The rice HIGH-TILLERING DWARF1 encoding an ortholog of Arabidopsis MAX3 is required for negative regulation of the outgrowth of axillary buds

Identification and functional analysis of strigolactone pathway genes regulating tillering traits in sugarcane

Saccharum officinarum and Saccharum spontaneum are two fundamental species of modern sugarcane cultivars, exhibiting divergent tillering patterns crucial for sugarcane architecture and yield. Strigolactones (SLs), a class of plant hormones, are considered to play a central role in shaping plant form and regulating tillering. Our study highlights the distinct tillering patterns observed between S. officinarum and S. spontaneum and implicates significant differences in SL levels in root exudates between the two species. Treatment with rac-GR24 (an artificial SL analog) suppressed tillering in S. spontaneum. Based on transcriptome analysis, we focused on two genes, TRANSCRIPTION ELONGATION FACTOR 1 (TEF1) and CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which show higher expression in S. spontaneum or S. officinarum, respectively. While the overexpression of SoCCA1 did not lead to significant phenotypic differences, overexpression of SsTEF1 in rice stimulated tillering and inhibited plant height, demonstrating its role in tillering regulation. However, the overexpression of suggests that SoCCA1 may not be the key regulator of sugarcane tillering. Yeast one-hybrid assays identified four transcription factors (TFs) regulating SsTEF1 and four and five TFs regulating SsCCA1 and SoCCA1. This study provides a theoretical foundation for deciphering the molecular mechanisms underlying the different tillering behaviors between S. officinarum and S. spontaneum, providing valuable insights for the molecular-based design of sugarcane breeding strategies.

Strigolactone and karrikin receptors regulate phytohormone biosynthetic and catabolic processes

KEY MESSAGE

Karrikin plays a more critical role in affecting the homeostasis of ABA and cytokinins, while strigolactones play a more critical role in influencing the homeostasis of jasmonic acid and gibberellins. Strigolactones (SLs) and karrikins (KARs) regulate plant growth and development through their crosstalk, and through the crosstalk between them and other phytohormones, such as abscisic acid (ABA) and auxin. However, how SL and KAR signaling pathways influence the levels of other phytohormones is still unknown. Here, we performed a comparative transcriptome analysis of the Arabidopsis thaliana double mutant dwarf14 karrikin-insensitive 2 (d14 kai2), deficient in SL and KAR perception, and the wild-type (WT) using their rosette leaves. Ten gene ontology terms related to phytohormones were enriched with differentially expressed genes derived from the 'd14 kai2 vs WT' comparison. Our data revealed that the levels of auxin, ABA and salicylic acid (SA) were higher in d14 and kai2 single and d14 kai2 mutant plants than in WT, which was consistent with the results of previous investigations. In contrast, the levels of cytokinins (CKs) were found to be lower in all single and double mutants than in WT. The levels of active gibberellins were lower in d14 and d14 kai2 mutants than in WT, while they were comparable in kai2 and WT plants. Similarly, the levels of jasmonic acid (JA) were lower in d14 and d14 kai2 plants, but higher in kai2 plants than in WT. Both transcriptome and qRT-PCR analyses indicated that SL and KAR signaling pathways affect the levels of auxin, SA, CKs, gibberellin 4 (GA4) and ABA by influencing the expression of their biosynthetic (in case of auxin, SA, GA4 and CKs) and catabolic (in case of ABA) genes. Collectively, our data demonstrated that KAI2 plays a more critical role in the homeostasis of ABA and CKs, while D14 plays a more critical role in the homeostasis of JA and gibberellins. Findings of this study indicate a complex and broad crosstalk among various phytohormones in plants, which can be considered for future exogenous applications and hormone engineering.

ZmCCD8 regulates sugar and amino acid accumulation in maize kernels via strigolactone signalling

How carbon (sucrose) and nitrogen (amino acid) accumulation is coordinatively controlled in cereal grains remains largely enigmatic. We found that overexpression of the strigolactone (SL) biosynthesis gene CAROTENOID CLEAVAGE DIOXYGENASE 8 (CCD8) resulted in greater ear diameter and enhanced sucrose and amino acid accumulation in maize kernels. Loss of ZmCCD8 function reduced kernel growth with lower sugar and amino acid concentrations. Transcriptomic analysis showed down-regulation of the transcription factors ZmMYB42 and ZmMYB63 in ZmCCD8 overexpression alleles and up-regulation in zmccd8 null alleles. Importantly, ZmMYB42 and ZmMYB63 were negatively regulated by the SL signalling component UNBRANCHED 3, and repressed expression of the sucrose transporters ZmSWEET10 and ZmSWEET13c and the lysine/histidine transporter ZmLHT14. Consequently, null alleles of ZmMYB42 or ZmMYB63 promoted accumulation of soluble sugars and free amino acids in maize kernels, whereas ZmLHT14 overexpression enhanced amino acid accumulation in kernels. Moreover, overexpression of the SL receptor DWARF 14B resulted in more sucrose and amino acid accumulation in kernels, down-regulation of ZmMYB42 and ZmMYB63 expression, and up-regulation of ZmSWEETs and ZmLHT14 transcription. Together, we uncover a distinct SL signalling pathway that regulates sucrose and amino acid accumulation in kernels. Significant association of two SNPs in the 5' upstream region of ZmCCD8 with ear and cob diameter implicates its potential in breeding toward higher yield and nitrogen efficiency.

A favorable natural variation in CCD7 from orchardgrass confers enhanced tiller number

Tiller number is a crucial determinant that significantly influences the productivity and reproductive capacity of forage. The regeneration potential, biomass production, and seed yield of perennial forage species are highly reliant on the development of tillering. Strigolactones (SLs) are recently discovered carotenoid-derived phytohormones that play a crucial role in the regulation of tillering in annual crops. However, the modulation of tiller growth in perennial forage by SLs remains insufficiently investigated. In this study, we identified two alleles of the SLs biosynthesis gene, DgCCD7A and DgCCD7D, which encode CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7), from two distinct subspecies of orchardgrass (Dactylis glomerata) exhibiting contrasting tillering phenotype and SLs content. The functionality of the DgCCD7A allele derived from high-tillering phenotypic orchardgrass was found to be diminished compared to that of DgCCD7D from the low-tillering type in rescuing the increased branching phenotype of CCD7-defective mutants in Arabidopsis and rice (Oryza sativa). Notably, the introduction of DgCCD7A in rice resulted in an increase in tiller number without significantly compromising grain yield. Moreover, we demonstrated that the L309P variation in DgCCD7A is a rare natural variant exclusively found in orchardgrass. Our findings revealed that DgCCD7A, a rare favorable natural variation of CCD7 in orchardgrass, holds significant potential for breeding application in improving the plant architecture of perennial forage and crops.

Regulatory mechanisms of strigolactone perception in rice

Strigolactones (SLs) are hormones essential for plant development and environmental responses. SL perception requires the formation of the complex composed of an SL receptor DWARF14 (D14), F-box protein D3, and transcriptional repressor D53, triggering ubiquitination and degradation of D53 to activate signal transduction. However, mechanisms of SL perception and their influence on plant architecture and environmental responses remain elusive and controversial. Here, we report that key residues at interfaces of the AtD14-D3-ASK1 complex are essential for the activation of SL perception, discover that overexpression of the D3-CTH motif negatively regulates SL perception to enhance tillering, and reveal the importance of phosphorylation and N-terminal disordered (NTD) domain in mediating ubiquitination and degradation of D14. Importantly, low nitrogen promotes phosphorylation and stabilization of D14 to repress rice tillering. These findings reveal a panorama of the activation, termination, and regulation of SL perception, which determines the plasticity of plant architecture in complex environments.

