Plant SWEETs: from sugar transport to plant-pathogen interaction and more unexpected physiological roles

The link between changing in host carbon allocation and resistance to Magnaporthe oryzae: a possible tactic for mitigating the rice blast fungus

One of the most destructive diseases affecting rice is rice blast, which is brought on by the rice blast fungus Magnaporthe oryzae. The preventive measures, however, are not well established. To effectively reduce the negative effects of rice blasts on crop yields, it is imperative to comprehend the dynamic interactions between pathogen resistance and patterns of host carbon allocation. This review explores the relationship between variations in carbon allocation and rice plants' ability to withstand the damaging effects of M. oryzae. The review highlights potential strategies for altering host carbon allocation including transgenic, selective breeding, crop rotation, and nutrient management practices as a promising avenue for enhancing rice blast resistance. This study advances our knowledge of the interaction between plants' carbon allocation and M. oryzae resistance and provides stakeholders and farmers with practical guidance on mitigating the adverse effects of the rice blast globally. This information may be used in the future to create varieties that are resistant to M. oryzae.

Functional analysis of the key BrSWEET genes for sugar transport involved in the Brassica rapa-Plasmodiophora brassicae interaction

Plasmodiophora brassicae, the causative agent of clubroot disease, establishes a long-lasting parasitic relationship with its host by inducing the expression of sugar transporters. Previous studies have indicated that most BrSWEET genes in Chinese cabbage are up-regulated upon infection with P. brassicae. However, the key BrSWEET genes responsive to P. brassicae have not been definitively identified. In this study, we selected five BrSWEET genes and conducted a functional analysis of them. These five BrSWEET genes showed a notable up-regulation in roots after P. brassicae inoculation. Furthermore, these BrSWEET proteins were localized to the plasma membrane. Yeast functional complementation assays confirmed transport activity for glucose, fructose, or sucrose in four BrSWEETs, with the exception of BrSWEET2a. Mutants and silenced plants of BrSWEET1a, -11a, and -12a showed lower clubroot disease severity compared to wild-type plants, while gain-of-function Arabidopsis thaliana plants overexpressing these three BrSWEET genes exhibited significantly higher disease incidence and severity. Our findings suggested that BrSWEET1a, BrSWEET11a, and BrSWEET12a play pivotal roles in P. brassicae-induced gall formation, shedding light on the role of sugar transporters in host-pathogen interactions.

The clade III subfamily of OsSWEETs directly suppresses rice immunity by interacting with OsHMGB1 and OsHsp20L

The clade III subfamily of OsSWEETs includes transmembrane proteins necessary for susceptibility to bacterial blight (BB). These genes are targeted by the specific transcription activator-like effector (TALE) of Xanthomonas oryzae pv. oryzae and mediate sucrose efflux for bacterial proliferation. However, the mechanism through which OsSWEETs regulate rice immunity has not been fully elucidated. Here, we demonstrated that the cytosolic carboxyl terminus of OsSWEET11a/Xa13 is required for complementing susceptibility to PXO99 in IRBB13 (xa13/xa13). Interestingly, the C-terminus of ZmXa13, the maize homologue of OsSWEET11a/Xa13, could perfectly substitute for the C-terminus of OsSWEET11a/Xa13. Furthermore, OsSWEET11a/Xa13 interacted with the high-mobility group B1 (OsHMGB1) protein and the small heat shock-like protein OsHsp20L through the same regions in the C-terminus. Consistent with the physical interactions, knockdown or knockout of either OsHMGB1 or OsHsp20L caused an enhanced PXO99-resistant phenotype similar to that of OsSWEET11a/OsXa13. Surprisingly, the plants in which OsHMGB1 or OsHsp20L was repressed developed increased resistance to PXO86, PXO61 and YN24, which carry TALEs targeting OsSWEET14/Xa41 or OsSWEET11a/Xa13. Additionally, OsHsp20L can interact with all six members of clade III OsSWEETs, whereas OsHMGB1 can interact with five other members in addition to OsSWEET12. Overall, we revealed that OsHMGB1 and OsHsp20L mediate conserved BB susceptibility by interacting with clade III OsSWEETs, which are candidates for breeding broad-spectrum disease-resistant rice.

The maize sugar transporters ZmSWEET15a and ZmSWEET15b positively regulate salt tolerance in plants

The SWEETs (sugars will eventually be exported transporter) family comprises a class of recently identified sugar transporters that play diverse roles in regulating plant development. Beyond those fundamental functions, emerging evidence suggests that SWEETs may also be involved in plant stress responses, such as salt tolerance. However, the specific role of maize SWEETs in regulating salt tolerance remains unexplored. In this study, we demonstrate that two maize SWEET family members, ZmSWEET15a and ZmSWEET15b, are typical sugar transporters with seven transmembrane helices localized in the cell membrane. The heterologous expression of ZmSWEET15a and ZmSWEET15b in the yeast mutant strain confirms their role as sucrose transporters. Overexpression of ZmSWEET15a and ZmSWEET15b in Arabidopsis resulted in improved NaCl resistance and significant increase in seed germination rate compared to the wild type. Furthermore, by generating maize knockout mutants, we observe that the absence of ZmSWEET15a and ZmSWEET15b affects both plant growth and grain development. The salt treatment results indicate that the knockout mutants of these two genes are more sensitive to salt stress. Comparative analyses revealed that wild-type maize plants outperformed the knockout mutants in terms of growth parameters and physiological indices. Our findings unravel a novel function of ZmSWEET15a and ZmSWEET15b in the salt stress response, offering a theoretical foundation for enhancing maize salt resistance.

Genome-wide analysis of the SWEET gene family in Hemerocallis citrina and functional characterization of HcSWEET4a in response to salt stress

Sugars will be eventually effluxed transporters (SWEETs) have been confirmed to play diverse physiological roles in plant growth, development and stress response. However, the characteristics and functions of the SWEET genes in Hemerocallis citrina remain unclear and poorly elucidated. In this study, the whole genome of Hemerocallis citrina was utilized to conduct bioinformatics analysis and a total of 19 HcSWEET genes were successfully identified. Analysis of the physicochemical properties indicated dominant differences among these HcSWEETs. A phylogenetic analysis revealed that HcSWEET proteins can be divided into 4 clades ranging from Clade I to IV, where proteins within the same clade exhibited shared conserved motifs and gene structures. Five to six exons were contained in the majority of HcSWEET genes, which were unevenly distributed across 11 chromosomes. The gene duplication analysis showed the presence of 4 gene pairs. Comparative syntenic maps revealed that the HcSWEET gene family might present more closed homology in monocotyledons than dicotyledons. Cis-acting element analysis of HcSWEET genes indicated key responsiveness to various hormones, light, and stresses. Additionally, transcriptome sequencing analysis suggested that most HcSWEET genes had a relatively higher expression in roots, and HcSWEET4a was significantly up-regulated under salt stress. Overexpression further verified the possibility that HcSWEET4a was involved in response to salt stress, which provides novel insights and facilitates in-depth studies of the functional analysis of HcSWEETs in resistance to abiotic stress.

Light signaling regulates root-knot nematode infection and development via HY5-SWEET signaling

BACKGROUND

Meloidogyne incognita is one of the most important plant-parasitic nematodes and causes tremendous losses to the agricultural economy. Light is an important living factor for plants and pathogenic organisms, and sufficient light promotes root-knot nematode infection, but the underlying mechanism is still unclear.

RESULTS

Expression level and genetic analyses revealed that the photoreceptor genes PHY, CRY, and PHOT have a negative impact on nematode infection. Interestingly, ELONGATED HYPOCOTYL5 (HY5), a downstream gene involved in the regulation of light signaling, is associated with photoreceptor-mediated negative regulation of root-knot nematode resistance. ChIP and yeast one-hybrid assays supported that HY5 participates in plant-to-root-knot nematode responses by directly binding to the SWEET negative regulatory factors involved in root-knot nematode resistance.

