Journal of Experimental Botany

Hybrid wheat breeding offers a promising route to enhance grain yield and yield stability through heterosis, yet hybrid grain production remains constrained by limited cross-pollination efficiency due to high rates of autogamy. To achieve cross-pollination in an autogamous species like wheat, pollen must shed outside the floret. This is typically assessed by scoring visual anther extrusion (VAEX), a key floral trait that sets the foundation for cross-pollination. However, VAEX explains only part of the variation in hybrid grain set. To address this, we analyzed floral structures and reproductive processes underlying cross-pollination efficiency in wheat. From 24 elite winter wheat genotypes, we developed traits describing anther extrusion kinetics, pollen release, and floral bract architecture. These traits showed substantial genotypic variation and high heritability. While VAEX alone explained approximately 49% of the variation in hybrid grain set, combined trait analyses explained up to 77%, demonstrating that hybrid grain production is governed by coordinated floral and reproductive trait interactions. Together, our analyses define a hierarchical trait architecture linking floral bract mechanics, anther extrusion dynamics, and pollen shedding to cross-fertilization success. This establishes a systems-level phenotyping framework for improving male parent selection in hybrid wheat breeding. HighlightHigh cross-pollination efficiency in wheat is a multi-factorial process that requires lighter floral bract architecture combined with adequate anther extrusion and pollen release for improving hybrid grain production.

Improving transpiration efficiency (TE) offers a pathway to increase yield in drought-prone environments. This study examined genotypic variation in TE and its physiological determinants across diverse wheat lines. An initial experiment with six cultivars was expanded to 105 genetically-diverse genotypes evaluated under well-watered conditions and fluctuating vapour pressure deficit (VPD). Using a high-throughput lysimeter platform, transpiration rates were recorded every 10 minutes and normalised daily at low VPD to account for genotypic variations in canopy size. TE was strongly associated with reduced normalised transpiration rate at high VPD (TRnorm-highVPD), while no significant relationship was found with maximum photosynthetic capacity. High-TE lines achieved either greater biomass with similar water use or similar biomass with lower water use, reflecting a water-saving strategy under high evaporative demand. A complementary experiment under low VPD revealed limited genotypic variation in intrinsic TE, reinforcing the value of TRnorm-highVPD as a screening trait. Consistent correlations between TE and TRnorm-highVPD across experiments highlight the potential of TRnorm-highVPD as a robust phenotyping target. Several high-TE lines outperformed modern cultivars, offering promising sources of novel alleles. These findings provide a scalable framework to identify drought-resilient genotypes and support breeding strategies to improve water productivity and achieve more crop per drop. HIGHLIGHTWheat genotypes with reduced normalised transpiration rate under high evaporative demand achieved greater transpiration efficiency. A new automatised high-throughput phenotyping framework is proposed to assist drought-resilient breeding.

The availability of ABA, a central component in the regulation of the dormancy cycle in grapevine buds, is controlled by coordinated and opposite regulation of NCED and ABA8OX expression, as shown during natural dormancy release and following treatment with Hydrogen Cyanamide (HC). This implies the existence of a shared regulatory entity, which serves as an upstream switch. A molecular switch for integrated and opposite regulation of NCED and ABA8OX was recently described in rice, involving a pair of bHLH transcription factors (OsSD6-OsICE2) that directly regulate ABA8OX3 expression and indirectly regulate NCED2 expression, by direct regulation of the expression of the NCED repressor OsbHLH048. Here, we tested whether expression of the Vitis homologs of the rice SD6 and ICE2 are regulated by dormancy release stimuli, and whether the direction of regulation agrees with that of ABA8OX. Treatment with two independent stimuli of bud break (HC and hypoxia), as well as natural dormancy release, resulted in upregulation of OsSD6 homologs and down regulation of OsICE2 homologs, in agreement with the rice model. In unexpected contrast, the homolog of OsbHLH048 was down-regulated. Our results suggest a grapevine model in which (1) the homologs of OsSD6 and OsICE2 act as direct activators and repressors of ABA8OX3 expression, as for rice, (2) they have opposed effects on the expression of an OsbHLH048 homolog, which serves as direct activator of NCED expression, as for Arabidopsis, and (3) together they act as a switch that allows removal of ABA repression, followed by meristem reactivation and bud break. HIGHLIGHTA molecular switch for integrated and opposite regulation of NCED and ABA8OX in rice seeds, operated by three bHLH transcription factors, is conserved in grapevine buds and regulates dormancy release