GA3-Induced SlXTH19 Expression Enhances Cell Wall Remodeling and Plant Height in Tomatoes

Plant height represents a pivotal agronomic trait for the genetic enhancement of crops. The plant cell wall, being a dynamic entity, is crucial in determining plant stature; however, the regulatory mechanisms underlying cell wall remodeling remain inadequately elucidated. This study demonstrates that the application of gibberellin 3 (GA3) enhances both plant height and cell wall remodeling in tomato (Solanum lycopersicum L.) plants. RNA sequencing (RNA-seq) results of GA3 treatment showed that the DEGs were mostly enriched for cell wall-related pathways; specifically, GA3 treatment elicited the expression of the cell wall-associated gene XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE 19 (SlXTH19), whose overexpression resulted in increased plant height. Comparative analyses revealed that SlXTH19-overexpressing lines exhibited larger cell dimensions and increased XTH activity, along with higher contents of lignin, cellulose, and hemicellulose, thereby underscoring the gene's role in maintaining cell wall integrity. Conversely, treatments with ethephon (ETH) and 1-Naphthaleneacetic acid (NAA) led to suppressed plant height and reduced SlXTH19 expression. Collectively, these findings illuminate a competitive interplay between GA and ethylene/auxin signaling pathways in regulating cell wall remodeling via SlXTH19 activation, ultimately influencing tomato plant height.

Regulation of tillering and panicle branching in rice and wheat

Branching is a critical aspect of plant architecture that significantly impacts the yield and adaptability of staple cereal crops like rice and wheat. Cereal crops develop tillers during the vegetative stage and panicle or spike branches during the reproductive stage, respectively, both of which are significantly impacted by hormones and genetic factors. Tillering and panicle branching are closely interconnected and exhibit high environmental plasticity. Here, we summarize the recent progress in genetic, hormonal, and environmental factors regulation in the branching of rice and wheat. This review not only provides a comprehensive overview of the current knowledge on branching mechanisms in rice and wheat, but also explores the prospects for future research aimed at optimizing crop architecture for enhanced productivity.

MEMBRANE PROTEIN 1 encoding an amino acid transporter confers resistance to blast fungus and leaf-blight bacterium in rice

Amino acid transporters (AATs) have been shown to be involved in immune responses during plant-pathogen interactions; however, the molecular mechanism by which they function in this process remains unclear. Here, we used a joint analysis of a genome-wide association study and quantitative trait locus (QTL) mapping to identify MEMBRANE PROTEIN 1, which acts as a QTL in rice against blast fungus. Heterogeneous expression of OsMP1 in yeast supported its function in transporting a wide range of amino acids, including Thr, Ser, Phe, His, and Glu. OsMP1 could also mediate 15N-Glu efflux and influx in Xenopus oocyte cells. The expression of OsMP1 was significantly induced by Magnaporthe oryzae in the resistant rice landrace Heikezijing, whereas no such induction was observed in the susceptible landrace Suyunuo. Overexpressing OsMP1 in Suyunuo enhanced disease resistance to blast fungus and leaf blight bacterium without resulting in a yield penalty. In addition, the overexpression of OsMP1 led to increased accumulation of Thr, Ser, Phe, and His in the leaves and this contributed to the reduced disease susceptibility, which was associated with up-regulation of the jasmonic acid pathway. Our results demonstrate the important role of OsMP1 in disease resistance in rice and provide a potential target for breeding more resistant cultivars without reducing yield.

Putative Allele of D10 Gene Alters Rice Tiller Response to Nitrogen

The number of tillers in rice significantly affects final yield, making it a key trait for breeding and nitrogen-efficient cultivation. By investigating agronomic characteristics, we analyzed phenotypic differences between the wild-type P47-1 and the mutant p47dt1, performing genetic analysis and gene mapping through population construction and BSA sequencing. The p47dt1 mutant, exhibiting dwarfism and multiple tillering, is controlled by a single gene, P47DT1, which is tightly linked to D10. A single base mutation (T to G) on chromosome 1 alters methionine to arginine, supporting D10 as the candidate gene for p47dt1. To investigate nitrogen response in tillering, KY131 (nitrogen-inefficient) and KY131OsTCP19-H (nitrogen-efficient) materials differing in TCP19 expression levels were analyzed. Promoter analysis of D10 identified TCP19 as a nitrogen-responsive transcription factor, suggesting D10's potential role in a TCP19-mediated nitrogen response pathway. Further analysis of P47-1, p47dt1, KY131, and KY131OsTCP19-H under different nitrogen concentrations revealed p47dt1's distinct tiller response to nitrogen, altered nitrogen content in stems and leaves, and changes in TCP19 expression. Additionally, D10 and TCP19 expression levels were lower in KY131OsTCP19-H than KY131 under identical conditions. In summary, P47DT1/D10 appears to modulate nitrogen response and distribution in rice, affecting tiller response, possibly under TCP19's regulatory influence.

Functions of exogenous strigolactone application and strigolactone biosynthesis genes GhMAX3/GhMAX4b in response to drought tolerance in cotton (Gossypium hirsutum L.)

BACKGROUND

Drought stress markedly constrains plant growth and diminishes crop productivity. Strigolactones (SLs) exert a beneficial influence on plant resilience to drought conditions. Nevertheless, the specific function of SLs in modulating cotton's response to drought stress remains to be elucidated.

RESULTS

In this study, we assess the impact of exogenous SL (rac-GR24) administration at various concentrations (0, 1, 5, 10, 20 µM) on cotton growth during drought stress. The findings reveal that cotton seedlings treated with 5 µM exogenous SL exhibit optimal mitigation of growth suppression induced by drought stress. Treatment with 5 µM exogenous SL under drought stress conditions enhances drought tolerance in cotton seedlings by augmenting photosynthetic efficiency, facilitating stomatal closure, diminishing reactive oxygen species (ROS) generation, alleviating membrane lipid peroxidation, enhancing the activity of antioxidant enzymes, elevating the levels of osmoregulatory compounds, and upregulating the expression of drought-responsive genes. The suppression of cotton SL biosynthesis genes, MORE AXILLARY GROWTH 3 (GhMAX3) and GhMAX4b, impairs the drought tolerance of cotton. Conversely, overexpression of GhMAX3 and GhMAX4b in respective Arabidopsis mutants ameliorates the drought-sensitive phenotype in these mutants.

CONCLUSION

These observations underscore that SLs significantly bolster cotton's resistance to drought stress.

Tomato SlARF5 participate in the flower organ initiation process and control plant height

Plant height is a critical agronomic trait closely linked to yield, primarily regulated by Gibberellins (GA) and auxins, which interact in complex ways. However, the mechanism underlying their interactions remain incompletely understood. In this study, we identified a tomato mutant exhibiting significantly reduced plant height. Through gene cloning and bulked segregant analysis (BSA) sequencing, we found that the mutant gene corresponds to the tomato auxin response factor gene SlARF5/MP. Here, we show that overexpression of SlARF5/MP significantly enhances plant height. Additionally, treatment with GA3 restored the plant height of the mutant to wild-type (WT) levels, indicating that GA content is a key factor influencing plant height. We also observed significant upregulation of GA-biosynthesis genes, including GA2-oxidases GA20ox3 and GA20ox4, as well as the GA3 biosynthesis gene GA3ox1, in SlARF5-overexpressing plants. Furthermore, we demonstrated that SlARF5 directly binds to SlGA2ox3, which mediates the conversion of GA3 to inactive GA, therebyregulating its expression. Our findings suggest that SlARF5 modulates GA3 metabolism by regulating GA synthesis genes, ultimately leading to alterations in plant height.

The role of strigolactones in resistance to environmental stress in plants

Abiotic stress impairs plant growth and development, thereby causing low yield and inferior quality of crops. Increasing studies reported that strigolactones (SL) are plant hormones that enhance plant stress resistance by regulating plant physiological processes and gene expressions. In this review, we introduce the response and regulatory role of SL in salt, drought, light, heat, cold and cadmium stresses in plants. This review also discusses how SL alleviate the damage of abiotic stress in plants, furthermore, introducing the mechanisms of SL enhancing plant stress resistance at the genetic level. Under abiotic stress, the exogenous SL analog GR24 can induce the biosynthesis of SL in plants, and endogenous SL can alleviate the damage caused by abiotic stress. SL enhanced the stress resistance of plants by protecting photosynthesis, enhancing the antioxidant capacity of plants and promoting the symbiosis between plants and arbuscular mycorrhiza (AM). SL interact with abscisic acid (ABA), salicylic acid (SA), auxin, cytokinin (CK), jasmonic acid (JA), hydrogen peroxide (H2O2) and other signal molecules to jointly regulate plant stress resistance. Lastly, both the importance of SL and their challenges for future work are outlined in order to further elucidate the specific mechanisms underlying the roles of SL in plant responses to abiotic stress.