CONCLUSIONS

This study elucidates the important role of light signaling pathways in plant resistance to nematodes, providing a new perspective for RKN resistance research.

Characterization and Expression Analysis of Sugar Transporters through Partial Least Square Structural Equation Model (PLS-SEM) Revealed Their Role in Pepper (Capsicum annuum L.)

Pepper (Capsicum annuum L.) is one of the most important economic crops in the world. By controlling the transport and distribution of photosynthetic products between cells and organs, sugar transporters are widely involved in growth and development, environmental adaptation, and microbial interactions. The present study was carried out at the genome-wide level to systematically characterize sugar transporters. As a result, 50 MST, 3 SUT, and 29 SWEET genes were identified and classified. The expression pattern of sugar transporters in pepper was analyzed by transcriptomic data. The expression properties of sugar transporters were further explored in pepper varieties with significant differences in weight, shape, and pungency. It was shown that the pepper sugar transporter genes had obvious spatiotemporal specific expression characteristics and exhibited variety-specific expression preferences. We focus on analyzing candidate genes that may be involved in fruit development and expansion. We further explore the response of pepper sugar transporters to adversity stress using a structural equation model. Finally, we found that the MST, SUT, and SWEET families are collectively involved in balancing pepper resistance to abiotic stress by coordinating the expression strengths of different family members. Our study may contribute to the functional study of pepper sugar transporter genes and create the prospect of utilizing sugar transporter gene resources to improve pepper variety.

Deep mutational scanning reveals sequence to function constraints for SWEET family transporters

Protein science is entering a transformative phase enabled by deep mutational scans that provide an unbiased view of the residue level interactions that mediate function. However, it has yet to be extensively used to characterize the mutational and evolutionary landscapes of plant proteins. Here, we apply the method to explore sequence-function relationships within the sugar transporter AtSWEET13. DMS results describe how mutational interrogation throughout different regions of the protein affects AtSWEET13 abundance and transport function. Our results identify novel transport-enhancing mutations that are validated using the FRET sensor assays. Extending DMS results to phylogenetic analyses reveal the role of transmembrane helix 4 (TM4) which makes the SWEET family transporters distinct from prokaryotic SemiSWEETs. We show that transmembrane helix 4 is intolerant to motif swapping with other clade-specific SWEET TM4 compositions, despite accommodating single point-mutations towards aromatic and charged polar amino acids. We further show that the transfer learning approaches based on physics and ML based In silico variant prediction tools have limited utility for engineering plant proteins as they were unable to reproduce our experimental results. We conclude that DMS can produce datasets which, when combined with the right predictive computational frameworks, can direct plant engineering efforts through derivative phenotype selection and evolutionary insights.

Potato E3 ubiquitin ligase StRFP1 positively regulates late blight resistance by degrading sugar transporters StSWEET10c and StSWEET11

Potato (Solanum tuberosum) is the fourth largest food crop in the world. Late blight, caused by oomycete Phytophthora infestans, is the most devastating disease threatening potato production. Previous research has shown that StRFP1, a potato Arabidopsis Tóxicos en Levadura (ATL) family protein, positively regulates late blight resistance via its E3 ligase activity. However, the underlying mechanism is unknown. Here, we reveal that StRFP1 is associated with the plasma membrane (PM) and undergoes constitutive endocytic trafficking. Its PM localization is essential for inhibiting P. infestans colonization. Through in vivo and in vitro assays, we investigated that StRFP1 interacts with two sugar transporters StSWEET10c and StSWEET11 at the PM. Overexpression (OE) of StSWEET10c or StSWEET11 enhances P. infestans colonization. Both StSWEET10c and StSWEET11 exhibit sucrose transport ability in yeast, and OE of StSWEET10c leads to an increased sucrose content in the apoplastic fluid of potato leaves. StRFP1 ubiquitinates StSWEET10c and StSWEET11 to promote their degradation. We illustrate a novel mechanism by which a potato ATL protein enhances disease resistance by degrading susceptibility (S) factors, such as Sugars Will Eventually be Exported Transporters (SWEETs). This offers a potential strategy for improving disease resistance by utilizing host positive immune regulators to neutralize S factors.

Co-overexpression of SWEET sucrose transporters modulates sucrose synthesis and defence responses to enhance immunity against bacterial blight in rice

Enhancing carbohydrate export from source to sink tissues is considered to be a realistic approach for improving photosynthetic efficiency and crop yield. The rice sucrose transporters OsSUT1, OsSWEET11a and OsSWEET14 contribute to sucrose phloem loading and seed filling. Crucially, Xanthomonas oryzae pv. oryzae (Xoo) infection in rice enhances the expression of OsSWEET11a and OsSWEET14 genes, and causes leaf blight. Here we show that co-overexpression of OsSUT1, OsSWEET11a and OsSWEET14 in rice reduced sucrose synthesis and transport leading to lower growth and yield but reduced susceptibility to Xoo relative to controls. The immunity-related hypersensitive response (HR) was enhanced in the transformed lines as indicated by the increased expression of defence genes, higher salicylic acid content and presence of HR lesions on the leaves. The results suggest that the increased expression of OsSWEET11a and OsSWEET14 in rice is perceived as a pathogen (Xoo) attack that triggers HR and results in constitutive activation of plant defences that are related to the signalling pathways of pathogen starvation. These findings provide a mechanistic basis for the trade-off between plant growth and immunity because decreased susceptibility against Xoo compromised plant growth and yield.

The Arabidopsis AtSWEET13 transporter discriminates sugars by selective facial and positional substrate recognition

Transporters are targeted by endogenous metabolites and exogenous molecules to reach cellular destinations, but it is generally not understood how different substrate classes exploit the same transporter's mechanism. Any disclosure of plasticity in transporter mechanism when treated with different substrates becomes critical for developing general selectivity principles in membrane transport catalysis. Using extensive molecular dynamics simulations with an enhanced sampling approach, we select the Arabidopsis sugar transporter AtSWEET13 as a model system to identify the basis for glucose versus sucrose molecular recognition and transport. Here we find that AtSWEET13 chemical selectivity originates from a conserved substrate facial selectivity demonstrated when committing alternate access, despite mono-/di-saccharides experiencing differing degrees of conformational and positional freedom throughout other stages of transport. However, substrate interactions with structural hallmarks associated with known functional annotations can help reinforce selective preferences in molecular transport.

Interplay between phosphorylation and oligomerization tunes the conformational ensemble of SWEET transporters

SWEET sugar transporters are desirable biotechnological targets for improving plant growth. One engineering strategy includes modulating how SWEET transporters are regulated. Phosphorylation and oligomerization have been shown to positively regulate SWEET function, leading to increased sugar transport activity. However, constitutive phosphorylation may not be beneficial to plant health under basal conditions. Structural and mechanistic understanding of the interplay between phosphorylation and oligomerization in functional regulation of SWEETs remains limited. Using extensive molecular dynamics simulations coupled with Markov state models, we demonstrate the thermodynamic and kinetic effects of SWEET phosphorylation and oligomerization using OsSWEET2b as a model. We report that the beneficial effects of these SWEET regulatory mechanisms bias outward-facing states and improved extracellular gating, which complement published experimental findings. Our results offer molecular insights to SWEET regulation and may guide engineering strategies throughout the SWEET transport family.

From acidity to sweetness: a comprehensive review of carbon accumulation in grape berries

Most of the carbon found in fruits at harvest is imported by the phloem. Imported carbon provide the material needed for the accumulation of sugars, organic acids, secondary compounds, in addition to the material needed for the synthesis of cell walls. The accumulation of sugars during fruit development influences not only sweetness but also various parameters controlling fruit composition (fruit "quality"). The accumulation of organic acids and sugar in grape berry flesh cells is a key process for berry development and ripening. The present review presents an update of the research on grape berry development, anatomical structure, sugar and acid metabolism, sugar transporters, and regulatory factors.