To investigate the interactive effects of elevated CO2 and heat stress (HS), we grew two contrasting wheat cultivars, early-maturing Scout and high-tillering Yitpi, under non-limiting water and nutrients at ambient (aCO2, 450 ppm) or elevated (eCO2, 650 ppm) CO2 and 22{degrees}C in the glasshouse. Plants were exposed to two 3-day HS cycles at the vegetative (38.1{degrees}C) and/or flowering (33.5{degrees}C) stage. At aCO2, both wheat cultivars showed similar responses of photosynthesis and mesophyll conductance to temperature and produced similar grain yield. Relative to aCO2, eCO2 enhanced photosynthesis rate and reduced stomatal conductance and maximal carboxylation rate (Vcmax). During HS, high temperature stimulated photosynthesis at eCO2 in both cultivars, while eCO2 stimulated photosynthesis in Scout. Electron transport rate (Jmax) was unaffected by any treatment. eCO2 equally enhanced biomass and grain yield of both cultivars in control, but not HS, plants. HS reduced biomass and yield of Scout at eCO2. Yitpi, the cultivar with higher grain nitrogen, underwent a trade-off between grain yield and nitrogen. In conclusion, eCO2 improved photosynthesis of control and HS wheat, and improved biomass and grain yield of control plants only. Under well-watered conditions, HS was not detrimental to photosynthesis or growth but precluded a yield response to eCO2. Key messageHigh temperatures increased photosynthetic rates only at eCO2 and photosynthesis was upregulated after recovery from heat stress at eCO2 in Scout suggesting that eCO2 increased optimum temperature of photosynthesis.

Stomatal traits balance carbon gain with water loss, yet their breeding potential in wheat remains underexploited. This study investigated physiological and anatomical stomatal responses alongside yield across two years of large-scale field trials under water-limitation and delayed sowing-induced heat exposure. Across both seasons, stomatal conductance (gs) declined under stress, reflecting strong environmental constraint on gas-exchange (water-limitation: -26.9%; heat: -13.8%). Partitioning responses by leaf surface and genotype identified the adaxial surface as the dominant contributor to gs variation and the most stress responsive. Despite increases in theoretical anatomical gas-exchange capacity (gsmax), gs-efficiency declined, indicating partial decoupling between structural potential and realised conductance. Drought reduced stomatal size while increasing density whereas heat increased size, suggesting stress-specific anatomical plasticity. Moderate-to-high heritability was observed for anatomical traits (Water-limitation: 0.13-0.57; Heat: 0.42-0.71), contrasting with lower and less stable heritability for gs (water-limitation: 0.13-0.41; heat: 0.13-0.50). Genome-wide-association-mapping identified 169 putative QTLs, predominantly for anatomical traits, including stable and co-localised pleiotropic loci. Fourteen sets of closely positioned markers were detected across seasons or studies, with stable regions on chromosomes 2B, 3B and 7B emerging as key loci. Focusing on stable loci controlling adaxial stomatal anatomy offers a realistic strategy to enhance wheat photosynthetic efficiency and climate resilience. HighlightAdaxial stomatal traits dominate gas exchange responses to heat and drought in wheat, with stable anatomical QTL identified on chromosomes 2B, 3B and 7B. Their stability across environments supports their relevance for crop improvement in water-limited and high temperature systems.

Rising night temperatures pose a significant threat to wheat productivity, yet the physiological basis of wheat adaptation to nocturnal warming remains poorly understood. We evaluated leaf photosynthetic and respiratory traits in ten Australian wheat cultivars released between 1901 and 2012 to warm nights under temperature-controlled environments. When exposed to warmer nights, rates of leaf net CO2 assimilation measured at 25 {degrees}C (Anet25) remained stable across cultivar release date despite declines in photosynthetic capacity (Vcmax and J1500) in newer cultivars. In most cultivars leaf respiratory CO2 release in the dark (Rdark) exhibited divergent thermal responses: warm nights suppressed temperature-normalised night Rdark (Rnight) but stimulated or maintained Rdark in the daytime (Rday). The results suggest that century-long, yield-focused selection may have inadvertently maintained Anet25 under warmer nights in modern cultivars through selection for more night-temperature sensitive but efficient photosynthetic capacity (i.e. greater return per protein investment) and overall reduced respiratory demand for maintenance of processes such as Rubisco protein turnover and synthesis. Our findings highlight trait-based targets for enhancing energy efficiency and climate resilience in wheat and opportunities to improve the parameterization of Rdark to warm nights in crop and Earth system models.

