The cytokinin response factors modulate root and shoot growth and promote leaf senescence in Arabidopsis

KNUCKLES regulates floral meristem termination by controlling auxin distribution and cytokinin activity

The termination of floral meristem (FM) activity is essential for the normal development of reproductive floral organs. During this process, KNUCKLES (KNU), a C2H2-type zinc finger protein, crucially regulates FM termination by directly repressing the expression of both the stem cell identity gene WUSCHEL (WUS) and the stem cell marker gene CLAVATA3 (CLV3) to abolish the WUS-CLV3 feedback loop required for FM maintenance. In addition, phytohormones auxin and cytokinin are involved in FM regulation. However, whether KNU modulates auxin and cytokinin activities for FM determinacy control remains unclear. Here, we show that the auxin distribution and the cytokinin activity mediated by KNU in Arabidopsis (Arabidopsis thaliana) promote the termination of FM during stage 6 of flower development. Mutation of KNU leads to altered distribution of auxin and cytokinin in the FM of a stage 6 floral bud. Moreover, KNU directly represses the auxin transporter gene PIN-FORMED1 (PIN1) and the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE7 (IPT7) via mediating H3K27me3 deposition on these 2 loci to regulate auxin and cytokinin activities. Our study presents a molecular regulatory network that elucidates how the transcriptional repressor KNU integrates and modulates the activities of auxin and cytokinin, thus securing the timed FM termination.

Controlled atmosphere as cold chain support for extending postharvest life in cabbage

Postharvest management of cabbage relies on high-intensity cooling to control postharvest physiology, minimising quality loss despite incurring significant energy and environmental costs. As an alternative, we hypothesised that controlled atmosphere (CA) could allow increased storage temperature by supporting physiological regulation, while maintaining quality and reducing energy demand. This study examined the effect CA (1.5 kPa CO2 and 6 kPa O2) at 5 or 10 °C on cabbage quality, with the aim of proposing a more sustainable and resilient supply chain. CA treatment was effective at reducing head respiration at higher temperature, with CA/10 °C treatment achieving lower respiration rates than Control/5 °C. Improved head colour retention and maintenance of stump quality were observed in cabbage under CA conditions. CA effects were seen also at a regulatory level; CA promoted an average of 25.4% reduction in abscisic acid accumulation potentially as part of a wider hypoxia stress response and was successful in decreasing expression of the senescence-coordinating transcription factor BoORE15. This finding was linked with a lower in downstream expression of pheophytinase and subtilisin protease. These results demonstrated that CA treatment fundamentally modified postharvest physiology in cabbage, which can be exploited to enable storage at warmer temperatures, contributing to supply chains with lower energy demand and its associated environmental benefits.

Unraveling the signaling pathways of phytohormones underlying salt tolerance in Elymus sibiricus: A transcriptomic and metabolomic approach

Understanding phytohormonal signaling is crucial for elucidating plant defense mechanisms against environmental stressors. However, knowledge regarding phytohormone-mediated tolerance pathways under salt stress in Elymus sibiricus, an important species for forage and ecological restoration, remains limited. In this study, transcriptomic and metabolomic approaches uncover the dynamics of phytohormonal signaling in Elymus sibiricus under salt stress. Notably, four hours after exposure to salt, significant activity was observed in the ABA, JA, IAA, and CTK pathways, with ABA, JA, JA-L-Ile, and IAA identified as key mediators in the response of Elymus sibiricus' to salinity. Moreover, SAPK3, Os04g0167900-like, CAT1, MKK2, and MPK12 were identified as potential central regulators within these pathways. The complex interactions between phytohormones and DEGs are crucial for facilitating the adaptation of Elymus sibiricus to saline environments. These findings enhance our understanding of the salt tolerance mechanisms in Elymus sibiricus and provide a foundation for breeding salt-resistant varieties.

How abiotic stresses trigger sugar signaling to modulate leaf senescence?

