دانلود رایگان ترجمه مقاله جذب نیتروژن و گوگرد در گیاهان و جلبک ها (نشریه الزویر 2014)

آسیمیلاسیون نیتروژن و سولفور در گیاهان و جلبک ها

Nitrogen and sulfur assimilation in plants and algae

Abstract

Nitrogen and sulfur are abundant constituents of plant and algal cells that are assimilated at the lowest oxidation number, as NH4+ and S2−, although they can (in the case of sulfur, usually must) be acquired with their highest oxidation number, as NO3− and SO42−. Some occasional differences and variants exists for transport and assimilation systems; the greatest differences in the way vascular plants and algae use N and S, however, most probably resides in regulation. For instance, nitrate assimilation in plants is strongly regulated by phosphorylation. In algae, redox regulation appears to be more important. Similarly, sulfate reduction has its main control step at the level of APS reductase in higher plants, whereas in algae a redox regulation has been recently been hypothesized for ATP sulfurylase, the first step in sulfate assimilation. Unfortunately, the information on the regulation of N and S acquisition and assimilation is limited to very few species (e.g. Chlamydomonas reinhardtii, Arabidopsis thaliana) this is especially true in the case of sulfur. This review attempts to highlight the points of divergence in N and S utilization by plants and algae, leaving aside the biochemical details and the features that do not show any obvious difference.

1- Objectives

In this review we attempt to provide an overall assessment of N and S assimilation in algae and plants. The task is not a simple one, given the patchiness of the information and the fact that most studies refer to few model organisms. Numerous reviews exist for nitrogen and sulfur metabolism in higher plants. Much less was published for algae (possibly with the exception of nitrogen metabolism in Chlamydomonas reinhardtii, e.g. Fernandez et al., 2009). For this reason we decided to put somewhat more emphasis on algae, while however always comparing them with the embryophytes. We integrated the biochemical and molecular information with ecological and evolutionary concepts; the latter aspects have often been disregarded in previous reviews, yet they are inextricably intertwined with phylogeny, gene expression and metabolic regulation and decisively concur to similarities and differences in the way photosynthetic organisms deal with N and S.

2- Sulfur and Nitrogen in plants and algae

Average stoichiometries of plants and algae show that N and S are among the most abundant component of photosynthetic cells (Giordano, 2013). The cost of the assimilation of N (especially) and S estimated on these stoichiometries is not trivial (Table 1). Under energy limitation, competition may occur among the intricately interconnected C, N and S assimilation pathways (Ruan, 2013; Fig. 1) and the extent of such competition may depend on the flexibility of cell stoichiometry, on the availability of these elements in the environment and on the degree by which their acquisition and assimilation can be modulated. The cost of the assimilation process increases for assimilation in shoots of vascular land plants, where nitrate and – especially – sulfate are commonly assimilated. The assimilation of nitrate and sulfate generates OH− (Raven and Smith, 1976; Raven, 1986; Andrews et al., 2013). Although the OH− generated in the roots is mostly released into the soil, most of the OH− that is produced as a consequence of shoot nitrate and sulfate assimilation is retained within the plants and must be neutralized by the production of organic acids (Raven and Smith, 1976; Andrews et al., 2009, 2013). The energy cost of producing the OH−-neutralizing organic acids outweighs energy saving from the more direct use of photosynthetically generated reductants with oxyanion reduction restricted to the photoperiod. Getting rid of excess acid produced as a consequence of ammonium and dinitrogen assimilation is much more difficult and it is restricted to roots, where ammonium or N2 can be assimilated with excretion of protons to the rooting medium (Raven and Smith, 1976; Raven, 1986; Andrews et al., 2009, 2013).