Cytokinin-dependent photorespiration and the protection of photosynthesis during water deficit

Abstract

We investigated the effects of P(SARK)IPT (for Senescence-Associated Receptor KinaseIsopentenyltransferase) expression and cytokinin production on several aspects of photosynthesis in transgenic tobacco (Nicotiana tabacum cv SR1) plants grown under optimal or restricted (30% of optimal) watering regimes. There were no significant differences in stomatal conductance between leaves from wild-type and transgenic P(SARK)-IPT plants grown under optimal or restricted watering. On the other hand, there was a significant reduction in the maximum rate of electron transport as well as the use of triose-phosphates only in wild-type plants during growth under restricted watering, indicating a biochemical control of photosynthesis during growth under water deficit. During water deficit conditions, the transgenic plants displayed an increase in catalase inside peroxisomes, maintained a physical association among chloroplasts, peroxisomes, and mitochondria, and increased the CO(2) compensation point, indicating the cytokinin-mediated occurrence of photorespiration in the transgenic plants. The contribution of photorespiration to the tolerance of transgenic plants to water deficit was also supported by the increase in transcripts coding for enzymes involved in the conversion of glycolate to ribulose-1,5-bisphosphate. Moreover, the increase in transcripts indicated a cytokinin-induced elevation in photorespiration, suggesting the contribution of photorespiration in the protection of photosynthetic processes and its beneficial role during water stress.

Figure 1.

Comparison of photosynthetic parameters in wild-type plants (WT) and two transgenic lines expressing P_SARK∷IPT measured over 75 d of growth under optimal conditions (1,000 mL d⁻¹; black symbols) or reduced watering conditions (300 mL d⁻¹; white symbols). The eighth fully expanded leaf was used for measurements using a LI-6400 gas-exchange system with a fixed chamber CO₂ concentration and light and temperature as described in “Materials and Methods.” Values are means ± se (n = 15).

Figure 2.

CO₂ assimilation rates at different intercellular CO₂ concentrations (A/Ci curves) of wild-type plants (WT) and two transgenic lines expressing P_SARK∷IPT grown under optimal conditions (1,000 mL d⁻¹; A) or reduced watering conditions (300 mL d⁻¹; B–F) over 70 d. Wild-type plants growing under optimal watering were the control for every measurement. The eighth fully expanded leaf was used for measurements using a LI-6400 gas-exchange system with a fixed chamber CO₂ concentration and light and temperature as described in “Materials and Methods.” Values are means ± se (n = 15).

Figure 3.

Vcmax (A), Jmax (B), and TPU (C) in wild-type plants (WT) and two transgenic lines expressing P_SARK∷IPT grown under optimal conditions (1,000 mL d⁻¹) or reduced watering conditions (300 mL d⁻¹). The eighth fully expanded leaf was used for measurements using a LI-6400 gas-exchange system with a fixed chamber CO₂ concentration and light and temperature as described in “Materials and Methods.” Values are means ± se (n = 15).

Figure 4.

Electron micrographs of chloroplasts and surrounding organelles from wild-type plants (WT; A and C) and transgenic plants expressing P_SARK∷IPT (B and D) grown under optimal conditions (1,000 mL d⁻¹; A and B) or reduced watering conditions (300 mL d⁻¹; C and D). C, Chloroplast; M, mitochondria; P, peroxisome; S, starch. Micrographs are from the eighth fully expanded leaf of 40- to 45-d-old plants. The crystalloids visible inside peroxisomes in transgenic plants are catalase. The micrographs are representative of four independent experiments (n = 4). Bars = 1 μm.

Figure 5.

The Γ of wild-type plants (WT) and transgenic P_SARK∷IPT plants grown for 70 d under optimal conditions (1,000 mL d⁻¹) or reduced watering conditions (300 mL d⁻¹). The eighth fully expanded leaf was used for measurements using a LI-6400 gas-exchange system with a fixed chamber CO₂ concentration and light and temperature as described in “Materials and Methods.” Values are means ± se (n = 15).

Figure 6.

Photorespiration in wild-type plants (WT) and transgenic plants expressing P_SARK∷IPT grown under optimal conditions (1,000 mL d⁻¹) or reduced watering conditions (300 mL d⁻¹). A, A schematic representation of the respiratory pathway of C₃ plants. 1, RbcS; 2, PGPase; 3, GO; 4, GDC; 5 SHMT; 6, GK. B, Relative expression of selected transcripts associated with photorespiration (1–6 as shown in A). C to E, Concentrations of selected photorespiratory metabolites. Analyses were performed in the eighth fully expanded leaf of individual plants. Values are means ± se (n = 6). D.W., Dry weight.