Spatiotemporal transcriptomic atlas of rhizome formation in Oryza longistaminata

Rhizomes are modified stems that grow underground and produce new individuals genetically identical to the mother plant. Recently, a breakthrough has been made in efforts to convert annual grains into perennial ones by utilizing wild rhizomatous species as donors, yet the developmental biology of this organ is rarely studied. Oryza longistaminata, a wild rice species featuring strong rhizomes, provides a valuable model for exploration of rhizome development. Here, we first assembled a double-haplotype genome of O. longistaminata, which displays a 48-fold improvement in contiguity compared to the previously published assembly. Furthermore, spatiotemporal transcriptomics was performed to obtain the expression profiles of different tissues in O. longistaminata rhizomes and tillers. Two spatially reciprocal cell clusters, the vascular bundle 2 cluster and the parenchyma 2 cluster, were determined to be the primary distinctions between the rhizomes and tillers. We also captured meristem initiation cells in the sunken area of parenchyma located at the base of internodes, which is the starting point for rhizome initiation. Trajectory analysis further indicated that the rhizome is regenerated through de novo generation. Collectively, these analyses revealed a spatiotemporal transcriptional transition underlying the rhizome initiation, providing a valuable resource for future perennial crop breeding.

Unveiling the complexity of strigolactones: exploring structural diversity, biosynthesis pathways, and signaling mechanisms

Strigolactone is the collective name for compounds containing a butenolide as a part of their structure, first discovered as compounds that induce seed germination of root parasitic plants. They were later found to be rhizosphere signaling molecules that induce hyphal branching of arbuscular mycorrhizal fungi, and, finally, they emerged as a class of plant hormones. Strigolactones are found in root exudates, where they display a great variability in their chemical structure. Their structure varies among plant species, and multiple strigolactones can exist in one species. Over 30 strigolactones have been identified, yet the chemical structure of the strigolactone that functions as an endogenous hormone and is found in the above-ground parts of plants remains unknown. We discuss our current knowledge of the synthetic pathways of diverse strigolactones and their regulation, as well as recent progress in identifying strigolactones as plant hormones. Strigolactone is perceived by the DWARF14 (D14), receptor, an α/β hydrolase which originated by gene duplication of KARRIKIN INSENSITIVE 2 (KAI2). D14 and KAI2 signaling pathways are partially overlapping paralogous pathways. Progress in understanding the signaling mechanisms mediated by two α/β hydrolase receptors as well as remaining challenges in the field of strigolactone research are reviewed.

Potential of rice tillering for sustainable food production

Tillering, also known as shoot branching, is a fundamental trait for cereal crops such as rice to produce sufficient panicle numbers. Effective tillering that guarantees successful panicle production is essential for achieving high crop yields. Recent advances in molecular biology have revealed the mechanisms underlying rice tillering; however, in rice breeding and cultivation, there remain limited genes or alleles suitable for effective tillering and high yields. A recently identified quantitative trait locus (QTL) called MORE PANICLES 3 (MP3) has been cloned as a single gene and shown to promote tillering and to moderately increase panicle number. This gene is an ortholog of the maize domestication gene TB1, and it has the potential to increase grain yield under ongoing climate change and in nutrient-poor environments. This review reconsiders the potential and importance of tillering for sustainable food production. Thus, I provide an overview of rice tiller development and the currently understood molecular mechanisms that underly it, focusing primarily on the biosynthesis and signaling of strigolactones, effective QTLs, and the importance of MP3 (TB1). The possible future benefits in using promising QTLs such as MP3 to explore agronomic solutions under ongoing climate change and in nutrient-poor environments are also highlighted.

Auxins and grass shoot architecture: how the most important hormone makes the most important plants

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.

The Complex Interplay between Arbuscular Mycorrhizal Fungi and Strigolactone: Mechanisms, Sinergies, Applications and Future Directions

Plants, the cornerstone of life on Earth, are constantly struggling with a number of challenges arising from both biotic and abiotic stressors. To overcome these adverse factors, plants have evolved complex defense mechanisms involving both a number of cell signaling pathways and a complex network of interactions with microorganisms. Among these interactions, the relationship between symbiotic arbuscular mycorrhizal fungi (AMF) and strigolactones (SLs) stands as an important interplay that has a significant impact on increased resistance to environmental stresses and improved nutrient uptake and the subsequent enhanced plant growth. AMF establishes mutualistic partnerships with plants by colonizing root systems, and offers a range of benefits, such as increased nutrient absorption, improved water uptake and increased resistance to both biotic and abiotic stresses. SLs play a fundamental role in shaping root architecture, promoting the growth of lateral roots and regulating plant defense responses. AMF can promote the production and release of SLs by plants, which in turn promote symbiotic interactions due to their role as signaling molecules with the ability to attract beneficial microbes. The complete knowledge of this synergy has the potential to develop applications to optimize agricultural practices, improve nutrient use efficiency and ultimately increase crop yields. This review explores the roles played by AMF and SLs in plant development and stress tolerance, highlighting their individual contributions and the synergistic nature of their interaction.

Low phosphorus promotes NSP1-NSP2 heterodimerization to enhance strigolactone biosynthesis and regulate shoot and root architecture in rice

Phosphorus is an essential macronutrient for plant development and metabolism, and plants have evolved ingenious mechanisms to overcome phosphate (Pi) starvation. However, the molecular mechanisms underlying the regulation of shoot and root architecture by low phosphorus conditions and the coordinated utilization of Pi and nitrogen remain largely unclear. Here, we show that Nodulation Signaling Pathway 1 (NSP1) and NSP2 regulate rice tiller number by promoting the biosynthesis of strigolactones (SLs), a class of phytohormones with fundamental effects on plant architecture and environmental responses. We found that NSP1 and NSP2 are induced by Oryza sativa PHOSPHATE STARVATION RESPONSE2 (OsPHR2) in response to low-Pi stress and form a complex to directly bind the promoters of SL biosynthesis genes, thus markedly increasing SL biosynthesis in rice. Interestingly, the NSP1/2-SL signaling module represses the expression of CROWN ROOTLESS 1 (CRL1), a newly identified early SL-responsive gene in roots, to restrain lateral root density under Pi deficiency. We also demonstrated that GR244DO treatment under normal conditions inhibits the expression of OsNRTs and OsAMTs to suppress nitrogen absorption but enhances the expression of OsPTs to promote Pi absorption, thus facilitating the balance between nitrogen and phosphorus uptake in rice. Importantly, we found that NSP1p:NSP1 and NSP2p:NSP2 transgenic plants show improved agronomic traits and grain yield under low- and medium-phosphorus conditions. Taken together, these results revealed a novel regulatory mechanism of SL biosynthesis and signaling in response to Pi starvation, providing genetic resources for improving plant architecture and nutrient-use efficiency in low-Pi environments.

Unlocking the Multifaceted Mechanisms of Bud Outgrowth: Advances in Understanding Shoot Branching

Shoot branching is a complex and tightly regulated developmental process that is essential for determining plant architecture and crop yields. The outgrowth of tiller buds is a crucial step in shoot branching, and it is influenced by a variety of internal and external cues. This review provides an extensive overview of the genetic, plant hormonal, and environmental factors that regulate shoot branching in several plant species, including rice, Arabidopsis, tomato, and wheat. We especially highlight the central role of TEOSINTE BRANCHED 1 (TB1), a key gene in orchestrating bud outgrowth. In addition, we discuss how the phytohormones cytokinins, strigolactones, and auxin interact to regulate tillering/branching. We also shed light on the involvement of sugar, an integral component of plant development, which can impact bud outgrowth in both trophic and signaling ways. Finally, we emphasize the substantial influence of environmental factors, such as light, temperature, water availability, biotic stresses, and nutrients, on shoot branching. In summary, this review offers a comprehensive evaluation of the multifaced regulatory mechanisms that underpin shoot branching and highlights the adaptable nature of plants to survive and persist in fluctuating environmental conditions.

