Beyond Porosity: 3D Leaf Intercellular Airspace Traits That Impact Mesophyll Conductance

The leaf intercellular airspace (IAS) is generally considered to have high conductance to CO2 diffusion relative to the liquid phase. While previous studies accounted for leaf-level variation in porosity and mesophyll thickness, they omitted 3D IAS traits that potentially influence IAS conductance (gIAS). Here, we reevaluated the standard equation for gIAS by incorporating tortuosity, lateral path lengthening, and IAS connectivity. We measured and spatially mapped these geometric IAS traits for 19 Bromeliaceae species with Crassulacean acid metabolism (CAM) or C3 photosynthetic pathways using x-ray microcomputed tomography imaging and a novel computational approach. We found substantial variation in porosity (0.04-0.73 m3 m-3), tortuosity (1.09-3.33 m2 m-2), lateral path lengthening (1.12-3.19 m m-1), and IAS connectivity (0.81-0.97 m2 m-2) across all bromeliad leaves. The revised gIAS model predicted significantly lower gIAS in CAM (0.01-0.19 mol m-2 s-1 bar-1) than in C3 (0.41-2.38 mol m-2 s-1 bar-1) plants due to a coordinated decline in these IAS traits. Our reevaluated equation also generally predicted lower gIAS values than the former one. Moreover, we observed high spatial heterogeneity in these IAS geometric traits throughout the mesophyll, especially within CAM leaves. Our data show that IAS traits that better capture the 3D complexity of leaves strongly influence gIAS and that the impact of the IAS on mesophyll conductance should be carefully considered with respect to leaf anatomy. We provide a simple function to estimate tortuosity and lateral path lengthening in the absence of access to imaging tools such as x-ray microcomputed tomography or other novel 3D image-processing techniques.

Inhibition of OsSWEET11 function in mesophyll cells improves resistance of rice to sheath blight disease

Pathogen-host interaction is a complicated process; pathogens mainly infect host plants to acquire nutrients, especially sugars. Rhizoctonia solani, the causative agent of sheath blight disease, is a major pathogen of rice. However, it is not known how this pathogen obtains sugar from rice plants. In this study, we found that the rice sugar transporter OsSWEET11 is involved in the pathogenesis of sheath blight disease. Quantitative real-time polymerase chain reaction (qRT-PCR) and β-d-glucuronidase expression analyses showed that R. solani infection significantly enhanced OsSWEET11 expression in leaves amongst the clade III SWEET members. The analyses of transgenic plants revealed that Ossweet11 mutants were less susceptible, whereas plants overexpressing OsSWEET11 were more susceptible, to sheath blight compared with wild-type controls, but the yield of OsSWEET11 mutants and overexpressors was reduced. SWEETs become active on oligomerization. Split-ubiquitin yeast two-hybrid, bimolecular fluorescence complementation and co-immunoprecipitation assays showed that mutated OsSWEET11 interacted with normal OsSWEET11. In addition, expression of conserved residue mutated AtSWEET1 inhibited normal AtSWEET1 activity. To analyse whether inhibition of OsSWEET11 function in mesophyll cells is related to defence against this disease, mutated OsSWEET11 was expressed under the control of the Rubisco promoter, which is specific for green tissues. The resistance of transgenic plants to sheath blight disease, but not other disease, was improved, whereas yield production was not obviously affected. Overall, these results suggest that R. solani might acquire sugar from rice leaves by the activation of OsSWEET11 expression. The plants can be protected from infection by manipulation of the expression of OsSWEET11 without affecting the crop yield.

Ratio of sugar concentrations in the phloem sap and the cytosol of mesophyll cells in different tree species as an indicator of the phloem loading mechanism

Sucrose concentration in phloem sap was several times higher than in the cytosol of mesophyll cells. The results suggest that phloem loading involves active steps in the analyzed tree species. Phloem loading in source leaves is a key step for carbon partitioning and passive symplastic loading has been proposed for several tree species. However, experimental evidence to prove the potential for sucrose diffusion from mesophyll to phloem is rare. Here, we analyzed three tree species (two angiosperms, Fagus sylvatica, Magnolia kobus, and one gymnosperm, Gnetum gnemon) to investigate the proposed phloem loading mechanism. For this purpose, the minor vein structure and the sugar concentrations in phloem sap as well as in the subcellular compartments of mesophyll cells were investigated. The analyzed tree species belong to the open type minor vein subcategory. The sucrose concentration in the cytosol of mesophyll cells ranged between 75 and 165 mM and was almost equal to the vacuolar concentration. Phloem sap could be collected from F. sylvatica and M. kobus and the concentration of sucrose in phloem sap was about five- and 11-fold higher, respectively, than in the cytosol of mesophyll cells. Sugar exudation of cut leaves was decreased by p-chloromercuribenzenesulfonic acid, an inhibitor of sucrose-proton transporter. The results suggest that phloem loading of sucrose in the analyzed tree species involves active steps, and apoplastic phloem loading seems more likely.

