Interactions with neighbouring plants have profound effects on plant growth, especially in crop-weed interactions, which can cause major crop-losses in agricultural systems. Black-grass ( Alopecurus myosuroides ) is the most problematic weed for UK agriculture, causing dramatic yield losses in winter wheat, but the basis of its competitive advantage is not clear. We aimed to fundamentally reappraise the nature of wheat – black-grass competition. Here, we show that black-grass is slow to establish, and requires long periods to demonstrate any competitive advantage over wheat. We show that black-grass grows significantly faster under winter conditions than wheat, and has much more vigorous root growth, collectively suggesting that prolonged root growth over winter is key to black-grass’s competitive advantage over wheat. We tested the competitiveness of a range of wheat and barley germplasm against black-grass in three distinct environmental set-ups, and show that competitiveness under controlled conditions reflects competitiveness under field conditions. In controlled conditions, we identified significant variation in crop varietal competitiveness against black-grass, in both ability to suppress black-grass growth, and to continue to grow in the presence of black-grass (tolerance). We further identified that this competition is not a zero-sum game, with loss in crop biomass not necessarily equal to gain in black-grass biomass. Finally, we found that the black-grass suppression, but not tolerance, is correlated with crop root growth, supporting our root growth hypothesis for black-grass competition. Overall, our results suggest that breeding for increased root growth and/or winter growth rate can create more competitive wheat cultivars for grassweed suppression. ### Competing Interest Statement The authors have declared no competing interest. Agriculture and Horticulture Development Board, https://ror.org/010gf7388

Strigolactones are phytohormones that regulate shoot branching and promote root interactions with arbuscular mycorrhizal fungi. In seed plants, strigolactone perception begins with the enzyme-receptor DWARF14 (D14), an α/β-hydrolase that is believed to have evolved via gene duplication from the karrikin receptor, KARRIKIN INSENSITIVE2 (KAI2). D14 and KAI2 are highly homologous at the sequence and structural levels, and both proteins bind and cleave similar butenolide compounds. Nevertheless, the two signalling pathways are distinct, as the activity of one receptor cannot compensate for the loss of the other. Here, we apply ancestral sequence reconstruction to generate a D14 protein representative of seed plants to study the evolution of substrate specificity, and to explore desirable traits for protein engineering. We describe the structure, as well as the in vitro and in planta activity of ancestral D14, showing that substrate specificity does not meaningfully differ from that of Arabidopsis thaliana D14. We also demonstrate that ancestral D14 shows higher recombinant yields, greatly increased thermostability, and enhanced catalytic activity relative to D14 from A. thaliana . This work provides insight into the evolution of phytohormone signalling and presents a robust scaffold for the application of D14-type proteins in synthetic biology. SIGNIFICANCE STATEMENT Ancestral sequence reconstruction of the strigolactone receptor DWARF14 (D14) reveals that its substrate specificity has remained largely conserved in seed plants. Additionally, the ancestral reconstruction exhibits superior recombinant yields, thermostability, and catalytic activity relative to Arabidopsis thaliana D14, providing both evolutionary insights and promising utility for synthetic biology applications. ### Competing Interest Statement The authors have declared no competing interest. Australian Research Council, DE240101210 Biotechnology and Biological Sciences Research Council, BB/R00398X/1

