Exploring natural variation of photosynthetic, primary metabolism and growth parameters in a large panel of Capsicum chinense accessions

Understanding source-sink interactions: Progress in model plants and translational research to crops

Agriculture is facing a massive increase in demand per hectare as a result of an ever-expanding population and environmental deterioration. While we have learned much about how environmental conditions and diseases impact crop yield, until recently considerably less was known concerning endogenous factors, including within-plant nutrient allocation. In this review, we discuss studies of source-sink interactions covering both fundamental research in model systems under controlled growth conditions and how the findings are being translated to crop plants in the field. In this respect we detail efforts aimed at improving and/or combining C₃, C₄, and CAM modes of photosynthesis, altering the chloroplastic electron transport chain, modulating photorespiration, adopting bacterial/algal carbon-concentrating mechanisms, and enhancing nitrogen- and water-use efficiencies. Moreover, we discuss how modulating TCA cycle activities and primary metabolism can result in increased rates of photosynthesis and outline the opportunities that evaluating natural variation in photosynthesis may afford. Although source, transport, and sink functions are all covered in this review, we focus on discussing source functions because the majority of research has been conducted in this field. Nevertheless, considerable recent evidence, alongside the evidence from classical studies, demonstrates that both transport and sink functions are also incredibly important determinants of yield. We thus describe recent evidence supporting this notion and suggest that future strategies for yield improvement should focus on combining improvements in each of these steps to approach yield optimization.

Lack of coordination between stomatal and vein traits provides functional benefits to the dioecious tropical tree Myrsine coriacea

Climate change will affect the distribution of many tropical plant species. However, the understanding of how dioecious tropical species cope with different environmental conditions is still limited. To address this issue, we investigated how secondary trait attributes in populations of the dioecious tropical tree Myrsine coriacea change along an altitudinal gradient. Eighty individual plants (40 male and 40 female) were selected among seven natural populations. Leaf variation in morphological and stomatal traits, and carbon and nitrogen isotopic compositions were analyzed. Female plants had greater isotopic leaf carbon composition (δ¹³ C) and nitrogen content than male plants, increasing their carboxylation capacity. Plants of both sexes had smaller stomata, greater water-use efficiency (greater δ¹³ C), and greater nitrogen isotopic composition (δ¹⁵ N) at higher altitudes. They also showed lower δ¹⁵ N and had greater carbon: nitrogen ratios at lower altitudes. There was a lack of coordination between stomatal and vein traits, which was compensated for by variation in specific leaf areas. This mechanism was essential for increasing plant performance under the limiting conditions found by the species at higher altitudes.

Metabolic shifts during fruit development in pungent and non-pungent peppers

Fruit pungency is caused by the accumulation of capsaicinoids, secondary metabolites whose relation to primary metabolism remains unclear. We have selected ten geographically diverse accessions of Capsicum chinense Jacq with different pungency levels. A detailed metabolic profile was conducted in the fruit placenta and pericarp at 20, 45, and 60 days after anthesis aiming at increasing our understanding of the metabolic changes in these tissues across fruit development and their potential connection to capsaicin metabolism. Overall, despite the variation in fruit pungency among the ten accessions, the composition and metabolite levels in both placenta and pericarp were uniformly stable across accessions. Most of the metabolite variability occurred between the fruit developmental stages rather than among the accessions. Interestingly, different metabolite adjustments in the placenta were observed among pungent and non-pungent accessions, which seem to be related to differences in the genetic background. Furthermore, we observed high coordination between metabolites and capsaicin production in C. chinense fruits, suggesting that pungency in placenta is adjusted with primary metabolism.

Exogenous ethylene reduces growth via alterations in central metabolism and cell wall composition in tomato (Solanum lycopersicum)

Ethylene is a gaseous hormone with a well-established role in the regulation of plant growth and development. However, its role in the modulation of carbon assimilation and central metabolism remains unclear. Here, we investigated the morphophysiological and biochemical responses of tomato plants (Solanum lycopersicum) following the application of ethylene in the form of ethephon (CEPA - 2-chloroethylphosphonic acid), forcing the classical triple response phenotype. CEPA-treated plants were characterized by growth inhibition, as revealed by significant reductions in both shoot and root dry weights, coupled with a reduced number of leaves and lower specific leaf area. Growth inhibition was associated with a reduction in carbon assimilation due to both lower photosynthesis rates and stomatal conductance, coupled with impairments in carbohydrate turnover. Furthermore, exogenous ethylene led to the accumulation of cell wall compounds (i.e., cellulose and lignin) and phenolics, indicating that exposure to exogenous ethylene also led to changes in specialized metabolism. Collectively, our findings demonstrate that exogenous ethylene disrupts plant growth and leaf structure by affecting both central and specialized metabolism, especially that involved in carbohydrate turnover and cell wall biosynthesis, ultimately leading to metabolic responses that mimic stress situations.

Physiological and metabolic bases of increased growth in the tomato ethylene-insensitive mutant Never ripe: extending ethylene signaling functions

The tomato mutant Never ripe (Nr), a loss-of-function for the ethylene receptor SlETR3, shows enhanced growth, associated with increased carbon assimilation and a rewiring of the central metabolism. Compelling evidence has demonstrated the importance of ethylene during tomato fruit development, yet its role on leaf central metabolism and plant growth remains elusive. Here, we performed a detailed characterization of Never ripe (Nr) tomato, a loss-of-function mutant for the ethylene receptor SlETR3, known for its fruits which never ripe. However, besides fruits, the Nr gene is also constitutively expressed in vegetative tissues. Nr mutant showed a growth enhancement during both the vegetative and reproductive stage, without an earlier onset of leaf senescence, with Nr plants exhibiting a higher number of leaves and an increased dry weight of leaves, stems, roots, and fruits. At metabolic level, Nr also plays a significant role with the mutant showing changes in carbon assimilation, carbohydrates turnover, and an exquisite reprogramming of a large number of metabolite levels. Notably, the expression of genes related to ethylene signaling and biosynthesis are not altered in Nr. We assess our results in the context of those previously published for tomato fruits and of current models of ethylene signal transduction, and conclude that ethylene insensitivity mediated by Nr impacts the whole central metabolism at vegetative stage, leading to increased growth rates.

