Revisiting growth-defence trade-offs and breeding strategies in crops

Rapid reprogramming and stabilization of homoeolog expression bias in hexaploid wheat biparental populations

BACKGROUND

Differences in the relative level of expression of homoeologs, known as homoeolog expression bias, are widely observed in allopolyploids. While the evolution of homoeolog expression bias through hybridization has been characterized, on shorter timescales such as those found in crop breeding programs, the extent to which homoeolog expression bias is preserved or altered between generations remains elusive.

RESULTS

Here we use biparental mapping populations of hexaploid wheat (Triticum aestivum) with a common "Paragon" parent to explore the inheritance of homoeolog expression bias in the F5 generation. We found that homoeolog expression bias is inherited for 26-27% of triads in both populations. Most triads conserved a similar homoeolog expression bias pattern as one or both parents. Inherited patterns were largely driven by changes in the expression of one homoeolog, allowing homoeolog expression bias in subsequent generations to match parental expression. Novel patterns of homoeolog expression bias occurred more frequently in the biparental population from a landrace × elite cross, than in the population with two elite parents.

CONCLUSIONS

These results demonstrate that there is significant reprogramming and stabilization of homoeolog expression bias within a small number of generations that differs significantly based on the parental lines used in the crossing.

Mixed DAMP/MAMP oligosaccharides promote both growth and defense against fungal pathogens of cucumber

Plants recognize a variety of environmental molecules, thereby triggering appropriate responses to biotic or abiotic stresses. Substances containing microbes-associated molecular patterns (MAMPs) and damage-associated molecular patterns (DAMPs) are representative inducers of pathogen resistance and damage repair, thus treatment of healthy plants with such substances can pre-activate plant immunity and cell repair functions. In this study, the effects of DAMP/MAMP oligosaccharides mixture (Oligo-Mix) derived from plant cell wall (cello-oligosaccharide and xylo-oligosaccharide), and fungal cell wall (chitin-oligosaccharide) were examined in cucumber. Treatment of cucumber with Oligo-Mix promoted root germination and plant growth, along with increased chlorophyll contents in the leaves. Oligo-Mix treatment also induced typical defense responses such as MAP kinase activation and callose deposition in leaves. Pretreatment of Oligo-Mix enhanced disease resistance of cucumber leaves against pathogenic fungi Podosphaera xanthii (powdery mildew) and Colletotrichum orbiculare (anthracnose). Oligo-Mix treatment increased the induction of hypersensitive cell death around the infection site of pathogens, which inhibited further infection and the conidial formation of pathogens on the cucumber leaves. RNA-seq analysis revealed that Oligo-Mix treatment upregulated genes associated with plant structural reinforcement, responses to abiotic stresses and plant defense. These results suggested that Oligo-Mix has beneficial effects on growth and disease resistance in cucumber, making it a promising biostimulant for agricultural application.

SEOR2 in Arabidopsis mediates Ca2+ dependent defense against phytoplasmas and reduction of plant growth

The Arabidopsis seor1ko line, which expresses the protein AtSEOR2 free of its bond with AtSEOR1, exhibits a lower phytoplasma titre as compared to wild type plants. In search for mechanism(s) underlying potential SEOR2-mediated defense responses the transcriptome of healthy wild type and Atseor1ko plants was disclosed by RNA sequencing. Comparative transcriptome analysis revealed 1036 differentially expressed genes (DEGs, 893 up- and 143 down-regulated) between the Atseor1ko line and the wild type. Sequence annotation and classification of the up-regulated genes identified "plant-pathogen interaction" among the most enriched clusters. The "plant-pathogen interaction" cluster included genes encoding members of the protein kinase superfamily, actors in calcium/calmodulin signaling transduction and WRKY transcription factors. An interaction network analysis and a host-phytoplasma interaction map demonstrated that AtSEOR2 protein could interact with the calcium-binding proteins CAM2 and TCH3. The latter one also turned out to be an indirect target of the SAP54CY phytoplasma effector, which suggests a SEOR2-mediated role of TCH3 in balancing nutrient investments in plant defense and plant growth.