Because water availability is the most important environmental factor limiting crop production, improving water use efficiency, the amount of carbon fixed per water used, is a major target for crop improvement. In rice, the genetic bases of transpiration efficiency, the derivation of water use efficiency at the whole-plant scale, and its putative component trait transpiration restriction under high evaporative demand, remain unknown. These traits were measured in a panel of 147 African rice Oryza glaberrima genotypes, known as potential sources of tolerance genes to biotic and abiotic stresses. Our results reveal that higher transpiration efficiency is associated with transpiration restriction in African rice. Detailed measurements in a subset of highly differentiated genotypes confirmed these associations and suggested that the root to shoot ratio played an important role in transpiration restriction. Genome wide association studies identified marker-trait associations for transpiration response to evaporative demand, transpiration efficiency and its residuals, that links to genes involved in water transport and cell wall patterning. Our data suggest that root shoot partitioning is an important component of transpiration restriction that has a positive effect on transpiration efficiency in African rice. Both traits are heritable and define targets for breeding rice with improved water use strategies.

Jasmonates (JAs) are plant hormones which regulate biosynthesis of many secondary metabolites, such as glucosinolates (GLSs), through JAs-responsive transcription factors (TFs). The JAs-responsive CYP83B1 gene, has been shown to catalyze the conversion of indole-3-acetaldoxime (IAOx) to indolic glucosinolates (IGLSs). However, little is known about the regulatory mechanism of CYP83B1 gene expression by JAs. In yeast one-hybrid screens using the CYP83B1 promoter as bait we isolated two JAs-responsive TFs ERF109 and MYB51 that are involved in JAs-regulated IGLS biosynthesis. Furthermore, using a yeast two-hybrid assay, we identified ERF109 as an interacting partner of MYB51, and Jasmonate ZIM-domain (JAZ) proteins as interactors of MYB51, and BTB/POZ-MATH (BPM) proteins as interactors of ERF109. Both JAZ and BPM proteins are necessary for the full repression of the ERF109-MYB51-MYC3 ternary complex activity on CYP83B1 gene expression and JA-regulated IGLS biosynthesis. Biochemical analysis showed that the 26S proteasome-mediated degradation of ERF109 protein is mediated by a CRL3BPM E3 ligase independently of JA signaling. Genetic and physiological evidence shows that MYB51 acts as an adaptor and activator to bridge the interaction with the co-activators MYC3 and ERF109, for synergistically activating the CYP83B1 gene expression, and all three factors are essential and exert a coordinated control in JAs-induced IGLS biosynthesis. Overall, this study provides insights into the molecular mechanisms of JAs-responsive ERF109-MYB51-MYC3 ternary complexes in controlling JAs-regulated GLSs biosynthesis, which provides a better understanding of plant secondary metabolism. One-sentence summaryThe JA-responsive ERF109-MYB51-MYC3 ternary complex controls JAs-regulated GLSs biosynthesis.

Root growth angle is a key determinant of root system architecture, nutrient capture efficiency and therefore yield. Yet the mechanisms governing non-vertical growth in cereal roots remain poorly understood. Here, we investigated if cereal roots maintain Gravitropic Setpoint Angles (GSAs) and the hormonal regulatory processes underpinning GSA maintenance in cereals. Firstly, we found that both wheat seminal roots and rice crown roots actively return toward their original growth angles following displacement, consistent with true GSA maintenance. Next, we show that removal of a stable reference to gravity through clinorotation resulted in a characteristic outward curvature in all root types, indicating the presence of an antigravitropic offset similar to that described in Arabidopsis. Exogenous auxin treatment induced steeper rooting in both species, suggesting conserved hormonal regulatory mechanisms of GSA in both monocots and dicots. Interestingly, lateral root GSAs displayed species-specific differences: wheat laterals returned to their GSAs more effectively than rice laterals, which showed slower and incomplete responses. Together, these findings establish that cereal roots maintain GSAs through gravity-dependent and auxin-regulated mechanisms, providing a novel framework for understanding and manipulating root system architecture in monocot crops.