Correlative inhibition mechanisms allow plants to regulate the number of organs they produce, but act as a developmental limitation on crop yields. Previously, trade-offs have been observed in wheat and barley between the size of individual compound inflorescences (spikes) and the total number of spikes produced by each plant. We aimed to test the hypothesis that these trade-offs represent an uncharacterized form of correlative inhibition in grass reproductive architecture. We show that ‘high spikelet’ lines with spikes containing enhanced numbers of spikelets have reduced tillering and retarded development of later-initiated spikes. Moreover, we show that this effect is driven by the presence of developing, but not mature, spikelets in early-initiated spikes, thus representing a form of correlative inhibition. We show that cytokinin treatment delays the development of spike meristems and formation of spikelets, and as such, can promote the development of additional spikes in high spikelet lines, by providing a longer window for tillering to occur. Our results thus identify cytokinin as a potential target for breeding crops with enhanced yield. Furthermore, our results provide a developmental basis for previously observed yield trade-offs in cereal crops, and illustrate the importance of understanding developmental timing when attempting to breed higher-yielding crops. ### Competing Interest Statement The authors have declared no competing interest. Biotechnology and Biological Sciences Research Council, https://ror.org/00cwqg982, BB/X001423/1 Natural Environment Research Council, NE/S007458/1

Agricultural intensification coupled with changing climate are causing soils to become increasingly vulnerable to stresses such as drought, soil erosion, and compaction. The mechanisms by which roots detect and respond to soil stresses remain poorly understood. Recent breakthroughs show that roots release volatile and soluble hormone signals into the surrounding soil, then monitor their levels to sense soil stresses. Our review discusses how hormones can act 'outside the plant' as 'rhizocrine' signals that function to improve plant resilience to different soil stresses. We also propose a novel signalling paradigm which we term 'root RADAR' where 'rhizocrine' levels change in soil in response to environmental stresses, feeding back to roots and triggering adaptive responses.

Wood formation is crucial for plant growth, enabling water and nutrient transport through vessel elements, derived from cambium stem cells (CSCs). CSCs produce vascular cell types in a bidirectional manner, but their regulation and cell fate trajectories remain unclear. Here, using single-cell transcriptome analysis in Arabidopsis thaliana, we reveal that the strigolactone (SL) signalling pathway negatively regulates vessel element formation, impacting plant water usage. While SL signalling is generally active in differentiating vascular tissues, it is low in developing vessels and CSCs, where it modulates cell fate decisions and drought response. SL-dependent changes in vessel element formation directly affect transpiration rates via stomata, underscoring the importance of vascular tissue composition in water balance. Our findings demonstrate the role of structural alignment in water-transport tissues under unstable water conditions, offering insights for enhancing drought resistance in plants through long-term modulation of vascular development.

Plants, though sessile, can detect and respond to their neighbors through chemical signals such as strigolactones (SLs). We investigated how SL synthesis and perception affect the climbing behavior of Pisum sativum by analyzing wild-type plants and two SL-related mutants-rms1-1 (SL-deficient) and rms3-1 (SL-insensitive) - grown either alone or paired with a plant of a different genotype but of the same genetic background. Using 3D kinematic analysis, we quantified the circumnutation and attachment dynamics. Our results show that social context significantly modulated climbing behavior. rms1-1 mutants, although unable to grasp the support, showed increased movement velocity in social conditions, suggesting enhanced exploratory behavior. In contrast, rms3-1 mutants exhibited slower, disoriented movements when paired, indicating impaired neighbor perception. Wild-type plants successfully grasped the support in all conditions but altered their behavior socially, increasing movement velocity with a more careful approaching phase. These results show that SL-mediated signaling, through both emission and perception, shapes context-dependent climbing strategies in pea plants.

Many plants show strong heteroblastic changes in the shape and size of organs as they transition from juvenile to reproductive age. Most attention has been focused on heteroblastic development in leaves, but we wanted to understand heteroblastic changes in reproductive organ size. We therefore studied the progression of reproductive development in the model plant Arabidopsis thaliana, and found strong reductions in the size of flowers, fruit, seed, and internodes during development. These did not arise from correlative inhibition by older fruits, or from changes in inflorescence meristem size, but seemed to stem from changes in the size of floral organ primordia themselves. We hypothesized that environmental conditions might influence this heteroblastic pattern and found that the ambient temperature during organ initiation strongly influences organ size. We show that this temperature-dependent heteroblasty is dependent on FLOWERING LOCUS T (FT)-mediated signal integration, adding to the repertoire of developmental processes regulated by this pathway. Our results demonstrate that rising global temperatures will not affect just fertility, as is widely described, but also the size and seed number of fruits produced. However, we also show that such effects are not hard-wired, and that selective breeding for FT expression during reproductive development could mitigate such effects.