Diffusion of CO2 across the Mesophyll-Bundle Sheath Cell Interface in a C4 Plant with Genetically Reduced PEP Carboxylase Activity

Phosphoenolpyruvate carboxylase (PEPC), localized to the cytosol of the mesophyll cell, catalyzes the first carboxylation step of the C₄ photosynthetic pathway. Here, we used RNA interference to target the cytosolic photosynthetic PEPC isoform in Setaria viridis and isolated independent transformants with very low PEPC activities. These plants required high ambient CO₂ concentrations for growth, consistent with the essential role of PEPC in C₄ photosynthesis. The combination of estimating direct CO₂ fixation by the bundle sheath using gas-exchange measurements and modeling C₄ photosynthesis with low PEPC activity allowed the calculation of bundle sheath conductance to CO₂ diffusion (g_bs ) in the progeny of these plants. Measurements made at a range of temperatures suggested no or negligible effect of temperature on g_bs depending on the technique used to calculate g_bs Anatomical measurements revealed that plants with reduced PEPC activity had reduced cell wall thickness and increased plasmodesmata (PD) density at the mesophyll-bundle sheath (M-BS) cell interface, whereas we observed little difference in these parameters at the mesophyll-mesophyll cell interface. The increased PD density at the M-BS interface was largely driven by an increase in the number of PD pit fields (cluster of PDs) rather than an increase in PD per pit field or the size of pit fields. The correlation of g_bs with bundle sheath surface area per leaf area and PD area per M-BS area showed that these parameters and cell wall thickness are important determinants of g_bs It is intriguing to speculate that PD development is responsive to changes in C₄ photosynthetic flux.

From reconstruction to C4 metabolic engineering: A case study for overproduction of polyhydroxybutyrate in bioenergy grasses

The compartmentalization of C₄ plants increases photosynthetic efficiency, while constraining how material and energy must flow in leaf tissues. To capture this metabolic phenomenon, a generic plant metabolic reconstruction was replicated into four connected spatiotemporal compartments, namely bundle sheath (B) and mesophyll (M) across the day and night cycle. The C₄ leaf model was used to explore how amenable polyhydroxybutyrate (PHB) production is with these four compartments working cooperatively. A strategic pattern of metabolite conversion and exchange emerged from a systems-level network that has very few constraints imposed; mainly the sequential two-step carbon capture in mesophyll, then bundle sheath and photosynthesis during the day only. The building of starch reserves during the day and their mobilization during the night connects day and night metabolism. Flux simulations revealed that PHB production did not require rerouting of metabolic pathways beyond what is already utilised for growth. PHB yield was sensitive to photoassimilation capacity, availability of carbon reserves, ATP maintenance, relative photosynthetic activity of B and M, and type of metabolites exchanged in the plasmodesmata, but not sensitive towards compartmentalization. Hence, the compartmentalization issues currently encountered are likely to be kinetic or thermodynamic limitations rather than stoichiometric.

Mesophyll Cells Are the Main Site of Abscisic Acid Biosynthesis in Water-Stressed Leaves

The hormone abscisic acid (ABA) plays a critical role in enhancing plant survival during water deficit. Recent molecular evidence suggests that ABA is synthesized in the phloem companion cells and guard cells. However, the nature of cell turgor and water status in these two cell types cannot easily account for the rapid, water status-triggered ABA biosynthesis observed in leaves. Here, we utilize the unique foliar anatomies of an angiosperm (Hakea lissosperma) and four conifer species (Saxegothaea conspicua, Podocarpus latifolius, Cephalotaxus harringtonii, and Amentotaxus formosana) in which the mesophyll can be isolated from the vascular tissue to identify the main site of ABA biosynthesis in water-stressed leaves. In all five species tested, considerable ABA biosynthesis occurred in mesophyll tissue that had been separated from vascular tissue. In addition, the removal of the epidermis from the mesophyll in two conifer species had no impact on the observed increase in ABA levels under water deficit. Our results suggest that mesophyll cells are the predominant location of water deficit-triggered ABA biosynthesis in the leaf.

