Stopping citrus greening with peptide therapy

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Citrus greening disease (also known as Huanglongbing) has had a huge impact on citrus fruit production worldwide, with Florida particularly hard hit. The disease is caused by insect-vector-spread bacteria, including Candidatus Liberibacter asiaticus (CLas). There is some genetic variability in susceptibility, which led Zhao et al. to investigate the role of an E3-ubiquitin ligase, PUB21, in disease susceptibility. Higher levels of PUB21, through both natural variation and induced expression, increased susceptibility. They also identified a naturally occurring dominant-negative PUB21 allele that encodes a variant without ubiquitin-ligase activity, and showed that it positively correlates with disease resistance. When they screened for potential targets of PUB21 ubiquitination, they identified the transcription factor MYB2, which regulates signaling by the defense hormone jasmonate. They also identified a bacterial effector that promotes the interaction between PUB21 and MYB2, leading to increased susceptibility. Finally, they identified an antiproteolysis peptide (APP), APP3-14, that disrupts the interaction between PUB21 and MYB2 in the presence of the bacterial effector. Remarkably, application of APP3-14 to infected trees increases MYB2 expression and jasmonate defenses, thereby decreasing bacterial growth and disease progression. This exciting work opens many avenues for control of citrus greening disease. (Summary by Mary Williams @PlantTeaching.bsky.social) Science 10.1126/science.adq7203

How plant vampires strike: Mechanosensitive channels in haustorium formation

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While the existence of vampires is up for debate, their botanical counterparts are very real – and much easier to find. Parasitic plants, such as Cuscuta campestris, wrap around their hosts, draining them of water and nutrients, and causing serious economic losses in agriculture. The first critical step in their attack is the formation of a haustorium – a specialized organ that attaches to the host and facilitates resource theft. Although light and mechanical cues are known to trigger haustorium primordium development, the underlying molecular mechanisms remain unclear. To address this, Park and colleagues focused on mechanosensitive ion channels (MSCs) and identified CcMCA1 as a key player in prehaustorium development. Silencing CcMCA1 significantly reduced haustorium formation, mimicking the effects of chemical inhibitors. This was accompanied by a downregulation of genes associated with haustorium development, some of which are also involved in auxin and ethylene signaling. However, the precise roles of phytohormones in this process remain to be elucidated. This study sheds light on the molecular machinery behind parasitic plant development and reveals promising targets for eliminating these “plant vampires”! (Summary by Ching Chan @ntnuchanlab) Plant Cell Physiol. 10.1093/pcp/pcaf009

Plant Science Research Weekly: April 25th, 2025

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Review: Cycad, chemicals, and coevolution

Cycads are an ancient lineage of gymnosperms with fascinating ecological interactions. In a recent review, Salzman et al. examine the various adaptations of cycads, from attracting pollinators to repelling parasites, focusing on the roles of their wide array of specialized metabolites. A somewhat unique trait in gymnosperms, most cycads have obligate insect pollination. To attract pollinators, cycads release a wide array of volatile organic compounds (VOCs) in their cones. As dioecious plants, they utilize daily fluctuations of VOCs to attract pollinators to their male cones to gather pollen, then repel them to encourage them towards female cones. Certain genera of cycads are thermogenic, with the ability to generate heat in their cones to attract insect pollinators. Aside from pollinator interactions, cycads also produce a myriad of phytotoxins in their foliage, which has driven tight coevolutionary relationships with specialized insects that have adapted to withstand these chemical defenses. Their use of secondary metabolites extends to below ground tissue where they secrete metabolites to attract nitrogen-fixing mutualists and boost their metabolic activity. Because they retain many ancient traits not found in most gymnosperms, cycads present a valuable opportunity to help bridge the gap between living and early seed plants, offering key insights into the evolutionary history of plant-animal interactions. (Summary by Xavier Ozowara [email protected]). New Phytologist https://doi.org/10.1111/nph.70109