Plants have evolved the adaptive capacity to mitigate the negative effect of external adversities at chemical, molecular, cellular, and physiological levels. This capacity is conferred by triggering the coordinated action of internal regulatory factors, in which sugars play an essential role in the regulating chloroplast degradation and leaf senescence under various stresses. In this review, we summarize the recent findings on the senescent-associated changes in carbohydrate metabolism and its relation to chlorophyl degradation, oxidative damage, photosynthesis inhibition, programmed cell death (PCD), and sink-source relation as affected by abiotic stresses. The action of sugar signaling in regulating the initiation and progression of leaf senescence under abiotic stresses involves interactions with various plant hormones, reactive oxygen species (ROS) burst, and protein kinases. This discussion aims to elucidate the complex regulatory network and molecular mechanisms that underline sugar-induced leaf senescence in response to various abiotic stresses. The imperative role of sugar signaling in regulating plant stress responses potentially enables the production of crop plants with modified sugar metabolism. This, in turn, may facilitate the engineering of plants with improved stress responses, optimal life span and higher yield achievement.

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

CRF transcription factors in the trade-off between abiotic stress response and plant developmental processes

Climate change-induced environmental stress significantly affects crop yield and quality. In response to environmental stressors, plants use defence mechanisms and growth suppression, creating a resource trade-off between the stress response and development. Although stress-responsive genes have been widely engineered to enhance crop stress tolerance, there is still limited understanding of the interplay between stress signalling and plant growth, a research topic that can provide promising targets for crop genetic improvement. This review focuses on Cytokinin Response Factors (CRFs) transcription factor's role in the balance between abiotic stress adaptation and sustained growth. CRFs, known for their involvement in cytokinin signalling and abiotic stress responses, emerge as potential targets for delaying senescence and mitigating yield penalties under abiotic stress conditions. Understanding the molecular mechanisms regulated by CRFs paves the way for decoupling stress responses from growth inhibition, thus allowing the development of crops that can adapt to abiotic stress without compromising development. This review highlights the importance of unravelling CRF-mediated pathways to address the growing need for resilient crops in the face of evolving climatic conditions.

TARGET OF MONOPTEROS: key transcription factors orchestrating plant development and environmental response

Plants have an incredible ability to sustain root and vascular growth after initiation of the embryonic root and the specification of vascular tissue in early embryos. Microarray assays have revealed that a group of transcription factors, TARGET OF MONOPTEROS (TMO), are important for embryonic root initiation in Arabidopsis. Despite the discovery of their auxin responsiveness early on, their function and mode of action remained unknown for many years. The advent of genome editing has accelerated the study of TMO transcription factors, revealing novel functions for biological processes such as vascular development, root system architecture, and response to environmental cues. This review covers recent achievements in understanding the developmental function and the genetic mode of action of TMO transcription factors in Arabidopsis and other plant species. We highlight the transcriptional and post-transcriptional regulation of TMO transcription factors in relation to their function, mainly in Arabidopsis. Finally, we provide suggestions for further research and potential applications in plant genetic engineering.

GWAS supported by computer vision identifies large numbers of candidate regulators of in planta regeneration in Populus trichocarpa

Plant regeneration is an important dimension of plant propagation and a key step in the production of transgenic plants. However, regeneration capacity varies widely among genotypes and species, the molecular basis of which is largely unknown. Association mapping methods such as genome-wide association studies (GWAS) have long demonstrated abilities to help uncover the genetic basis of trait variation in plants; however, the performance of these methods depends on the accuracy and scale of phenotyping. To enable a large-scale GWAS of in planta callus and shoot regeneration in the model tree Populus, we developed a phenomics workflow involving semantic segmentation to quantify regenerating plant tissues over time. We found that the resulting statistics were of highly non-normal distributions, and thus employed transformations or permutations to avoid violating assumptions of linear models used in GWAS. We report over 200 statistically supported quantitative trait loci (QTLs), with genes encompassing or near to top QTLs including regulators of cell adhesion, stress signaling, and hormone signaling pathways, as well as other diverse functions. Our results encourage models of hormonal signaling during plant regeneration to consider keystone roles of stress-related signaling (e.g. involving jasmonates and salicylic acid), in addition to the auxin and cytokinin pathways commonly considered. The putative regulatory genes and biological processes we identified provide new insights into the biological complexity of plant regeneration, and may serve as new reagents for improving regeneration and transformation of recalcitrant genotypes and species.