Investigation of the Inhibitory Effects of Illicium verum Essential Oil Nanoemulsion on Fusarium proliferatum via Combined Transcriptomics and Metabolomics Analysis

Fusarium proliferatum is the main pathogen that causes Panax notoginseng root rot. The shortcomings of strong volatility and poor water solubility of Illicium verum essential oil (EO) limit its utilization. In this study, we prepared traditional emulsion (BDT) and nanoemulsion (Bneo) of I. verum EO by ultrasonic method with Tween-80 and absolute ethanol as solvents. The chemical components of EO, BDT, and Bneo were identified by gas chromatography-mass spectrometry (GC-MS) and the antifungal activity and mechanism were compared. The results show that Bneo has good stability and its particle size is 34.86 nm. The contents of (-) -anethole and estragole in Bneo were significantly higher than those in BDT. The antifungal activity against F. proliferatum was 5.8-fold higher than BDT. In the presence of I. verum EO, the occurrence of P. notoginseng root rot was significantly reduced. By combining transcriptome and metabolomics analysis, I. verum EO was found to be involved in the mutual transformation of pentose and glucuronic acid, galactose metabolism, streptomycin biosynthesis, carbon metabolism, and other metabolic pathways of F. proliferatum, and it interfered with the normal growth of F. proliferatum to exert antifungal effects. This study provide a theoretical basis for expanding the practical application of Bneo.

Ethylene inhibits photosynthesis via temporally distinct responses in tomato plants

Ethylene is a volatile plant hormone that regulates many developmental processes and responses toward (a)biotic stress. Studies have shown that high levels of ethylene repress vegetative growth in many important crops, including tomato (Solanum lycopersicum), possibly by inhibiting photosynthesis. We investigated the temporal effects of ethylene on young tomato plants using an automated ethylene gassing system to monitor the physiological, biochemical, and molecular responses through time course RNA-seq of a photosynthetically active source leaf. We found that ethylene evokes a dose-dependent inhibition of photosynthesis, which can be characterized by 3 temporally distinct phases. The earliest ethylene responses that marked the first phase and occurred a few hours after the start of the treatment were leaf epinasty and a decline in stomatal conductance, which led to lower light perception and CO2 uptake, respectively, resulting in a rapid decline of soluble sugar levels (glucose, fructose). The second phase of the ethylene effect was marked by low carbohydrate availability, which modulated plant energy metabolism to adapt by using alternative substrates (lipids and proteins) to fuel the TCA cycle. Long-term continuous exposure to ethylene led to the third phase, characterized by starch and chlorophyll breakdown, which further inhibited photosynthesis, leading to premature leaf senescence. To reveal early (3 h) ethylene-dependent regulators of photosynthesis, we performed a ChIP-seq experiment using anti-ETHYLENE INSENSITIVE 3-like 1 (EIL1) antibodies and found several candidate transcriptional regulators. Collectively, our study revealed a temporal sequence of events that led to the inhibition of photosynthesis by ethylene and identified potential transcriptional regulators responsible for this regulation.

Sugar transporters spatially organize microbiota colonization along the longitudinal root axis of Arabidopsis

Plant roots are functionally heterogeneous in cellular architecture, transcriptome profile, metabolic state, and microbial immunity. We hypothesized that axial differentiation may also impact spatial colonization by root microbiota along the root axis. We developed two growth systems, ArtSoil and CD-Rhizotron, to grow and then dissect Arabidopsis thaliana roots into three segments. We demonstrate that distinct endospheric and rhizosphere bacterial communities colonize the segments, supporting the hypothesis of microbiota differentiation along the axis. Root metabolite profiling of each segment reveals differential metabolite enrichment and specificity. Bioinformatic analyses and GUS histochemistry indicate microbe-induced accumulation of SWEET2, 4, and 12 sugar uniporters. Profiling of root segments from sweet mutants shows altered spatial metabolic profiles and reorganization of endospheric root microbiota. This work reveals the interdependency between root metabolites and microbial colonization and the contribution of SWEETs to spatial diversity and stability of microbial ecosystem.

Transcriptional responses to salinity-induced changes in cell wall morphology of the euryhaline diatom Pleurosira laevis

Diatoms are unicellular algae with morphologically diverse silica cell walls, which are called frustules. The mechanism of frustule morphogenesis has attracted attention in biology and nanomaterials engineering. However, the genetic regulation of the morphology remains unclear. We therefore used transcriptome sequencing to search for genes involved in frustule morphology in the centric diatom Pleurosira laevis, which exhibits morphological plasticity between flat and domed valve faces in salinity 2 and 7, respectively. We observed differential expression of transposable elements (TEs) and transporters, likely due to osmotic response. Up-regulation of mechanosensitive ion channels and down-regulation of Ca2+-ATPases in cells with flat valves suggested that cytosolic Ca2+ levels were changed between the morphologies. Calcium signaling could be a mechanism for detecting osmotic pressure changes and triggering morphological shifts. We also observed an up-regulation of ARPC1 and annexin, involved in the regulation of actin filament dynamics known to affect frustule morphology, as well as the up-regulation of genes encoding frustule-related proteins such as BacSETs and frustulin. Taken together, we propose a model in which salinity-induced morphogenetic changes are driven by upstream responses, such as the regulation of cytosolic Ca2+ levels, and downstream responses, such as Ca2+-dependent regulation of actin dynamics and frustule-related proteins. This study highlights the sensitivity of euryhaline diatoms to environmental salinity and the role of active cellular processes in controlling gross valve morphology under different osmotic pressures.

The AT-hook protein AHL29 promotes Bacillus subtilis colonization by suppressing SWEET2-mediated sugar retrieval in Arabidopsis roots

Beneficial Bacillus subtilis (BS) symbiosis could combat root pathogenesis, but it relies on root-secreted sugars. Understanding the molecular control of sugar flux during colonization would benefit biocontrol applications. The SWEET (Sugar Will Eventually Be Exported Transporter) uniporter regulates microbe-induced sugar secretion from roots; thus, its homologs may modulate sugar distribution upon BS colonization. Quantitative polymerase chain reaction revealed that gene transcripts of SWEET2, but not SWEET16 and 17, were significantly induced in seedling roots after 12 h of BS inoculation. Particularly, SWEET2-β-glucuronidase fusion proteins accumulated in the apical mature zone where BS abundantly colonized. Yet, enhanced BS colonization in sweet2 mutant roots suggested a specific role for SWEET2 to constrain BS propagation, probably by limiting hexose secretion. By employing yeast one-hybrid screening and ectopic expression in Arabidopsis protoplasts, the transcription factor AHL29 was identified to function as a repressor of SWEET2 expression through the AT-hook motif. Repression occurred despite immunity signals. Additionally, enhanced SWEET2 expression and reduced colonies were specifically detected in roots of BS-colonized ahl29 mutant. Taken together, we propose that BS colonization may activate repression of AHL29 on SWEET2 transcription that would be enhanced by immunity signals, thereby maintaining adequate sugar secretion for a beneficial Bacillus association.

Structure, evolution, and roles of SWEET proteins in growth and stress responses in plants

Carbohydrates are exported by the SWEET family of transporters, which is a novel class of carriers that can transport sugars across cell membranes and facilitate sugar's long-distance transport from source to sink organs in plants. SWEETs play crucial roles in a wide range of physiologically important processes by regulating apoplastic and symplastic sugar concentrations. These processes include host-pathogen interactions, abiotic stress responses, and plant growth and development. In the present review, we (i) describe the structure and organization of SWEETs in the cell membrane, (ii) discuss the roles of SWEETs in sugar loading and unloading processes, (iii) identify the distinct functions of SWEETs in regulating plant growth and development including flower, fruit, and seed development, (iv) shed light on the importance of SWEETs in modulating abiotic stress resistance, and (v) describe the role of SWEET genes during plant-pathogen interaction. Finally, several perspectives regarding future investigations for improving the understanding of sugar-mediated plant defenses are proposed.