Precise genome editing of Dense and Erect Panicle 1 promotes rice sheath blight resistance and yield production in japonica rice

The primary goals of crop breeding are to enhance yield and improve disease resistance. However, the "trade-off" mechanism, in which signalling pathways for resistance and yield are antagonistically regulated, poses challenges for achieving both simultaneously. Previously, we demonstrated that knock-out mutants of the Dense and Erect Panicle 1 (DEP1) gene can significantly enhance rice resistance to sheath blight (ShB), and we mapped DEP1's association with panicle length. In this study, we discovered that dep1 mutants significantly reduced rice yield. Nonetheless, truncated DEP1 was able to achieve both ShB resistance and yield increase in japonica rice. To further explore the function of truncated DEP1 in promoting yield and ShB resistance, we generated CRISPR/Cas9-mediated genome editing mutants, including a full-length deletion mutant of DEP1, named dep1, and a truncated version, dep1-cys. Upon inoculation with Rhizoctonia solani, the dep1-cys mutant demonstrated stronger ShB resistance than the dep1 mutant. Additionally, dep1-cys increased yield per plant, whereas dep1 reduced it. Compared to the full DEP1 protein, the truncated DEP1 (dep1-cys) demonstrated a decreased interaction affinity with IDD14 and increased affinity with IDD10, which are known to positively and negatively regulate ShB resistance through the activation of PIN1a and ETR2, respectively. The dep1-cys mutant exhibited higher PIN1a and lower ETR2 expression than wild-type plants, suggesting that dep1-cys modulated IDD14 and IDD10 interactions to regulate PIN1a and ETR2, thereby enhancing ShB resistance. Overall, these data indicate that precise genome editing of DEP1 could simultaneously improve both ShB resistance and yield, effectively mitigating trade-off regulation in rice.

Arabidopsis OTU2 deubiquitinates cysteine protease RD21A to enhance clubroot resistance

Clubroot is a major threat to cruciferous crops worldwide, largely due to the complex pathogenesis of its causal agent, Plasmodiophora brassicae, and the limited availability of genetic resistance in plants. Previous research has shown that P. brassicae secretes the E3 ubiquitin ligase PbE3-2, which targets and degrades the Arabidopsis thaliana cysteine protease RD21A to facilitate infection. In this study, we identified a plant defense mechanism that counteracts this pathogen virulence strategy. We found that the A. thaliana deubiquitinating enzyme OTU2, whose expression is upregulated during infection, interacts with RD21A. Notably, OTU2 stabilized RD21A by deubiquitination and inhibited the interaction between PbE3-2 and RD21A. Furthermore, OTU2 overexpression enhanced A. thaliana resistance to P. brassicae in an RD21A-dependent manner. Collectively, our findings demonstrate that OTU2 deubiquitinates RD21A, protecting it from PbE3-2-mediated degradation and thereby mitigating P. brassicae virulence. This study provides new insights into plant immune mechanisms and offers potential strategies for developing clubroot-resistant crops.

The PopbZIP2-PopMYB4 regulatory module enhances disease resistance in poplars by modulating proanthocyanidin accumulation

Anthracnose, caused by Colletotrichum gloeosporioides, is a significant fungal disease that affects poplar trees globally, leading to reduced yields and substantial economic losses. Proanthocyanidins (PAs) play a key role in resistance to fungal pathogens; however, the mechanisms by which PAs mediate resistance to anthracnose in poplar remain poorly understood. In this study, we identified PopbZIP2, a transcription factor-encoding gene that was initially expressed in infected leaves and subsequently in uninfected leaves in response to C. gloeosporioides infection. As a transcriptional activator, PopbZIP2 can bind to the promoters of target genes PopGRF3 and PopAPA1, increasing proanthocyanidin levels in cells to enhance defense against pathogens. It is noteworthy that the PopAPA1 protein can directly inhibit pathogen growth. We further demonstrated that PopMYB4 can interact with PopbZIP2, reducing its promoter binding activity and thereby inhibiting the expression of PopGRF3 and PopAPA1. Overexpression of PopMYB4 led to sensitivity to the pathogen C. gloeosporiodes. Under normal conditions, the soluble and insoluble proanthocyanidin contents in PopMYB4 transgenic plants were significantly lower compared to the control. The dual regulation of immune responses by the PopMYB4-PopbZIP2 module unveils a novel regulatory mechanism in Populus, enhancing our understanding of the complex networks governing immune responses.