A cytokinin response factor PtCRF1 is involved in the regulation of wood formation in poplar

Wood formation is a complex developmental process under the control of multiple levels of regulatory transcriptional network and hormone signals in trees. It is well known that cytokinin (CK) signaling plays an important role in maintaining the activity of the vascular cambium. The CK response factors (CRFs) encoding a subgroup of AP2 transcription factors have been identified to mediate the CK-dependent regulation in different plant developmental processes. However, the functions of CRFs in wood development remain unclear. Here, we characterized the function of PtCRF1, a CRF transcription factor isolated from poplar, in the process of wood formation. The PtCRF1 is preferentially expressed in secondary vasculature, especially in vascular cambium and secondary phloem, and encodes a transcriptional activator. Overexpression of PtCRF1 in transgenic poplar plants led to a significant reduction in the cell layer number of vascular cambium. The development of wood tissue was largely promoted in the PtCRF1-overexpressing lines, while it was significantly compromised in the CRISPR/Cas9-generated double mutant plants of PtCRF1 and its closest homolog PtCRF2. The RNA sequencing (RNA-seq) and quantitative reverse transcription PCR (RT-qPCR) analyses showed that PtCRF1 repressed the expression of the typical CK-responsive genes. Furthermore, bimolecular fluorescence complementation assays revealed that PtCRF1 competitively inhibits the direct interactions between histidine phosphotransfer proteins and type-B response regulator by binding to PtHP protein. Collectively, these results indicate that PtCRF1 negatively regulates CK signaling and is required for woody cell differentiation in poplar.

Overexpression of SlCRF6 in tomato inhibits leaf development and affects plant morphology

Cytokinin response factors (CRFs) are transcription factors (TFs) that are specific to plants and have diverse functions in plant growth and stress responses. However, the precise roles of CRFs in regulating tomato plant architecture and leaf development have not been comprehensively investigated. Here, we identified a novel CRF, SlCRF6, which is involved in the regulation of plant growth via the gibberellin (GA) signaling pathway. SlCRF6-overexpressing (SlCRF6-OE) plants displayed pleiotropic phenotypic changes, including reduced internode length and leaf size, which caused dwarfism in tomato plants. This dwarfism could be alleviated by application of exogenous GA3. Remarkably, quantitative real-time PCR (qRTPCR), a dual luciferase reporter assay and a yeast one-hybrid (Y1H) assay revealed that SlCRF6 promoted the expression of SlDELLA (a GA signal transduction inhibitor) in vivo. Furthermore, transgenic plants displayed variegated leaves and diminished chlorophyll content, resulting in decreased photosynthetic efficiency and less starch than in wild-type (WT) plants. The results of transient expression assays and Y1H assays indicated that SlCRF6 suppressed the expression of SlPHAN (leaf morphology-related gene). Collectively, these findings suggest that SlCRF6 plays a crucial role in regulating tomato plant morphology, leaf development, and the accumulation of photosynthetic products.

Dissecting chickpea genomic loci associated with the root penetration responsive traits in compacted soil

