David L. Rayle

The in-vitro acid-growth response: Relation to in-vivo growth responses and auxin action

SummaryWe have examined in detail the characteristics of the hydrogen-ion extension response in frozen-thawed Avena coleoptile sections (in-vitro acid-growth response). These data allow us to compare the in vitro response with the in-vivo extension responses initiated by auxin and hydrogen ions. The in-vitro response has three characteristics in common with the in-vivo responses: a similar Q10 (3–4 between 15 and 25°C, but almost 1 between 25 and 35°); a minimum yield stress; and a lack of stored growth (i.e., an inability to induce a potential for growth during periods of reduced wall tension). Both the in-vivo and in-vitro acid-growth responses have a threshold pH of about 4.5 and give an optimum response at pH values of 3 and below. These similarities suggest that the in-vitro and in-vivo acid-growth responses have a common wall-loosening and wall-extension mechanism. We have also examined the effects of Pronase, sodium lauryl sulfate (SLS), elevated temperatures, calcium, and potassium ions on the in-vitro acid-growth response. We suggest that hydrogen ions do not activate wall-associated enzymes, but act to hydrolyze non-enzymatically some acid-labile linkages in the cell wall. Furthermore, we suggest that auxin induces cell elongation either by causing the release of hydrogen ions from the protoplast or by causing the appearance in the wall of an enzyme which can hydrolyse the acid-labile linkages.

Characterization of the growth and auxin physiology of roots of the tomato mutant, diageotropica

Roots of the tomato (Lycopersicon esculentum, Mill.) mutant diageotropica (dgt) exhibit an altered phenotype. These roots are agravitropic and lack lateral roots. Relative to wild-type (VFN8) roots, dgt roots are less sensitive to growth inhibition by exogenously applied IAA and auxin transport inhibitors (phytotropins), and the roots exhibit a reduction in maximal growth inhibition in response to ethylene. However, IAA transport through roots, binding of the phytotropin, tritiated naphthylphthalamic acid ([3H]NPA), to root microsomal membranes, NPA-sensitive IAA uptake by root segments, and uptake of [3H]NPA into root segments are all similar in mutant and wild-type roots. We speculate that the reduced sensitivity of dgt root growth to auxin-transport inhibitors and ethylene is an indirect result of the reduction in sensitivity to auxin in this single gene, recessive mutant. We conclude that dgt roots, like dgt shoots, exhibit abnormalities indicating they have a defect associated with or affecting a primary site of auxin perception or action.

The diageotropica mutant of tomato lacks high specific activity auxin binding sites

Tomato plants homozygous for the diageotropica (dgt) mutation exhibit morphological and physiological abnormalities which suggest that they are unable to respond to the plant growth hormone auxin (indole-3-acetic acid). The photoaffinity auxin analog [3H]5N3-IAA specifically labels a polypeptide doublet of 40 and 42 kilodaltons in membrane preparations from stems of the parental variety, VFN8, but not from stems of plants containing the dgt mutation. In roots of the mutant plants, however, labeling is indistinguishable from that in VFN8. These data suggest that the two polypeptides are part of a physiologically important auxin receptor system, which is altered in a tissue-specific manner in the mutant.

The pH profile for acid-induced elongation of coleoptile and epicotyl sections is consistent with the acid-growth theory

The acid-growth theory predicts that a solution with a pH identical to that of the apoplast of auxintreated tissues (4.5–5.0) should induce elongation at a rate comparable to that of auxin. Different pH profiles for elongation have been obtained, however, depending on the type of pretreatment between harvest of the sections and the start of the pH-incubations. To determine the acid sensitivity under in vivo conditions, oat (Avena sativa L.) coleoptile, maize (Zea mays L.) coleoptile and pea (Pisum sativum L.) epicotyl sections were abraded so that exogenous buffers could penetrate the free space, and placed in buffered solutions of pH 3.5–6.5 without any preincubation. The extension, without auxin, was measured over the first 3 h. Experiments conducted in three laboratories produced similar results. For all three species, sections placed in buffer without pretreatment elongated at least threefold faster at pH 5.0 than at 6.0 or 6.5, and the rate elongation at pH 5.0 was comparable to that induced by auxin. Pretreatment of abraded sections with pH-6.5 buffer or distilled water adjusted to pH 6.5 or above gave similar results. We conclude that the pH present in the apoplast of auxin-treated coleoptile and stems is sufficiently low to account for the initial growth response to auxin.

