Overexpression of an Agave Phosphoenolpyruvate Carboxylase Improves Plant Growth and Stress Tolerance

It has been challenging to simultaneously improve photosynthesis and stress tolerance in plants. Crassulacean acid metabolism (CAM) is a CO2-concentrating mechanism that facilitates plant adaptation to water-limited environments. We hypothesized that the ectopic expression of a CAM-specific phosphoenolpyruvate carboxylase (PEPC), an enzyme that catalyzes primary CO2 fixation in CAM plants, would enhance both photosynthesis and abiotic stress tolerance. To test this hypothesis, we engineered a CAM-specific PEPC gene (named AaPEPC1) from Agave americana into tobacco. In comparison with wild-type and empty vector controls, transgenic tobacco plants constitutively expressing AaPEPC1 showed a higher photosynthetic rate and biomass production under normal conditions, along with significant carbon metabolism changes in malate accumulation, the carbon isotope ratio δ13C, and the expression of multiple orthologs of CAM-related genes. Furthermore, AaPEPC1 overexpression enhanced proline biosynthesis, and improved salt and drought tolerance in the transgenic plants. Under salt and drought stress conditions, the dry weight of transgenic tobacco plants overexpressing AaPEPC1 was increased by up to 81.8% and 37.2%, respectively, in comparison with wild-type plants. Our findings open a new door to the simultaneous improvement of photosynthesis and stress tolerance in plants.

Transformation of the Plastid Genome in Tobacco: The Model System for Chloroplast Genome Engineering

The protocol we report here is based on biolistic delivery of transforming DNA to tobacco leaves, selection of transplastomic clones by spectinomycin or kanamycin resistance and regeneration of plants with uniformly transformed plastid genomes. Because the plastid genome of Nicotiana tabacum derives from Nicotiana sylvestris, and the two genomes are highly conserved, vectors developed for N. tabacum can be used in N. sylvestris. The tissue culture responses of N. tabacum cv. Petit Havana and N. sylvestris accession TW137 are similar. Plastid transformation in a subset of N. tabacum cultivars and in Nicotiana benthamiana requires adjustment of the tissue culture protocol. We describe updated vectors targeting insertions in the unique and repeated regions of the plastid genome, vectors suitable for regulated gene expression by the engineered PPR10 RNA binding protein as well as systems for marker gene excision.

Genome-wide identification and characterisation of Aquaporins in Nicotiana tabacum and their relationships with other Solanaceae species

BACKGROUND

Cellular membranes are dynamic structures, continuously adjusting their composition, allowing plants to respond to developmental signals, stresses, and changing environments. To facilitate transmembrane transport of substrates, plant membranes are embedded with both active and passive transporters. Aquaporins (AQPs) constitute a major family of membrane spanning channel proteins that selectively facilitate the passive bidirectional passage of substrates across biological membranes at an astonishing 108 molecules per second. AQPs are the most diversified in the plant kingdom, comprising of five major subfamilies that differ in temporal and spatial gene expression, subcellular protein localisation, substrate specificity, and post-translational regulatory mechanisms; collectively providing a dynamic transportation network spanning the entire plant. Plant AQPs can transport a range of solutes essential for numerous plant processes including, water relations, growth and development, stress responses, root nutrient uptake, and photosynthesis. The ability to manipulate AQPs towards improving plant productivity, is reliant on expanding our insight into the diversity and functional roles of AQPs.

RESULTS

We characterised the AQP family from Nicotiana tabacum (NtAQPs; tobacco), a popular model system capable of scaling from the laboratory to the field. Tobacco is closely related to major economic crops (e.g. tomato, potato, eggplant and peppers) and itself has new commercial applications. Tobacco harbours 76 AQPs making it the second largest characterised AQP family. These fall into five distinct subfamilies, for which we characterised phylogenetic relationships, gene structures, protein sequences, selectivity filter compositions, sub-cellular localisation, and tissue-specific expression. We also identified the AQPs from tobacco's parental genomes (N. sylvestris and N. tomentosiformis), allowing us to characterise the evolutionary history of the NtAQP family. Assigning orthology to tomato and potato AQPs allowed for cross-species comparisons of conservation in protein structures, gene expression, and potential physiological roles.

CONCLUSIONS

This study provides a comprehensive characterisation of the tobacco AQP family, and strengthens the current knowledge of AQP biology. The refined gene/protein models, tissue-specific expression analysis, and cross-species comparisons, provide valuable insight into the evolutionary history and likely physiological roles of NtAQPs and their Solanaceae orthologs. Collectively, these results will support future functional studies and help transfer basic research to applied agriculture.

Genome-wide identification and characterization of ABA receptor PYL/RCAR gene family reveals evolution and roles in drought stress in Nicotiana tabacum

BACKGROUND

Abscisic acid (ABA) is an important phytohormone for plant growth, development and responding to stresses such as drought, salinity, and pathogen infection. Pyrabactin Resistance 1 (PYR1)/PYR1-Like (PYL)/Regulatory Component of ABA Receptor (RCAR) (hereafter referred to as PYLs) has been identified as the ABA receptors. The PYL family members have been well studied in many plants. However, the members of PYL family have not been systematically identified at genome level in cultivated tobacco (Nicotiana tabacum) and its two ancestors. In this study, the phylogenic relationships, chromosomal distribution, gene structures, conserved motifs/regions, and expression profiles of NtPYLs were analyzed.