Streptomyces-triggered coordination between rhizosphere microbiomes and plant transcriptome enables watermelon Fusarium wilt resistance

The use of microbial inoculant is a promising strategy to improve plant health, but their efficiency often faces challenges due to difficulties in successful microbial colonization in soil environments. To this end, the application of biostimulation products derived from microbes is expected to resolve these barriers via direct interactions with plants or soil pathogens. However, their effectiveness and mechanisms for promoting plant growth and disease resistance remain elusive. In this study, we showed that root irrigation with the extracts of Streptomyces ahygroscopicus strain 769 (S769) solid fermentation products significantly reduced watermelon Fusarium wilt disease incidence by 30% and increased the plant biomass by 150% at a fruiting stage in a continuous cropping field. S769 treatment led to substantial changes in both bacterial and fungal community compositions, and induced a highly interconnected microbial association network in the rhizosphere. The root transcriptome analysis further suggested that S769 treatment significantly improved the expression of the MAPK signalling pathway, plant hormone signal transduction and plant-pathogen interactions, particular those genes related to PR-1 and ethylene, as well as genes associated with auxin production and reception. Together, our study provides mechanistic and empirical evidences for the biostimulation products benefiting plant health through coordinating plant and rhizosphere microbiome interaction.

The Function of Root Exudates in the Root Colonization by Beneficial Soil Rhizobacteria

Soil-beneficial microbes in the rhizosphere play important roles in improving plant growth and health. Root exudates play key roles in plant-microbe interactions and rhizobacterial colonization. This review describes the factors influencing the dynamic interactions between root exudates and the soil microbiome in the rhizosphere, including plant genotype, plant development, and environmental abiotic and biotic factors. We also discuss the roles of specific metabolic mechanisms, regulators, and signals of beneficial soil bacteria in terms of colonization ability. We highlight the latest research progress on the roles of root exudates in regulating beneficial rhizobacterial colonization. Organic acids, amino acids, sugars, sugar alcohols, flavonoids, phenolic compounds, volatiles, and other secondary metabolites are discussed in detail. Finally, we propose future research objectives that will help us better understand the role of root exudates in root colonization by rhizobacteria and promote the sustainable development of agriculture and forestry.

Damage-Associated Molecular Patterns and Systemic Signaling

Cellular damage inflicted by wounding, pathogen infection, and herbivory releases a variety of host-derived metabolites, degraded structural components, and peptides into the extracellular space that act as alarm signals when perceived by adjacent cells. These so-called damage-associated molecular patterns (DAMPs) function through plasma membrane localized pattern recognition receptors to regulate wound and immune responses. In plants, DAMPs act as elicitors themselves, often inducing immune outputs such as calcium influx, reactive oxygen species generation, defense gene expression, and phytohormone signaling. Consequently, DAMP perception results in a priming effect that enhances resistance against subsequent pathogen infections. Alongside their established function in local tissues, recent evidence supports a critical role of DAMP signaling in generation and/or amplification of mobile signals that induce systemic immune priming. Here, we summarize the identity, signaling, and synergy of proposed and established plant DAMPs, with a focus on those with published roles in systemic signaling.

Phytophagy impacts the quality and quantity of plant carbon resources acquired by mutualistic arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi associate with the roots of many plant species, enhancing their hosts access to soil nutrients whilst obtaining their carbon supply directly as photosynthates. AM fungi often face competition for plant carbon from other organisms. The mechanisms by which plants prioritise carbon allocation to mutualistic AM fungi over parasitic symbionts remain poorly understood. Here, we show that host potato plants (Solanum tuberosum cv. Désirée) selectively allocate carbon resources to tissues interacting with AM fungi rather than those interacting with phytophagous parasites (the nematode Globodera pallida). We found that plants reduce the supply of hexoses but maintain the flow of plant-derived fatty acids to AM fungi when concurrently interacting with parasites. Transcriptomic analysis suggest that plants prioritise carbon transfer to AM fungi by maintaining expression of fatty acid biosynthesis and transportation pathways, whilst decreasing the expression of mycorrhizal-induced hexose transporters. We also report similar findings from a different plant host species (Medicago truncatula) and phytophagous pest (the aphid Myzus persicae). These findings suggest a general mechanism of plant-driven resource allocation in scenarios involving multiple symbionts.

Review: Defense responses in sweetpotato (Ipomoea batatas L.) against biotic stress

Sweetpotato (Ipomoea batatas L.) is regarded as amongst the world's most important crops for food, vegetable, forage, and raw material for starch and alcohol production. Since pest attack and disease infection are the main limiting aspects frequently causing the yield loss and quality degradation of sweetpotato, it is a great demand to develop the effective defense strategies for maintaining productivity. In the past decade, many studies have focused on dynamic analysis at the physiological, biochemical, and molecular responses of sweetpotatoes to environmental challenges. This review offers an overview of the defense mechanisms against biotic stresses in sweetpotato observed so far, particularly insect herbivory and pathogen infections. The defenses of sweetpotato include the regulation of the toxic and anti-digestive proteins, plant-derived compounds, physical barrier formation, and sugar distribution. Ipomoelin and sporamin have been extensively researched for the defense against herbivore wounding. Herbivory-induced plant volatiles, chlorogenic acid, and latex phytochemicals play important roles in defenses for insect herbivory. Induction of IbSWEET10 reduces sugar content to mediate F. oxysporum resistance. Therefore, these researches provide the genetic strategies for improving resistance bioengineering and breeding of sweetpotato crops and future prospects for research in this field.

The Arabidopsis SWEET1 and SWEET2 uniporters recognize similar substrates while differing in subcellular localization

Sugars Will Eventually be Exported Transporters (SWEETs) are central for sugar allocation in plants. The SWEET family has approximately 20 homologs in most plant genomes, and despite extensive research on their structures and molecular functions, it is still unclear how diverse SWEETs recognize different substrates. Previous work using SweetTrac1, a biosensor constructed by the intramolecular fusion of a conformation-sensitive fluorescent protein in the plasma membrane transporter SWEET1 from Arabidopsis thaliana, identified common features in the transporter's substrates. Here, we report SweetTrac2, a new biosensor based on the Arabidopsis vacuole membrane transporter SWEET2, and use it to explore the substrate specificity of this second protein. Our results show that SWEET1 and SWEET2 recognize similar substrates but some with different affinities. Sequence comparison and mutagenesis analysis support the conclusion that the differences in affinity depend on nonspecific interactions involving previously uncharacterized residues in the substrate-binding pocket. Furthermore, SweetTrac2 can be an effective tool for monitoring sugar transport at vacuolar membranes that would be otherwise challenging to study.

Genetic Basis of Grain Size and Weight in Rice, Wheat, and Barley

Grain size is a key component of grain yield in cereals. It is a complex quantitative trait controlled by multiple genes. Grain size is determined via several factors in different plant development stages, beginning with early tillering, spikelet formation, and assimilates accumulation during the pre-anthesis phase, up to grain filling and maturation. Understanding the genetic and molecular mechanisms that control grain size is a prerequisite for improving grain yield potential. The last decade has brought significant progress in genomic studies of grain size control. Several genes underlying grain size and weight were identified and characterized in rice, which is a model plant for cereal crops. A molecular function analysis revealed most genes are involved in different cell signaling pathways, including phytohormone signaling, transcriptional regulation, ubiquitin-proteasome pathway, and other physiological processes. Compared to rice, the genetic background of grain size in other important cereal crops, such as wheat and barley, remains largely unexplored. However, the high level of conservation of genomic structure and sequences between closely related cereal crops should facilitate the identification of functional orthologs in other species. This review provides a comprehensive overview of the genetic and molecular bases of grain size and weight in wheat, barley, and rice, focusing on the latest discoveries in the field. We also present possibly the most updated list of experimentally validated genes that have a strong effect on grain size and discuss their molecular function.