OsMDH12: A Peroxisomal Malate Dehydrogenase Regulating Tiller Number and Salt Tolerance in Rice

Salinity is an important environmental factor influencing crop growth and yield. Malate dehydrogenase (MDH) catalyses the reversible conversion of oxaloacetate (OAA) to malate. While many MDHs have been identified in various plants, the biochemical function of MDH in rice remains uncharacterised, and its role in growth and salt stress response is largely unexplored. In this study, the biochemical function of OsMDH12 was determined, revealing its involvement in regulating tiller number and salt tolerance in rice. OsMDH12 localises in the peroxisome and is expressed across various organs. In vitro analysis confirmed that OsMDH12 converts OAA to malate. Seedlings of OsMDH12-overexpressing (OE) plants had shorter shoot lengths and lower fresh weights than wild-type (WT) plants, while osmdh12 mutants displayed the opposite. At maturity, OsMDH12-OE plants had fewer tillers than WT, whereas osmdh12 mutants had more, suggesting OsMDH12's role in tiller number regulation. Moreover, OsMDH12-OE plants were sensitive to salt stress, but osmdh12 mutants showed enhanced salt tolerance. The Na+/K+ content ratio increased in OsMDH12-OE plants and decreased in osmdh12 mutants, suggesting that OsMDH12 might negatively affect salt tolerance through influencing the Na+/K+ balance. These findings hint at OsMDH12's potential as a genetic tool to enhance rice growth and salt tolerance.

Strigolactones and Shoot Branching: What Is the Real Hormone and How Does It Work?

There have been substantial advances in our understanding of many aspects of strigolactone regulation of branching since the discovery of strigolactones as phytohormones. These include further insights into the network of phytohormones and other signals that regulate branching, as well as deep insights into strigolactone biosynthesis, metabolism, transport, perception and downstream signaling. In this review, we provide an update on recent advances in our understanding of how the strigolactone pathway co-ordinately and dynamically regulates bud outgrowth and pose some important outstanding questions that are yet to be resolved.

Advances in cloning functional genes for rice yield traits and molecular design breeding in China

Rice, a major food crop in China, contributes significantly to international food stability. Advances in rice genome sequencing, bioinformatics, and transgenic techniques have catalyzed Chinese researchers' discovery of novel genes that control rice yield. These breakthroughs in research also encompass the analysis of genetic regulatory networks and the establishment of a new framework for molecular design breeding, leading to numerous transformative findings in this field. In this review, some breakthroughs in rice yield traits and a series of achievements in molecular design breeding in China in recent years are presented; the identification and cloning of functional genes related to yield traits and the development of molecular markers of rice functional genes are summarized, with the intention of playing a reference role in the following molecular design breeding work and how to further improve rice yield.

MORE PANICLES 3, a natural allele of OsTB1/FC1, impacts rice yield in paddy fields at elevated CO2 levels

Improving crop yield potential through an enhanced response to rising atmospheric CO₂ levels is an effective strategy for sustainable crop production in the face of climate change. Large-sized panicles (containing many spikelets per panicle) have been a recent ideal plant architecture (IPA) for high-yield rice breeding. However, few breeding programs have proposed an IPA under the projected climate change. Here, we demonstrate through the cloning of the rice (Oryza sativa) quantitative trait locus for MORE PANICLES 3 (MP3) that the improvement in panicle number increases grain yield at elevated atmospheric CO₂ levels. MP3 is a natural allele of OsTB1/FC1, previously reported as a negative regulator of tiller bud outgrowth. The temperate japonica allele advanced the developmental process in axillary buds, moderately promoted tillering, and increased the panicle number without negative effects on the panicle size or culm thickness in a high-yielding indica cultivar with large-sized panicles. The MP3 allele, containing three exonic polymorphisms, was observed in most accessions in the temperate japonica subgroups but was rarely observed in the indica subgroup. No selective sweep at MP3 in either the temperate japonica or indica subgroups suggested that MP3 has not been involved and utilized in artificial selection during domestication or breeding. A free-air CO₂ enrichment experiment revealed a clear increase of grain yield associated with the temperate japonica allele at elevated atmospheric CO₂ levels. Our findings show that the moderately increased panicle number combined with large-sized panicles using MP3 could be a novel IPA and contribute to an increase in rice production under climate change with rising atmospheric CO₂ levels.

SlZF3 regulates tomato plant height by directly repressing SlGA20ox4 in the gibberellic acid biosynthesis pathway

Plant height is an important target trait for crop genetic improvement. Our previous work has identified a salt-tolerant C2H2 zinc finger, SlZF3, and its overexpression lines also showed a semi-dwarf phenotype, but the molecular mechanism remains to be elucidated. Here, we characterized the dwarf phenotype in detail. The dwarfism is caused by a decrease in stem internode cell elongation and deficiency of bioactive gibberellic acids (GAs), and can be rescued by exogenous GA3 treatment. Gene expression assays detected reduced expression of genes in the GA biosynthesis pathway of the overexpression lines, including SlGA20ox4. Several protein-DNA interaction methods confirmed that SlZF3 can directly bind to the SlGA20ox4 promoter and inhibit its expression, and the interaction can also occur for SlKS and SlKO. Overexpression of SlGA20ox4 in the SlZF3-overexpressing line can recover the dwarf phenotype. Therefore, SlZF3 regulates plant height by directly repressing genes in the tomato GA biosynthesis pathway.

Fine Mapping of the Affecting Tillering and Plant Height Gene CHA-1 in Rice

The plant architecture of rice is an important factor affecting yield. Strigolactones (SLs) are newly discovered carotenoid-derived plant hormones that play an important role in rice plant architecture. In this study, a high-tillering dwarf mutant, CHA-1, was identified by spatial mutagenesis. CHA-1 was located in the region of 31.52-31.55 MB on chromosome 1 by map-based cloning. Compared with the wild-type THZ, the CHA-1 mutant showed that ACCAC replaced TGGT in the coding region of the candidate gene LOC_Os01g54810, leading to premature termination of expression. Genetic complementation experiments proved that LOC_Os01g54810 was CHA-1, which encodes a putative member of Class III lipase. Expression analysis showed that CHA-1 was constitutively expressed in various organs of rice. Compared with those in THZ, the expression levels of the D17 and D10 genes were significantly downregulated in the CHA-1 mutant. In addition, the concentrations of ent-2'-epi-5-deoxystrigol (epi-5DS) in the root exudates of the CHA-1 mutant was significantly reduced compared with that of THZ, and exogenous application of GR24 inhibited the tillering of the CHA-1 mutant. These results suggest that CHA-1 influences rice architecture by affecting SL biosynthesis.

Can the Wild Perennial, Rhizomatous Rice Species Oryza longistaminata be a Candidate for De Novo Domestication?

As climate change intensifies, the development of resilient rice that can tolerate abiotic stresses is urgently needed. In nature, many wild plants have evolved a variety of mechanisms to protect themselves from environmental stresses. Wild relatives of rice may have abundant and virtually untapped genetic diversity and are an essential source of germplasm for the improvement of abiotic stress tolerance in cultivated rice. Unfortunately, the barriers of traditional breeding approaches, such as backcrossing and transgenesis, make it challenging and complex to transfer the underlying resilience traits between plants. However, de novo domestication via genome editing is a quick approach to produce rice with high yields from orphans or wild relatives. African wild rice, Oryza longistaminata, which is part of the AA-genome Oryza species has two types of propagation strategies viz. vegetative propagation via rhizome and seed propagation. It also shows tolerance to multiple types of abiotic stress, and therefore O. longistaminata is considered a key candidate of wild rice for heat, drought, and salinity tolerance, and it is also resistant to lodging. Importantly, O. longistaminata is perennial and propagates also via rhizomes both of which are traits that are highly valuable for the sustainable production of rice. Therefore, O. longistaminata may be a good candidate for de novo domestication through genome editing to obtain rice that is more climate resilient than modern elite cultivars of O. sativa.

