotic solutes. Fusicoccin-induced proton excretion is only slightly inhibited by ABA. In contrast to ABA, growth inhibition by cycloheximide (CHI) is always much larger than the concomitant reduction of ΔE pl , indicating that a further growth parameter is also involved in the inhibition of cell growth by CHI. ΔE el is not affected by either IAA, ABA, or CHI. It is concluded that δE pl as determined by the applied method, represents a relative measure of the actual in-vivo extensibility of the growing cell wall at the very moment when the tissue is killed, rather than an average extensibility accumulated over some immediate-past period of time as suggested by Cleland (1984, Planta 160, 514-520). Hence, we further draw the conclusion that IAA and ABA control of cell growth can entirely be attributed to a modulation of cell wall extensibility by these hormones in maize coleoptiles.

Initial phases of indoleacetic acid induced growth in coleoptile segments of Avena sativa L

The intial phases of auxin-induced growth in coleoptile segments of Avena sativa L. were investigated using a high resolution growth recording technique, based on an angular position sensing transducer. The first response to the hormone is a slight, transient reduction of the growth rate lasting about 5 min. After this phase growth rate increases to a maximum. The duration of the increase and the maximum clearly depend on the concentration of the hormone. With increasing auxin concentration the duration of the growth rate increase is reduced from about 80 min in 10(-9) M indoleacetic acid (IAA) to about 14 min in 10(-4) M IAA. After the maximum the growth rate declines. Looking at the maximum of the growth rate, we obtained a dose-response curve with a sharp increase between 10(-9) M and 10(-6) M IAA and a slight decline between 10(-6) M and 10(-4) M IAA. This result is confirmed by growth rates measured one and two hours after the application of the hormone.

Rapid growth responses of corn root segments: Effect of pH on elongation

The effect of pH on the elongation rate of 2-mm-long subapical segments from corn roots (Zea mays L.) was investigated under aerobic (O2) and anaerobic (N2) conditions using a specially designed apparatus. The maximum elongation rate was obtained with a citrate-phosphate buffered solution of pH 3.4 or a non-buffered solution of pH 3.2. Acid stimulation of the elongation rate occurred in the zone of maximum elongation in the root, the 3 mm region 1 mm behind the apex. The acid response can be rapidly and repeatedly reinitiated by alternating between pH 4.0 and 7.0 media. Low pH can increase the elongation rate under N2 conditions, but the response is only ca. 50% that of the increase observed under O2 conditions. Reinitiation of the acid response does not occur when solutions are gassed with N2. The data suggest that the response of root segments to H ions is similar to that reported in the literature for shoot tissue. However, the magnitude of the root response is much higher than that of shoot tissue. Also, there is evidence that some aspects of the mode of action of H ions on wall loosening in root tissue may differ from that in shoot tissue. Corn root segments, though lacking a cuticle, do not respond to low pH in the manner reported for peeled stem tissue, the cuticle and epidermis of which have been removed. For these reasons rapid-growth responses of roots and shoots may not be strictly comparable.

Acid-induced growth and the geotropic response of the wheat node

Growth is initiated in segments of the leaf sheath base of Triticum aestivum by gravitational stimulation and by incubation in buffer solutions of pH 3. Both responses involve only an increase in segment length, are rapidly terminated on removal of the stimulus, have a similar Q10 and are dependent upon cell turgor. They differ, however, in that the response to acid solutions is rapid and unaffected by anoxia. Acid-induced growth can be stopped and started repeatedly by changing the pH, an increase in pH from 3 to 5 or 7 being sufficient to terminate the response. The maximum growth induced by low pH is not increased by simultaneous stimulation by gravity.

The effects of temperature and IAA concentration on the latent period for IAA-induced rapid growth of Avena coleoptile segments

Indoleacetic acid buffered at pH 7.0 induces a high growth rate in Avena coleoptile segments after a latent period, the duration of which is dependent upon both IAA concentration and temperature. A minimum latent period of 7.3 min is observed at 25° C with 10(-3) M IAA in phosphate buffer at pH 7.0.In contrast, 5×10(-3) M IAA made up in 0.01 M KH2PO4 alone, promotes elongation almost immediately, regardless of whether the segments have been previously incubated in 0.01 M KH2PO4 at pH 4.7, or phosphate buffer at pH 7.0. This immediate response is unaffected by 10(-4) M KCN which abolishes the rapid growth induced by 5×10(-3) M IAA buffered at pH 7.0 but does not affect the immediate appearance of low-pH-induced growth. Since we consistently find solutions of 5×10(-3) M IAA in 0.01 M KH2PO4 to have a pH of 3.5, our results indicate that the immediate growth response elicited by this solution is attributable to its low pH rather than to the presence of IAA as previously reported in the literature.