A sweet story from Phytophthora-soybean interaction

Phytopathogenic microbes obtain nutrients from host plants to support their growth and metabolism. A recent study by Zhu et al. revealed that the oomycete pathogen Phytophthora sojae upregulates the activity of soybean trehalose 6-phosphate synthase 6 (GmTPS6) and increases trehalose accumulation (through an effector PsAvh413) to promote nutritional gain.

Genome-Wide Characterization of the Maize (Zea mays L.) WRKY Transcription Factor Family and Their Responses to Ustilago maydis

Members of the WRKY transcription factor (TF) family are unique to plants and serve as important regulators of diverse physiological processes, including the ability of plants to manage biotic and abiotic stressors. However, the functions of specific WRKY family members in the context of maize responses to fungal pathogens remain poorly understood, particularly in response to Ustilago maydis (DC.) Corda (U. maydis), which is responsible for the devastating disease known as corn smut. A systematic bioinformatic approach was herein employed for the characterization of the maize WRKY TF family, leading to the identification of 120 ZmWRKY genes encoded on 10 chromosomes. Further structural and phylogenetic analyses of these TFs enabled their classification into seven different subgroups. Segmental duplication was established as a major driver of ZmWRKY family expansion in gene duplication analyses, while the Ka/Ks ratio suggested that these ZmWRKY genes had experienced strong purifying selection. When the transcriptional responses of these genes to pathogen inoculation were evaluated, seven U. maydis-inducible ZmWRKY genes were identified, as validated using a quantitative real-time PCR approach. All seven of these WKRY proteins were subsequently tested using a yeast one-hybrid assay approach, which revealed their ability to directly bind the ZmSWEET4b W-box element, thereby controlling the U. maydis-inducible upregulation of ZmSWEET4b. These results suggest that these WRKY TFs can control sugar transport in the context of fungal infection. Overall, these data offer novel insight into the evolution, transcriptional regulation, and functional characteristics of the maize WRKY family, providing a basis for future research aimed at exploring the mechanisms through which these TFs control host plant responses to common smut and other fungal pathogens.

MdSWEET23, a sucrose transporter from apple (Malus × domestica Borkh.), influences sugar metabolism and enhances cold tolerance in tomato

Photosynthetic products in most fleshy fruits are unloaded via the apoplasmic pathway. Sugar transporters play an important role in the apoplasmic unloading pathway and are involved in sugar transport for fruit development. The MdSWEET23, cloned from ''Hanfu'' apple (Malus × domestica Borkh.) fruits, belongs to Clade III of the SWEET family. Subcellular localization revealed that MdSWEET23 is localized on the plasma membrane. β-glucuronidase activity assays showed that MdSWEET23 was primarily expressed in the sepal and carpel vascular bundle of apple fruits. Heterologous expression assays in yeast showed that MdSWEET23 functions in sucrose transport. The overexpression of MdSWEET23 in the ''Orin" calli increased the soluble sugar content. The silencing of MdSWEET23 significantly reduced the contents of sucrose and sorbitol in apple fruits. Ectopic overexpression of MdSWEET23 in tomato altered sugar metabolism and distribution in leaves and fruits, causing a reduction in photosynthetic rates and plant height, enhanced cold stress tolerance, and increased the content of sucrose, fructose, and glucose in breaking color fruits, but did not increase sugar sink potency of tomato fruits.

Pineapple SWEET10 is a glucose transporter

SWEET transporters are a unique class of sugar transporters that play vital roles in various developmental and physiological processes in plants. While the functions of SWEETs have been well established in model plants such as Arabidopsis, their functions in economically important fruit crops like pineapple have not been well studied. Here we aimed to investigate the substrate specificity of pineapple SWEETs by comparing the protein sequences of known glucose and sucrose transporters in Arabidopsis with those in pineapple. Our genome-wide approach and 3D structure comparison showed that the Arabidopsis SWEET8 homolog in pineapple, AcSWEET10, shares similar sequences and protein properties responsible for glucose transport. To determine the functional conservation of AcSWEET10, we tested its ability to complement glucose transport mutants in yeast and analyzed its expression in stamens and impact on the microspore phenotype and seed set in transgenic Arabidopsis. The results showed that AcSWEET10 is functionally equivalent to AtSWEET8 and plays a critical role in regulating microspore formation through the regulation of the Callose synthase5 (CalS5), which highlights the importance of SWEET transporters in pineapple. This information could have important implications for improving fruit crop yield and quality by manipulating SWEET transporter activity.

Maize plant expresses SWEET transporters differently when interacting with Trichoderma asperellum and Fusarium verticillioides, two fungi with different lifestyles

Most Trichoderma species are beneficial fungi that promote plant growth and resistance, while Fusarium genera cause several crop damages. During the plant-fungi interaction there is a competition for sugars in both lifestyles. Here we analyzed the plant growth promotion and biocontrol activity of T. asperellum against F. verticillioides and the effect of both fungi on the expression of the maize diffusional sugar transporters, the SWEETs. The biocontrol activity was done in two ways, the first was by observing the growth capacity of both fungus in a dual culture. The second one by analyzing the infection symptoms, the chlorophyl content and the transcript levels of defense genes determined by qPCR in plants with different developmental stages primed with T. asperellum conidia and challenged with F. verticillioides. In a dual culture, T. asperellum showed antagonist activity against F. verticillioides. In the primed plants a delay in the infection disease was observed, they sustained chlorophyll content even after the infection, and displayed upregulated defense-related genes. Additionally, the T. asperellum primed plants had longer stems than the nonprimed plants. SWEETs transcript levels were analyzed by qPCR in plants primed with either fungus. Both fungi affect the transcript levels of several maize sugar transporters differently. T. asperellum increases the expression of six SWEETs on leaves and two at the roots and causes a higher exudation of sucrose, glucose, and fructose at the roots. On the contrary, F. verticillioides reduces the expression of the SWEETs on the leaves, and more severely when a more aggressive strain is in the plant. Our results suggest that the plant is able to recognize the lifestyle of the fungi and respond accordingly by changing the expression of several genes, including the SWEETs, to establish a new sugar flux.

Arabidopsis TGA256 Transcription Factors Suppress Salicylic-Acid-Induced Sucrose Starvation

Salicylic acid (SA) is produced by plants in response to pathogen infection. SA binds the NONEXPRESSOR OF PATHOGENESIS-RELATED GENES (NPR) family of receptors to regulate both positive (NPR1) and negative (NPR3/4) plant immune responses by interacting with the clade II TGACG (TGA) motif-binding transcription factors (TGA2, TGA5, and TGA6). Here, we report that the principal metabolome-level response to SA treatment in Arabidopsis is a reduction in sucrose and other free sugars. We observed nearly identical effects in the tga256 triple mutant, which lacks all clade II TGA transcription factors. The tga256 mutant presents reduced leaf blade development and elongated hypocotyls, roots, and petioles consistent with sucrose starvation. No changes were detected in auxin levels, and mutant seedling growth could be restored to that of wild-type by sucrose supplementation. Although the retrograde signal 2-C-methyl-D-erythritol-2,4-cyclodiphosphate is known to stimulate SA biosynthesis and defense signaling, we detected no negative feedback by SA on this or any other intermediate of the 2-C-methyl-D-erythritol-4-phosphate pathway. Trehalose, a proxy for the sucrose regulator trehalose-6-phosphate (T6P), was highly reduced in tga256, suggesting that defense-related reductions in sugar availability may be controlled by changes in T6P levels. We conclude that the negative regulatory roles of TGA2/5/6 include maintaining sucrose levels in healthy plants. Disruption of TGA2/5/6-NPR3/4 inhibitory complexes by mutation or SA triggers sucrose reductions in Arabidopsis leaves, consistent with the 'pathogen starvation' hypothesis. These findings highlight sucrose availability as a mechanism by which TGA2/5/6 balance defense and development.

Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria

Plant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.

Interaction between sugar transport and plant development

Endogenous programs and constant interaction with the environment regulate the development of the plant organism and its individual organs. Sugars are necessary building blocks for plant and organ growth and at the same time act as critical integrators of the metabolic state into the developmental program. There is a growing recognition that the specific type of sugar and its subcellular or tissue distribution is sensed and translated to developmental responses. Therefore, the transport of sugars across membranes is a key process in adapting plant organ properties and overall development to the nutritional state of the plant. In this review, we discuss how plants exploit various sugar transporters to signal growth responses, for example, to control the development of sink organs such as roots or fruits. We highlight which sugar transporters are involved in root and shoot growth and branching, how intracellular sugar allocation can regulate senescence, and, for example, control fruit development. We link the important transport processes to downstream signaling cascades and elucidate the factors responsible for the integration of sugar signaling and plant hormone responses.

Identification and expression analysis of the SWEET genes in radish reveal their potential functions in reproductive organ development

BACKGROUND

Sugars produced by photosynthesis provide energy for biological activities and the skeletons for macromolecules; they also perform multiple physiological functions in plants. Sugar transport across plasma membranes mediated by the Sugar Will Eventually be Exported Transporter (SWEET) genes substantially affects these processes. However, the evolutionary dynamics and function of the SWEET genes are largely unknown in radish, an important Brassicaceae species.

METHODS AND RESULTS

Genome-wide identification and analysis of the RsSWEET genes from the recently updated radish reference genome was conducted using bioinformatics methods. The tissue-specific expression was analyzed using public RNA-seq data, and the expression levels in the bud, stamens, pistils, pericarps and seeds at 15 and 30 days after flowering (DAF) were determined by RT‒qPCR. Thirty-seven RsSWEET genes were identified and named according to their Arabidopsis homologous. They are unevenly distributed across the nine radish chromosomes and were further divided into four clades by phylogenetic analysis. There are 5-7 transmembrane domains and at least one MtN3_slv domain in the RsSWEETs. RNA-seq and RT‒qPCR revealed that the RsSWEETs exhibit higher expression levels in the reproductive organs, indicating that these genes might play vital roles in reproductive organ development. RsSWEET15.1 was found to be especially expressed in siliques according to the RNA-seq data, and the RT‒qPCR results further confirmed that it was most highly expressed levels in the seeds at 30 DAF, followed by the pericarp at 15 DAF, indicating that it is involved in seed growth and development.

CONCLUSIONS

This study suggests that the RsSWEET genes play vital roles in reproductive organ development and provides a theoretical basis for the future functional analysis of RsSWEETs in radish.

Functional Analysis of Plant Monosaccharide Transporters Using a Simple Growth Complementation Assay in Yeast

The study of genes and their products is an essential prerequisite for fundamental research. Characterization can be achieved by analyzing mutants or overexpression lines or by studying the localization and substrate specificities of the resulting proteins. However, functional analysis of specific proteins in complex eukaryotic organisms can be challenging. To overcome this, the use of heterologous systems to express genes and analyze the resulting proteins can save time and effort. Yeast is a preferred heterologous model organism: it is easy to transform, and tools for genomics, engineering, and metabolomics are already available. Here, we describe a well-established and simple method to analyze the activity of plant monosaccharide transporters in the baker's yeast, Saccharomyces cerevisiae, using a simple growth complementation assay. We used the famous hexose-transport-deficient yeast strain EBY.VW4000 to express candidate plant monosaccharide transporters and analyzed their transport activity. This assay does not require any radioactive labeling of substrates and can be easily extended for quantitative analysis using growth curves or by analyzing the transport rates of fluorescent substrates like the glucose analog 2-NBDG. Finally, to further simplify the cloning of potential candidate transporters, we provide level 0 modular cloning (MoClo) modules for efficient and simple Golden Gate cloning. This approach provides a convenient tool for the functional analysis of plant monosaccharide transporters in yeast. Key features Comprehensive, simple protocol for analysis of plant monosaccharide transporters in yeast Includes optional MoClo parts for cloning with Golden Gate method Includes protocol for the production and transformation of competent yeast cells Does not require hazardous solutions, radiolabeled substrates, or specialized equipment.

Starving the enemy: how plant and microbe compete for sugar on the border

As the primary energy source for a plant host and microbe to sustain life, sugar is generally exported by Sugars Will Eventually be Exported Transporters (SWEETs) to the host extracellular spaces or the apoplast. There, the host and microbes compete for hexose, sucrose, and other important nutrients. The host and microbial monosaccharide transporters (MSTs) and sucrose transporters (SUTs) play a key role in the "evolutionary arms race". The result of this competition hinges on the proportion of sugar distribution between the host and microbes. In some plants (such as Arabidopsis, corn, and rice) and their interacting pathogens, the key transporters responsible for sugar competition have been identified. However, the regulatory mechanisms of sugar transporters, especially in the microbes require further investigation. Here, the key transporters that are responsible for the sugar competition in the host and pathogen have been identified and the regulatory mechanisms of the sugar transport have been briefly analyzed. These data are of great significance to the increase of the sugar distribution in plants for improvement in the yield.

Phytophthora sojae boosts host trehalose accumulation to acquire carbon and initiate infection

Successful infection by pathogenic microbes requires effective acquisition of nutrients from their hosts. Root and stem rot caused by Phytophthora sojae is one of the most important diseases of soybean (Glycine max). However, the specific form and regulatory mechanisms of carbon acquired by P. sojae during infection remain unknown. In the present study, we show that P. sojae boosts trehalose biosynthesis in soybean through the virulence activity of an effector PsAvh413. PsAvh413 interacts with soybean trehalose-6-phosphate synthase 6 (GmTPS6) and increases its enzymatic activity to promote trehalose accumulation. P. sojae directly acquires trehalose from the host and exploits it as a carbon source to support primary infection and development in plant tissue. Importantly, GmTPS6 overexpression promoted P. sojae infection, whereas its knockdown inhibited the disease, suggesting that trehalose biosynthesis is a susceptibility factor that can be engineered to manage root and stem rot in soybean.

The brassinosteroid-responsive protein OCTOPUS is a novel regulator of Arabidopsis thaliana immune signaling

Phloem is a critical tissue for transport of photosynthates and extracellular signals in vascular plants. However, it also represents an ideal environment for pathogens seeking access to valuable host nutrients. Although many vascular pathogens induce economically relevant crop damage, there is still little known about the mechanisms by which immune signaling operates through the phloem. An existing phosphoproteomic dataset was mined to identify proteins that were both phosphorylated in response to the defense-elicitor flagellin (flg22) and expressed in vascular cells. A single candidate, OCTOPUS (OPS), is polarly associated with the plasma membrane of sieve element cells and has been characterized as an inhibitor of brassinosteroid insensitive-2 in promotion of brassinosteroid-related phytohormone signaling. The observation that OPS is differentially phosphorylated in response to flg22 led us to the examine whether OPS may also regulate flg22-induced immune signaling. Two independent alleles of ops exhibited enhanced immunity outputs across multiple signaling branches of PAMP-triggered immunity (PTI), constitutively and in response to flg22 treatment. Together with our observation that interactions between OPS and brassinosteroid insensitive-2 were disrupted by induction of salicylic acid and depletion of brassinosteriod, these data support a model whereby OPS modulates brassinolide and immune signaling to control downstream responses. We present OPS as a novel addition to the list of proteins with documented roles in PAMP-PTI signaling. These results further indicate that immune signaling in the phloem may be a significant and unique component of the host detection and response to pathogens in vascular plants.

