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

Engineering nitrogen and carbon fixation for next-generation plants

Improving plant nitrogen (N) and carbon (C) acquisition and assimilation is a major challenge for global agriculture, food security, and ecological sustainability. Emerging synthetic biology techniques, including directed evolution, artificial intelligence (AI)-guided enzyme design, and metabolic engineering, have opened new avenues for optimizing nitrogenase to fix atmospheric N2 in plants, engineering Rhizobia or other nitrogen-fixing bacteria for symbiotic associations with both legume and nonlegume crops, and enhancing carbon fixation to improve photosynthetic efficiency and source-to-sink assimilate fluxes. Here, we discuss the potential for engineering nitrogen fixation and carbon fixation mechanisms in plants, from rational and AI-driven optimization of nitrogen and carbon fixation cycles. Furthermore, we discuss strategies for modifying source-to-sink relationships to promote robust growth in extreme conditions, such as arid deserts, saline-alkaline soils, or even extraterrestrial environments like Mars. The combined engineering of N and C pathways promises a new generation of crops with enhanced productivity, resource-use efficiency, and resilience. Finally, we explore future perspectives, focusing on the integration of enzyme engineering via directed evolution and computational design to accelerate metabolic innovation in plants.

Tonoplast sugar transporters coordinately regulate tomato fruit development and quality

Fruit yield and quality are antagonistically regulated traits in tomato. An excessive focus on increasing yield often leads to a decline in quality. Achieving a delicate balance between high yield and desirable fruit quality is a challenging aspect of tomato breeding. In this study, we discovered that disrupting the function of the tomato tonoplast sugar transporter 3a (TST3a) significantly enhances both fruit weight and flavor. Three TSTs have been identified in tomato, namely SlTST1, SlTST3a, and SlTST3b; they possess the same sugar transport specificity for fructose and glucose and redundantly control cell expansion during fruit development. Sugar accumulation in sltst mutants correlates significantly with fruit size and flavor. The enlarged fruits in sltst3a mutants result from sugar accumulation due to the increased abundance of SlTST1 at the tonoplast coupled with the highest sugar transport capacity of SlTST1 among the three SlTSTs. Further experiments established that SlTST3a prevents the localization of SlTST1 to the tonoplast by inhibiting its interaction with VH1-interacting kinase (SlVIK). Mutation of SlTST3a in cultivated tomato can enhance both tomato fruit size and sugar content. Our findings offer potential avenues for simultaneously improving fruit quality and yield, providing valuable insights into the mechanisms underlying sugar storage during fruit development.

Gene Pyramiding Strategies for Sink Size and Source Capacity for High-Yield Japonica Rice Breeding

In Japan, high-yielding indica rice cultivars such as 'Habataki', 'Takanari', and 'Hokuriku 193' have been bred, and many genes related to the high-yield traits have been isolated from these and other indica cultivars. Many such genes are expected to be effective in increasing the yield of japonica rice, including those that increase sink size. It has been expected that high-yielding japonica rice could be bred by introducing sink-size genes into the genetic background of japonica cultivars such as 'Koshihikari', which show strong cold tolerance, have good taste characteristics, and fetch a high price. However, the corresponding near-isogenic lines did not necessarily produce high yields when tested in the field. In this review, we summarize information on the major high-yield-related rice genes and discuss pyramiding strategies to further increase the yield of japonica rice. In parallel with increasing sink size, source capacity needs to be increased by increasing photosynthetic rate per unit leaf area (single leaf photosynthesis), improving canopy structure, and increasing translocation capacity during the ripening stage. To implement these strategies, innovative breeding methodologies that efficiently produce the combinations of desired alleles are required.

A Guide to Metabolic Network Modeling for Plant Biology

Plants produce a diverse array of compounds that play crucial roles in growth, in development, and in responses to abiotic and biotic stresses. Understanding the fluxes within metabolic pathways is essential for guiding strategies aimed at directing metabolism for crop improvement and the plant natural product industry. Over the past decade, metabolic network modeling has emerged as a predominant tool for the integration, quantification, and prediction of the spatial and temporal distribution of metabolic flows. In this review, we present the primary methods for constructing mathematical models of metabolic systems and highlight recent achievements in plant metabolism using metabolic modeling. Furthermore, we discuss current challenges in applying network flux analysis in plants and explore the potential use of machine learning technologies in plant metabolic modeling. The practical application of mathematical modeling is expected to provide significant insights into the structure and regulation of plant metabolic networks.