A plant kinesin-microtubule module governs chromosome alignment in mitosis

During cell division, it’s essential that each new cell receives a complete set of chromosomes. To ensure this, chromosomes first align at the center of the cell, where structures called microtubules connect to a region on each chromosome known as the centromere. This alignment step—called chromosome congression—is crucial for proper chromosome separation. In this study, Chen et al. used a chemical genetic screen in Arabidopsis thaliana to identify mutants that are hypersensitive to a drug (propyzamide) that disrupts microtubules. One of these mutants, named propyzamide oversensitive3-1 (pos3-1), was found to have defects in cell division. The mutated gene encodes POS3, a motor protein that shares features with an animal protein called CENP-E, which helps chromosomes align during mitosis. The authors show that POS3 binds to microtubules and dynamically localizes to the kinetochores—the protein structures on chromosomes where microtubules attach. Loss of POS3 results in delayed mitosis, improper alignment of chromosomes, and errors in chromosome number in daughter cells (a condition called aneuploidy). Unexpectedly, the researchers discovered that POS3 physically interacts with MOR1, a microtubule polymerase previously known to support microtubule growth and organization. MOR1 helps POS3 localize correctly to the kinetochore. When both POS3 and MOR1 are mutated, cells show even more severe division errors, indicating these two proteins work together to coordinate chromosome alignment and segregation. This work uncovers a plant-specific system involving a kinesin motor (POS3) and a microtubule-regulating protein (MOR1) that together ensure accurate chromosome behavior during mitosis. It advances our understanding of how plants organize their mitotic machinery in the absence of structures like centrosomes, which are present in animal cells. (Summary by Muhammad Aamir Khan @MAKNature1998)  Plant Cell  10.1093/plcell/koaf053

Breaking the breeding cycle: Parthenogenesis paves the way for faster sunflower breeding

Hybrid crops are key to modern agriculture, offering higher yields, improved stress resilience, and greater uniformity through heterosis (“hybrid vigor”), where offspring from two different inbred lines outperform their parents. However, producing homozygous inbred lines for stable hybrid seed production traditionally takes at least six generations of self-pollination, slowing breeding progress. Doubled haploid (DH) technology offers a faster solution by generating homozygous lines in a single generation through haploid embryo formation and genome doubling. While DH systems are established in maize, their use in other crops has been limited by inefficiency and genotype dependence. In a serendipitous discovery, Lv et al. found that sunflower (Helianthus annuus) can produce haploid embryos spontaneously through parthenogenesis, which is when embryos form from unfertilized eggs.. Testing a chemical method used in maize to induce haploids, they observed viable seeds on emasculated, unpollinated sunflower plants. These seeds germinated into plants with smaller leaves and reduced stomatal size, confirming they were true haploids from unfertilized egg cells. Genetic analysis confirmed these seeds were maternally derived, lacking paternal DNA. By combining chemical emasculation, high-intensity light to stimulate parthenogenesis, and genome doubling, they developed a highly efficient, non-transgenic DH pipeline, reducing breeding timelines from six years to just ten months. This breakthrough not only accelerates sunflower breeding plans but also opens the door to transferring DH technology across species. Understanding the genetic and cellular mechanisms behind this process could eventually allow parthenogenesis to be induced in other crops, revolutionizing plant breeding and accelerating crop improvement. (See also the News and Views by Todesco and Reisburg 10.1038/d41586-025-00904-8) (Summary by Gourav Arora @gourav_arora_g)  Nature 10.1038/s41586-025-08798-2

“Tea rice”- the catechin fortified rice

Rice feeds nearly half of the world population. Rice grains are rich in starch but low in micronutrients and bioactive compounds such as flavonoids. By contrast, tea leaves are rich in antioxidants such as catechins but low in calories. Zhu and colleagues combine the advantages of rice and tea by producing “tea rice”. They transformed six catechin synthesis genes from tea plant (Camellia sinensis) and montbretia (Crocosmia × crocosmiiflora) into rice plants and drove their expressions in the endosperm. The transgenic rice grains had increased catechins as well as other flavonoids including kaempferol and quercetin. Consistently, the transgenic rice grains also had increased antioxidating and radical-scavenging capacities. The antioxidating environment subsequently favours the accumulation of B-vitamins in the rice grains. The study sets a successful example of genetic engineering for metabolic flux redirection and multiple health-beneficial compound accumulation in rice.  (Summary by Yee-Shan Ku @YeeShanKu1) Plant Biotechnol. J.  10.1111/pbi.70060