Camellia oleifera CoSWEET10 Is Crucial for Seed Development and Drought Resistance by Mediating Sugar Transport in Transgenic Arabidopsis

Sugar transport from the source leaf to the sink organ is critical for seed development and crop yield, as well as for responding to abiotic stress. SWEETs (sugar will eventually be exported transporters) mediate sugar efflux into the reproductive sink and are therefore considered key candidate proteins for sugar unloading during seed development. However, the specific mechanism underlying the sugar unloading to seeds in Camellia oleifera remains elusive. Here, we identified a SWEET gene named CoSWEET10, which belongs to Clade III and has high expression levels in the seeds of C. oleifera. CoSWEET10 is a plasma membrane-localized protein. The complementation assay of CoSWEET10 in SUSY7/ura3 and EBY.VW4000 yeast strains showed that CoSWEET10 has the ability to transport sucrose, glucose, and fructose. Through the C. oleifera seeds in vitro culture, we found that the expression of CoSWEET10 can be induced by hexose and sucrose, and especially glucose. By generating the restoration lines of CoSWEET10 in Arabidopsis atsweet10, we found that CoSWEET10 restored the seed defect phenotype of the mutant by regulating soluble sugar accumulation and increased plant drought tolerance. Collectively, our study demonstrates that CoSWEET10 plays a dual role in promoting seed development and enhancing plant drought resistance as a sucrose and hexose transporter.

Unraveling the diversity and functions of sugar transporters for sustainable management of wheat rust

Plant diseases threaten global food security by reducing the production and quality of produce. Identification of disease resistance sources and their utilization in crop improvement is of paramount significance. However, constant evolution and occurrence of new, more aggressive and highly virulent pathotypes disintegrates the resistance of cultivars and hence demanding the steady stream of disease resistance cultivars as the most sustainable way of disease management. In this context, molecular tools and technologies facilitate an efficient and rational engineering of crops to develop cultivars having resistance to multiple pathogens and pathotypes. Puccinia spp. is biotrophic fungi that interrupt crucial junctions for causing infection, thus risking nutrient access of wheat plants and their subsequent growth. Sugar is a major carbon source taken from host cells by pathogens. Sugar transporters (STPs) are key players during wheat-rust interactions that regulate the transport, exchange, and allocation of sugar at plant-pathogen interfaces. Intense competition for accessing sugars decides fate of incompatibility or compatibility between host and the pathogen. The mechanism of transport, allocation, and signaling of sugar molecules and role of STPs and their regulatory switches in determining resistance/susceptibility to rusts in wheat is poorly understood. This review discusses the molecular mechanisms involving STPs in distribution of sugar molecules for determination of rust resistance/susceptibility in wheat. We also present perspective on how detailed insights on the STP's role in wheat-rust interaction will be helpful in devising efficient strategies for wheat rust management.

Secondary metabolites in grapevine: crosstalk of transcriptional, metabolic and hormonal signals controlling stress defence responses in berries and vegetative organs

Abiotic stresses, such as temperature, heat waves, water limitation, solar radiation and the increase in atmospheric CO₂ concentration, significantly influence the accumulation of secondary metabolites in grapevine berries at different developmental stages, and in vegetative organs. Transcriptional reprogramming, miRNAs, epigenetic marks and hormonal crosstalk regulate the secondary metabolism of berries, mainly the accumulation of phenylpropanoids and of volatile organic compounds (VOCs). Currently, the biological mechanisms that control the plastic response of grapevine cultivars to environmental stress or that occur during berry ripening have been extensively studied in many worlds viticultural areas, in different cultivars and in vines grown under various agronomic managements. A novel frontier in the study of these mechanisms is the involvement of miRNAs whose target transcripts encode enzymes of the flavonoid biosynthetic pathway. Some miRNA-mediated regulatory cascades, post-transcriptionally control key MYB transcription factors, showing, for example, a role in influencing the anthocyanin accumulation in response to UV-B light during berry ripening. DNA methylation profiles partially affect the berry transcriptome plasticity of different grapevine cultivars, contributing to the modulation of berry qualitative traits. Numerous hormones (such as abscisic and jasmomic acids, strigolactones, gibberellins, auxins, cytokynins and ethylene) are involved in triggering the vine response to abiotic and biotic stress factors. Through specific signaling cascades, hormones mediate the accumulation of antioxidants that contribute to the quality of the berry and that intervene in the grapevine defense processes, highlighting that the grapevine response to stressors can be similar in different grapevine organs. The expression of genes responsible for hormone biosynthesis is largely modulated by stress conditions, thus resulting in the numeourous interactions between grapevine and the surrounding environment.

SWEET11b transports both sugar and cytokinin in developing barley grains

Even though Sugars Will Eventually be Exported Transporters (SWEETs) have been found in every sequenced plant genome, a comprehensive understanding of their functionality is lacking. In this study, we focused on the SWEET family of barley (Hordeum vulgare). A radiotracer assay revealed that expressing HvSWEET11b in African clawed frog (Xenopus laevis) oocytes facilitated the bidirectional transfer of not only just sucrose and glucose, but also cytokinin. Barley plants harboring a loss-of-function mutation of HvSWEET11b could not set viable grains, while the distribution of sucrose and cytokinin was altered in developing grains of plants in which the gene was knocked down. Sucrose allocation within transgenic grains was disrupted, which is consistent with the changes to the cytokinin gradient across grains, as visualized by magnetic resonance imaging and Fourier transform infrared spectroscopy microimaging. Decreasing HvSWEET11b expression in developing grains reduced overall grain size, sink strength, the number of endopolyploid endosperm cells, and the contents of starch and protein. The control exerted by HvSWEET11b over sugars and cytokinins likely predetermines their synergy, resulting in adjustments to the grain's biochemistry and transcriptome.

CRISPRi in Xanthomonas demonstrates functional convergence of transcription activator-like effectors in two divergent pathogens

Functional analysis of large gene families in plant pathogens can be cumbersome using classical insertional mutagenesis. Additionally, Cas9 toxicity has limited the application of CRISPR-Cas9 for directed mutagenesis in bacteria. Here, we successfully applied a CRISPR interference strategy to investigate the cryptic role of the transcription activator-like effector (tale) multigene family in several plant-pathogenic Xanthomonas bacterial species, owing to their contribution to pathogen virulence. Single guide RNAs (sgRNAs) designed against Xanthomonas phaseoli pv manihotis tale conserved gene sequences efficiently silenced expression of all tales, with concomitant decrease in virulence and TALE-induced host gene expression. The system is readily translatable to other Xanthomonas species infecting rice, citrus, Brassica, and cassava, silencing up to 16 tales in a given strain using a single sgRNA. Complementation with plasmid-borne designer tales lacking the sgRNA-targeted sequence restored molecular and virulence phenotypes in all pathosystems. Our results evidenced that X. campestris pv campestris CN08 tales are relevant for symptom development in cauliflower. They also show that the MeSWEET10a sugar transporter is surprisingly targeted by the nonvascular cassava pathogen X. cassavae, highlighting a new example of TALE functional convergence between phylogenetically distant Xanthomonas. Overall, this novel technology provides a platform for discovery and rapid functional understanding of highly conserved gene families.

The Dynamic Transcription Activator-Like Effector Family of Xanthomonas

Transcription activator-like effectors (TALEs) are bacterial proteins that are injected into the eukaryotic nucleus to act as transcriptional factors and function as key virulence factors of the phytopathogen Xanthomonas. TALEs are translocated into plant host cells via the type III secretion system and induce the expression of host susceptibility (S) genes to facilitate disease. The unique modular DNA binding domains of TALEs comprise an array of nearly identical direct repeats that enable binding to DNA targets based on the recognition of a single nucleotide target per repeat. The very nature of TALE structure and function permits the proliferation of TALE genes and evolutionary adaptations in the host to counter TALE function, making the TALE-host interaction the most dynamic story in effector biology. The TALE genes appear to be a relatively young effector gene family, with a presence in all virulent members of some species and absent in others. Genome sequencing has revealed many TALE genes throughout the xanthomonads, and relatively few have been associated with a cognate S gene. Several species, including Xanthomonas oryzae pv. oryzae and X. citri pv. citri, have near absolute requirement for TALE gene function, while the genes appear to be just now entering the disease interactions with new fitness contributions to the pathogens of tomato and pepper among others. Deciphering the simple and effective DNA binding mechanism also has led to the development of DNA manipulation tools in fields of gene editing and transgenic research. In the three decades since their discovery, TALE research remains at the forefront of the study of bacterial evolution, plant-pathogen interactions, and synthetic biology. We also discuss critical questions that remain to be addressed regarding TALEs.