Cell wall properties in Oryza sativa influence mesophyll CO2 conductance

Diffusion of CO₂ from the leaf intercellular air space to the site of carboxylation (g_m ) is a potential trait for increasing net rates of CO₂ assimilation (A_net ), photosynthetic efficiency, and crop productivity. Leaf anatomy plays a key role in this process; however, there are few investigations into how cell wall properties impact g_m and A_net . Online carbon isotope discrimination was used to determine g_m and A_net in Oryza sativa wild-type (WT) plants and mutants with disruptions in cell wall mixed-linkage glucan (MLG) production (CslF6 knockouts) under high- and low-light growth conditions. Cell wall thickness (T_cw ), surface area of chloroplast exposed to intercellular air spaces (S_c ), leaf dry mass per area (LMA), effective porosity, and other leaf anatomical traits were also analyzed. The g_m of CslF6 mutants decreased by 83% relative to the WT, with c. 28% of the reduction in g_m explained by S_c . Although A_net /LMA and A_net /Chl partially explained differences in A_net between genotypes, the change in cell wall properties influenced the diffusivity and availability of CO₂ . The data presented here indicate that the loss of MLG in CslF6 plants had an impact on g_m and demonstrate the importance of cell wall effective porosity and liquid path length on g_m .

Leaf blade structure of Verbesina macrophylla (Cass.) F. S. Blake (Asteraceae): ontogeny, duct secretion mechanism and essential oil composition

Secretory structures are common in Asteraceae, where they exhibit a high degree of morphological diversity. The species Verbesina macrophylla, popularly known as assa-peixe, is native to Brazil where it is widely used for medicinal purposes. Despite its potential medical importance, there have been no studies of the anatomy of this species, especially its secretory structures and secreted compounds. This study examined leaves of V. macrophylla with emphasis on secretory structures and secreted secondary metabolites. Development of secretory ducts and the mechanism of secretion production are described for V. macrophylla using ultrastructure, yield and chemical composition of its essential oils. Verbesina macrophylla has a hypostomatic leaf blade with dorsiventral mesophyll and secretory ducts associated with vascular bundles of schizogenous origin. Histochemistry identified the presence of lipids, terpenes, alkaloids and mucopolysaccharides. Ultrastructure suggests that the secretion released into the duct lumen is produced in plastids of transfer cells, parenchymal sheath cells and stored in vacuoles in these cells and duct epithelial cells. The essential oil content was 0.8%, and its major components were germacrene D, germacrene D-4-ol, β-caryophyllene, bicyclogermacrene and α-cadinol. Secretory ducts of V. macrophylla are squizogenous. Substances identified in tissues suggest that both secretions stored in the ducts and in adjacent parenchyma cells are involved in chemical defence. The essential oil is rich in sesquiterpenes, with germacrene D and its derivatives being notable components.

Eudicots from severely phosphorus-impoverished environments preferentially allocate phosphorus to their mesophyll

Plants allocate nutrients to specific leaf cell types, with commelinoid monocots preferentially allocating phosphorus (P) to the mesophyll and calcium (Ca) to the epidermis, whereas the opposite is thought to occur in eudicots. However, Proteaceae from severely P-impoverished habitats present the same P-allocation pattern as monocots. This raises the question of whether preferential P allocation to mesophyll cells is a phylogenetically conserved trait, exclusive to commelinoid monocots and a few Proteaceae, or a trait that has evolved multiple times to allow plants to cope with very low soil P availability. We analysed the P-allocation patterns of 16 species from 10 genera, eight families and six orders within three major clades of eudicots across different P-impoverished environments in Australia and Brazil, using elemental X-ray mapping to quantitatively determine leaf cell-specific nutrient concentrations. Many of the analysed species showed P-allocation patterns that differed substantially from that expected for eudicots. Instead, P-allocation patterns were strongly associated with the P availability in the natural habitat of the species, suggesting a convergent evolution of P-allocation patterns at the cellular level, with P limitation as selective pressure and without a consistent P-allocation pattern within eudicots. Here, we show that most eudicots from severely P-impoverished environments preferentially allocated P to their mesophyll. We surmise that this preferential P allocation to photosynthetically active cells might contribute to the very high photosynthetic P-use efficiency of species adapted to P-impoverished habitats.

