Once bound to the ABP1 receptor, a signal must be transmitted to the rest of the cell to cause the specific changes in metabolism that result in altered growth. Brummell and Hall (1987) consider that for a relatively small number of auxin molecules binding to a relatively small number of receptors to produce a major effect, some sort of rapid amplification reaction is necessary. They suggest that both signal amplification and transduction are achieved through changes in intracellular concentrations of calcium ions with important roles for inositol triphosphate (IP3) and diacylglycerol (DG) as additional secondary messengers.
The cytoplasmic concentration of free calcium ions in a 'resting' cell is kept low, in the region of 10t?-10-8M, due to its continual active removal into the cell wall or into organelles, such as the endoplasmic reticulum and vacuole. However, when the cell is stimulated by auxin binding at receptors, a transient increase in cytoplasmic calcium ions (to 10-5-10-6M) results, which is then able to bind to a calcium-binding protein (calmodu-lin) and this complex may stimulate protein kinases. These in turn phosphorylate, and hence activate, key enzymes.
The link between auxin receptor occupancy and cytoplasmic calcium ion concentration is thought to involve the hydrolysis of phosphatidyl inositol bisphosphate (PIP2 ) at the plasmalemma, which results in the production of IP3 and DG. IP3 can mobilise calcium ions from the endoplasmic reticulum, and is certainly metabolised in the presence of auxin (Ettlinger and Lehle, 1988), and DG may also activate calcium-dependent protein kinases.
A unifying, but still highly speculative, view of the consequences of binding to an auxin receptor in young tissues is presented in Figure 7.7. Auxin binding to the plasmalemmal receptor causes hydrolysis of PIP2 to IP3 and DG. IP3 causes enhanced mobilisation and cytoplasmic accumulation of calcium ions, which together with DG activates protein kinases, and the activation of other key enzymes follows. The elevated cytoplasmic concentration of calcium ions is reduced to resting state levels by transport into the vacuole or endoplasmic reticulum. Transport into the vacuole is in exchange for protons (H+ ) which are then exported out of the cell by an H+ - ATPase.
We now know that auxin also binds to a soluble receptor in the nucleus where specific genomes become derepressed and specific mRNA sequences are synthesised. These messengers are translated at the endoplasmic reticulum to products, for example, involved in new cell wall synthesis (Figure 7.7). This view is supported by the observation by many workers that specific mRNA synthesis can be detected within minutes after auxin application. These models are of real value for an appreciation of auxin-type herbicide action because they identify primary and measurable responses to auxin receptor occupancy, namely selective gene expression and enhanced proton (H+ ) - efflux.
Useful information has been gained from studies of auxin-induced H+-efflux. It is now well established that plant cells maintain large differences in electrical potential and pH across their intracellular compartments (Figure 7.8) . Indeed, the electrogenic gradient generated by H+ pumping provides the driving force for the transport of various solutes, including anions, cations, amino acids, sugars and auxins. These ATPases are associated with the plasmalemma and tonoplast and are the subjects of much current research. It has been known for some time that auxins are able to induce cell elongation in young tissues that is associated with H+-efflux. Indeed, the traditional bioassays of measuring the elongation or curvature of intact tissues or tissue segments after a 24-h incubation was a cornerstone of auxin discovey. However, the relationship between auxin concentration and growth in these bioassays is poorly defined, such that a very large change in exogenous auxin is often necessary to cause a measurable difference in, for example, growth rate. Furthermore, most auxin bioassays have poor, or at least variable, sensitivity usually caused by poor or slow penetration of the exogenous auxin. Indeed, mecoprop was initially overlooked as a potential auxin herbicide because of its relative inactivity in the oat straight growth test (as cited in Kirby 1980). On the other hand, the measurement of rates of H+ - efflux from sensitive tissues is rapid, linear with time, and highly dependent on auxin concentration. In oat coleoptile tissues at least, it can be considered a primary auxin response in ion transport, which occurs as a direct consequence of auxin-receptor complex formation. By measuring H+-efflux from oat segments, Fitzsimons (1989) and Fitzsimons et al. t1988) derived sensitivity parameters for a wide range of auxin-type herbicides
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