MAIN CONCLUSION

Soil compaction reduces root exploration in chickpea. We found genes related to root architectural traits in chickpea that can help understand and improve root growth in compacted soils. Soil compaction is a major concern for modern agriculture, as it constrains plant root growth, leading to reduced resource acquisition. Phenotypic variation for root system architecture (RSA) traits in compacted soils is present for various crops; however, studies on genetic associations with these traits are lacking. Therefore, we investigated RSA traits in different soil compaction levels and identified significant genomic associations in chickpea. We conducted a Genome-Wide Association Study (GWAS) of 210 chickpea accessions for 13 RSA traits under three bulk densities (BD) (1.1BD, 1.6BD, and 1.8BD). Soil compaction decreases root exploration by reducing 12 RSA traits, except average diameter (AD). Further, AD is negatively correlated with lateral root traits, and this correlation increases in 1.8BD, suggesting the negative effect of AD on lateral root traits. Interestingly, we identified probable candidate genes such as GLP3 and LRX for lateral root traits and CRF1-like for total length (TL) in 1.6BD soil. In heavy soil compaction, DGK2 is associated with lateral root traits. Reduction in laterals during soil compaction is mainly due to delayed seedling establishment, thus making lateral root number a critical trait. Interestingly, we also found a higher contribution of the  GxE component of the number of root tips (Tips) to the total variation than the other lateral traits. We also identified a pectin esterase, PPE8B, associated with Tips in high soil compaction and a significantly associated SNP with the relative change in Tips depicting a trade-off between Tips and AD. Identified genes and loci would help develop soil-compaction-resistant chickpea varieties.

Involvement of plant signaling network and cell metabolic homeostasis in nitrogen deficiency-induced early leaf senescence

Nitrogen (N) is a basic building block that plays an essential role in the maintenance of normal plant growth and its metabolic functions through complex regulatory networks. Such the N metabolic network comprises a series of transcription factors (TFs), with the coordinated actions of phytohormone and sugar signaling to sustain cell homeostasis. The fluctuating N concentration in plant tissues alters the sensitivity of several signaling pathways to stressful environments and regulates the senescent-associated changes in cellular structure and metabolic process. Here, we review recent advances in the interaction between N assimilation and carbon metabolism in response to N deficiency and its regulation to the nutrient remobilization from source to sink during leaf senescence. The regulatory networks of N and sugar signaling for N deficiency-induced leaf senescence is further discussed to explain the effects of N deficiency on chloroplast disassembly, reactive oxygen species (ROS) burst, asparagine metabolism, sugar transport, autophagy process, Ca2+ signaling, circadian clock response, brassinazole-resistant 1 (BZRI), and other stress cell signaling. A comprehensive understanding for the metabolic mechanism and regulatory network underlying N deficiency-induced leaf senescence may provide a theoretical guide to optimize the source-sink relationship during grain filling for the achievement of high yield by a selection of crop cultivars with the properly prolonged lifespan of functional leaves and/or by appropriate agronomic managements.

Weed-induced changes in the maize root transcriptome reveal transcription factors and physiological processes impacted early in crop-weed interactions

A new paradigm suggests weeds primarily reduce crop yield by altering crop developmental and physiological processes long before the weeds reduce resources through competition. Multiple studies have implicated stress response pathways are activated when crops such as maize are grown in close proximity with weeds during the first 4-8 weeks of growth-the point at which weeds have their greatest impact on subsequent crop yields. To date, these studies have mostly focused on the response of above-ground plant parts and have not examined the early signal transduction processes associated with maize root response to weeds. To investigate the impact of signals from a below-ground competitor on the maize root transcriptome when most vulnerable to weed pressure, a system was designed to expose maize to only below-ground signals. Gene set enrichment analyses identified over-represented ontologies associated with oxidative stress signalling throughout the time of weed exposure, with additional ontologies associated with nitrogen use and transport and abscisic acid (ABA) signalling, and defence responses being enriched at later time points. Enrichment of promoter motifs indicated over-representation of sequences known to bind FAR-RED IMPAIRED RESPONSE 1 (FAR1), several AP2/ERF transcription factors and others. Likewise, co-expression networks were identified using Weighted-Gene Correlation Network Analysis (WGCNA) and Spatiotemporal Clustering and Inference of Omics Networks (SC-ION) algorithms. WGCNA highlighted the potential roles of several transcription factors including a MYB 3r-4, TB1, WRKY65, CONSTANS-like5, ABF3, HOMEOBOX 12, among others. These studies also highlighted the role of several specific proteins involved in ABA signalling as being important for the initiation of the early response of maize to weeds. SC-ION highlighted potential roles for NAC28, LOB37, NAC58 and GATA2 transcription factors, among many others.