RESULTS

We identified 29, 11, 16 PYLs in the genomes of allotetraploid N. tabacum, and its two diploid ancestors N. tomentosiformis and N. sylvestris, respectively. The phylogenetic analysis revealed that NtPYLs can be divided into three subfamilies, and each NtPYL has one counterpart in N. sylvestris or N. tomentosiformis. Based on microarray analysis of NtPYL transcripts, four NtPYLs (from subfamily II, III), and five NtPYLs (from subfamily I) are highlighted as potential candidates for further functional characterization in N. tabacum seed development, response to ABA, and germination, and resistance to abiotic stresses, respectively. Interestingly, the expression profiles of members in the same NtPYL subfamily showed somehow similar patterns in tissues at different developmental stages and in leaves of seedlings under drought stress, suggesting particular NtPYLs might have multiple functions in both plant development and drought stress response.

CONCLUSIONS

NtPYLs are highlighted for important functions in seed development, germination and response to ABA, and particular in drought tolerance. This work will not only shed light on the PYL family in tobacco, but also provides some valuable information for functional characterization of ABA receptors in N. tabacum.

Identification and Characterization of the WOX Family Genes in Five Solanaceae Species Reveal Their Conserved Roles in Peptide Signaling

Members of the plant-specific WOX (WUSCHEL-related homeobox) transcription factor family have been reported to play important roles in peptide signaling that regulates stem cell maintenance and cell fate specification in various developmental processes. Even though remarkable advances have been made in studying WOX genes in Arabidopsis, little is known about this family in Solanaceae species. A total of 45 WOX members from five Solanaceae species were identified, including eight members from Solanum tuberosum, eight from Nicotiana tomentosiformis, 10 from Solanum lycopersicum, 10 from Nicotiana sylvestris and nine from Nicotiana tabacum. The newly identified WOX members were classified into three clades and nine subgroups based on phylogenetic analysis using three different methods. The patterns of exon-intron structure and motif organization of the WOX proteins agreed with the phylogenetic results. Gene duplication events and ongoing evolution were revealed by additional branches on the phylogenetic tree and the presence of a partial WUS-box in some non-WUS clade members. Gene expression with or without CLE (clavata3 (clv3)/embryo surrounding region-related) peptide treatments revealed that tobacco WOX genes showed similar or distinct expression patterns compared with their Arabidopsis homologues, suggesting either functional conservation or divergence. Expression of Nicotiana tabacum WUSCHEL (NtabWUS) in the organizing center could rescue the wus-1 mutant phenotypes in Arabidopsis, implying conserved roles of the Solanaceae WOX proteins in peptide-mediated regulation of plant development.

Nicotiana sylvestris calcineurin B-like protein NsylCBL10 enhances salt tolerance in transgenic Arabidopsis

KEY MESSAGE

Nicotiana sylvestris calcineurin B-like protein NsylCBL10 improves tolerance to high-salt stress through better maintenance of Na (+) balance. The calcineurin B-like (CBL) proteins represent a unique group of plant calcium sensors and play an important role in regulating the response of a plant cell to the stress. Although many studies have been made in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa) and poplar (Populus trichocarpa), the characterization and elucidation of the functions of CBLs in tobacco have not yet been reported. In this study, NsylCBL10, a CBL gene showing higher similarities to other CBL10 genes, was cloned from Nicotiana sylvestris. NsylCBL10 is expressed in most of the tobacco tissues, and the protein targets to the plasma membrane specifically. Over-expression of NsylCBL10 enhanced the salt tolerance of Arabidopsis wild type plants greatly, and rescued the high-salt-sensitive phenotype of Arabidopsis cbl10 mutant. The analysis of ion content indicated that over-expressing NsylCBL10 in plants is able to maintain a lower Na(+)/K(+) ratio in roots and higher Na(+)/K(+) ratio in shoots, compared with cbl10 mutant. The results suggest that NsylCBL10 might play an important role in response to high salinity stress in N. sylvestris, by keeping a better ionic homeostasis to reduce the damage of toxic ion to the plant cell.