The Modulation of Growth and Metabolism in Solanum lycopersicum Contrast With the Leaf-Specific Regulation of Wild Tomato Species

Plant organs harbour diverse components that connect their physiology to the whole organism. The turnover of metabolites may be higher in some organs than in others, triggering differential growth patterns throughout the organism. We revealed that Solanum lycopersicum exhibits more coordinated growth and physiology across the entire plant compared to wild tomato species. Specifically, young leaves of S. lycopersicum develop more slowly than mature leaves, whereas wild species do not exhibit this pattern. Wild tomato Solanum pennellii displays young leaves with higher photosynthetic rates than mature leaves. Consequently, sucrose metabolism in S. pennellii is quite similar between young and mature leaves, while expression patterns of circadian clock genes differ significantly between leaves of different ages. Additionally, we demonstrated that introducing alleles related to tomato domestication into the wild tomato Solanum pimpinellifolium promotes coordinated growth between young and mature leaves, resulting in similar patterns to those observed in S. lycopersicum. Collectively, S. lycopersicum appears to exhibit more coordinated regulation of growth and metabolism, and understanding this process is likely fundamental to explaining its elevated harvest index.

PROCERA interacts with JACKDAW in gibberellin-enhanced source-sink sucrose partitioning in tomato

Proper regulation of the source-sink relationship is an effective way to increase crop yield. Gibberellin (GA) is an important regulator of plant growth and development, and physiological evidence has demonstrated that GA can promote source-sink sucrose partitioning. However, the underlying molecular mechanism remains unclear. Here, we used a combination of physiological and molecular approaches to identify the components involved in GA-enhanced source-sink sucrose partitioning in tomato (Solanum lycopersicum). GA treatment increased the sucrose export rate from source leaves and the sucrose level in young leaves (sink organ). GA-mediated enhancement of source-sink sucrose partitioning depended on SlPROCERA (SlPRO), the DELLA protein in tomato. Sucrose transporter 1 (SlSUT1) was involved in phloem sucrose loading. SlJACKDAW (SlJKD) was identified as an interaction partner of SlPRO. SlJKD negatively regulated the sucrose export rate from source leaves and could directly bind to the promoter of SlSUT1 and repress its expression, while SlPRO enhanced the transcription repression function of SlJKD. This study reveals the molecular mechanism by which GA promotes source-sink sucrose partitioning in tomato and provides potential targets for source-sink relationship optimization.

Sink-source driven metabolic acclimation of winter oilseed rape leaves (Brassica napus L.) to drought

The crop cycle of winter oilseed rape (WOSR) incorporates source-to-sink remobilisation during the vegetative stage as a principal factor influencing the ultimate seed yield. These processes are supported by the coordinated activity of the plant's central metabolism. However, climate change-induced drought will affect the metabolic acclimation of WOSR sink/source relationships at this vegetative stage, with consequences that remain to be determined. In this study, we subjected WOSR to severe soil dehydration for 18 days and analysed the physiological and metabolic acclimation of sink and source leaves along the kinetics in combination with measurements of enzymatic activities and transcript levels. Overall, the acclimation of WOSR to drought led to subtle regulations of central metabolism in relation to leaf growth and Pro-induced osmotic adjustment. Notably, sink leaves drastically reduced their growth and transiently accumulated starch. Subsequent starch degradation correlated with the induction of beta-amylases, sucrose transporters, pyrroline-5-carboxylate synthases and proline accumulation. The functioning of the tricarboxylic acid cycle was also altered in sink leaves, as evidenced by variations in citrate, malate and associated enzymatic activities. The metabolic origin of Pro in sink leaves is discussed in relation to Pro accumulation in source leaves and the up-regulation of amino acid permease 1 and glutamine synthetase genes.

The metal tolerance protein OsMTP11 facilitates cadmium sequestration in the vacuoles of leaf vascular cells for restricting its translocation into rice grains

Rice (Oryza sativa) provides >20% of the consumed calories in the human diet. However, rice is also a leading source of dietary cadmium (Cd) that seriously threatens human health. Deciphering the genetic network that underlies the grain-Cd accumulation will benefit the development of low-Cd rice and mitigate the effects of Cd accumulation in the rice grain. In this study, we identified a QTL gene, OsCS1, which is allelic to OsMTP11 and encodes a protein sequestering Cd in the leaf during vegetative growth and preventing Cd from being translocated to the grain after heading in rice. OsCS1 is predominantly expressed in leaf vascular parenchyma cells, where it binds to a vacuole-sorting receptor protein OsVSR2 and is translocated intracellularly from the trans-Golgi network to pre-vacuolar compartments and then to the vacuole. In this trafficking process, OsCS1 actively transports Cd into the endomembrane system and sequesters it in the vacuoles. There are natural variations in the promoter of OsCS1 between the indica and japonica rice subspecies. Duplication of a G-box-like motif in the promoter region of the superior allele of OsCS1 from indica rice enhances the binding of the transcription factor OsIRO2 to the OsCS1 promoter, thereby promoting OsCS1 expression. Introgression of this allele into commercial rice varieties could significantly lower grain-Cd levels compared to the inferior allele present in japonica rice. Collectively, our findings offer new insights into the genetic control of leaf-to-grain Cd translocation and provide a novel gene and its superior allele for the genetic improvement of low-Cd variety in rice.