The hidden architecture of symbiosis: Casparian strips in root and nodule integration

The Casparian strip (CS), a lignin-rich apoplastic barrier in the root endodermis, forms early in development and regulates solute flow between the soil and the vasculature. It allows plants to control ion uptake, defend against pathogens, and coordinate developmental signals. While its functions in these processes are well studied, its role in root nodule symbiosis has remained elusive. Recent findings by Defeng et al. reveal a strong link between CS formation and nodule development in Lotus japonicus. Mutants lacking key CS regulators (LjMYB36, LjSGN1, and LjSGN3) showed impaired lignin deposition, delayed barrier formation, and reduced nodule formation after inoculation with Mesorhizobium loti R7A. Notably, wild-type nodules contain an internal Casparian strip within their vascular endodermis, serving as a secondary barrier that regulates nutrient and signal exchange between the plant and symbiont. In CS-deficient nodules, this internal barrier was absent, leading to ion imbalances (notably sodium, potassium, molybdenum, and phosphorus). These mutants also failed to produce LjCEP1, a shoot-directed peptide induced under nitrogen starvation. Without LjCEP1, the shoot could not suppress LjTML (TOO MUCH LOVE), a negative regulator of nodulation, further impairing nodule formation. Together, these results redefine the Casparian strip as a dynamic regulator of symbiotic coordination and metabolic exchange. These findings have broad implications for enhancing nitrogen fixation in crops, potentially improving agricultural productivity and sustainability. (Summary by Gourav Arora @gourav_arora_g) Science 10.1126/science.ado8680

The elephant in the genome: Cryptic infection by giant viruses

Eukaryotic genomes contain many endogenous viral elements (EVEs), which are genetic footprints of past viral infections. With the discovery of Nucleocytoplasmic Large DNA Viruses (NCLDVs), or giant viruses, research has turned to exploring how giant endogenous viral elements (GEVEs) shape host genomes. Giant viruses are prevalent in unicellular green and brown algae, and some species of bryophytes and lycophytes. While viral latency is a well-known strategy in many viral lineages, how giant viruses use latency remains poorly understood. Erazo et al. explored the effect of latency in Chlamydomonas reinhardtii green algae. Using long-read sequencing and qPCR, they discovered active GEVEs in the host genome producing new viruses, without host death. The accumulation of new viruses during the stationary phase of algal growth indicates a cryptic infection strategy. Similar viral elements have also been found in Chlamydomonas and other green algae species field isolates, suggesting that latent infections may be a widespread strategy for giant viruses. This study highlights the importance of investigating GEVEs to uncover a hidden layer of virus-host interactions that has largely gone unnoticed, with potential implications for understanding genome evolution and the broader role of giant viruses in eukaryotic biology.(Summary by Xavier Ozowara [email protected]). Science 10.1126/science.ads6303

Friend or Foe? How fungi switch between helping and harming plants

Plants coexist and interact with various microorganisms in the soil environment, including fungi. These associated fungi can not only cause diseases but also establish symbiotic interactions that boost plant health. This is the case of the endophyte Colletotrichum tofieldiae, where some strains can improve the plant’s nutritional status by transferring phosphorus from the soil, while others can exhibit pathogenic behavior. In this study, Ujimatsu and colleagues investigated the genetic causes behind this continuum between beneficial and pathogenic lifestyles. They discovered that a transcription factor called CtBOT6, part of a gene cluster involved in producing sesquiterpene metabolites that increase fungal virulence, acts like a molecular switch.  Activation of this transcription factor in pathogenic isolates drives infection, while it remains inactive in beneficial ones. When CtBOT6 is overexpressed in normally non-expressing (beneficial) strains, it transforms the fungus into a necrotrophic pathogen. This shift activates virulence genes and triggers the production of a wide array of fungal compounds. The newly pathogenic fungus can now not only colonize roots but also invade and reproduce in leaves. These findings help explain how some endophytes switch between being plant allies and aggressors, providing insight into the genetic mechanisms that distinguish pathogenic and beneficial fungal lifestyles. (Summary by Carlos González Sanz @carlosgonzsanz) Curr. Biol. 10.1016/j.cub.2025.03.026