Genome-wide identification of MAXs genes for strigolactones synthesis/signaling in solanaceous plants and analysis of their potential functions in tobacco

The more axillary growth (MAX) gene family is a group of key genes involved in the synthesis and signal transduction of strigolactones (SLs) in plants. Although MAX genes play vital roles in plant growth and development, characterization of the MAX gene family has been limited in solanaceous crops, especially in tobacco. In this study, 74 members of the MAX family were identified in representative Solanaceae crops and classified into four groups. The physicochemical properties, gene structure, conserved protein structural domains, cis-acting elements, and expression patterns could be clearly distinguished between the biosynthetic and signal transduction subfamilies; furthermore, MAX genes in tobacco were found to be actively involved in the regulation of meristem development by responding to hormones. MAX genes involved in SL biosynthesis were more responsive to abiotic stresses than genes involved in SL signaling. Tobacco MAX genes may play an active role in stress resistance. The results of this study provide a basis for future in-depth analysis of the molecular mechanisms of MAX genes in tobacco meristem development and stress resistance.

Potentially Useful Dwarfing or Semi-dwarfing Genes in Rice Breeding in Addition to the sd1 Gene

The "Green revolution" gene sd1 has been used widely in the breeding of modern rice varieties for over half a century. The application of this gene has increased rice yields and thereby supported a significant proportion of the global population. The use of a single gene, however, has raised concerns in the scientific community regarding its durability, especially given the bottleneck in genetic background and the need for large input of fertilizer. New dwarfing or semi-dwarfing genes are needed to alleviate our dependence on the sole "Green revolution" gene. In the past few years, several new dwarfing and semi-dwarfing genes as well as their mutants have been reported. Here, we provide an extensive review of the recent discoveries concerning newly identified genes that are potentially useful in rice breeding, including methods employed to create and effectively screen new rice mutants, the phenotypic characteristics of the new dwarfing and semi-dwarfing mutants, potential values of the new dwarfing and semi-dwarfing genes in rice breeding, and potential molecular mechanisms associated with the newly identified genes.

Analysis of controlling genes for tiller growth of Psathyrostachys juncea based on transcriptome sequencing technology

BACKGROUND

Tillering is a complicated process in plant and is a significant trait that affects biomass and seed yield of bunch grass Psathyrostachys juncea, a typical perennial forage species. To clarify the regulatory mechanisms of tillering in P. juncea and to explore related candidate genes could be helpful to improve the seed and forage yield of perennial gramineous forages. We selected the tiller node tissues of P. juncea for transcriptome sequencing to determine the differentially expressed genes (DEG) between dense and sparse tillering genotypes. The metabolic pathway was studied, candidate genes were screened, and reference genes stability were evaluated.

RESULTS

The results showed that approximately 5466 DEGs were identified between the two genotypes with dense and sparse tillers of P. juncea, which significantly differed in tiller number. Tillering regulation pathways analysis suggested that DEGs closely related to the biosynthesis of three plant hormones, namely auxin (IAA), cytokinin (CTK), and strigolactones (SLs), while "biosynthesis of lignin" and "nitrogen metabolism" have remarkable differences between the dense and sparse tillering genotypes. Meanwhile, the reference gene Actin1, having the best stability, was screened from twelve genes with highest expression level and was used in verification of ten tillering related candidate genes.

CONCLUSIONS

The tillering mechanism of perennial grass P. juncea was expounded by transcriptome analysis of tiller node tissues. We demonstrated that dense-tillering genotypes may be distinguished by their low expression patterns of genes involved in SL, IAA, and high expression patterns of genes involved in CTK biosynthesis at the tillering stage, and nitrogen metabolism and lignin biosynthesis can also affect the number of tillers. Furthermore, the expression level of ten tillering related candidate genes were verified using Actin1 as reference gene. These candidate genes provide valuable breeding resources for marker assisted selection and yield traits improvement of P. juncea.

The tomato cytochrome P450 CYP712G1 catalyses the double oxidation of orobanchol en route to the rhizosphere signalling strigolactone, solanacol

Strigolactones (SLs) are rhizosphere signalling molecules and phytohormones. The biosynthetic pathway of SLs in tomato has been partially elucidated, but the structural diversity in tomato SLs predicts that additional biosynthetic steps are required. Here, root RNA-seq data and co-expression analysis were used for SL biosynthetic gene discovery. This strategy resulted in a candidate gene list containing several cytochrome P450s. Heterologous expression in Nicotiana benthamiana and yeast showed that one of these, CYP712G1, can catalyse the double oxidation of orobanchol, resulting in the formation of three didehydro-orobanchol (DDH) isomers. Virus-induced gene silencing and heterologous expression in yeast showed that one of these DDH isomers is converted to solanacol, one of the most abundant SLs in tomato root exudate. Protein modelling and substrate docking analysis suggest that hydroxy-orbanchol is the likely intermediate in the conversion from orobanchol to the DDH isomers. Phylogenetic analysis demonstrated the occurrence of CYP712G1 homologues in the Eudicots only, which fits with the reports on DDH isomers in that clade. Protein modelling and orobanchol docking of the putative tobacco CYP712G1 homologue suggest that it can convert orobanchol to similar DDH isomers as tomato.

Dwarf and High Tillering1 represses rice tillering through mediating the splicing of D14 pre-mRNA

Strigolactones (SLs) constitute a class of plant hormones that regulate many aspects of plant development, including repressing tillering in rice (Oryza sativa). However, how SL pathways are regulated is still poorly understood. Here, we describe a rice mutant dwarf and high tillering1 (dht1), which exhibits pleiotropic phenotypes (such as dwarfism and increased tiller numbers) similar to those of mutants defective in SL signaling. We show that DHT1 encodes a monocotyledon-specific hnRNP-like protein that acts as a previously unrecognized intron splicing factor for many precursor mRNAs (pre-mRNAs), including for the SL receptor gene D14. We find that the dht1 (DHT1I232F) mutant protein is impaired in its stability and RNA binding activity, causing defective splicing of D14 pre-mRNA and reduced D14 expression, and consequently leading to the SL signaling-defective phenotypes. Overall, our findings deepen our understanding of the functional diversification of hnRNP-like proteins and establish a connection between posttranscriptional splicing and SL signaling in the regulation of plant development.

Roles of auxin in the inhibition of shoot branching in 'Dugan' fir

Shoot branching substantially impacts vegetative and reproductive growth as well as wood characteristics in perennial woody species by shaping the shoot system architecture. Although plant hormones have been shown to play a fundamental role in shoot branching in annual species, their corresponding actions in perennial woody plants are largely unknown, in part due to the lack of branching mutants. Here, we demonstrated the role of plant hormones in bud dormancy transition toward activation and outgrowth in woody plants by comparing the physiological and molecular changes in the apical shoot stems of 'Yangkou' 020 fir and 'Dugan' fir, two Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) clones with normal and completely abolished branching phenotypes, respectively. Our studies showed that the defect in bud outgrowth was the cause of failed shoot branching in 'Dugan' fir whereas apically derived signals acted as triggers of this ectopic bud activity. Further studies indicated that auxin played a key role in inhibiting bud outgrowth in 'Dugan' fir. During bud dormancy release, the differential auxin resistant 1/Like AUX1 (AUX1/LAX) and PIN-formed (PIN) activity resulted in an ectopic auxin/indole-3-acetic acid (IAA) accumulation in the apical shoot stem of 'Dugan' fir, which could inhibit the cell cycle in the axillary meristem by decreasing cytokinin (CK) biosynthesis but increasing abscisic acid (ABA) production and response through the signaling pathway. In contrast, during bud activation and outgrowth, the striking increase in auxin biosynthesis and PIN activity in the shoot tip of 'Dugan' fir may trigger the correlative inhibition of axillary buds by modulating the polar auxin transport stream (PATS) and connective auxin transport (CAT) in shoots, and by influencing the biosynthesis of secondary messengers, including CK, gibberellin (GA) and ABA, thereby inducing the paradormancy of axillary buds in 'Dugan' fir by apical dominance under favorable conditions. The findings of this study provide important insights into the roles of plant hormones in bud outgrowth control in perennial woody plants.