Changes in chromatin accessibility between Arabidopsis stem cells and mesophyll cells illuminate cell type-specific transcription factor networks

Cell differentiation is driven by changes in the activity of transcription factors (TFs) and subsequent alterations in transcription. To study this process, differences in TF binding between cell types can be deduced by probing chromatin accessibility. We used cell type-specific nuclear purification followed by the assay for transposase-accessible chromatin (ATAC-seq) to delineate differences in chromatin accessibility and TF regulatory networks between stem cells of the shoot apical meristem (SAM) and differentiated leaf mesophyll cells in Arabidopsis thaliana. Chromatin accessibility profiles of SAM stem cells and leaf mesophyll cells were very similar at a qualitative level, yet thousands of regions having quantitatively different chromatin accessibility were also identified. Analysis of the genomic regions preferentially accessible in each cell type identified hundreds of overrepresented TF-binding motifs, highlighting sets of TFs that are probably important for each cell type. Within these sets, we found evidence for extensive co-regulation of target genes by multiple TFs that are preferentially expressed in each cell type. Interestingly, the TFs within each of these cell type-enriched sets also showed evidence of extensively co-regulating each other. We further found that preferentially accessible chromatin regions in mesophyll cells tended to also be substantially accessible in the stem cells, whereas the converse was not true. This observation suggests that the generally higher accessibility of regulatory elements in stem cells might contribute to their developmental plasticity. This work demonstrates the utility of cell type-specific chromatin accessibility profiling for the rapid development of testable models of regulatory control differences between cell types.

Co-Compartmentation of Terpene Biosynthesis and Storage via Synthetic Droplet

Traditional bioproduct engineering focuses on pathway optimization, yet is often complicated by product inhibition, downstream consumption, and the toxicity of certain products. Here, we present the co-compartmentation of biosynthesis and storage via a synthetic droplet as an effective new strategy to improve the bioproduct yield, with squalene as a model compound. A hydrophobic protein was designed and introduced into the tobacco chloroplast to generate a synthetic droplet for terpene storage. Simultaneously, squalene biosynthesis enzymes were introduced to chloroplasts together with the droplet-forming protein to co-compartmentalize the biosynthesis and storage of squalene. The strategy has enabled a record yield of squalene at 2.6 mg/g fresh weight without compromising plant growth. Confocal fluorescent microscopy imaging, stimulated Raman scattering microscopy, and droplet composition analysis confirmed the formation of synthetic storage droplet in chloroplast. The co-compartmentation of synthetic storage droplet with a targeted metabolic pathway engineering represents a new strategy for enhancing bioproduct yield.

Loss of LHCI system affects LHCII re-distribution between thylakoid domains upon state transitions

LHCI, the peripheral antenna system of Photosystem I, includes four light-harvesting proteins (Lhca1-Lhca4) in higher plants, all of which are devoid in the Arabidopsis thaliana knock-out mutant ΔLhca. PSI absorption cross-section was reduced in the mutant, thus affecting the redox balance of the photosynthetic electron chain and resulting in a more reduced PQ with respect to the wild type. ΔLhca plants developed compensatory response by enhancing LHCII binding to PSI. However, the amplitude of state transitions, as measured from changes of chlorophyll fluorescence in vivo, was unexpectedly low than the high level of PSI-LHCII supercomplex established. In order to elucidate the reasons for discrepancy, we further analyzed state transition in ΔLhca plants. The STN7 kinase was fully active in the mutant as judged from up-regulation of LHCII phosphorylation in state II. Instead, the lateral heterogeneity of thylakoids was affected by lack of LHCI, with LHCII being enriched in stroma membranes with respect to the wild type. Re-distribution of this complex affected the overall fluorescence yield of thylakoids already in state I and minimized changes in RT fluorescence yield when LHCII did connect to PSI reaction center. We conclude that interpretation of chlorophyll fluorescence analysis of state transitions becomes problematic when applied to mutants whose thylakoid architecture is significantly modified with respect to the wild type.