Overexpression of Cassava MeSTP7 Promotes Arabidopsis Seedling Development

The sugar transporter (STP) gene family is a key regulator of plant development, which is crucial for the efficient transport and utilization of sugars during plant growth and development. In this study, we identified the MeSTP7 gene, which is highly expressed in cassava fibrous roots, early storage roots, and under hormonal treatment, including IAA, MeJA, ABA, and GA3, and abiotic stressors, such as mannitol and NaCl. A strong response was observed with exoqenous IAA. Transfecting MeSTP7 into Arabidopsis promoted early seedling growth, particularly in lateral root development. The content of endogenous hormones (IAA and MeJA) as well as soluble sugars (sucrose, fructose, and glucose) was elevated in transgenic Arabidopsis. Hormone treatments with IAA, MeJA, GA3, and ABA on transgenic Arabidopsis revealed that transgenic Arabidopsis responded positively to added 20 μM IAA. They also exhibited co-induced regulation of lateral root formation by GA3, MeJA, and ABA. qRT-PCR analysis showed that overexpression of MeSTP7 upregulated the expression of IAA14, ARF7, and ARF19 in Arabidopsis. Under IAA treatment, the expression of these genes was similarly upregulated but downregulated under MeJA treatment. These results suggest that MeSTP7 may promote Arabidopsis seedling development by increasing the content of sucrose, glucose, and fructose in roots, which in turn influences IAA-based hormonal signaling.

Metabolic imaging in living plants: A promising field for chemical exchange saturation transfer (CEST) MRI

Magnetic resonance imaging (MRI) is a versatile technique in the biomedical field, but its application to the study of plant metabolism in vivo remains challenging because of magnetic susceptibility problems. In this study, we report the establishment of chemical exchange saturation transfer (CEST) for plant MRI. This method enables noninvasive access to the metabolism of sugars and amino acids in complex sink organs (seeds, fruits, taproots, and tubers) of major crops (maize, barley, pea, potato, sugar beet, and sugarcane). Because of its high signal detection sensitivity and low susceptibility to magnetic field inhomogeneities, CEST analyzes heterogeneous botanical samples inaccessible to conventional magnetic resonance spectroscopy. The approach provides unprecedented insight into the dynamics and distribution of sugars and amino acids in intact, living plant tissue. The method is validated by chemical shift imaging, infrared microscopy, chromatography, and mass spectrometry. CEST is a versatile and promising tool for studying plant metabolism in vivo, with many applications in plant science and crop improvement.

Source-sink synergy is the key unlocking sweet potato starch yield potential

Sweet potato starch is in high demand globally for food and industry. However, starch content is negatively correlated with fresh yield. It is urgent to uncover the genetic basis and molecular mechanisms underlying the starch yield of sweet potato. Here we systematically explore source-sink synergy-mediated sweet potato starch yield formation: the production, loading, and transport of photosynthates in leaves, as well as their unloading and allocation in storage roots, lead to starch content divergence between sweet potato varieties. Moreover, we find that six haplotypes of IbPMA1 encoding a plasma membrane H+-ATPase are significantly linked with starch accumulation. Overexpression of IbPMA1 in sweet potato results in significantly increased starch and sucrose contents, while its knockdown exhibits an opposing effect. Furthermore, a basic helix-loop-helix (bHLH) transcription factor IbbHLH49 directly targets IbPMA1 and activates its transcription. Overexpression of IbbHLH49 notably improves source-sink synergy-mediated fresh yield and starch accumulation in sweet potato. Both IbbHLH49 and IbPMA1 substantially influence sugar transport and starch biosynthesis in source and sink tissues. These findings expand our understanding of starch yield formation and provide strategies and candidate genes for high starch breeding in root and tuber crops.

