Unravelling mechanisms and impacts of day respiration in plant leaves: an introduction to a Virtual Issue

Leaf day respiration: More than just catabolic CO2 production in the light

Day respiration is a net flux resulting from several CO2‐generating and CO2‐fixing reactions, not only related to catabolism but also to anabolism. We review pieces of evidence that decarboxylating reactions are partly fed by carbon sources disconnected from current photosynthesis and how they reflect various metabolic pathways.

Laisk measurements in the nonsteady state: Tests in plants exposed to warming and variable CO2 concentrations

Light respiration (RL) is an important component of plant carbon balance and a key parameter in photosynthesis models. RL is often measured using the Laisk method, a gas exchange technique that is traditionally employed under steady-state conditions. However, a nonsteady-state dynamic assimilation technique (DAT) may allow for more rapid Laisk measurements. In 2 studies, we examined the efficacy of DAT for estimating RL and the parameter Ci* (the intercellular CO2 concentration where Rubisco's oxygenation velocity is twice its carboxylation velocity), which is also derived from the Laisk technique. In the first study, we compared DAT and steady-state RL and Ci* estimates in paper birch (Betula papyrifera) growing under control and elevated temperature and CO2 concentrations. In the second, we compared DAT-estimated RL and Ci* in hybrid poplar (Populus nigra L. × P. maximowiczii A. Henry "NM6") exposed to high or low CO2 concentration pre-treatments. The DAT and steady-state methods provided similar RL estimates in B. papyrifera, and we found little acclimation of RL to temperature or CO2; however, Ci* was higher when measured with DAT compared to steady-state methods. These Ci* differences were amplified by the high or low CO2 pre-treatments. We propose that changes in the export of glycine from photorespiration may explain these apparent differences in Ci*.

Shifts in carbon partitioning by photosynthetic activity increase terpenoid synthesis in glandular trichomes

Several commercially important secondary metabolites are produced and accumulated in high amounts by glandular trichomes, giving the prospect of using them as metabolic cell factories. Due to extremely high metabolic fluxes through glandular trichomes, previous research focused on how such flows are achieved. The question regarding their bioenergetics became even more interesting with the discovery of photosynthetic activity in some glandular trichomes. Despite recent advances, how primary metabolism contributes to the high metabolic fluxes in glandular trichomes is still not fully elucidated. Using computational methods and available multi-omics data, we first developed a quantitative framework to investigate the possible role of photosynthetic energy supply in terpenoid production and next tested experimentally the simulation-driven hypothesis. With this work, we provide the first reconstruction of specialised metabolism in Type-VI photosynthetic glandular trichomes of Solanum lycopersicum. Our model predicted that increasing light intensities results in a shift of carbon partitioning from catabolic to anabolic reactions driven by the energy availability of the cell. Moreover, we show the benefit of shifting between isoprenoid pathways under different light regimes, leading to a production of different classes of terpenes. Our computational predictions were confirmed in vivo, demonstrating a significant increase in production of monoterpenoids while the sesquiterpenes remained unchanged under higher light intensities. The outcomes of this research provide quantitative measures to assess the beneficial role of chloroplast in glandular trichomes for enhanced production of secondary metabolites and can guide the design of new experiments that aim at modulating terpenoid production.

Toward mechanistic modeling and rational engineering of plant respiration

Plant respiration not only provides energy to support all cellular processes, including biomass production, but also plays a major role in the global carbon cycle. Therefore, modulation of plant respiration can be used to both increase the plant yield and mitigate the effects of global climate change. Mechanistic modeling of plant respiration at sufficient biochemical detail can provide key insights for rational engineering of this process. Yet, despite its importance, plant respiration has attracted considerably less modeling effort in comparison to photosynthesis. In this update review, we highlight the advances made in modeling of plant respiration, emphasizing the gradual but important change from phenomenological to models based on first principles. We also provide a detailed account of the existing resources that can contribute to resolving the challenges in modeling plant respiration. These resources point at tangible improvements in the representation of cellular processes that contribute to CO2 evolution and consideration of kinetic properties of underlying enzymes to facilitate mechanistic modeling. The update review emphasizes the need to couple biochemical models of respiration with models of acclimation and adaptation of respiration for their effective usage in guiding breeding efforts and improving terrestrial biosphere models tailored to future climate scenarios.

15N-labelling of Leaves Combined with GC-MS Analysis as a Tool for Monitoring the Dynamics of Nitrogen Incorporation into Amino Acids

Labeling plant material such as detached leaves with ¹⁵NH₄⁺ is a very instrumental method for the characterization of metabolic pathways of mineral nitrogen assimilation and incorporation into amino acids. A procedure of labeling, followed by amino acid extraction, purification, and derivatization for gas chromatography coupled to mass spectrometry (GC/MS) analysis, is presented. The rationale of heavy isotope abundance calculations and amino acid quantification is detailed. This method is adaptable to various plant species and various kinds of investigations, such as elucidating physiological changes occurring as a result of gene mutations (overexpression or inhibition) in natural variants or genetically modified crops, or characterization of metabolic fluxes in genotypes exhibiting contrasted physiological or developmental adaptive responses to biotic and/or abiotic environmental stresses. Furthermore, the benefit of working on detached organs or pieces of organs is to investigate finely the metabolism of species that are not amenable to laboratory work, such as plants growing in natural environments or under agricultural conditions in the field.