Proteaceae from phosphorus-impoverished habitats preferentially allocate phosphorus to photosynthetic cells: An adaptation improving phosphorus-use efficiency

Plants allocate nutrients to specific leaf cell types; eudicots are thought to predominantly allocate phosphorus (P) to epidermal/bundle sheath cells. However, three Proteaceae species have been shown to preferentially allocate P to mesophyll cells instead. These Proteaceae species are highly adapted to P-impoverished habitats, with exceptionally high photosynthetic P-use efficiencies (PPUE). We hypothesized that preferential allocation of P to photosynthetic mesophyll cells is an important trait in species adapted to extremely P-impoverished habitats, contributing to their high PPUE. We used elemental X-ray mapping to determine leaf cell-specific nutrient concentrations for 12 Proteaceae species, from habitats of strongly contrasting soil P concentrations, in Australia, Brazil, and Chile. We found that only species from extremely P-impoverished habitats preferentially allocated P to photosynthetic mesophyll cells, suggesting it has evolved as an adaptation to their extremely P-impoverished habitat and that it is not a family-wide trait. Our results highlight the possible role of soil P in driving the evolution of ecologically relevant nutrient allocation patterns and that these patterns cannot be generalized across families. Furthermore, preferential allocation of P to photosynthetic cells may provide new and exciting strategies to improve PPUE in crop species.

Metabolic Flux Engineering of Cembratrien-ol Production in Both the Glandular Trichome and Leaf Mesophyll in Nicotiana tabacum

Cembratrien-ol synthase (CBTS) catalyzes the first step in cembranoid biosynthesis, producing cembratrien-ols in plant trichomes. In our previous study, microarray transcriptomes between leaves with trichomes and leaves without trichomes showed that an NtCBTS2 gene was expressed exclusively and abundantly in trichomes. Here, two NtCBTS2 isogenes (NtCBTS2a and NtCBTS2b), derived from a diploid genome donor, Nicotiana sylvestris, were identified from N. tabacum. Both genes were expressed primarily in trichomes, with relatively decreased transcription in flowers and stems, and faint expression in roots, and no expression was detected in leaves lacking trichomes. To demonstrate the feasibility of producing natural product cembratrien-ols in tobacco mesophylls, the mesophylls of 35S:NtCBTS2b transgenic tobacco plants were used in the analysis, suggesting that constitutive expression of NtCBTS2b led to the cembratrien-ol production in mesophylls. Overexpression of NtCBTS2b using either Cauliflower mosaic virus (CaMV) 35S or trichome-specific Cyt P450 oxygenase (CYP) promoters greatly increased aphid resistance by promoting the accumulation of CBT-ols, increased the secretory cell growth in glandular trichomes and increased the levels of various physiological measures, including sugar esters, gibberellins, and cembranoid production. Meanwhile, specifically overexpressing NtCBTS2b in glandular trichomes could most efficiently promote aphid resistance in tobacco plants. Notably, our results indicate the feasibility of utilizing bio-engineering to produce large amounts of CBT-ols, and modify significantly the composition of naturally produced CBT-ols and CBT-diols, thereby promoting aphid resistance in plants.

The response of mesophyll conductance to short-term variation in CO2 in the C4 plants Setaria viridis and Zea mays

Mesophyll conductance (gm) limits rates of C3 photosynthesis but little is known about its role in C4 photosynthesis. If gm were to limit C4 photosynthesis, it would likely be at low CO2 concentrations (pCO2). However, data on C4-gm across ranges of pCO2 are scarce. We describe the response of C4-gm to short-term variation in pCO2, at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite gm calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO2. In both species, gm increased with decreasing pCO2. Including a finite gm resulted in either no change or increased ϕ compared with values calculated with infinite gm depending on whether the observed 13C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of gm and ϕ. At pCO2 below ambient, the photosynthetic rate was limited by CO2 availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO2, CA further constrained photosynthesis. High gm could increase CO2 assimilation at low pCO2 and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.