Study on the damage and variation of Agropyron mongolicum induced by the combined action of discharge plasma and plasma-activated water

To investigate the effect of combined action of discharge plasma (DP) and plasma-activated water (PAW) in mutagenesis breeding, this study focuses on Agropyron mongolicum. We utilized high-voltage DC pulsed dielectric barrier discharge for seed treatment, alone and in combination with PAW. The research focused on germination rates, evolution of physicochemical properties of imbibition residual solution, reactive oxygen species (ROS), malondialdehyde (MDA), and volatile organic compounds (VOCs) to assess DP-induced damage and variability in Agropyron mongolicum. Results indicated that after 18 h of combined treatment, the germination rate of Agropyron mongolicum dropped to 29.67%, below the LD50 threshold. Treated seedlings exhibited elevated ROS and MDA levels compared to controls. The concentration of reactive nitrogen and oxygen species (RONS) in the imbibition residual solution of the combined treatment group was lower than in freshly prepared PAW, indicating RONS penetration into the seed embryo via water, leading to oxidative damage. Enhanced lateral root differentiation, early tillering, increased biomass, and albino variant plants were observed in the surviving seedlings post-treatment. Transmission electron microscope (TEM) and Gas Chromatography-Ion Mobility Spectrometry (GC-IMS) analysis confirmed that plasma treatment induced oxidative damage in Agropyron mongolicum. In conclusion, high-power, long-duration direct DP treatment caused oxidative damage and reduced germination rates in Agropyron mongolicum, with PAW intensifying these effects. PAW was identified as the main driver of variation and lethality, while DP played a supportive role. Combined DP and PAW treatment induced variations in Agropyron mongolicum, providing experimental evidence and theoretical insights for applying DP treatment in plant mutagenesis breeding.

Non-adapted bacterial infection suppresses plant reproduction

Environmental stressors, including pathogens, substantially affect the growth of host plants. However, how non-adapted bacteria influence nonhost plants has not been reported. Here, we reveal that infection of Arabidopsis flowers by Xanthomonas oryzae pv. oryzae PXO99A, a bacterial pathogen causing rice blight disease, suppresses ovule initiation and reduces seed number without causing visible disease symptoms. TleB, secreted by the type VI secretion system (T6SS), interacts with plant E3 ligase PUB14 and disrupts the function of the PUB14-BZR1 module, leading to decreased ovule initiation and seed yield. On the other site, PUB14 concurrently promotes TleB's degradation. Our findings indicate that bacterial infections in nonhost plants directly repress offspring production. The regulatory mechanism by effectors PUB14-BZR1 is widely present, suggesting that plants may balance reproduction and defense and produce fewer offspring to conserve resources, thus enabling them to remain in a standby mode prepared for enhanced resistance.

Ubiquitination of OsCSN5 by OsPUB45 activates immunity by modulating the OsCUL3a-OsNPR1 module

The COP9 signalosome (CSN) is a highly conserved protein complex in eukaryotes, with CSN5 serving as its critical catalytic subunit. However, the role of CSN5 in plant immunity is largely unexplored. Here, we found that suppression of OsCSN5 in rice enhances resistance against the fungal pathogen Magnaporthe oryzae and the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) without affecting growth. OsCSN5 is ubiquitinated and degraded by the E3 ligase OsPUB45. Overexpression of OsPUB45 increased resistance against M. oryzae and Xoo, while dysfunction of OsPUB45 decreased resistance. In addition, OsCSN5 stabilized OsCUL3a to promote the degradation of a positive regulator OsNPR1. Overexpression of OsPUB45 compromised accumulation of OsCUL3a, leading to stabilization of OsNPR1, whereas mutations in OsPUB45 destabilized OsNPR1. These findings suggest that OsCSN5 stabilizes OsCUL3a to facilitate the degradation of OsNPR1, preventing its constitutive activation without infection. Conversely, OsPUB45 promotes the degradation of OsCSN5, contributing to immunity activation upon pathogen infection.