The rhizosphere is a complex physical and chemical interface between plants and their underground environment, both biotic and abiotic. Plants exude a large number of chemicals into the rhizosphere in order to manipulate these biotic and abiotic components. Among such chemicals are strigolactones, ancient signalling molecules that in flowering plants act as both internal hormones and external rhizosphere signals. Plants exude strigolactones to communicate with their preferred symbiotic partners and neighbouring plants, but at least some classes of parasitic organisms are able to 'crack' these private messages and eavesdrop on the signals. In this review, we examine the intentional consequences of strigolactone exudation, and also the unintentional consequences caused by eavesdroppers. We examine the molecular mechanisms by which strigolactones act within the rhizosphere, and attempt to understand the enigma of the strigolactone molecular diversity synthesized and exuded into the rhizosphere by plants. We conclude by looking at the prospects of using improved understanding of strigolactones in agricultural contexts.

Cytokinin (CK) is a key plant hormone, but one whose effects are often misunderstood, partly due to reliance on older data from before the molecular genetic age of plant science. In this mini-review, we examine the role of CK in controlling the reproductive shoot architecture of flowering plants. We begin with a long overdue re-examination of the role of CK in shoot branching, and discuss the relatively paucity of genetic evidence that CK does play a major role in this process. We then examine the role of CK in determining the number of inflorescences, flowers, fruit and seed that plants initiate during reproductive development, and how these are arranged in space and time. The genetic evidence for a major role of CK in controlling these processes is much clearer, and CK has profound effects in boosting the size and number of most reproductive structures. Conversely, the attenuation of CK levels during the reproductive phase likely contributes to reduced organ size seen later in flowering, and the ultimate arrest of inflorescence meristems during end-of-flowering. We finish by discussing how this information can potentially be used to improve crop yields.

RsbQ from bacteria and KAI2 from plants are highly related α/β-hydrolase proteins with unknown ligands. In a new study, Melville, Kamran et al. attempt to understand the ligand binding of RsbQ using knowledge from studies of KAI2, with surprising results.

Strigolactones (SLs) are key regulators of shoot growth and responses to environmental stimuli. Numerous studies have indicated that nitrogen (N) limitation induces SL biosynthesis, suggesting that SLs may play a pivotal role in coordinating systemic responses to N availability, but this idea has not been clearly demonstrated. Here, we generated triple knockout mutants in the SL synthesis gene TaDWARF17 (TaD17) in bread wheat and investigated their phenotypic and transcriptional responses under N limitation, aiming to elucidate the role of SLs in the adaptation to N limitation. Tad17 mutants display typical SL mutant phenotypes, and fail to adapt their shoot growth appropriately to N. Despite exhibiting an increased tillering phenotype, Tad17 mutants continued to respond to N limitation by reducing tiller number, suggesting that SLs are not the sole regulators of tillering in response to N availability. RNA-seq analysis of basal nodes revealed that the loss of D17 significantly altered the transcriptional response of N-responsive genes, including changes in the expression profiles of key N response master regulators. Crucially, our findings suggest that SLs are required for the transcriptional downregulation of cytokinin (CK) synthesis and signalling in response to N limitation. Collectively, our results suggest that SLs are essential for the appropriate morphological and transcriptional adaptation to N limitation in wheat, and that the repressive effect of SLs on shoot growth is partly mediated by their repression of CK synthesis.