O_LIPearl millet is a key cereal for food security in drylands but its yield is strongly impacted by drought. We investigated how root anatomical traits contribute to mitigating the effects of vegetative drought stress in pearl millet. C_LIO_LIWe examined associations between root anatomical traits and agronomical performance in a pearl millet diversity panel under irrigated and vegetative drought stress treatments in field trials. The impact of associated anatomical traits on transpiration was assessed using subpanels grown in different soil within a greenhouse. C_LIO_LIIn the field, total metaxylem area was positively correlated with grain weight and its maintenance under drought. In the greenhouse, genotypes with larger metaxylem area grown in sandy soil exhibited a consumerist water use strategy under irrigation, which shifted to a conservative strategy under drought. Water savings was mediated by transpiration restriction under high evaporative demand. This mechanism was dependent on soil hydraulics as it was not observed in peat soil with higher hydraulic conductivity upon soil drying. C_LIO_LIWe propose that water savings under drought, mediated by large metaxylem area and its interaction with soil hydraulics, help mitigate vegetative drought stress. Our findings highlight the role of soil hydraulic properties in shaping plant hydraulics and drought tolerance. C_LI

Water deficiency at the reproductive stage of cereal crops mainly affects the development or function of male organs, which causes strong losses in grain yield. We investigated the effects of short-term drought on the post-meiotic maturation of stamens in barley cultivar Scarlett, where it leads to a stage-dependent decline in fertility. Water deficiency neither affects pollen viability nor the formation of trinuclear pollen. However, it completely blocks pollen starch accumulation. Metabolite profiling suggests that this is due to decreased sugar content at starch-filling stages, probably reflecting impaired carbon supply from photosynthetic tissues to anther sinks. Accordingly, transcriptomic analysis shows that drought reduces the expression of stamen sugar transporters. Moreover, drought causes a strong downregulation of the pollen transcriptional network of auxin signalling and central carbon metabolism genes that controls barley pollen starch production. This wider model of the molecular effects of water deficiency on cereal pollen provides a solid foundation to characterize tolerance mechanisms in potential drought-resistant germplasm.

Leaf expansion under drought drives the trade-off between water saving for later grain production and canopy photosynthesis. Fine-tuning leaf expansion could therefore become a target of genetic progress for drought-prone environments. However, its components (branching, leaf production and elongation) may have their own genetic variability and plasticity under drought, making hard to calibrate crop simulation models and specify breeding targets. In this study, we focused on the genetic diversity of bread wheat and durum wheat to determine the links and trade-offs between the underlying processes of leaf growth under drought and how it translates to leaf expansion at the whole plant and canopy level. For that, we used non-destructive imaging both in the field and controlled condition platforms to determine the dynamics of the components of shoot expansion and analyze their relative contribution to the genetic variability of whole-plant shoot expansion under drought. Results show that leaf expansion measured at plant level in controlled environment was associated with that measured at canopy level in the field, indicating that controlled phenotyping platforms can capture the genetic variability of growth in the field. Both whole-plant and canopy expansion were associated with tillering rate. In addition, the sensitivity of shoot growth and tillering to soil water deficit were correlated, indicating that both tillering ability and sensitivity to water deficit drive the genetic variability of shoot expansion. Overall, dissecting leaf expansion dynamics allowed determining the links between shoot expansion traits under drought, and provides key targets in phenotyping, modelling and breeding for drought environments.

The Multiple Synthetic Derivatives (MSD) population is a unique hexaploid wheat resource that captures extensive genetic diversity from Aegilops tauschii and exhibits wide variation in agronomic traits. However, root system architecture (RSA), a key determinant of resource acquisition and stress adaptation, remains poorly characterized in this population. Here, we established a practical phenotyping framework for RSA analysis and evaluated MSD417 as a representative genotype. A two-dimensional cultivation platform enabling continuous imaging of seedling root growth under controlled conditions was established to quantify RSA traits and their responses to high temperatures. MSD417 was compared with its recurrent parent, Norin 61 (N61). Under controlled conditions, MSD417 displayed greater total root length, root system width, and convex hull area than N61, indicating enhanced early root vigor. This genotype also exhibited a wider seminal root angle, suggesting improved horizontal soil exploration while maintaining root depth. High-temperature treatment reduced overall root growth and minimized genotypic differences, indicating that temperature stress constrains RSA expression. Microscopic observations further revealed a lower height-to-width ratio of coleorhiza tissue of MSD417, suggesting restricted downward expansion. Collectively, this study establishes a practical framework for RSA phenotyping and demonstrates the potential of Aegilops tauschii-derived germplasm to enhance wheat root-related adaptive traits.