RAV1 mediates cytokinin signaling for regulating primary root growth in Arabidopsis

Root growth dynamics is an outcome of complex hormonal crosstalk. The primary root meristem size, for example, is determined by antagonizing actions of cytokinin and auxin. Here we show that RAV1, a member of the AP2/ERF family of transcription factors, mediates cytokinin signaling in roots to regulate meristem size. The rav1 mutants have prominently longer primary roots, with a meristem that is significantly enlarged and contains higher cell numbers, compared with wild-type. The mutant phenotype could be restored on exogenous cytokinin application or by inhibiting auxin transport. At the transcript level, primary cytokinin-responsive genes like ARR1, ARR12 were significantly downregulated in the mutant root, indicating impaired cytokinin signaling. In concurrence, cytokinin induced regulation of SHY2, an Aux/IAA gene, and auxin efflux carrier PIN1 was hindered in rav1, leading to altered auxin transport and distribution. This effectively altered root meristem size in the mutant. Notably, CRF1, another member of the AP2/ERF family implicated in cytokinin signaling, is transcriptionally repressed by RAV1 to promote cytokinin response in roots. Further associating RAV1 with cytokinin signaling, our results demonstrate that cytokinin upregulates RAV1 expression through ARR1, during post-embryonic root development. Regulation of RAV1 expression is a part of secondary cytokinin response that eventually represses CRF1 to augment cytokinin signaling. To conclude, RAV1 functions in a branch pathway downstream to ARR1 that regulates CRF1 expression to enhance cytokinin action during primary root development in Arabidopsis.

Spatiotemporal dynamics of the tomato fruit transcriptome under prolonged water stress

Water availability influences all aspects of plant growth and development; however, most studies of plant responses to drought have focused on vegetative organs, notably roots and leaves. Far less is known about the molecular bases of drought acclimation responses in fruits, which are complex organs with distinct tissue types. To obtain a more comprehensive picture of the molecular mechanisms governing fruit development under drought, we profiled the transcriptomes of a spectrum of fruit tissues from tomato (Solanum lycopersicum), spanning early growth through ripening and collected from plants grown under varying intensities of water stress. In addition, we compared transcriptional changes in fruit with those in leaves to highlight different and conserved transcriptome signatures in vegetative and reproductive organs. We observed extensive and diverse genetic reprogramming in different fruit tissues and leaves, each associated with a unique response to drought acclimation. These included major transcriptional shifts in the placenta of growing fruit and in the seeds of ripe fruit related to cell growth and epigenetic regulation, respectively. Changes in metabolic and hormonal pathways, such as those related to starch, carotenoids, jasmonic acid, and ethylene metabolism, were associated with distinct fruit tissues and developmental stages. Gene coexpression network analysis provided further insights into the tissue-specific regulation of distinct responses to water stress. Our data highlight the spatiotemporal specificity of drought responses in tomato fruit and indicate known and unrevealed molecular regulatory mechanisms involved in drought acclimation, during both vegetative and reproductive stages of development.

An APETALA2/ethylene responsive factor transcription factor GmCRF4a regulates plant height and auxin biosynthesis in soybean

Plant height is one of the key agronomic traits affecting soybean yield. The cytokinin response factors (CRFs), as a branch of the APETALA2/ethylene responsive factor (AP2/ERF) super gene family, have been reported to play important roles in regulating plant growth and development. However, their functions in soybean remain unknown. This study characterized a soybean CRF gene named GmCRF4a by comparing the performance of the homozygous Gmcrf4a-1 mutant, GmCRF4a overexpression (OX) and co-silencing (CS) lines. Phenotypic analysis showed that overexpression of GmCRF4a resulted in taller hypocotyls and epicotyls, more main stem nodes, and higher plant height. While down-regulation of GmCRF4a conferred shorter hypocotyls and epicotyls, as well as a reduction in plant height. The histological analysis results demonstrated that GmCRF4a promotes epicotyl elongation primarily by increasing cell length. Furthermore, GmCRF4a is required for the expression of GmYUCs genes to elevate endogenous auxin levels, which may subsequently enhance stem elongation. Taken together, these observations describe a novel regulatory mechanism in soybean, and provide the basis for elucidating the function of GmCRF4a in auxin biosynthesis pathway and plant heigh regulation in plants.