A frameshift mutation in JAZ10 resolves the growth versus defense dilemma in rice

CRISPR-Cas9 genome editing systems have revolutionized plant gene functional studies by enabling the targeted introduction of insertion-deletions (INDELs) via the nonhomologous end-joining (NHEJ) pathway. Frameshift-inducing INDELs can introduce a premature termination codon and, in other instances, can lead to the appearance of new proteins. Here, we found that mutations in the rice jasmonate (JA) signaling gene OsJAZ10 by CRISPR-Cas9-based genome editing did not affect canonical JA signaling. However, a type of mutant with an INDEL that yielded a novel frameshift protein named FJ10 (Frameshift mutation of JAZ10), exhibited enhanced rice growth and increased resistance to brown planthopper attacks. Overexpression of FJ10 in wild-type plants phenocopies OsJAZ10 frameshift mutants. Further characterization revealed that FJ10 interacts with Slender Rice 1 (OsSLR1) and F-box/Kelch 16 (OsFBK16). These interactions disrupt the function of OsSLR1 in suppressing gibberellin-mediated growth and the function of OsFBK16 in repressing lignin-mediated defense responses, respectively. Field experiments with FJ10-expressing plants demonstrate that this protein uncouples the growth-defense tradeoff, opening broad avenues to obtain cultivars with enhanced yield without compromised defenses.

Rice E3 ubiquitin ligases: From key modulators of host immunity to potential breeding applications

To combat pathogen attacks, plants have developed a highly advanced immune system, which requires tight regulation to initiate robust defense responses while simultaneously preventing autoimmunity. The ubiquitin-proteasome system (UPS), which is responsible for degrading excess or misfolded proteins, has vital roles in ensuring strong and effective immune responses. E3 ligases, as key UPS components, play extensively documented roles in rice immunity by modulating the ubiquitination and degradation of downstream substrates involved in various immune signaling pathways. Here, we summarize the crucial roles of rice E3 ligases in both pathogen/microbe/damage-associated molecular pattern-triggered immunity and effector-triggered immunity, highlight the molecular mechanisms by which E3 ligases function in rice immune signaling, and emphasize the functions of E3 ligases as targets of pathogen effectors for pathogenesis. We also discuss potential strategies for application of immunity-associated E3 ligases in breeding of disease-resistant rice varieties without growth penalty. This review provides a comprehensive and updated understanding of the sophisticated and interconnected regulatory functions of E3 ligases in rice immunity and in balancing immunity with growth and development.

Transcriptome Analysis Reveals the Molecular Mechanisms of Carrot Adaptation to Alternaria Leaf Blight

Carrot (Daucus carota L.) is an important vegetable crop that is rich in carotenoids and is widely cultivated throughout the world. Alternaria leaf blight (ALB), caused by infection with Alternaria dauci (A. dauci), is the most serious fungal disease in carrot production. Although several quantitative trait loci associated with ALB resistance have been identified, the genetic mechanisms underlying this resistance remain largely unelucidated. The aim of the present study was to clarify the infection mode of A. dauci and examine the molecular mechanisms underlying carrot cultivar adaptation to ALB by RNA sequencing. Microscopic observation revealed that A. dauci invades leaf tissues by entering through stomata, and resistant germplasms may significantly inhibit the infection and colonization of A. dauci. In addition, transcriptomic analyses were performed to detect the key pathways and genes associated with the differential responses between ALB-resistant (HB55) and ALB-susceptible (14088) carrot cultivars. These results suggest that the secondary metabolic process, phenylpropanoid biosynthesis, and tyrosine metabolism might play important roles in the resistance of carrots to A. dauci. Three candidate genes (LOC108208301, LOC108215577, and LOC108224339) that were specifically upregulated in the resistant carrot cultivar 'HB55' after A. dauci infection were identified as the key resistance response genes. These findings provide insights into the resistance mechanism of carrots to ALB, as well as key candidate genes and information on expression regulation for the molecular breeding of carrot disease resistance.