Using precision phenotyping to inform de novo domestication

An update on the use of precision phenotyping to assess the potential of lesser cultivated species as candidates for de novo domestication or similar development for future agriculture.

Source Strength Modulates Fruit Set by Starch Turnover and Export of Both Sucrose and Amino Acids in Pepper

Fruit set is an important yield-related parameter, which varies drastically due to genetic and environmental factors. Here, two commercial cultivars of Capsicum chinense (Biquinho and Habanero) were evaluated in response to light intensity (unshaded and shaded) and N supply (deficiency and sufficiency) to understand the role of source strength on fruit set at the metabolic level. We assessed the metabolic balance of primary metabolites in source leaves during the flowering period. Furthermore, we investigated the metabolic balance of the same metabolites in flowers to gain more insights into their influence on fruit set. Genotype and N supply had a strong effect on fruit set and the levels of primary metabolites, whereas light intensity had a moderate effect. Higher fruit set was mainly related to the export of both sucrose and amino acids from source leaves to flowers. Additionally, starch turnover in source leaves, but not in flowers, had a central role on the sucrose supply to sink organs at night. In flowers, our results not only confirmed the role of the daily supply of carbohydrates on fruit set but also indicated a potential role of the balance of amino acids and malate.

Nitrogen differentially modulates photosynthesis, carbon allocation and yield related traits in two contrasting Capsicum chinense cultivars

Yield-related traits of Capsicum chinense are highly dependent on coordination between vegetative and reproductive growth, since the formation of reproductive tissues occurs iteratively in new sympodial bifurcations. In this study, we used two C. chinense cultivars (Biquinho and Habanero), contrasting for fruit size and fruit set, to investigate the responses of nitrogen (N) deficiency and excess on growth, photosynthesis, carbon (C) and N metabolisms as well as yield-related traits. Both cultivars increased biomass allocation to leaves in conditions of higher N supply and exhibited a parabolic behavior for fruit biomass allocation. Plants growing under N-deficiency produced a lower number of flowers and heavier fruits. Contrarily, plants under high N condition tended to decrease their CO₂ assimilation rate, harvest index and fruit weight. Biquinho, the cultivar with lower fruit size and higher fruit set, was initially less affected by excess of N due to its continuous formation of new reproductive sinks in relation to Habanero (which has lower fruit set and higher fruit size). The results suggest that N amount influences sucrose supply to different organs and can differentially affect yield-related traits between Capsicum cultivars with contrasting source-sink relations.

Capsaicinoids: Pungency beyond Capsicum

Capsaicinoids are metabolites responsible for the appealing pungency of Capsicum (chili pepper) species. The completion of the Capsicum annuum genome has sparked new interest into the development of biotechnological applications involving the manipulation of pungency levels. Pungent dishes are already part of the traditional cuisine in many countries, and numerous health benefits and industrial applications are associated to capsaicinoids. This raises the question of how to successfully produce more capsaicinoids, whose biosynthesis is strongly influenced by genotype-environment interactions in fruits of Capsicum. In this Opinion article we propose that activating the capsaicinoid biosynthetic pathway in a more amenable species such as tomato could be the next step in the fascinating story of pungent crops.

The genetic architecture of photosynthesis and plant growth-related traits in tomato

To identify genomic regions involved in the regulation of fundamental physiological processes such as photosynthesis and respiration, a population of Solanum pennellii introgression lines was analyzed. We determined phenotypes for physiological, metabolic, and growth related traits, including gas exchange and chlorophyll fluorescence parameters. Data analysis allowed the identification of 208 physiological and metabolic quantitative trait loci with 33 of these being associated to smaller intervals of the genomic regions, termed BINs. Eight BINs were identified that were associated with higher assimilation rates than the recurrent parent M82. Two and 10 genomic regions were related to shoot and root dry matter accumulation, respectively. Nine genomic regions were associated with starch levels, whereas 12 BINs were associated with the levels of other metabolites. Additionally, a comprehensive and detailed annotation of the genomic regions spanning these quantitative trait loci allowed us to identify 87 candidate genes that putatively control the investigated traits. We confirmed 8 of these at the level of variance in gene expression. Taken together, our results allowed the identification of candidate genes that most likely regulate photosynthesis, primary metabolism, and plant growth and as such provide new avenues for crop improvement.

Natural genetic variation for morphological and molecular determinants of plant growth and yield

The rates of increase in yield of the main commercial crops have been steadily falling in many areas worldwide. This generates concerns because there is a growing demand for plant biomass due to the increasing population. Plant yield should thus be improved in the context of climate change and decreasing natural resources. It is a major challenge which could be tackled by improving and/or altering light-use efficiency, CO2 uptake and fixation, primary metabolism, plant architecture and leaf morphology, and developmental plant processes. In this review, we discuss some of the traits which could lead to yield increase, with a focus on how natural genetic variation could be harnessed. Moreover, we provide insights for advancing our understanding of the molecular aspects governing plant growth and yield, and propose future avenues for improvement of crop yield. We also suggest that knowledge accumulated over the last decade in the field of molecular physiology should be integrated into new ideotypes.