Molecular framework integrating nitrate sensing in root and auxin-guided shoot adaptive responses

Mineral nutrition is one of the key environmental factors determining plant development and growth. Nitrate is the major form of macronutrient nitrogen that plants take up from the soil. Fluctuating availability or deficiency of this element severely limits plant growth and negatively affects crop production in the agricultural system. To cope with the heterogeneity of nitrate distribution in soil, plants evolved a complex regulatory mechanism that allows rapid adjustment of physiological and developmental processes to the status of this nutrient. The root, as a major exploitation organ that controls the uptake of nitrate to the plant body, acts as a regulatory hub that, according to nitrate availability, coordinates the growth and development of other plant organs. Here, we identified a regulatory framework, where cytokinin response factors (CRFs) play a central role as a molecular readout of the nitrate status in roots to guide shoot adaptive developmental response. We show that nitrate-driven activation of NLP7, a master regulator of nitrate response in plants, fine tunes biosynthesis of cytokinin in roots and its translocation to shoots where it enhances expression of CRFs. CRFs, through direct transcriptional regulation of PIN auxin transporters, promote the flow of auxin and thereby stimulate the development of shoot organs.

Know when and how to die: gaining insights into the molecular regulation of leaf senescence

Senescence is the ultimate phase in the life cycle of leaves which is crucial for recycling of nutrients to maintain plant fitness and reproductive success. The earliest visible manifestation of leaf senescence is their yellowing, which usually commences with the breakdown of chlorophyll. The degradation process involves a gradual and highly coordinated disassembly of macromolecules resulting in the accumulation of nutrients, which are subsequently mobilized from the senescing leaves to the developing organs. Leaf senescence progresses under overly tight genetic and molecular control involving a well-orchestrated and intricate network of regulators that coordinate spatio-temporally with the influence of both internal and external cues. Owing to the advancements in omics technologies, the availability of mutant resources, scalability of molecular analyses methodologies and the advanced capacity to integrate multidimensional data, our understanding of the genetic and molecular basis of leaf ageing has greatly expanded. The review provides a compilation of the multitier regulation of senescence process and the interrelation between the environment and the terminal phase of leaf development. The knowledge gained would benefit in devising the strategies for manipulation of leaf senescence process to improve crop quality and productivity.

New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones

Leaf senescence is the last stage of leaf development, manifested by leaf yellowing due to the loss of chlorophyll, along with the degradation of macromolecules and facilitates nutrient translocation from the sink to the source tissues, which is essential for the plants' fitness. Leaf senescence is controlled by a sophisticated genetic network that has been revealed through the study of the molecular mechanisms of hundreds of senescence-associated genes (SAGs), which are involved in multiple layers of regulation. Leaf senescence is primarily regulated by plant age, but also influenced by a variety of factors, including phytohormones and environmental stimuli. Phytohormones, as important signaling molecules in plant, contribute to the onset and progression of leaf senescence. Recently, peptide hormones have been reported to be involved in the regulation of leaf senescence, enriching the significance of signaling molecules in controlling leaf senescence. This review summarizes recent advances in the regulation of leaf senescence by classical and peptide hormones, aiming to better understand the coordinated network of different pathways during leaf senescence.

PUCHI represses early meristem formation in developing lateral roots of Arabidopsis thaliana

Lateral root organogenesis is a key process in the development of a plant's root system and its adaptation to the environment. During lateral root formation, an early phase of cell proliferation first produces a four-cell-layered primordium, and only from this stage onwards is a root meristem-like structure, expressing root stem cell niche marker genes, being established in the developing organ. Previous studies reported that the gene regulatory network controlling lateral root formation is organized into two subnetworks whose mutual inhibition may contribute to organ patterning. PUCHI encodes an AP2/ERF transcription factor expressed early during lateral root primordium development and required for correct lateral root formation. To dissect the molecular events occurring during this early phase, we generated time-series transcriptomic datasets profiling lateral root development in puchi-1 mutants and wild types. Transcriptomic and reporter analyses revealed that meristem-related genes were expressed ectopically at early stages of lateral root formation in puchi-1 mutants. We conclude that, consistent with the inhibition of genetic modules contributing to lateral root development, PUCHI represses ectopic establishment of meristematic cell identities at early stages of organ development. These findings shed light on gene network properties that orchestrate correct timing and patterning during lateral root formation.