Stopping citrus greening with peptide therapy

Citrus greening disease (also known as Huanglongbing) has had a huge impact on citrus fruit production worldwide, with Florida particularly hard hit. The disease is caused by insect-vector-spread bacteria, including Candidatus Liberibacter asiaticus (CLas). There is some genetic variability in susceptibility, which led Zhao et al. to investigate the role of an E3-ubiquitin ligase, PUB21, in disease susceptibility. Higher levels of PUB21, through both natural variation and induced expression, increased susceptibility. They also identified a naturally occurring dominant-negative PUB21 allele that encodes a variant without ubiquitin-ligase activity, and showed that it positively correlates with disease resistance. When they screened for potential targets of PUB21 ubiquitination, they identified the transcription factor MYB2, which regulates signaling by the defense hormone jasmonate. They also identified a bacterial effector that promotes the interaction between PUB21 and MYB2, leading to increased susceptibility. Finally, they identified an antiproteolysis peptide (APP), APP3-14, that disrupts the interaction between PUB21 and MYB2 in the presence of the bacterial effector. Remarkably, application of APP3-14 to infected trees increases MYB2 expression and jasmonate defenses, thereby decreasing bacterial growth and disease progression. This exciting work opens many avenues for control of citrus greening disease. (Summary by Mary Williams @PlantTeaching.bsky.social) Science 10.1126/science.adq7203

How plant vampires strike: Mechanosensitive channels in haustorium formation

While the existence of vampires is up for debate, their botanical counterparts are very real – and much easier to find. Parasitic plants, such as Cuscuta campestris, wrap around their hosts, draining them of water and nutrients, and causing serious economic losses in agriculture. The first critical step in their attack is the formation of a haustorium – a specialized organ that attaches to the host and facilitates resource theft. Although light and mechanical cues are known to trigger haustorium primordium development, the underlying molecular mechanisms remain unclear. To address this, Park and colleagues focused on mechanosensitive ion channels (MSCs) and identified CcMCA1 as a key player in prehaustorium development. Silencing CcMCA1 significantly reduced haustorium formation, mimicking the effects of chemical inhibitors. This was accompanied by a downregulation of genes associated with haustorium development, some of which are also involved in auxin and ethylene signaling. However, the precise roles of phytohormones in this process remain to be elucidated. This study sheds light on the molecular machinery behind parasitic plant development and reveals promising targets for eliminating these “plant vampires”! (Summary by Ching Chan @ntnuchanlab) Plant Cell Physiol. 10.1093/pcp/pcaf009

 

Review: Celebrating 150 years of Arabidopsis genetics

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The first known report of an Arabidopsis thaliana mutant appeared approximately 150 years ago (1873). In the intervening years, Arabidopsis has become an essential model for plant genetic research, driving groundbreaking discoveries across multiple disciplines. In a recent review, Yaschenko et al. provide a broad and succinct examination of how Arabidopsis has contributed more than its weight in scientific advancement. Its small genome, short life cycle, and ease of genetic manipulation have made it an ideal model organism, leading to major advancements in understanding plant growth, development, disease resistance, and stress tolerance. The ability to translate these findings to economically important crops has kept it as an essential tool in addressing agricultural challenges facing the world. Beyond plant biology, Arabidopsis has contributed greatly to medical research by identifying orthologs linked to human diseases, aiding in the discovery of genetic markers and biological processes relevant to potential treatments. The wealth of genomic data from Arabidopsis has also advanced biotechnology, leading to the development of tools like inducible expression systems, optogenetics, and protein interaction regulators. As research continues to evolve, Arabidopsis remains an ever-important model organism, offering critical insights into fundamental biological mechanisms that have applications beyond plant biology. (Summary by Xavier Ozowara [email protected]). Plant Cell 10.1093/plcell/koae065