UMAMIT44 is a key player in glutamate export from Arabidopsis chloroplasts

Selective partitioning of amino acids among organelles, cells, tissues, and organs is essential for cellular metabolism and plant growth. Nitrogen assimilation into glutamine and glutamate and de novo biosynthesis of most protein amino acids occur in chloroplasts; therefore, various transport mechanisms must exist to accommodate their directional efflux from the stroma to the cytosol and feed the amino acids into the extraplastidial metabolic and long-distance transport pathways. Yet, Arabidopsis (Arabidopsis thaliana) transporters functioning in plastidial export of amino acids remained undiscovered. Here, USUALLY MULTIPLE ACIDS MOVE IN AND OUT TRANSPORTER 44 (UMAMIT44) was identified and shown to function in glutamate export from Arabidopsis chloroplasts. UMAMIT44 controls glutamate homeostasis within and outside of chloroplasts and influences nitrogen partitioning from leaves to sinks. Glutamate imbalances in chloroplasts and leaves of umamit44 mutants impact cellular redox state, nitrogen and carbon metabolism, and amino acid (AA) and sucrose supply of growing sinks, leading to negative effects on plant growth. Nonetheless, the mutant lines adjust to some extent by upregulating alternative pathways for glutamate synthesis outside the plastids and by mitigating oxidative stress through the production of other amino acids and antioxidants. Overall, this study establishes that the role of UMAMIT44 in glutamate export from chloroplasts is vital for controlling nitrogen availability within source leaf cells and for sink nutrition, with an impact on growth and seed yield.

Transcriptional dynamics in source-sink tissues identifies molecular factors regulating the corm development process in saffron (Crocus sativus L.)

AIMS

Geophytic plants have evolved to develop underground storage organs (USO) in the active growing season to withstand harsh environments as well as to coordinate growth and reproduction when conditions are favourable. Saffron is an autumn flowering geophyte and an expensive spice crop restricted to certain geographical locations in the world. Saffron, being sterile, does not produce seeds and thus propagates only through corms, the quality of which determines its yield. Corm development in saffron is unexplored and the underlying molecular mechanism is still elusive. In this study, we performed an extensive characterisation of the transcriptional dynamics in the source (leaf) and sink (corm) tissues during corm development in saffron.

KEY RESULTS

Via morphological and transcriptome studies, we identified molecular factors regulating corm development process in saffron, which defined corm development into three stages: the initiation stage demonstrates enhanced vegetative growth aboveground and swelling of shoot base belowground due to active cell division & carbohydrate storage; the bulking stage comprises of increased source and sink strength, active photosynthesis, circadian gating and starch accumulation; the maturation stage represents reduced source and sink strength, lowered photosynthesis, sugar transport, starch synthesis and cell cycle arrest.

UTILITY

The global view of transcriptional changes in source and sink identifies similar and new molecular factors involved in the saffron corm development process compared to USO formation in other geophytes and provides a valuable resource for dissecting the molecular network underlying the corm development. We propose a hypothetical model based on data analysis, of how molecular factors via environmental cues can regulate the corm development process in saffron.

Plant Heterotrophic Cultures: No Food, No Growth

Plant cells are capable of uptaking exogenous organic substances. This inherited trait allows the development of heterotrophic cell cultures in various plants. The most common of them are Nicotiana tabacum and Arabidopsis thaliana. Plant cells are widely used in academic studies and as factories for valuable substance production. The repertoire of compounds supporting the heterotrophic growth of plant cells is limited. The best growth of cultures is ensured by oligosaccharides and their cleavage products. Primarily, these are sucrose, raffinose, glucose and fructose. Other molecules such as glycerol, carbonic acids, starch, and mannitol have the ability to support growth occasionally, or in combination with another substrate. Culture growth is accompanied by processes of specialization, such as elongation growth. This determines the pattern of the carbon budget. Culture ageing is closely linked to substrate depletion, changes in medium composition, and cell physiological rearrangements. A lack of substrate leads to starvation, which results in a decrease in physiological activity and the mobilization of resources, and finally in the loss of viability. The cause of the instability of cultivated cells may be the non-optimal metabolism under cultural conditions or the insufficiency of internal regulation.