Rapid auxin- and fusicoccin-enhanced Rb(+) uptake and malate synthesis in Avena coleoptile sections.

The short-term effects of auxin (indole-3-acetic acid) and fusicoccin (FC) on Rb+ uptake and malate accumulation in Avena sativa L. coleoptile sections have been investigated. FC stimulates 86Rb+ uptake within 1 min while auxin-enhanced uptake begins after a 15–20-min lag period. Auxin has little or no effect on 86Rb+ uptake at external pHs of 6.0 or less, but substantial auxin effects can be observed in the range of pH 6.5 to 7.5. Competition studies indicate that the uptake mechanism is specific for Rb+ and K+. After 3 h of auxin treatment the total amount of malate in the coleoptile sections is doubled compared to control sections. FC causes a doubling of malate levels within 60 min of treatment. Auxin-induced malate accumulation exhibits a sensitivity to inhibitors and pH which is similar to that observed for the H+-extrusion and Rb+-uptake responses. Both auxin- and FC-enhanced malate accumulation are stimulated by monovalent cations but this effect is not specific for K+.

Absence of auxin-induced stored growth in Avena coleoptiles and its implication concerning the mechanism of wall extension.

SummaryWe have reinvestigated the ability of Avena coleoptiles to undergo auxin-induced stored growth (stored growth is defined as the ability of a cell to store up a potential for extension during periods of reduced turgor which can be converted into extra extension upon restoration of normal turgor). We could detect little or no stored growth, with either moderate (1–2 bar) or more severe (3–5 bar) reductions in turgor, and with varying periods (10–100 min) of reduced turgor. Earlier reports of a stored growth potential (e.g., Cleland and Bonner, 1956) are shown to be in error, in that the apparent growth potential is probably an artifact of the use of argon or nitrogen as an inhibitor of auxin action. The absence of stored growth reported here is not due to a direct inhibitory effect of the osmoticum itself on auxin action, since coleoptiles can extend in response to auxin even in the presence of mannitol if an external force is applied to the section to replace the normal turgor. These results show that the two components of cell-wall extension, wall loosening and wall extension, usually are inseparable. Two possible explanations are considered; the walls may be extending by the process of chemical creep, or the wall loosening may only occur when the load-bearing bonds are under tension.

Calcium bridges are not load-bearing cell-wall bonds in Avena coleoptiles

I examined the ability of frozen-thawed Avena sativa L. coleoptile sections under applied load to extend in response to the calcium chelators ethyleneglycol-bis-(β-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) and 2-[(20bis-[carboxymethyl] amino-5-methylphenoxy)methyl]-6-methoxy-8-bis [carboxymethyl]aminoquinoline (Quin II). Addition of 5 mM EGTA to weakly buffered (0.1 mM, pH 6.2) solutions of 2(N-morpholino) ethanesulfonic acid (Mes) initiated rapid extension and wall acidification. When the buffer strength was increased (e.g. from 20 to 100 mM Mes, pH 6.2) EGTA did not initiate extension nor did it cause wall acidification. At 5 mM Quin II failed to stimulate cell extension or wall acidification at all buffer molarities tested (0.1 to 100 mM Mes). Both chelators rapidly and effectively removed Ca2+ from Avena sections. These data indicate that Ca2+ chelation per se does not result in loosening of Avena cells walls. Rather, EGTA promotes wall extension indirectly via wall acidification.