Engineering a conserved immune coreceptor into a primed state enhances fungal resistance in crops without growth penalty

Plants must tactically balance immunity and growth when combating lethal pathogens like fungi. CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1), a conserved cell-surface co-receptor for the fungal elicitor chitin, enables plants to induce chitin-triggered immunity to counteract fungal invasion. Previously, we reported that bacterial infection can prime CERK1 through juxtamembrane (JM) phosphorylation to enhance fungal resistance, which only occurs in Arabidopsis (Arabidopsis thaliana) and its close relatives in Brassicaceae. Here, we aim to transfer the priming mechanism of Arabidopsis CERK1 (AtCERK1) to crop CERK1 via JM substitution. We revealed in protoplasts that the entire AtCERK1 JM variable region (AtJM) is essential for imparting the bacterial elicitor flg22-induced primed state to the Nicotiana benthamiana CERK1 (NbCERK1). The NbCERK1 chimera containing AtJM (NbCERK1AtJM) and similarly constructed rice (Oryza sativa) OsCERK1AtJM could undergo flg22-induced JM phosphorylation and confer enhanced antifungal immunity upon bacterial coinfection. Moreover, the NbCERK1AtJM+3D derivative with AtJM phosphomimetic mutations to mimic a constant primed state and similarly constructed OsCERK1AtJM+3D were sufficient to mediate strengthened chitin responses and fungal resistance in transgenic plants independent of bacterial infection. Importantly, no growth and reproduction defects were observed in these plants. Taken together, this study demonstrates that manipulating the primed state of a cell-surface immune receptor offers an effective approach to improve disease resistance in crops without compromising growth and yield and showcases how fundamental insights in plant biology can be translated into crop breeding applications.

Manipulating hormones to mitigate trade-offs in crops

Addressing trait coupling due to gene pleiotropy presents challenges in conventional breeding system. However, targeted hormonal manipulation and precise genetic engineering designs hold promise to alleviate trade‐offs and unlock the potential of crops for multiple desirable traits.

Comparative Transcriptomic Analysis Reveals Domestication and Improvement Patterns of Broomcorn Millet (Panicum miliaceum L.)

Broomcorn millet (Panicum miliaceum L.) is one of the earliest crops, domesticated nearly 8000 years ago in northern China. It gradually spread across the entire Eurasian continent, as well as to America and Africa, with recent improvement in various reproductive and vegetative traits. To identify the genes that were selected during the domestication and improvement processes, we performed a comparative transcriptome analysis based on wild types, landraces, and improved cultivars of broomcorn millet at both seeding and filling stages. The variations in gene expression patterns between wild types and landraces and between landraces and improved cultivars were further evaluated to explore the molecular mechanisms underlying the domestication and improvement of broomcorn millet. A total of 2155 and 3033 candidate genes involved in domestication and a total of 84 and 180 candidate genes related to improvement were identified at seedling and filling stages of broomcorn millet, respectively. The annotation results suggested that the genes related to metabolites, stress resistance, and plant hormones were widely selected during both domestication and improvement processes, while some genes were exclusively selected in either domestication or improvement stages, with higher selection pressure detected in the domestication process. Furthermore, some domestication- and improvement-related genes involved in stress resistance either lost their functions or reduced their expression levels due to the trade-offs between stress resistance and productivity. This study provided novel genetic materials for further molecular breeding of broomcorn millet varieties with improved agronomic traits.

Epistasis and pleiotropy-induced variation for plant breeding

Epistasis refers to nonallelic interaction between genes that cause bias in estimates of genetic parameters for a phenotype with interactions of two or more genes affecting the same trait. Partitioning of epistatic effects allows true estimation of the genetic parameters affecting phenotypes. Multigenic variation plays a central role in the evolution of complex characteristics, among which pleiotropy, where a single gene affects several phenotypic characters, has a large influence. While pleiotropic interactions provide functional specificity, they increase the challenge of gene discovery and functional analysis. Overcoming pleiotropy-based phenotypic trade-offs offers potential for assisting breeding for complex traits. Modelling higher order nonallelic epistatic interaction, pleiotropy and non-pleiotropy-induced variation, and genotype × environment interaction in genomic selection may provide new paths to increase the productivity and stress tolerance for next generation of crop cultivars. Advances in statistical models, software and algorithm developments, and genomic research have facilitated dissecting the nature and extent of pleiotropy and epistasis. We overview emerging approaches to exploit positive (and avoid negative) epistatic and pleiotropic interactions in a plant breeding context, including developing avenues of artificial intelligence, novel exploitation of large-scale genomics and phenomics data, and involvement of genes with minor effects to analyse epistatic interactions and pleiotropic quantitative trait loci, including missing heritability.