Review. Genetic switchboards: Rewiring plant traits with synthetic circuits

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The expression of a transgene in plants can impose a significant stress, sometimes referred to as metabolic burden. Synthetic gene circuits offer a precise approach to engineering plant traits by regulating gene expression through programmable operations. This review by Lloyd et al. examines the core principles and components of these circuits, including sensors, integrators, and actuators. While natural gene regulatory networks have evolved for survival and been further modified by selective breeding, synthetic circuits provide targeted control over gene expression. These circuits function through logical operations (e.g., AND, OR, NOR gates) and require orthogonality, a principle that relies on genetic parts designed to interact strongly with each other while minimizing unintended interactions with other cellular components. The authors outline synthetic gene circuit architecture: sensors detect molecular or environmental inputs via inducible promoters, though stability challenges must be addressed. Integrators process signals using engineered promoters, recombinases, or CRISPR repressors, enabling logic-based regulation. Actuators execute the response, modifying cell function, such as controlling endogenous genes or influencing metabolic pathways. Bacterial allosteric transcription factors (aTFs) offer a promising means of combining sensing of specific metabolites and regulated gene expression but require further optimization to function efficiently in plant systems. A major challenge in plant synthetic biology is the long development time compared to bacteria, where rapid design-build-test-learn (DBTL) cycles enable faster refinement. Transient expression systems can accelerate testing before stable transformation. Other challenges include inefficient gene targeting, lack of standardized DNA delivery methods, and whole-plant regeneration constraints. Advances in computational modeling, high-throughput screening, and targeted transgene integration will be critical for progress. Overcoming these obstacles will unlock new plant traits, improve crop resilience, and enhance fundamental plant research. (Summary by Elisa De Meo) Plant J. 10.1111/tpj.70090.

Cells are larger than life when ExPOSEd

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Cells are as small as life gets, but can be much larger than they appear if given room for expansion. This is possible with expansion microscopy, a technique that enables three dimensional cell imaging by physically expanding cellular components for visualization. Although expansion microscopy has been used in several eukaryotic systems, it is only recently being extended to plant cells, which present challenges due to their cell walls. To address this problem, Cox et al. introduce ExPOSE, an expansion microscopy technique that has been optimized for plant protoplasts – cells without cell walls. The cell wall of maize and Arabidopsis leaf cells were enzymatically digested to isolate protoplasts which were then fixed, treated with a protein-binding anchor, and embedded in a swellable hydrogel overnight. This process resulted in an average physical expansion of more than 10-fold. With the cell physically zoomed in, nothing stays hidden. ExPOSE allows high-resolution visualization of cell components such as protein localization within mitochondrial matrices which are normally invisible in unexpanded cells. The authors also used ExPOSE to observe DNA architecture, detect individual mRNA foci, resolve the spatial resolution of tightly packed proteins, and capture subtle co-localization changes – all using a standard confocal microscope. Perhaps one of ExPOSE’s standout advantages is its application in studying biomolecular condensates, a use not previously reported with expansion microscopy. If single cell study is the target, then there’s room for expansion with ExPOSE. (Summary by Irene I. Ikiriko @ireneikiriko) Plant J. 10.1111/tpj.70049

Expanding the resolution limits of conventional microscopy in whole plant tissues