OsIDD2, a zinc finger and INDETERMINATE DOMAIN protein, regulates secondary cell wall formation

Previously, we found 123 transcription factors (TFs) as candidate regulators of secondary cell wall (SCW) formation in rice by using phylogenetic and co-expression network analyses. Among them, we examined in this work the role of OsIDD2, a zinc finger and indeterminate domain (IDD) family TF. Its overexpressors showed dwarfism, fragile leaves, and decreased lignin content, which are typical phenotypes of plants defective in SCW formation, whereas its knockout plants showed slightly increased lignin content. The RNA-seq and quantitative reverse transcription polymerase chain reaction analyses confirmed that some lignin biosynthetic genes were downregulated in the OsIDD2-overexpressing plants, and revealed the same case for other genes involved in cellulose synthesis and sucrose metabolism. The transient expression assay using rice protoplasts revealed that OsIDD2 negatively regulates the transcription of genes involved in lignin biosynthesis, cinnamyl alcohol dehydrogenase 2 and 3 (CAD2 and 3), and sucrose metabolism, sucrose synthase 5 (SUS5), whereas an AlphaScreen assay, which can detect the interaction between TFs and their target DNA sequences, directly confirmed the interaction between OsIDD2 and the target sequences located in the promoter regions of CAD2 and CAD3. Based on these observations, we conclude that OsIDD2 is negatively involved in SCW formation and other biological events by downregulating its target genes.

Quantitative mesophyll parameters rather than whole-leaf traits predict response of C3 steppe plants to aridity

We studied the impact of aridity on leaf and mesophyll traits in dominant and very abundant plant species of Eurasian steppe plant communities. We covered a 500-km latitudinal gradient across three vegetation zones in the Volga region of southern European Russia. Whole-leaf traits, volumetric fractions of leaf tissues, quantitative parameters of photosynthetic cells and chloroplasts, and chlorophyll, carbon (C) and nitrogen (N) contents were analyzed and related to plant functional type (PFT), type of mesophyll anatomy, phylogeny and climate aridity. The proportions of prevailing PFTs in the communities, such as C₃ monocots, C₃ dicots with dorsiventral and isopalisade anatomy and C₄ dicots, changed with increasing aridity which influenced the whole-leaf parameters and tissue composition in the leaf. Leaf mass per unit area and leaf thickness slightly increased along the aridity gradient, but the most significant changes were observed in the mesophyll. Mesophyll cell surface area, chloroplast number and chloroplast surface area per unit leaf area were higher in C₃ plants growing in the desert steppe compared with those of the forest steppe, while chlorophyll content per single chloroplast and per unit N content as well chlorophyll a/b ratio decreased. Our results identify a suite of mesophyll traits as a typical 'syndrome' of increasingly drought-adapted steppe plants.

Calibration matters: On the procedure of using the chlorophyll fluorescence method to estimate mesophyll conductance

Estimates of mesophyll conductance (g_m), when calculated from chlorophyll fluorescence, are uncertain, especially when the photosystem II (PSII) operating efficiency is measured from the traditional single saturation pulse methodology. The multiphase flash method has recently been recommended to replace the single saturation pulse method, allowing a more reliable estimation of g_m. Also, many researchers still directly use the PSII operating efficiency to derive linear electron transport rate J (that is required to estimate g_m), without appropriate calibration using measurements under non-photorespiratory conditions. Here we demonstrate for tomato and rice that (i) using the multiphase flash method did not yield realistic estimates of g_m if no calibration was conducted; and (ii) using the single saturation pulse method still gave reasonable estimates of g_m when calibration based on the non-photorespiratory measurements was properly conducted. Therefore, conducting calibration based on data under non-photorespiratory conditions was indispensable for a reliable estimation of g_m, regardless whether the multiphase flash or the single saturation pulse method was used for measuring the PSII operating efficiency. Other issues related to the procedure of using the chlorophyll fluorescence method to estimate g_m were discussed.

Toward multifaceted roles of sucrose in the regulation of stomatal movement

Plant atmospheric CO₂ fixation depends on the aperture of stomatal pores at the leaf epidermis. Stomatal aperture or closure is regulated by changes in the metabolism of the two surrounding guard cells, which respond directly to environmental and internal cues such as mesophyll-derived metabolites. Sucrose has been shown to play a dual role during stomatal movements. The sucrose produced in the mesophyll cells can be transported to the vicinity of the guard cells via the transpiration stream, inducing closure in periods of high photosynthetic rate. By contrast, sucrose breakdown within guard cells sustains glycolysis and glutamine biosynthesis during light-induced stomatal opening. Here, we provide an update regarding the role of sucrose in the regulation of stomatal movement highlighting recent findings from metabolic and systems biology studies. We further explore how sucrose-mediated mechanisms of stomatal movement regulation could be useful to understand evolution of stomatal physiology among different plant groups.