Plant phosphate status influences root biotic interactions

Phosphorus (P) deficiency stress in combination with biotic stress(es) severely impacts crop yield. Plant responses to P deficiency overlapping with that of other stresses exhibit a high degree of complexity involving different signaling pathways. On the one hand, plants engage with rhizosphere microbiome/arbuscular mycorrhizal fungi for improved phosphate (Pi) acquisition and plant stress response upon Pi deficiency; on the other hand, this association is gets disturbed under Pi sufficiency. This nutrient-dependent response is highly regulated by the phosphate starvation response (PSR) mediated by the master regulator, PHR1, and its homolog, PHL. It is interesting to note that Pi status (deficiency/sufficiency) has a varying response (positive/negative) to different biotic encounters (beneficial microbes/opportunistic pathogens/insect herbivory) through a coupled PSR-PHR1 immune system. This also involves crosstalk among multiple players including transcription factors, defense hormones, miRNAs, and Pi transporters, among others influencing the plant-biotic-phosphate interactions. We provide a comprehensive view of these key players involved in maintaining a delicate balance between Pi homeostasis and plant immunity. Finally, we propose strategies to utilize this information to improve crop resilience to Pi deficiency in combination with biotic stresses.

An Agrobacterium-Mediated Transient Expression Method for Functional Assay of Genes Promoting Disease in Monocots

Agrobacterium-mediated transient expression (AMTE) has been widely used for high-throughput assays of gene function in diverse plant species. However, its application in monocots is still limited due to low expression efficiency. Here, by using histochemical staining and a quantitative fluorescence assay of β-glucuronidase (GUS) gene expression, we investigated factors affecting the efficiency of AMTE on intact barley plants. We found prominent variation in GUS expression levels across diverse vectors commonly used for stable transformation and that the vector pCBEP produced the highest expression. Additionally, concurrent treatments of plants with one day of high humidity and two days of darkness following agro-infiltration also significantly increased GUS expression efficiency. We thus established an optimized method for efficient AMTE on barley and further demonstrated its efficiency on wheat and rice plants. We showed that this approach could produce enough proteins suitable for split-luciferase assays of protein-protein interactions on barley leaves. Moreover, we incorporated the AMTE protocol into the functional dissection of a complex biological process such as plant disease. Based on our previous research, we used the pCBEP vector to construct a full-length cDNA library of genes upregulated during the early stage of rice blast disease. A subsequent screen of the library by AMTE identified 15 candidate genes (out of ~2000 clones) promoting blast disease on barley plants. Four identified genes encode chloroplast-related proteins: OsNYC3, OsNUDX21, OsMRS2-9, and OsAk2. These genes were induced during rice blast disease; however, constitutive overexpression of these genes conferred enhanced disease susceptibility to Colletotrichum higginsianum in Arabidopsis. These observations highlight the power of the optimized AMTE approach on monocots as an effective tool for facilitating functional assays of genes mediating complex processes such as plant-microbe interactions.

AtSWEET11 and AtSWEET12 transporters function in tandem to modulate sugar flux in plants

The sugar will eventually be exported transporter (SWEET) members in Arabidopsis, AtSWEET11 and AtSWEET12 are the important sucrose efflux transporters that act synergistically to perform distinct physiological roles. These two transporters are involved in apoplasmic phloem loading, seed filling, and sugar level alteration at the site of pathogen infection. Here, we performed the structural analysis of the sucrose binding pocket of AtSWEET11 and AtSWEET12 using molecular docking followed by rigorous molecular dynamics (MD) simulations. We observed that the sucrose molecule binds inside the central cavity and in the middle of the transmembrane (TM) region of AtSWEET11 and AtSWEET12, that allows the alternate access to the sucrose molecule from either side of the membrane during transport. Both AtSWEET11 and AtSWEET12, shares the similar amino acid residues that interact with sucrose molecule. Further, to achieve more insights on the role of these two transporters in other plant species, we did the phylogenetic and the in-silico analyses of AtSWEET11 and AtSWEET12 orthologs from 39 economically important plants. We reported the extensive information on the gene structure, protein domain and cis-acting regulatory elements of AtSWEET11 and AtSWEET12 orthologs from different plants. The cis-elements analysis indicates the involvement of AtSWEET11 and AtSWEET12 orthologs in plant development and also during abiotic and biotic stresses. Both in silico and in planta expression analysis indicated AtSWEET11 and AtSWEET12 are well-expressed in the Arabidopsis leaf tissues. However, the orthologs of AtSWEET11 and AtSWEET12 showed the differential expression pattern with high or no transcript expression in the leaf tissues of different plants. Overall, these results offer the new insights into the functions and regulation of AtSWEET11 and AtSWEET12 orthologs from different plant species. This might be helpful in conducting the future studies to understand the role of these two crucial transporters in Arabidopsis and other crop plants.

Management of Reniform Nematode in Cotton Using Winter Crop Residue Amendments Under Greenhouse Conditions

Rotylenchulus reniformis (reniform nematode, RN) is among the most important nematodes affecting cotton. Cultural practices, such as rotation and soil amendment, are established methods for managing RN. Management may be enhanced if crop residue has biofumigant properties against RN. The objective was to evaluate the efficacy of winter crop amendments for managing RN in the greenhouse. Reniform nematode-infested soil was amended with dry or fresh organic matter (OM, 2% w/w) from winter crops - canola, carinata, hairy vetch, oat, or no crop. Cotton was subsequently grown in this soil. Independent of the crop, dry OM amendments were more effective than no amendment at managing RN, while fresh OM amendments were not. Soil and root RN abundances and reproduction factors were generally lower in Trials 1 and 3 for dry OM than fresh OM amendments or control without OM. In Trial 2, none of the OM treatments reduced RN parameters compared with no OM control. In general, when compared to plants without RN or OM, RN did not produce significant changes in growth parameters but did affect physiology (Soil Plant Analysis Development, or SPAD, values). In conclusion, dry OM amendments can help manage RN, crop growth does not always relate to RN abundances, and SPAD values could help indicate RN presence.

Terpenoid Transport in Plants: How Far from the Final Picture?

Contrary to the biosynthetic pathways of many terpenoids, which are well characterized and elucidated, their transport inside subcellular compartments and the secretion of reaction intermediates and final products at the short- (cell-to-cell), medium- (tissue-to-tissue), and long-distance (organ-to-organ) levels are still poorly understood, with some limited exceptions. In this review, we aim to describe the state of the art of the transport of several terpene classes that have important physiological and ecological roles or that represent high-value bioactive molecules. Among the tens of thousands of terpenoids identified in the plant kingdom, only less than 20 have been characterized from the point of view of their transport and localization. Most terpenoids are secreted in the apoplast or stored in the vacuoles by the action of ATP-binding cassette (ABC) transporters. However, little information is available regarding the movement of terpenoid biosynthetic intermediates from plastids and the endoplasmic reticulum to the cytosol. Through a description of the transport mechanisms of cytosol- or plastid-synthesized terpenes, we attempt to provide some hypotheses, suggestions, and general schemes about the trafficking of different substrates, intermediates, and final products, which might help develop novel strategies and approaches to allow for the future identification of terpenoid transporters that are still uncharacterized.