Strigolactones and Cytokinin Interaction in Buds in the Control of Rice Tillering

Shoot branching is among the most crucial morphological traits in rice (Oryza sativa L.) and is physiologically modulated by auxins, cytokinins (CKs), and strigolactones (SLs) cumulatively in rice. A number of studies focused on the interplay of these three hormones in regulating rice tiller extension. The present study primarily aimed at determining the impact of different treatments, which were used to regulate rice tiller and axillary bud development on node 2 at the tillering stage and full heading stage, respectively. Transcription levels of several genes were quantified through qRT-PCR analysis, and an endogenous auxin and four types of CKs were determined through LC-MS/MS. Both nutrient deficiency and exogenous SL supply were found to inhibit rice tiller outgrowth by reducing the CK content in the tiller buds. Furthermore, supplying the inhibitor of both exogenous SLs and endogenous SL synthesis could also affect the expression level of OsCKX genes but not the OsIPT genes. Comparison of OsCKX gene expression pattern under exogenous SL and CK supply suggested that the induction of OsCKX expression was most likely via a CK-induced independent pathway. These results combined with the expression of CK type-A RR genes in bud support a role for SLs in regulating bud outgrowth through the regulation of local CK levels. SL functioned antagonistically with CK in regulating the outgrowth of buds on node 2, by promoting the OsCKX gene expression in buds.

Assessment of Rice Sheath Blight Resistance Including Associations with Plant Architecture, as Revealed by Genome-Wide Association Studies

BACKGROUND

Sheath blight (ShB) disease caused by Rhizoctonia solani Kühn, is one of the most economically damaging rice (Oryza sativa L.) diseases worldwide. There are no known major resistance genes, leaving only partial resistance from small-effect QTL to deploy for cultivar improvement. Many ShB-QTL are associated with plant architectural traits detrimental to yield, including tall plants, late maturity, or open canopy from few or procumbent tillers, which confound detection of physiological resistance.

RESULTS

To identify QTL for ShB resistance, 417 accessions from the Rice Diversity Panel 1 (RDP1), developed for association mapping studies, were evaluated for ShB resistance, plant height and days to heading in inoculated field plots in Arkansas, USA (AR) and Nanning, China (NC). Inoculated greenhouse-grown plants were used to evaluate ShB using a seedling-stage method to eliminate effects from height or maturity, and tiller (TN) and panicle number (PN) per plant. Potted plants were used to evaluate the RDP1 for TN and PN. Genome-wide association (GWA) mapping with over 3.4 million SNPs identified 21 targeted SNP markers associated with ShB which tagged 18 ShB-QTL not associated with undesirable plant architecture traits. Ten SNPs were associated with ShB among accessions of the Indica subspecies, ten among Japonica subspecies accessions, and one among all RDP1 accessions. Across the 18 ShB QTL, only qShB4-1 was not previously reported in biparental mapping studies and qShB9 was not reported in the GWA ShB studies. All 14 PN QTL overlapped with TN QTL, with 15 total TN QTL identified. Allele effects at the five TN QTL co-located with ShB QTL indicated that increased TN does not inevitably increase disease development; in fact, for four ShB QTL that overlapped TN QTL, the alleles increasing resistance were associated with increased TN and PN, suggesting a desirable coupling of alleles at linked genes.

CONCLUSIONS

Nineteen accessions identified as containing the most SNP alleles associated with ShB resistance for each subpopulation were resistant in both AR and NC field trials. Rice breeders can utilize these accessions and SNPs to develop cultivars with enhanced ShB resistance along with increased TN and PN for improved yield potential.

Genetic Dissection of Rice Ratooning Ability Using an Introgression Line Population and Substitution Mapping of a Pleiotropic Quantitative Trait Locus qRA5

Ratooning ability is a key factor that influences ratoon rice yield, in the area where light and temperature are not enough for second season rice. In the present study, an introgression line population derived from Minghui 63 as the recipient parent and 02428 as the donor parent was developed, and a high-density bin map containing 4568 bins was constructed. Nine ratooning-ability-related traits were measured, including maximum tiller number, panicle number, and grain yield per plant in the first season and ratoon season, as well as three secondary traits, maximum tiller number ratio, panicle number ratio, and grain yield ratio. A total of 22 main-effect QTLs were identified and explained for 3.26–18.63% of the phenotypic variations in the introgression line population. Three genomic regions, including 14.12–14.65 Mb on chromosome 5, 4.64–5.76 Mb on chromosome 8, and 10.64–15.52 Mb on chromosome 11, were identified to simultaneously control different ratooning-ability-related traits. Among them, qRA5 in the region of 14.12–14.65 Mb on chromosome 5 was validated for its pleiotropic effects on maximum tiller number and panicle number in the first season, as well as its maximum tiller number ratio, panicle number ratio, and grain yield ratio. Moreover, qRA5 was independent of genetic background and delimited into a 311.16 kb region by a substitution mapping approach. These results will help us better understand the genetic basis of rice ratooning ability and provide a valuable gene resource for breeding high-yield ratoon rice varieties.

The Genetic Components of a Natural Color Palette: A Comprehensive List of Carotenoid Pathway Mutations in Plants

Carotenoids comprise the most widely distributed natural pigments. In plants, they play indispensable roles in photosynthesis, furnish colors to flowers and fruit and serve as precursor molecules for the synthesis of apocarotenoids, including aroma and scent, phytohormones and other signaling molecules. Dietary carotenoids are vital to human health as a source of provitamin A and antioxidants. Hence, the enormous interest in carotenoids of crop plants. Over the past three decades, the carotenoid biosynthesis pathway has been mainly deciphered due to the characterization of natural and induced mutations that impair this process. Over the year, numerous mutations have been studied in dozens of plant species. Their phenotypes have significantly expanded our understanding of the biochemical and molecular processes underlying carotenoid accumulation in crops. Several of them were employed in the breeding of crops with higher nutritional value. This compendium of all known random and targeted mutants available in the carotenoid metabolic pathway in plants provides a valuable resource for future research on carotenoid biosynthesis in plant species.

How Strigolactone Shapes Shoot Architecture

Despite its central role in the control of plant architecture, strigolactone has been recognized as a phytohormone only 15 years ago. Together with auxin, it regulates shoot branching in response to genetically encoded programs, as well as environmental cues. A central determinant of shoot architecture is apical dominance, i.e., the tendency of the main shoot apex to inhibit the outgrowth of axillary buds. Hence, the execution of apical dominance requires long-distance communication between the shoot apex and all axillary meristems. While the role of strigolactone and auxin in apical dominance appears to be conserved among flowering plants, the mechanisms involved in bud activation may be more divergent, and include not only hormonal pathways but also sugar signaling. Here, we discuss how spatial aspects of SL biosynthesis, transport, and sensing may relate to apical dominance, and we consider the mechanisms acting locally in axillary buds during dormancy and bud activation.

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

Harmony but Not Uniformity: Role of Strigolactone in Plants

Strigolactones (SLs) represent an important new plant hormone class marked by their multifunctional roles in plants and rhizosphere interactions, which stimulate hyphal branching in arbuscular mycorrhizal fungi (AMF) and seed germination of root parasitic plants. SLs have been broadly implicated in regulating root growth, shoot architecture, leaf senescence, nodulation, and legume-symbionts interaction, as well as a response to various external stimuli, such as abiotic and biotic stresses. These functional properties of SLs enable the genetic engineering of crop plants to improve crop yield and productivity. In this review, the conservation and divergence of SL pathways and its biological processes in multiple plant species have been extensively discussed with a particular emphasis on its interactions with other different phytohormones. These interactions may shed further light on the regulatory networks underlying plant growth, development, and stress responses, ultimately providing certain strategies for promoting crop yield and productivity with the challenges of global climate and environmental changes.