On the contributions of photorespiration and compartmentation to the contrasting intramolecular 2H profiles of C3 and C4 plant sugars

Compartmentation of C₄ photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C₃ plants is predicted to have hydrogen isotopic consequences for metabolites at both molecular and site-specific levels. Molecular-level evidence was recently reported (Zhou et al., 2016), but evidence at the site-specific level is still lacking. We propose that such evidence exists in the contrasting ²H distribution profiles of glucose samples from naturally grown C₃, C₄ and CAM plants: photorespiration contributes to the relative ²H enrichment in H⁵ and relative ²H depletion in H¹ & H⁶ (the average of the two pro-chiral Hs and in particular H^6,pro-R) in C₃ glucose, while ²H-enriched C₃ mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H⁴; export of (transferable hydrogen atoms of) NADPH from C₄ mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of ²H-relatively unenriched BS cellular water contribute to the relative depletion of H⁴ & H⁵ respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C₄ bundle sheath and mesophyll cells contributes to the relative enrichment in H¹ & H⁶ (in particular H^6,pro-R) in C₄ glucose.

De novo assembly and comparative analysis of the transcriptome of embryogenic callus formation in bread wheat (Triticum aestivum L.)

BACKGROUND

During asexual reproduction the embryogenic callus can differentiate into a new plantlet, offering great potential for fostering in vitro culture efficiency in plants. The immature embryos (IMEs) of wheat (Triticum aestivum L.) are more easily able to generate embryogenic callus than mature embryos (MEs). To understand the molecular process of embryogenic callus formation in wheat, de novo transcriptome sequencing was used to generate transcriptome sequences from calli derived from IMEs and MEs after 3d, 6d, or 15d of culture (DC).

RESULTS

In total, 155 million high quality paired-end reads were obtained from the 6 cDNA libraries. Our de novo assembly generated 142,221 unigenes, of which 59,976 (42.17%) were annotated with a significant Blastx against nr, Pfam, Swissprot, KOG, KEGG, GO and COG/KOG databases. Comparative transcriptome analysis indicated that a total of 5194 differentially expressed genes (DEGs) were identified in the comparisons of IME vs. ME at the three stages, including 3181, 2085 and 1468 DEGs at 3, 6 and 15 DC, respectively. Of them, 283 overlapped in all the three comparisons. Furthermore, 4731 DEGs were identified in the comparisons between stages in IMEs and MEs. Functional analysis revealed that 271transcription factor (TF) genes (10 overlapped in all 3 comparisons of IME vs. ME) and 346 somatic embryogenesis related genes (SSEGs; 35 overlapped in all 3 comparisons of IME vs. ME) were differentially expressed in at least one comparison of IME vs. ME. In addition, of the 283 overlapped DEGs in the 3 comparisons of IME vs. ME, excluding the SSEGs and TFs, 39 possessed a higher rate of involvement in biological processes relating to response to stimuli, in multi-organism processes, reproductive processes and reproduction. Furthermore, 7 were simultaneously differentially expressed in the 2 comparisons between the stages in IMEs, but not MEs, suggesting that they may be related to embryogenic callus formation. The expression levels of genes, which were validated by qRT-PCR, showed a high correlation with the RNA-seq value.

CONCLUSIONS

This study provides new insights into the role of the transcriptome in embryogenic callus formation in wheat, and will serve as a valuable resource for further studies addressing embryogenic callus formation in plants.

Biochemical and transcriptomic analyses of drought stress responses of LY1306 tobacco strain

This study aimed to investigate drought resistance of the LY1306 tobacco strain. Seedlings of tobacco strains LY1306, ZhongYan 100 (ZY100) and Hong Hua Da Jin Yuan (HHDJY) were treated with polyethylene glycol (PEG)-6000 to induce osmotic stress. As validation, water-deficit-induced drought treatments, including mild drought (MD; watering 1.5 L/week) and severe drought (SD, without watering) were carried out. Changes in cell morphology, leaf water potential, antioxidant enzyme activity, as well as contents of malondialdehyde (MDA) and proline were determined for each treatment. Transcriptome sequencing was performed for the seedlings treated with 15% PEG-6000. No obvious changes were observed in morphology of LY1306 and ZY100 under osmotic or drought stress; whereas, visible wilting was observed in HHDJY. Superoxide dismutase and peroxidase activities of LY1036 and ZY100 under osmotic stress were significantly higher than those of HHDJY. Under SD, the MDA content of LY1306 was significantly lower and the proline content of LY1306 was significantly higher than that of HHDJY. Differential genes between LY1306, ZY100 and HHDJY were enriched in functions about alpha-linolenic acid, and arginine and proline metabolisms. LY1306 could increase its antioxidant enzyme activities and proline accumulation in response to drought stress, probably by regulating drought resistance-related pathways and genes.