This study provides a thorough exploration of the mechanisms regulating the onset of flowering in peach trees, a process principally governed by bud-dormancy. We applied untargeted metabolomics combined with a comprehensive series of molecular and biochemical experiments to scrutinize the variations in bloom times among different peach cultivars. The impact of exogenous chemical stimuli, specifically ethephon (ET) and abscisic acid (ABA), on bloom times was also evaluated. Our study revealed that the ET-induced delay in bloom time was associated with higher levels of proanthocyanidin (PA) compared to anthocyanins (ACNs) during endodormancy. Furthermore, fluctuations in the PA/ACNs ratio during dormancy demonstrated a strong correlation with the chill requirements and bloom dates of 12 distinct peach genotypes. The research further uncovers the crucial role of ABA in regulating the biosynthesis of PAs and ACNs during peach tree dormancy. Intriguingly, the exogenous application of ABA during endodormancy resulted in a reduction of PA content, leading to an earlier bloom time. We also observed variations in DAM gene expression between early- and late-blooming cultivars. The late-blooming cultivars exhibited higher transcript levels of DAM genes, elevated PA levels, and lower ABA levels compared to their early-blooming counterparts. Importantly, our study proposes PAs and ACNs as quantitative marker metabolites for endo- and ecodormancy phases. This innovative finding paves the way for developing more accurate chill and heat requirement models, thereby enabling a more precise understanding and projection of the impacts of global climate change on the phenology of tree fruit species.

Seed dormancy and timing of its release is important developmental transition determining the survival of individual as well as population and species. We used Medicago truncatula as model to study legume seed dormancy in ecological and genomics context. The effect of oscillating temperatures as one of the dormancy release factor was tested over the period of 88 days on the set of 178 accessions originating from variable environmental conditions of Mediterranean basin. Phenotypic plasticity of final dormancy was significantly correlated with increased aridity, suggesting that plastic responses to external stimuli provide seeds with strong bet-hedging capacity and the potential to cope with high levels of environmental heterogeneity. Genome-wide association analysis identified candidate genes associated with dormancy release related to secondary metabolites synthesis, hormone regulation and modification of the cell wall likely mediating seed coat permeability and ultimately imbibition and germination. HighlightMedicago seed dormancy was correlated with increased aridity of the environment, suggesting that plastic responses provide seeds with a bet-hedging capacity. Genome-wide association analysis identified candidate genes associated with release from dormancy.

For wheat (Triticum aestivum), sustained crop yield at limited soil water has been linked to osmotic adjustment (OA) as one of the main drivers to minimize drought-induced reductions in leaf hydration status and growth. Canada Western Red Spring (CWRS) cultivars are typically grown in rainfed areas in northern regions with milder climates, but ongoing climate change has increased the frequency and intensity of drought event questioning how successful they are in tolerating drought. The extent of OA and its relation to stomatal behavior, leaf roll, and kernel development under periods of drought remain elusive for CWRS. For several commercially used Canada Western Red Spring (CWRS) wheat cultivars ( Superb, Stettler, AAC Viewfield), OA was not found to be a mechanism contributing to drought tolerance. In contrast, we found that sustained kernel weight during periods of relatively low soil water content was linked to tight stomatal behavior (i.e., efficient transition from onset to full stomatal closure) and early leaf roll (i.e., reductions in flag leaf width). Moreover, leaf hydration status ({Theta}RWC) marked the onset of drought-induced losses in kernel weight in all three cultivars. In conclusion, CWRS wheat lacks OA but leaf stomatal behavior and leaf rolling aid in securing leaf hydration status and kernel weight under drought. One-sentence summarySelect wheat cultivars maintain leaf hydration status and yield by early leaf roll and rapid stomatal closure in the absence of osmotic adjustment and isohydric behavior.