Auxin Interactions with Other Hormones in Plant Development

Auxin is a crucial growth regulator that governs plant development and responses to environmental perturbations. It functions at the heart of many developmental processes, from embryogenesis to organ senescence, and is key to plant interactions with the environment, including responses to biotic and abiotic stimuli. As remarkable as auxin is, it does not act alone, but rather solicits the help of, or is solicited by, other endogenous signals, including the plant hormones abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellic acid, jasmonates, salicylic acid, and strigolactones. The interactions between auxin and other hormones occur at multiple levels: hormones regulate one another's synthesis, transport, and/or response; hormone-specific transcriptional regulators for different pathways physically interact and/or converge on common target genes; etc. However, our understanding of this crosstalk is still fragmentary, with only a few pieces of the gigantic puzzle firmly established. In this review, we provide a glimpse into the complexity of hormone interactions that involve auxin, underscoring how patchy our current understanding is.

Transgenic rice plants expressing the α-L-arabinofuranosidase of Coprinopsis cinerea exhibit strong dwarfism and markedly enhanced tillering

Lignocellulosic materials are potential renewable sources of fermentable sugars for bioethanol production. In this study, we used the CcAbf62A gene encoding CcAbf62A, a putative extracellular α-L-arabinofuranosidase, cloned from the mycotrophic basidiomycete Coprinopsis cinerea. CcAbf62A acts on arabinoxylan, the major hemicellulose of grasses, releasing arabinose. CcAbf62A was introduced into rice with the aim of enhancing delignification efficiency and the availability of lignocellulosic materials without reducing lignin content. Among the 32 lines of regenerated transgenic rice, 13 exhibited markedly disrupted elongation growth and excessive tillering (dwarf), seven showed delayed elongation growth (retarded-growth), and 12 showed phenotypes similar to those of control plants (normal). Additionally, the dwarf lines showed reduced acclimation. RT-PCR analysis revealed that dwarf lines had higher levels of CcAbf62A expression than retarded-growth and normal lines. Although the lignin content of transgenic rice plants expressing CcAbf62A did not differ significantly from that of control rice plants, dwarf lines were characterized by delayed deposition of lignin in the culms compared with the controls. The reduced acclimation ability of dwarf lines is believed to be associated with increased water loss and reduced water conductivity concomitant with delayed lignin deposition. Contrary to expectations, the alkaline delignification rates of dwarf and retarded-growth Abf lines were slightly lower than those of control rice plants. Our findings indicate that CcAbf62A reduces ferulate-lignin cross-links by detaching arabinose side chains from arabinoxylan and increases the relative abundance of alkaline-resistant benzyl ether cross-links. CcAbf62A is anticipated to provide new approaches for breeding plants containing altered lignocellulosic materials or lodging-resistant crops.

Mechanical stimulation in wheat triggers age- and dose-dependent alterations in growth, development and grain characteristics

BACKGROUND AND AIMS

Wheat crops are exposed to a range of mechanical stimulations in their natural environment, yet we know very little about their response to such conditions. The aim of this study was to better understand the effect of mechanical stimulation on wheat growth and development, stem mechanical properties and grain measures. We focused on the following questions: (1) Does plant age affect the response to mechanical stimulation? (2) Is there a minimum threshold for the perception of mechanical stimuli? (3) Is the effect of manual brushing different to natural wind stimulation?

METHODS

For age- and dose-response experiments, wheat plants were grown under controlled glasshouse conditions with brushing treatments applied using a purpose-built rig. The results of the controlled experiments are compared with those from an outside experiment where wheat plants were exposed to natural wind, with or without additional brushing. Detailed phenotypic measurements were conducted and treatment effects on grain characteristics were determined using micro-computed tomography imaging.

KEY RESULTS

Two-week-old wheat plants were particularly sensitive to mechanical stimulation by controlled brushing treatments. Amongst others, plants exhibited a large reduction in height and grain yield, and an increase in tillers, above-ground biomass and stiffness of stem segments. Plants responded significantly to doses as small as one daily brushstroke. Outdoor experiments by and large confirmed results from controlled environment experiments.

CONCLUSIONS

The morphological and developmental response to mechanical brushing treatment, in relation to vegetative above-ground biomass and grain yield, is dependent on plant age as well as the dose of the treatments. This study shows that mechanical stimulation of wheat impacts on a multitude of agriculturally relevant traits and provides a much needed advancement of our understanding of wheat thigmomorphogenesis and the potential applications of mechanical conditioning to control relevant traits.

Integration of the SMXL/D53 strigolactone signalling repressors in the model of shoot branching regulation in Pisum sativum

DWARF53 (D53) in rice (Oryza sativa) and its homologs in Arabidopsis (Arabidopsis thaliana), SUPPRESSOR OF MAX2-LIKE 6 (SMXL6), SMXL7 and SMXL8, are well established negative regulators of strigolactone (SL) signalling in shoot branching regulation. Little is known of pea (Pisum sativum) homologs and whether D53 and related SMXLs are specific to SL signalling pathways. Here, we identify two allelic pea mutants, dormant3 (dor3), and demonstrate through gene mapping and sequencing that DOR3 corresponds to a homolog of D53 and SMXL6/SMXL7, designated PsSMXL7. Phenotype analysis, gene expression, protein and hormone quantification assays were performed to determine the role of PsSMXL7 in regulation of bud outgrowth and the role of PsSMXL7 and D53 in integrating SL and cytokinin (CK) responses. Like D53 and related SMXLs, we show that PsSMXL7 can be degraded by SL and induces feedback upregulation of PsSMXL7 transcript. Here we reveal a system conserved in pea and rice, whereby CK also upregulates PsSMXL7/D53 transcripts, providing a clear mechanism for SL and CK cross-talk in the regulation of branching. To further deepen our understanding of the branching network in pea, we provide evidence that SL acts via PsSMXL7 to modulate auxin content via PsAFB5, which itself regulates expression of SL biosynthesis genes. We therefore show that PsSMXL7 is key to a triple hormone network involving an auxin-SL feedback mechanism and SL-CK cross-talk.

Carotenoid Cleavage Dioxygenase Genes of Chimonanthus praecox, CpCCD7 and CpCCD8, Regulate Shoot Branching in Arabidopsis

Strigolactones (SLs) regulate plant shoot development by inhibiting axillary bud growth and branching. However, the role of SLs in wintersweet (Chimonanthus praecox) shoot branching remains unknown. Here, we identified and isolated two wintersweet genes, CCD7 and CCD8, involved in the SL biosynthetic pathway. Quantitative real-time PCR revealed that CpCCD7 and CpCCD8 were down-regulated in wintersweet during branching. When new shoots were formed, expression levels of CpCCD7 and CpCCD8 were almost the same as the control (un-decapitation). CpCCD7 was expressed in all tissues, with the highest expression in shoot tips and roots, while CpCCD8 showed the highest expression in roots. Both CpCCD7 and CpCCD8 localized to chloroplasts in Arabidopsis. CpCCD7 and CpCCD8 overexpression restored the phenotypes of branching mutant max3-9 and max4-1, respectively. CpCCD7 overexpression reduced the rosette branch number, whereas CpCCD8 overexpression lines showed no phenotypic differences compared with wild-type plants. Additionally, the expression of AtBRC1 was significantly up-regulated in transgenic lines, indicating that two CpCCD genes functioned similarly to the homologous genes of the Arabidopsis. Overall, our study demonstrates that CpCCD7 and CpCCD8 exhibit conserved functions in the CCD pathway, which controls shoot development in wintersweet. This research provides a molecular and theoretical basis for further understanding branch development in wintersweet.

Genome-Wide Association Study of the Genetic Basis of Effective Tiller Number in Rice

BACKGROUND

Effective tiller number (ETN) has a pivotal role in determination of rice (Oryza sativa L.) grain yield. ETN is a complex quantitative trait regulated by both genetic and environmental factors. Despite multiple tillering-related genes have been cloned previously, few of them have been utilized in practical breeding programs.