Metabolism within the specialized guard cells of plants

Contents 1018 I. 1018 II. 1019 III. 1022 IV. 1025 V. 1026 VI. 1029 1030 References 1030 SUMMARY: Stomata are leaf epidermal structures consisting of two guard cells surrounding a pore. Changes in the aperture of this pore regulate plant water-use efficiency, defined as gain of C by photosynthesis per leaf water transpired. Stomatal aperture is actively regulated by reversible changes in guard cell osmolyte content. Despite the fact that guard cells can photosynthesize on their own, the accumulation of mesophyll-derived metabolites can seemingly act as signals which contribute to the regulation of stomatal movement. It has been shown that malate can act as a signalling molecule and a counter-ion of potassium, a well-established osmolyte that accumulates in the vacuole of guard cells during stomatal opening. By contrast, their efflux from guard cells is an important mechanism during stomatal closure. It has been hypothesized that the breakdown of starch, sucrose and lipids is an important mechanism during stomatal opening, which may be related to ATP production through glycolysis and mitochondrial metabolism, and/or accumulation of osmolytes such as sugars and malate. However, experimental evidence supporting this theory is lacking. Here we highlight the particularities of guard cell metabolism and discuss this in the context of the guard cells themselves and their interaction with the mesophyll cells.

Components of mesophyll resistance and their environmental responses: A theoretical modelling analysis

Mesophyll resistance (r_m ), stomatal resistance, and biochemical limitations are recognized as three critical factors limiting leaf photosynthesis. Contrary to the expectation of being a constant, r_m not only varies with light and CO₂ conditions but also shows different responses among species. To elucidate the mechanistic basis of these responses, we derived an analytical model of r_m , which incorporates various anatomical and biochemical factors including permeabilities of cell wall and chloroplast envelope to CO₂ and HCO₃^- , carbonic anhydrase activities in cytosol and stroma, Rubisco activities, and relative location of mitochondria and chloroplast. The robustness of this model was confirmed by comparing the predicted r_m and its components to numerical models developed at cell and leaf levels, which incorporate detailed 3-dimensional cell and leaf anatomies, CO₂ hydration and diffusion processes from intercellular air space to stroma, and CO₂ fixation by Rubisco. A combination of these model analyses shows that the varying r_m is influenced by four biochemical factors: (a) nonuniform photosynthesis status across the leaf, (b) photorespiration and respiration, (c) bicarbonate leakage on the chloroplast envelope, and (d) hydration activity in cytosol and stroma. This study provides a theoretical framework to study components of r_m and their responses to environmental perturbations.

Leaf photosynthetic rate and mesophyll cell anatomy changes during ontogenesis in backcrossed indica × japonica rice inbred lines

The high-yielding indica rice variety, 'Takanari', has the high rate of leaf photosynthesis compared with the commercial japonica varieties. Among backcrossed inbred lines from a cross between 'Takanari' and a japonica variety, 'Koshihikari', two lines, BTK-a and BTK-b, showed approximately 20% higher photosynthetic rate than that of 'Takanari' for a flag leaf at full heading. This is a highest recorded rate of rice leaf photosynthesis. Here, the timing and cause of the increased leaf photosynthesis in the BTK lines were investigated by examining the photosynthesis and related parameters, as well as mesophyll cell anatomy during ontogenesis. Their photosynthetic rate was greater than that of 'Takanari' in the 13th leaf, as well as the flag leaf, but there were no differences in the 7th and 10th leaves. There were no consistent differences in the stomatal conductance, or the leaf nitrogen and Rubisco contents in the 13th and flag leaves. The total surface area of mesophyll cells per leaf area (TA_mes) in the 13th and flag leaves increased significantly in the BTK lines due to the increased number and developed lobes of mesophyll cells compared with in 'Takanari'. The mesophyll conductance (g _m) became greater in the BTK lines compared with 'Takanari' in the flag leaves but not in the 10th leaves. A close correlation was observed between TA_mes and g _m. We concluded that the increased mesophyll conductance through the development of mesophyll cells during the reproductive period is a probable cause of the greater photosynthetic rate in the BTK lines.