Two Classes Of Feedback Agents

The chemical nature of the agents responsible for the negative feedback of plant N fluxes is not completely resolved, although it is essential for these agents, as well as the mechanisms by which they act, to be identified, if the prospect of altering N influx (or net N acquisition) is to be achieved. Therefore, this subject deserves further attention here. Feedback agents are likely to fall into two categories, the transported substrates themselves, or the N metabolites produced from these substrates (chapters 1 and 2). Correlative evidence that NH4+ accumulation in the cytosol of root cells feeds back negatively on NH4+ influx into these cells was found by Wang et al. (1993a), who grew rice plants hydroponically, in complete nutrient solutions with three NH+ regimes (2, 100, and 1000 pM). As shown in Figure 2, the influx of NH4+ at a given external concentration declined with increasing steady-state concentration of NH4+ applied during growth; the cytosolic pool size of NH4+ was positively correlated with the external NH4+ concentration. This pattern has been seen in other plants, including white spruce (Kronzucker et al. 1995c) and A. thaliana (Rawat et al. 1999). However, other evidence shows that high tissue levels, or exogenous application, of certain amino acids, in particular glutamine and asparagine, are also correlated with decreased NH4+ influx across the plasma membrane (Lee et al. 1992, Causin and Barneix 1993, Wieneke and Roeb 1998, Rawat et al. 1999, chapter 2). In Arabidopsis, this down-regulation of influx was attributed mainly to a decrease in transcript abundance of AMT1 effected by glutamine. An increase in cytosolic NH4+ concentration, achieved by blocking the assimilation of NH4+ by use of the glutamine synthetase inhibitor methionine sulfoximine (MSX), did have a small inhibitory effect on NH4+ influx while not down-regulating AMT1 transcription (Rawat et al. 1999). However, this effect is by no means universal; indeed, in most cases MSX treatment results in both increased cytosolic NH4+ concentration and increased NH4+ influx (Wieneke and Roeb 1998). The difficulties in interpreting results obtained by use of MSX were discussed by Lee and Ayling (1993), Kronzucker et al. (1995d) and Glass et al. (1997).

In the case of NO3-, a situation similar to that shown in Figure 2 has been observed (Kronzucker et al. 1995b); again, there appears to be an inverse correlation between cytosolic NO3- pool size and NO3- influx. The possibility that nitrate down-regulates its own influx is also supported by experiments in which such down-regulation was observed in plants made deficient in nitrate reduction and assimilation (and hence deficient in downstream N metabolites), either through mutation or via the application of chemical inhibitors (Ingemarsson et al. 1987, Warner and Huffaker 1989, Mattsson et al. 1991, King et al. 1993). However, in some instances, such as over the course of induction of NO3- transport in white spruce, the highest NO3- influx was associated with the largest cytosolic NO3- pools (Kronzucker et al. 1995a). Alternative possibilities for down-regulatory agents include the nitrate pool in the vacuole, and the ammonium or amino acid pools in various compartments. The down-regulatory role of N metabolites on NO3- fluxes is supported by numerous studies (Cooper and Clarkson 1989, Lee et al. 1992, Imsande and Touraine 1994, Muller et al. 1995, Fraisier et al. 2000, Vidmar et al. 2000, Forde 2002, chapter 1). It is important to note, however, that in the cases of both NO3- and NH4+ fluxes, studies using exogenously applied amino acids as putative down-regulators are generally conducted against a backdrop of poorly understood uptake processes for the amino acids themselves. This is a subject that has only recently been making substantial progress (Persson and Nasholm 2003), but has potentially great importance given the high soil concentrations of amino acids in certain environments (Henry and Jefferies 2002).

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

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