Improvement of Quality and Disease Resistance for a Heavy-Panicle Hybrid Restorer Line, R600, in Rice (Oryza sativa L.) by Gene Pyramiding Breeding

The utilization of heavy-panicle hybrid rice exemplifies the successful integration of architectural enhancement and heterosis, which has been widely adopted in the southwest rice-producing area of China. Iterative improvement in disease resistance and grain quality of heavy-panicle hybrid rice varieties is crucial to promote their sustainable utilization. Here, we performed a molecular design breeding strategy to introgress beneficial alleles of broad-spectrum disease resistance and grain quality into a heavy-panicle hybrid backbone restorer line Shuhui 600 (R600). We successfully developed introgression lines through marker-assisted selection to pyramid major genes (Wxb + ALKA-GC + Pigm + Xa23) derived from three parents (Huanghuazhan, I135, I488), which significantly enhance grain quality and confer resistance to rice blast and bacterial blight (BB). The improved parental R600 line (iR600) exhibited superior grain quality and elevated disease resistance while maintaining the heavy-panicle architecture and high-yield capacity of R600. Moreover, the iR600 was crossed with male sterility line 608A to obtain a new heavy-panicle hybrid rice variety with excellent eating and cooking quality (ECQ) and high yield potential. This study presents an effective breeding strategy for rice breeders to expedite the improvement of grain quality and disease resistance in heavy-panicle hybrid rice.

Alleviating protein-condensation-associated damage at the endoplasmic reticulum enhances plant disease tolerance

Disease tolerance is an essential defense strategy against pathogens, alleviating tissue damage regardless of pathogen multiplication. However, its genetic and molecular basis remains largely unknown. Here, we discovered that protein condensation at the endoplasmic reticulum (ER) regulates disease tolerance in Arabidopsis against Pseudomonas syringae. During infection, Hematopoietic protein-1 (HEM1) and Bax-inhibitor 1 (BI-1) coalesce into ER-associated condensates facilitated by their phase-separation behaviors. While BI-1 aids in clearing these condensates via autophagy, it also sequesters lipid-metabolic enzymes within condensates, likely disturbing lipid homeostasis. Consequently, mutations in hem1, which hinder condensate formation, or in bi-1, which prevent enzyme entrapment, enhance tissue-damage resilience, and preserve overall plant health during infection. These findings suggest that the ER is a crucial hub for maintaining cellular homeostasis and establishing disease tolerance. They also highlight the potential of engineering disease tolerance as a defense strategy to complement established resistance mechanisms in combating plant diseases.

Engineering disease-resistant plants with alternative translation efficiency by switching uORF types through CRISPR

Engineering disease-resistant plants can be a powerful solution to the issue of food security. However, it requires addressing two fundamental questions: what genes to express and how to control their expressions. To find a solution, we screen CRISPR-edited upstream open reading frame (uORF) variants in rice, aiming to optimize translational control of disease-related genes. By switching uORF types of the 5'-leader from Arabidopsis TBF1, we modulate the ribosome accessibility to the downstream firefly luciferase. We assume that by switching uORF types using CRISPR, we could generate uORF variants with alternative translation efficiency (CRISPR-aTrE-uORF). These variants, capable of boosting translation for resistance-associated genes and dampening it for susceptible ones, can help pinpoint previously unidentified genes with optimal expression levels. To test the assumption, we screened edited uORF variants and found that enhanced translational suppression of the plastic glutamine synthetase 2 can provide broad-spectrum disease resistance in rice with minimal fitness costs. This strategy, which involves modifying uORFs from none to some, or from some to none or different ones, demonstrates how translational agriculture can speed up the development of disease-resistant crops. This is vital for tackling the food security challenges we face due to growing populations and changing climates.