The Sink-Source Relationship in Cucumber (Cucumis sativus L.) Is Modulated by DNA Methylation

The optimization of the sink-source relationship is of great importance for crop yield regulation. Cucumber is a typical raffinose family oligosaccharide (RFO)-transporting crop. DNA methylation is a common epigenetic modification in plants, but its role in sink-source regulation has not been demonstrated in RFO-translocating species. Here, whole-genome bisulfite sequencing (WGBS-seq) was conducted to compare the nonfruiting-node leaves (NFNLs) and leaves of fruit setting (FNLs) at the 12th node by removing all female flowers in other nodes of the two treatments. We found considerable differentially methylated genes enriched in photosynthesis and carbohydrate metabolic processes. Comparative transcriptome analysis between FNLs and NFNLs indicated that many differentially expressed genes (DEGs) with differentially methylated regions were involved in auxin, ethylene and brassinolide metabolism; sucrose metabolism; and RFO synthesis pathways related to sink-source regulation. Moreover, DNA methylation levels of six sink-source-related genes in the pathways mentioned above decreased in leaves after 5-aza-dC-2'-deoxycytidine (5-Aza-dC, a DNA methyltransferase inhibitor) treatment on FNLs, and stachyose synthase (CsSTS) gene expression, enzyme activity and stachyose content in RFO synthesis pathway were upregulated, thereby increasing fruit length and dry weight. Taken together, our findings proposed an up-to-date inference for the potential role of DNA methylation in the sink-source relationship, which will provide important references for further exploring the molecular mechanism of DNA methylation in improving the yield of RFO transport plants.

Multi-Omics Analysis Reveals Synergistic Enhancement of Nitrogen Assimilation Efficiency via Coordinated Regulation of Nitrogen and Carbon Metabolism by Co-Application of Brassinolide and Pyraclostrobin in Arabidopsis thaliana

Improving nitrogen (N) assimilation efficiency without yield penalties is important to sustainable food security. The chemical regulation approach of N assimilation efficiency is still less explored. We previously found that the co-application of brassinolide (BL) and pyraclostrobin (Pyr) synergistically boosted biomass and yield via regulating photosynthesis in Arabidopsis thaliana. However, the synergistic effect of BL and Pyr on N metabolism remains unclear. In this work, we examined the N and protein contents, key N assimilatory enzyme activities, and transcriptomic and metabolomic changes in the four treatments (untreated, BL, Pyr, and BL + Pyr). Our results showed that BL + Pyr treatment synergistically improved N and protein contents by 56.2% and 58.0%, exceeding the effects of individual BL (no increase) or Pyr treatment (36.4% and 36.1%). Besides synergistically increasing the activity of NR (354%), NiR (42%), GS (62%), and GOGAT (62%), the BL + Pyr treatment uniquely coordinated N metabolism, carbon utilization, and photosynthesis at the transcriptional and metabolic levels, outperforming the effects of individual BL or Pyr treatments. These results revealed that BL + Pyr treatments could synergistically improve N assimilation efficiency through improving N assimilatory enzyme activities and coordinated regulation of N and carbon metabolism. The identified genes and metabolites also informed potential targets and agrochemical combinations to enhance N assimilation efficiency.

Circadian regulation of metabolism across photosynthetic organisms

Circadian regulation produces a biological measure of time within cells. The daily cycle in the availability of light for photosynthesis causes dramatic changes in biochemical processes in photosynthetic organisms, with the circadian clock having crucial roles in adaptation to these fluctuating conditions. Correct alignment between the circadian clock and environmental day-night cycles maximizes plant productivity through its regulation of metabolism. Therefore, the processes that integrate circadian regulation with metabolism are key to understanding how the circadian clock contributes to plant productivity. This forms an important part of exploiting knowledge of circadian regulation to enhance sustainable crop production. Here, we examine the roles of circadian regulation in metabolic processes in source and sink organ structures of Arabidopsis. We also evaluate possible roles for circadian regulation in root exudation processes that deposit carbon into the soil, and the nature of the rhythmic interactions between plants and their associated microbial communities. Finally, we examine shared and differing aspects of the circadian regulation of metabolism between Arabidopsis and other model photosynthetic organisms, and between circadian control of metabolism in photosynthetic and non-photosynthetic organisms. This synthesis identifies a variety of future research topics, including a focus on metabolic processes that underlie biotic interactions within ecosystems.

Plant biochemical genetics in the multiomics era

Our understanding of plant biology has been revolutionized by modern genetics and biochemistry. However, biochemical genetics can be traced back to the foundation of Mendelian genetics; indeed, one of Mendel's milestone discoveries of seven characteristics of pea plants later came to be ascribed to a mutation in a starch branching enzyme. Here, we review both current and historical strategies for the elucidation of plant metabolic pathways and the genes that encode their component enzymes and regulators. We use this historical review to discuss a range of classical genetic phenomena including epistasis, canalization, and heterosis as viewed through the lens of contemporary high-throughput data obtained via the array of approaches currently adopted in multiomics studies.