RESULTS

In this study, we conducted a genome-wide association study (GWAS) for ETN using a panel of 490 rice accessions derived from the 3 K rice genomes project. Thirty eight ETN-associated QTLs were identified, interestingly, four of which colocalized with the OsAAP1, DWL2, NAL1, and OsWRKY74 gene previously reported to be involved in rice tillering regulation. Haplotype (Hap) analysis revealed that Hap5 of OsAAP1, Hap3 and 6 of DWL2, Hap2 of NAL1, and Hap3 and 4 of OsWRKY74 are favorable alleles for ETN. Pyramiding favorable alleles of all these four genes had more enhancement in ETN than accessions harboring the favorable allele of only one gene. Moreover, we identified 25 novel candidate genes which might also affect ETN, and the positive association between expression levels of the OsPILS6b gene and ETN was validated by RT-qPCR. Furthermore, transcriptome analysis on data released on public database revealed that most ETN-associated genes showed a relatively high expression from 21 days after transplanting (DAT) to 49 DAT and decreased since then. This unique expression pattern of ETN-associated genes may contribute to the transition from vegetative to reproductive growth of tillers.

CONCLUSIONS

Our results revealed that GWAS is a feasible way to mine ETN-associated genes. The candidate genes and favorable alleles identified in this study have the potential application value in rice molecular breeding for high ETN and grain yield.

Overexpression of MtRAV3 enhances osmotic and salt tolerance and inhibits growth of Medicago truncatula

Related to ABI3/VP1 (RAV) transcription factors play important roles in regulating plant growth and stress tolerance, which have been studied in many plant species, but have remained largely unidentified in legumes. To functionally characterize RAV in legumes, MtRAV3 from legume model plant Medicago truncatula was isolated and its function was investigated by overexpressing MtRAV3 in M. truncatula. Expression analysis demonstrated that MtRAV3 was markedly induced by NaCl and polyethylene glycol (PEG). MtRAV3 overexpression enhanced tolerance of transgenic M. truncatula to mannitol, drought and salt stresses, and induced the expression of adversity-related genes, including MtWRKY76, MtMYB61, cold-acclimation specific protein 31 (MtCAS31), alternative oxidase 1 (MtAOX1) and ethylene response factor 1 (MtERF1). There were lower relative electrolyte leakage and higher chlorophyll content of leaves in the MtRAV3 overexpression plants than in wild type plants under both salt and drought stress. MtRAV3 overexpression M. truncatula were featured by some phenotypes of dwarfing, late flowering, more branches, smaller flower and leaf organs. Further investigations showed that the expression levels of DWARF14 (MtD14), CAROTENOID CLEAVAGE DIOXYGENASES 7 (MtCCD7) and GA3-oxidase1 (MtGA3ox1), which related to dwarf and branch phenotype, were obviously reduced, as well as MtGA3ox1' (MTR_1g011580), GA20-oxidase1 (MtGA20ox1), MtGA20ox1' (MTR_1g102070) and GA20-oxidase2 (MtGA20ox2) involved in gibberellins (GAs) pathway. Overall, our results revealed that MtRAV3 exerted an important role in adversity response and plant growth, was a multifunctional gene in M. truncatula, which provided reference for genetic improvement of alfalfa (Medicago sativa).

Novel QTL Associated with Shoot Branching Identified in Doubled Haploid Rice (Oryza sativa L.) under Low Nitrogen Cultivation

Shoot branching is considered as an important trait for the architecture of plants and contributes to their growth and productivity. In cereal crops, such as rice, shoot branching is controlled by many factors, including phytohormones signaling networks, operating either in synergy or antagonizing each other. In rice, shoot branching indicates the ability to produce more tillers that are essential for achieving high productivity and yield potential. In the present study, we evaluated the growth and development, and yield components of a doubled haploid population derived from a cross between 93-11 (P1, indica) and Milyang352 (P2, japonica), grown under normal nitrogen and low nitrogen cultivation open field conditions. The results of the phenotypic evaluation indicated that parental lines 93-11 (P1, a high tillering indica cultivar) and Milyang352 (P2, a low tillering japonica cultivar) showed distinctive phenotypic responses, also reflected in their derived population. In addition, the linkage mapping and quantitative trait locus (QTL) analysis detected three QTLs associated with tiller number on chromosome 2 (qTNN2-1, 130 cM, logarithm of the odds (LOD) 4.14, PVE 14.5%; and qTNL2-1, 134 cM, LOD: 6.05, PVE: 20.5%) and chromosome 4 (qTN4-1, 134 cM, LOD 3.92, PVE 14.5%), with qTNL2-1 having the highest phenotypic variation explained, and the only QTL associated with tiller number under low nitrogen cultivation conditions, using Kompetitive Allele-Specific PCR (KASP) and Fluidigm markers. The additive effect (1.81) of qTNL2-1 indicates that the allele from 93-11 (P1) contributed to the observed phenotypic variation for tiller number under low nitrogen cultivation. The breakthrough is that the majority of the candidate genes harbored by the QTLs qTNL2-1 and qTNN4-1 (here associated with the control of shoot branching under low and normal nitrogen cultivation, respectively), were also proposed to be involved in plant stress signaling or response mechanisms, with regard to their annotations and previous reports. Therefore, put together, these results would suggest that a possible crosstalk exists between the control of plant growth and development and the stress response in rice.

Association of TaD14-4D, a Gene Involved in Strigolactone Signaling, with Yield Contributing Traits in Wheat

Tillering is a crucial agronomic trait of wheat; it determines yield and plant architecture. Strigolactones (SLs) have been reported to inhibit plant branching. D14, a receptor of SLs, has been described to affect tillering in rice, yet it has seldomly been studied in wheat. In this study, three TaD14 homoeologous genes, TaD14-4A, TaD14-4B, and TaD14-4D, were identified. TaD14-4A, TaD14-4B, and TaD14-4D were constitutively expressed, and TaD14-4D had a higher expression level in most tissues. TaD14 proteins were localized in both cytoplasm and nucleus. An SNP and a 22 bp insertion/deletion (Indel) at the exon regions of TaD14-4D were detected, forming three haplotypes, namely 4D-HapI, 4D-HapII, and 4D-HapIII. Due to the frameshift mutation in the coding region of 4D-HapII, the interaction of 4D-HapII with TaMAX2 and TaD53 was blocked, which led to the blocking of SL signal transduction. Based on the two variation sites, two molecular markers, namely dCAPS-250 and Indel-747, were developed. Association analysis suggested that haplotypes of TaD14-4D were associated with effective tillering number (ETN) and thousand kernel weight (TKW) simultaneously in four environments. The favorable haplotype 4D-HapIII underwent positive selection in global wheat breeding. This study provides insights into understanding the function of natural variations of TaD14-4D and develops two useful molecular markers for wheat breeding.

Cadmium stress suppresses the tillering of perennial ryegrass and is associated with the transcriptional regulation of genes controlling axillary bud outgrowth

Perennial ryegrass (Lolium perenne L.), a grass species with superior tillering capacity, plays a potential role in the phytoremediation of cadmium (Cd)-contaminated soils. Tiller production is inhibited in response to serious Cd stress. However, the regulatory mechanism of Cd stress-induced inhibition of tiller development is not well documented. To address this issue, we investigated the phenotype, the expression levels of genes involved in axillary bud initiation and bud outgrowth, and endogenous hormone biosynthesis and signaling pathways in seedlings of perennial ryegrass under Cd stress. The results showed that the number of tillers and axillary buds in the Cd-treated seedlings decreased by 67% and 21%, respectively. The suppression of tiller production in the Cd-treated seedlings was more closely associated with the inhibition of axillary bud outgrowth than with bud initiation. Cd stress upregulated the expression level of genes related to axillary bud dormancy and downregulated bud activity genes. Additionally, genes involved in strigolactone biosynthesis and signaling, auxin transport and signaling, and cytokinin degradation were upregulated in Cd-treated seedlings, and cytokinin biosynthesis gene expression were decreased by Cd stress. The content of zeatin in the Cd-treated pants was significantly reduced by 69~85% compared to the control plants. The content of indole-3-acetic acid (IAA) remains constant under Cd stress. Overall, Cd stress induced axillary bud dormancy and subsequently inhibited axillary bud outgrowth. The decrease of zeatin content and upregulation of genes involved in strigolactone signaling and bud dormancy might be responsible for the inhibition of axillary bud outgrowth.