Within-plant relationships among wounding, jasmonic acid, and nicotine implications for defence in Nicotiana sylvestris

In Nicotiana sylvestris Spegazzini and Comes (Solanaceae), we examined the relationships among wounding, endogenous leaf jasmonic acid (JA) pools, and whole-plain (WP) nicotine accumulation over a range of wounding intensities and spatial distributions, in order to explore optimal defence (OD) theory predictions. We quantitatively wounded one or four leaves and then quantified: (1) JA in damaged and undamaged leaves 90 min after wounding; (2) WP nicotine concentration after 5 d (the times when JA and nicotine attain the largest wound-induced concentrations). We find: (1) statistically significant, positive relationships on a leaf-by-leaf basis among the number of leaf punctures, endogenous leaf JA, and WP nicotine accumulation; (2) that young, undamaged leaves have a higher concentration of JA than do older, undamaged leaves, and produce a greater amount of JA per puncture than older leaves, but that all leaves have the same JA content (ng JA per leaf); and (3) that a damaged leaf produces less JA when other leaves in the canopy are wounded than when it is the onh wounded leaf in the canopy, but that when it is the only wounded leaf, the phylotactically adjacent, undamaged leaves do not increase their JA concentrations. The observation that younger leaves produce more JA per puncture than do older leaves is consistent with OD theory predictions. The observation that a small amount of damage localized to a single leaf is as effective as a larger amount of damage dispersed across the canopy in increasing leaf JA and WP nicotine accumulation shows the plant's ability to differentiate between dispersed and localized damage. Because the quantity of JA in a wounded leaf 90 min after wounding is a reliable indicator of the WP nicotine response to wounding, this trait provides insight into how plants integrate information about environmental insults and tailor their defence responses.

Autotoxicity and chemical defense: nicotine accumulation and carbon gain in solanaceous plants

We quantified the accumulation of and tolerance to exogenously-fed nicotine by monitoring photosynthetic capacity and growth in two nicotine producing species of Solanaceous plants (Nicotiana sylvestris andN. glauca) as well as two Solanaceous species (Datura stramonium andLycopersicon esculentum) that do not produce nicotine to examine the relationship between tolerence and the ability to produce nicotine in defensive quantities. SinceN. sylvestris uses nicotine as an inducible defense, we examined whether nicotine tolerance is induced by damage to examine further the relationship between nicotine tolerence and synthesis. All species were grown in a 1 mM nicotine-containing hydroponic solution. Reductions in the photosynthetic capacity of nicotine-fed plants were found in all species tested. Nicotine-producing species showed no greater tolerance as measured by photosynthetic capacity than the two non-producing species. Leaf damage marginally increased the tolerence ofN. sylvestris to exogeneouslyfed nicotine suggesting that photosynthetic tolerance is coordinated with nicotine production in this nicotine-producing species.N. glauca plants regained photosynthetic capacity after their accumulated nicotine was demethylated to form nornicotine. Leaf nicotine pools in the other three species did not decrease, suggesting that for these species alkaloid metabolism does not play a major role in tolerance. Tolerance, as measured by biomass gained, was higher in the two non-producing species than in the nicotine-producing species suggesting that nicotine may also be functioning as a growth regulator. These results do not support the hypothesis that tolerance is as important as biosynthetic ability in determining which species accumulate defensively significant quantities of nicotine.

Short-term damage-induced increases in tobacco alkaloids protect plants

Leaf damage significantly increases the alkaloid content in undamaged leaves on damaged field-grown wild tobacco plants. Although field-grown pot-bound plants fail to exhibit the same damage-induced increase in alkaloid content, the ability to respond to leaf damage is restored 6 days after removing plants from their pots. Freshly hatched Manduca sexta larvae reared individually in the laboratory on the high-alkaloid foliage of damaged plants released from their pots gain less weight and eat less (57.2% and 45.7% of controls, respectively) than larvae fed low-alkaloid foliage from undamaged released plants. Moreover, larvae grow equally well on the foliage of damaged and undamaged pot-bound plants. The higher chlorophyll contents characteristic of damaged released plants did not negate the effects of the increased alkaloid contents on larval growth. Undamaged leaves from undamaged field-grown plants stem-fed nicotine solutions had elevated leaf nicotine and nornicotine contents. Larvae reared on these artificially induced leaves gain only 38.5% of the weight gained by larvae reared on low-alkaloid foliage. These results demonstrate that damage-induced increases in leaf alkaloids protect induced foliage from attack and are sufficient to explain the decreased growth of caterpillars on the foliage of damaged plants.

The alkaloidal responses of wild tobacco to real and simulated herbivory

I compared the induced alkaloidal response in undamaged leaves of plants subjected to herbivory by the larvae of Manduca sexta and to different simulations of this herbivory; all herbivory treatments removed similar amounts of leaf mass. Although larval feeding induced a significant increase (2.2x) in alkaloid concentrations compared to undamaged plants, the alkaloid responses to larval feeding were significantly lower than the responses to an herbivory simulation (4x controls) which involved removing the same amount of leaf area from the same positions on the leaf, over a similar time period. Moreover, another herbivory simulation, identical in amount of leaf mass removed and duration of damage to the larval feeding, but without regard to spatial array of leaf damage, resulted in an alkaloidal response (5.5x controls) higher still than the previous herbivory simulation. In a second experiment the importance of leaf vein damage on the induced alkaloidal response was examined. Here, leaf removal that involved cutting leaf tissues from between secondary veins before removing the midrib, resulted in alkaloidal responses that were significantly lower (1.7x controls) than responses from leaf removal that involved cutting both veins and midribs along with the intervein tissues (2.6x controls). Vein damage alone did not produce a significant response. These results indicate that herbivory is difficult to simulate: that how a leaf is damaged can be as important as the magnitude of leaf damage in determining a plant's response to damage.