Strigolactones (SLs) are signaling compounds made by plants. They play a crucial role in acting as long-distance signals from root to shoot to coordinate shoot growth with root environmental conditions. Here, we test whether and how SLs play a role in the climbing behavior of pea plants by studying the circumnutation of the tendrils using three-dimensional (3D) kinematical analysis. To assess this, we compare the typical behavior of P. sativum, a wild-type plant that produces and perceives SLs, with mutants defective in SLs synthesis or signaling, known as ramosus(rms) mutants. The results indicate that mutant plants seem unable to locate and grasp a potential support. Their movement appears to be disoriented and much less energized. We contend that this research opens new avenues for exploring SLs’ role in plant behavior, a novel lens through which the role of SLs in root-to-shoot communication can be observed and analyzed.

Plants are able to actively detect and respond to the presence in neighboring plants, in order to optimize their physiology to promote survival and reproduction despite the presence of competing organisms. A key but still poorly understood mechanism for neighbor detection is through the perception of root exudates. In this review, we explore recent findings on the role of root exudates in plant-plant interactions, focusing both on general interactions and also the highly specialized example of root parasite-host plant interactions.

The ability of plants to detect their environment, including other organisms within that environment, is unquestioned. The ability of plants to differentially process, integrate and respond to complex combinations of environmental information could perhaps be described as cognition. But no amount of evidence for cognitive abilities in plants equates to evidence for plant sentience. Nor is plant sentience required to understand or interpret the behaviour of plants in their environment.

Plants integrate environmental information into their developmental program throughout their lifetime. Light and temperature are particularly critical cues for plants to correctly time developmental transitions. Here, we investigated the role of photo-thermal cues in the regulation of the end-of-flowering developmental transition in the model plant Arabidopsis (Arabidopsis thaliana). We found that increased day length and higher temperature during flowering promote earlier inflorescence arrest by accelerating the rate at which the inflorescence meristem (IM) initiates floral primordia. Specifically, we show that plants arrest at a photo-thermal threshold and demonstrate that this photo-thermally mediated arrest is mediated by the floral integrator FLOWERING LOCUS T (FT), a known activator of flowering. FT expression increased over the duration of flowering, peaking during IM arrest, and we show that this is necessary and sufficient for photo-thermally induced arrest. Our data demonstrate the role of light and temperature, through FT, as key regulators of end-of-flowering. Overall, our results have important implications for understanding and modulating the flowering duration of crop species in changing light and temperature conditions in a warming global climate.

Wood formation is fundamental for the remarkable growth of plant bodies by continuously providing cells for long-distance transport of water and nutrients[1][1]–[3][2]. Vessel elements, the water transporting units within woody tissues, are generated from a cylindrical domain of cambium stem cells (CSCs) producing different vascular cell types in a bidirectional manner[4][3]–[6][4]. However, knowledge on the regulation of CSC activity is limited with unclear cell fate trajectories as the most obscure aspect in this context. Here, via revealing transcriptome signatures of CSCs and their derivatives with single cell resolution in Arabidopsis thaliana , we discover that the strigolactone (SL) signalling pathway modulates cell type composition in vascular tissues and thereby increases drought resistance. In particular, we find that SL signalling negatively regulates vessel element formation and thereby plant water usage. SL signalling is generally associated with differentiating vascular tissues but low in developing vessels and in CSCs implying a local role during fate decisions in CSC-derived cells. Highlighting the importance of vascular tissue composition for the overall plant water balance, alteration of vessel element formation has a direct impact on transpiration rates through leaf stomata. Our results demonstrate the importance of structural alignment of water transporting tissues to unstable water regimes and provide perspectives for a long-term modulation of drought resistance in plants.### Competing Interest StatementThe authors have declared no competing interest. [1]: #ref-1 [2]: #ref-3 [3]: #ref-4 [4]: #ref-6