On the rate of phytoplankton respiration in the light

The rate of algal and cyanobacterial respiration in the light is an important ecophysiological term that remains to be completely characterized and quantified. To address this issue, we exploited process-specific decarboxylation rates from flux balance analysis and isotopically nonstationary metabolic flux analysis. Our study, based on published data, suggested that decarboxylation is about 22% of net CO2 assimilation when the tricarboxylic acid cycle is completely open (characterized by the commitment of alpha ketoglutarate to amino acid synthesis and very low rates of succinate formation). This estimate was supported by calculating the decarboxylation rates required to synthesize the major components of biomass (proteins, lipids, and carbohydrates) at their typical abundance. Of the 22 CO2 molecules produced by decarboxylation (normalized to net assimilation = 100), approximately 13 were from pyruvate and 3 were from isocitrate. The remaining six units of decarboxylation were in the amino acid synthesis pathways outside the tricarboxylic acid cycle. A small additional flux came from photorespiration, decarboxylations of six phosphogluconate in the oxidative pentose phosphate pathway, and decarboxylations in the syntheses of lower-abundance compounds, including pigments and ribonucleic acids. This general approach accounted for the high decarboxylation rates in algae and cyanobacteria compared to terrestrial plants. It prompts a simple speculation for the origin of the Kok effect and helps constrain the photoautotrophic respiration rate, in the light, in the euphotic zone of the ocean and lakes.

Drought exerts a greater influence than growth temperature on the temperature response of leaf day respiration in wheat (Triticum aestivum)

We assessed how the temperature response of leaf day respiration (R_d ) in wheat responded to contrasting water regimes and growth temperatures. In Experiment 1, well-watered and drought-stressed conditions were imposed on two genotypes; in Experiment 2, the two water regimes combined with high (HT), medium (MT) and low (LT) growth temperatures were imposed on one of the genotypes. R_d was estimated from simultaneous gas exchange and chlorophyll fluorescence measurements at six leaf temperatures (T_leaf ) for each treatment, using the Yin method for nonphotorespiratory conditions and the nonrectangular hyperbolic fitting method for photorespiratory conditions. The two genotypes responded similarly to growth and measurement conditions. Estimates of R_d for nonphotorespiratory conditions were generally higher than those for photorespiratory conditions, but their responses to T_leaf were similar. Under well-watered conditions, R_d and its sensitivity to T_leaf slightly acclimated to LT, but did not acclimate to HT. Temperature sensitivities of R_d were considerably suppressed by drought, and the suppression varied among growth temperatures. Thus, it is necessary to quantify interactions between drought and growth temperature for reliably modelling R_d under climate change. Our study also demonstrated that the Kok method, one of the currently popular methods for estimating R_d , underestimated R_d significantly.

The crucial roles of mitochondria in supporting C4 photosynthesis

C₄ photosynthesis involves a series of biochemical and anatomical traits that significantly improve plant productivity under conditions that reduce the efficiency of C₃ photosynthesis. We explore how evolution of the three classical biochemical types of C₄ photosynthesis (NADP-ME, NAD-ME and PCK types) has affected the functions and properties of mitochondria. Mitochondria in C₄ NAD-ME and PCK types play a direct role in decarboxylation of metabolites for C₄ photosynthesis. Mitochondria in C₄ PCK type also provide ATP for C₄ metabolism, although this role for ATP provision is not seen in NAD-ME type. Such involvement has increased mitochondrial abundance/size and associated enzymatic capacity, led to changes in mitochondrial location and ultrastructure, and altered the role of mitochondria in cellular carbon metabolism in the NAD-ME and PCK types. By contrast, these changes in mitochondrial properties are absent in the C₄ NADP-ME type and C₃ leaves, where mitochondria play no direct role in photosynthesis. From an eco-physiological perspective, rates of leaf respiration in darkness vary considerably among C₄ species but does not differ systematically among the three C₄ types. This review outlines further mitochondrial research in key areas central to the engineering of the C₄ pathway into C₃ plants and to the understanding of variation in rates of C₄ dark respiration.

Growth and photosynthetic traits differ between shoots originated from axillary buds or from adventitious buds in Populus balsamifera L. cuttings

Bud development influences shoot branching and the plasticity and adaptability of plants. To explore the differences of post-embryonic development of different types of buds, shoots originated from adventitious buds and axillary buds of cuttings in two populations of balsam poplar (Populus balsamifera L.) were investigated for differences in leaf morphology, photosynthetic and growth characteristics, and the effects of a carbonic anhydrase (CA) inhibitor on CA activity, photosynthesis and mesophyll conductance (g_m ). The results showed that axillary buds produced ovate first few leaves and longer shoots while adventitious buds produced lanceolate first few leaves with higher specific leaf area (SLA). There were no significant differences in leaf area-based photosynthetic rate (A_n ), maximum carboxylation rate (V_cmax ), and maximum electron transport rate (J_max ) between shoots originated from the two bud types. Based on the principal component analysis, shoots of adventitious bud origin grouped on daytime respiration and SLA, while cuttings from axillary buds clustered toward the opposite direction of quantum yield and light saturation point. Shoots originated from different types of buds had different growth rates and biomass, but the direction of the differences varied with the population of the mother tree. The two populations differed in A_n , g_m , and relationships between CA, A_n , and g_m . There were differences in post-embryonic growth traits of shoots from axillary buds and those from adventitious buds, which may be an adaptive strategy for regeneration under different light conditions.