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How can we precisely image plant tissues in super-resolution when approaching the optical limits of conventional microscopes? One solution lies in expansion microscopy, a technique that embeds tissue samples in an expandable hydrogel that proportionally increases the distances between structures, allowing the user to image them clearly on microscopes that would be diffraction-limited at such fine scales. While this technique has been applied to many fields of biology, and even plant protoplasts (see “Cells are larger than life when ExPOSEd“), whole plant tissues present particular challenges due to their rigid and cohesive cell walls. In this paper, Gallei, Truckenbrodt and colleagues describe PlantEx, a plant-specific expansion microscopy protocol that includes a cell wall digestion step crafted to address these challenges. This process is demonstrated with Arabidopsis thaliana root tissue and the results confirmed to introduce no significant distortion to tissue architecture. PlantEx is also combined with stimulated emission depletion microscopy to further increase resolution and enable subcellular imaging. PlantEx can only be performed on fixed tissues and would require calibration for application to other plant species and tissue types, but it has transformative potential in increasing ease and accessibility of super-resolution imaging for plant biology. (Summary by Elise Krespan) Plant Cell 10.1093/plcell/koaf006

CA-nundrum: How a spontaneous mutation in carbonic anhydrase uncouples leaf δ13C, WUE and C4 photosynthesis

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With climate change, drought is expected to happen more frequently, making supplemental irrigation increasingly necessary to sustain crop productivity. One target trait to improve climate resilience is water use efficiency (WUE), defined by the ratio of carbon assimilation to water used by the plant. However, directly measuring WUE is time consuming and low-throughput, highlighting the need for proxy traits that enable large-scale phenotyping. Leaf tissue stable carbon isotope composition (δ13Cleaf) is commonly used as a proxy for WUE because it reflects CO2 availability and carbon metabolism. In this study, Twohey et al. investigated two maize lines, OQ414 and LH82, which showed similar morphological characteristics but very different δ13Cleaf values. A biparental mapping F2 population derived from these two lines showed a simple dominance segregation pattern for δ13Cleaf. Genetic mapping identified a major QTL on chromosome 3, located within 1 LOD of the carbonic anhydrase 1 (cah1) and carbonic anhydrase 2 (cah2) loci. Complementation test confirmed that the phenotype observed was due to a mutation in cah1. Sequencing revealed that OQ414 harbors a deletion in exons 5-10 and part of exon 11 in cah1. Surprisingly, this mutation led to significantly increased carbonic anhydrase (CA) content and photosynthetic rates. The altered domain structure in OQ414 is 222 amino acids shorter than wildtype which resembles a functional splice variant. In conclusion, this study demonstrated that mutation in cah1 uncoupled the relationship between δ13C and WUE in C4 plants. The findings underscore the complexity of using δ13Cleaf as a proxy for breeding WUE in C4 species. This paper also reports, for the first time, a mutation in CA gene that does not impair, but rather enhances, CA activity. (Summary by Mae Mercado) bioRxiv https://doi.org/10.1101/2025.02.20.639358

Lights, camera, pectin: Bringing hypocotyl elongation out of the dark

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Light is a powerful signal, shaping plant development and growth. However, the cellular mechanisms that translate light signals into precise developmental responses are still being unravelled. The Arabidopsis hypocotyl (the embryonic stem, situated underneath the cotyledons in seedlings) rapidly restricts elongation upon light exposure. Zhang et al. combined molecular and mechanical techniques to uncover a succinct model for how this inhibition occurs in the dark-to-light transition. Initially, using time-lapse photography, they identified a key regulator inhibiting hypocotyl elongation: ELONGATED HYPOCOTYL 5 (HY5). Through chemical quantification and immunohistochemical analysis of the cell walls they found that, unlike in wild-type plants, there is no change in the accumulation of pectin in the hy5 mutant. Further, Raman microscopy showed that pectin is polarised to transverse walls of cells following light treatment – a pattern absent in hy5. This polarisation of pectin is due to the asymmetric localisation of GALACTOTRANSFERASE9 (GALT9) to the transverse cell walls. GALT9 is upregulated in the absence of miR775 – which is suppressed by stabilized HY5. This polarisation of pectin increases the elastic modulus (measured using Atomic Force Microscopy) of transverse cell walls in a wild-type system in response to light, therefore inhibiting elongation of the hypocotyl. These results offer new mechanical insight into the inhibition of hypocotyl growth in response to light, unveiling pectin and GALT9 as key players in this process. (Summary by Kes Maio @kesmaio.bsky.social) Curr. Biol. 10.1016/j.cub.2024.12.026