Wheat (Triticum aestivum L.) plays a vital role in global food security, and understanding its root traits is essential for improving water uptake under varying environmental conditions. This study investigates how breeding over a century has influenced root morphological and hydraulic properties in six German winter wheat cultivars released between 1895 and 2002. Field and hydroponic experiments were used to measure root diameter, root number, branching density, and whole root system hydraulic conductance (Krs). Results showed a significant decline in root axes number and Krs over time, while root diameter remained stable across cultivars. Additionally, dynamic functional-structural modeling using the whole-plant model CPlantBox was employed to simulate the development of Krs with root system growth, revealing that older cultivars consistently had higher hydraulic conductance than modern ones. The combined approach of field phenotyping and modeling provided a comprehensive view of the changes in root traits with breeding. These findings suggest that breeding may have unintentionally favored cultivars with smaller root systems and more conservative water uptake strategies, under the high-input, high-density conditions of modern agriculture. The lessons from this study may inform future breeding efforts aimed at optimizing wheat root systems, helping to develop cultivars with water uptake better tailored to locally changing environmental conditions.

Grapevine (Vitis vinifera L.) buds undergo seasonal dormancy to survive unfavourable conditions and synchronize growth with environmental cues. While dormancy transitions have been widely studied in temperate woody perennials, the physiological and metabolic dynamics underlying these transitions in grapevine remain poorly understood. Our study investigates seasonal changes in the physiology and metabolism of perennating buds in V. vinifera cv. Cabernet Sauvignon, focusing on dormancy depth, respiration, sugar metabolism, and cell cycle activity. We identified three distinct phases of bud quiescence: (i) para-dormancy (early summer), characterized by active metabolism and high levels of tricarboxylic acid cycle intermediates, shikimate, and myo-inositol; (ii) endo-dormancy (late summer to autumn), where dormancy depth peaked in late summer and was marked by reduced respiration and bud water content, and a sharp decline in hexose levels with a concomitant increase in raffinose; and (iii) eco-dormancy (winter to spring), featuring increased respiration, sugar mobilization (notably sucrose, glucose, and trehalose), and reactivation of cell division, with a shift of cells into the G2 phase. Just prior to bud burst, we observed a significant accumulation of sugar-phosphates, providing evidence that supports their role in promoting bud burst also in grapevine. Our findings provide new insights into the biochemical and physiological regulation of bud dormancy and bud burst, contributing to a deeper understanding of dormancy transitions in perennial crops.

The architecture of the rice inflorescence is an important determinant of seed yield. The length of the inflorescence and the number of branches are among the key factors determining the amount of spikelets, and thus seeds, that will develop. Especially the timing of the identity transition from indeterminate branch meristem to determinate spikelet meristem regulates the complexity of the inflorescence. In this context, the ALOG gene TAWAWA1 (TAW1) has been shown to delay the transition to determinate spikelet development in rice. Recently, by combining precise laser microdissection of inflorescence meristems with RNA-seq we observed that two ALOG genes, Oryza sativa OsG1-like 1 (OsG1L1) and OsG1L2, have an expression profile similar to TAW1. Here we report that osg1l1 and osg1l2 loss-of-function CRISPR mutants have similar phenotypes as the taw1 mutant, suggesting that these genes might act on related pathways during inflorescence development. Transcriptome analysis of the osg1l2 mutant suggested interactions of OsG1L2 with other known inflorescence architecture regulators and the datasets were also used for the construction of a gene regulatory network (GRN) proposing interactions between genes potentially involved in controlling inflorescence development in rice. The spatio-temporal expression profiling and phenotypical analysis of CRISPR loss-of-function mutants of the homeodomain-leucine zipper transcription factor gene OsHOX14 suggest that the proposed GRN indeed serves as a valuable resource for the identification of new players involved in rice inflorescence development. One-sentence summaryOsG1L1 and OsG1L2 control panicle architecture through delaying the transition from indeterminate branch- to determinate spikelet-meristem identity.

Lateral roots are essential components of the plant edaphic interface, contributing to water and nutrient uptake, biotic and abiotic interactions, stress survival, and plant anchorage. We have identified the TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE 3 (TTL3) being related to lateral root emergence and later development. TTL3 interacts with microtubules and potentially interconnects cytoskeletal function with the brassinosteroid signalling pathway. Loss of function of TTL3 leads to a reduced number of emerged lateral roots due to delayed development of lateral root primordia. Lateral root growth of the ttl3 mutant is less sensitive to BR treatment. Timing and spatial distribution of TTL3 expression is consistent with its role in development of lateral root primordia before their emergence and subsequent development into lateral roots. TTL3 is a novel component of the root system morphogenesis regulatory network.