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. 2004 Aug 3;101(31):11506-10.
doi: 10.1073/pnas.0404388101. Epub 2004 Jul 22.

Nitrate assimilation in plant shoots depends on photorespiration

Shimon Rachmilevitch  1 , Asaph B Cousins, Arnold J Bloom
Affiliations

Nitrate assimilation in plant shoots depends on photorespiration

Shimon Rachmilevitch et al. Proc Natl Acad Sci U S A. .

Abstract

Photorespiration, a process that diminishes net photosynthesis by approximately 25% in most plants, has been viewed as the unfavorable consequence of plants having evolved when the atmosphere contained much higher levels of carbon dioxide than it does today. Here we used two independent methods to show that exposure of Arabidopsis and wheat shoots to conditions that inhibited photorespiration also strongly inhibited nitrate assimilation. Thus, nitrate assimilation in both dicotyledonous and monocotyledonous species depends on photorespiration. This previously undescribed role for photorespiration (i) explains several responses of plants to rising carbon dioxide concentrations, including the inability of many plants to sustain rapid growth under elevated levels of carbon dioxide; and (ii) raises concerns about genetic manipulations to diminish photorespiration in crops.

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Figures

Fig. 1.
Fig. 1.
[Formula: see text] reductase activity (μmol of [Formula: see text] generated per g of fresh mass per min) as a function of plant age (d) in leaves of a wild-type A. thaliana cv. Columbia (WT), a transgenic line harboring the chimeric gene Lhch1*3::Nia1*2 (OE), and a genotype (nia1 nia2) with mutations in both structural genes for [Formula: see text] reductase (Mut). Because [Formula: see text] reductase is regulated through phosphorylation, leaf tissue was assayed under conditions that either dephosphorylated the enzyme (fully activated) or did not change its phosphorylation (in vivo). Shown are the mean ± SE (n = 5-8 plants).
Fig. 2.
Fig. 2.
Changes in assimilatory quotient with the shift from [Formula: see text] to [Formula: see text] (ΔAQ) as a function of photosynthetic PFD in shoots of A. thaliana cv. Columbia. Thirty-six-day-old wild-type plants (A), 48-d-old wild-type plants (B), a genotype with mutations in the two structural genes for [Formula: see text] reductase (nia1 nia2) (C), and a transgenic line harboring the chimeric gene Lhch1*3::Nia1*2 (D). The plants were grown under ambient CO2 (360 μmol·mol-1) and measured under ambient CO2 and O2 (360 μmol·mol-1 CO2 and 21% O2; circles), elevated CO2 (720 μmol·mol-1 CO2 and 21% O2; triangles), or low O2 (360 μmol·mol-1 CO2 and 2% O2; squares). Shown are the mean ± SE, n = 5-8 plants.
Fig. 3.
Fig. 3.
Changes in assimilatory quotient with the shift from [Formula: see text] to [Formula: see text] (ΔAQ) as a function of photosynthetic PFD in shoots of wheat (T. aestivum cv. Veery 10). The plants were grown under ambient CO2 (360 μmol·mol-1) and measured under ambient CO2 and O2 (360 μmol·mol-1 CO2 and 21% O2; circles), elevated CO2 (700 μmol·mol-1 CO2 and 21% O2; triangles), or low O2 (360 μmol·mol-1 CO2 and 2% O2; squares). Shown are the mean ± SE, n = 5-8 plants. The data for ambient CO2 and O2 and elevated CO2 and ambient O2 have been published (15).
Fig. 4.
Fig. 4.
In wild-type Arabidopsis and wheat, [Formula: see text] uptake as the amount of [Formula: see text] depleted from a medium and [Formula: see text] assimilation as the difference between the rates of net [Formula: see text] uptake and net accumulation of free [Formula: see text] in plant tissues. Thirty-six-d-old Arabidopsis plants (A) or 10-d-old wheat (B) were exposed to either 360 μmol·mol-1 CO2 and 21% O2 (gray), 720 μmol·mol-1 CO2 and 21% O2 (black), or 360 μmol·mol-1 CO2 and 2% O2 (white). Shown are the mean ± SE (n = 13-16). Treatments labeled with different letters differ significantly (P ≤ 0.05). The light levels were 500 and 1,000 μmol·m-2·s-1 PAR for Arabidopsis and wheat, respectively.
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