To maximize reproductive success, flowering plants must correctly time entry and exit from the reproductive phase. While much is known about mechanisms that regulate initiation of flowering, end-of-flowering remains largely uncharacterized. End-of-flowering in Arabidopsis (Arabidopsis thaliana) consists of quasi-synchronous arrest of inflorescences, but it is unclear how arrest is correctly timed with respect to environmental stimuli and reproductive success. Here, we showed that Arabidopsis inflorescence arrest is a complex developmental phenomenon, which includes the arrest of the inflorescence meristem (IM), coupled with a separable "floral arrest" of all unopened floral primordia; these events occur well before visible inflorescence arrest. We showed that global inflorescence removal delays both IM and floral arrest, but that local fruit removal only delays floral arrest, emphasizing their separability. We tested whether cytokinin regulates inflorescence arrest, and found that cytokinin signaling dynamics mirror IM activity, while cytokinin treatment can delay both IM and floral arrest. We further showed that gain-of-function cytokinin receptor mutants can delay IM and floral arrest; conversely, loss-of-function mutants prevented the extension of flowering in response to inflorescence removal. Collectively, our data suggest that the dilution of cytokinin among an increasing number of sink organs leads to end-of-flowering in Arabidopsis by triggering IM and floral arrest.

The fruit and seed produced by a small number of crop plants provide the majority of food eaten across the world. Given the growing global population, there is a pressing need to increase yields of these crops without using more land or more chemical inputs. Many of these crops display prominent 'fruit-flowering feedbacks', in which fruit produced early in sexual reproductive development can inhibit the production of further fruit by a range of mechanisms. Understanding and over-coming these feedbacks thus presents a plausible route to increasing crop yields 'for free'. In this review, we define three key types of fruit-flowering feedback, and examine how frequent they are, and their effects on reproduction in a wide range of both wild and cultivated species. We then assess how these phenomenologically distinct phenomena might arise from conserved phytohormonal signalling events, particularly the export of auxin from growing organs. Finally, we offer some thoughts on the evolutionary basis for these self-limiting sexual reproductive patterns, and whether they are also present in the cereal crops that fundamentally underpin global diets.

Cereals are a group of grasses cultivated by humans for their grain. It is from these cereal grains that the majority of all calories consumed by humans are derived. The production of these grains is the result of the development of a series of hierarchical reproductive structures that form the distinct shoot architecture of the grasses. Being spatiotemporally complex, the coordination of grass shoot development is tightly controlled by a network of genes and signals, including the key phytohormone auxin. Hormonal manipulation has therefore been identified as a promising potential approach to increasing cereal crop yields and therefore ultimately global food security. Recent work translating the substantial body of auxin research from model plants into cereal crop species is revealing the contribution of auxin biosynthesis, transport, and signalling to the development of grass shoot architecture. This review discusses this still-maturing knowledge base and examines the possibility that changes in auxin biology could have been a causative agent in the evolution of differences in shoot architecture between key grass species, or could underpin the future selective breeding of cereal crops.

Photomorphogenic remodelling of seedling growth is a key developmental transition in the plant life cycle. The α/β-hydrolase signalling protein KARRIKIN-INSENSITIVE2 (KAI2), a close homologue of the strigolactone receptor DWARF14 (D14), is involved in this process, but it is unclear how the effects of KAI2 on development are mediated. Here, using a combination of physiological, pharmacological, genetic and imaging approaches in Arabidopsis thaliana (Heynh.) we show that kai2 phenotypes arise because of a failure to downregulate auxin transport from the seedling shoot apex towards the root system, rather than a failure to respond to light per se. We demonstrate that KAI2 controls the light-induced remodelling of the PIN-mediated auxin transport system in seedlings, promoting a reduction in PIN7 abundance in older tissues, and an increase of PIN1/PIN2 abundance in the root meristem. We show that removing PIN3, PIN4 and PIN7 from kai2 mutants, or pharmacological inhibition of auxin transport and synthesis, is sufficient to suppress most kai2 seedling phenotypes. We conclude that KAI2 regulates seedling morphogenesis by its effects on the auxin transport system. We propose that KAI2 is not required for the light-mediated changes in PIN gene expression but is required for the appropriate changes in PIN protein abundance within cells.