Isoprene enhances leaf cytokinin metabolism and induces early senescence

Isoprene, a major biogenic volatile hydrocarbon of climate-relevance, indisputably mitigates abiotic stresses in emitting plants. However functional relevance of constitutive isoprene emission in unstressed plants remains contested. Isoprene and cytokinins (CKs) are synthesized from a common substrate and pathway in chloroplasts. It was postulated that isoprene emission may affect CK-metabolism. Using transgenic isoprene-emitting (IE) Arabidopsis and isoprene nonemitting (NE) RNA-interference grey poplars (paired with respective NE and IE genotypes), the life of individual IE and NE leaves from emergence to abscission was followed under stress-free conditions. We monitored plant growth rate, aboveground developmental phenotype, modelled leaf photosynthetic energy status, quantified the abundance of leaf CKs, analysed Arabidopsis and poplar leaf transcriptomes by RNA-sequencing in presence and absence of isoprene during leaf senescence. Isoprene emission by unstressed leaves enhanced the abundance of CKs (isopentenyl adenine and its precursor) by > 200%, significantly upregulated genes coding for CK-synthesis, CK-signalling and CK-degradation, hastened plant development, increased chloroplast metabolic rate, altered photosynthetic energy status, induced early leaf senescence in both Arabidopsis and poplar. IE leaves senesced sooner even in decapitated poplars where source-sink relationships and hormone homeostasis were perturbed. Constitutive isoprene emission significantly accelerates CK-led leaf and organismal development and induces early senescence independent of growth constraints. Isoprene emission provides an early-riser evolutionary advantage and shortens lifecycle duration to assist rapid diversification in unstressed emitters.

Molecular analysis of nematode-responsive defence genes CRF1, WRKY45, and PR7 in Solanum lycopersicum tissues during the infection of plant-parasitic nematode species of the genus Meloidogyne

Several pathogens, including nematodes, have severe effects on plant development and growth, and immense populations of parasitic nematodes may cause plant death and crop loss. Obligate plant-parasitic nematodes and root-knot nematodes belonging to the genus Meloidogyne are significant parasites in crops. During nematode infection, damage-associated molecular patterns play a role in the activation of plant defence responses to pathogens. Several genes are involved in Meloidogyne parasitism. However, the expression of nematode-responsive genes CRF1, WRKY45, and PR7 during infection with different parasitic nematode species is not well understood. Therefore, this study aimed to reveal plant responses to differential gene expression of nematode-responsive genes in tomato plants, and their relationship to nematode reproduction and comparative phylogeny. Molecular methods for gene expression, greenhouse work for nematode reproduction, and phylogenetic analysis were used to determine nematode-plant interactions. The results revealed that differential gene expression of CRF1, WRKY45, and PR7 depended on the nematode species. The relative CRF1 gene expression reached its highest level at 3 dpi, following nematode infection. In conclusion, plant defense responses disturbed the expression of nematode-responsive genes, and the differential expression of nematode-responsive genes was affected by nematode species and nematode parasitism.

Early Sucrose Degradation and the Dominant Sucrose Cleavage Pattern Influence Lycoris sprengeri Bulblet Regeneration In Vitro

Bulblet formation and development determine the quantitative and qualitative traits, respectively, of bulb yield for most flowering bulbs. For Lycoris species, however, the underlying molecular mechanism remains elusive. Here, clonal bulblets of Lycoris sprengeri (Ls) derived from the same probulb were used as explants to establish efficient and inefficient in vitro regeneration systems by adjusting the 6-benzyladenine (BA) concentrations in media. BA application did not change the biological processes among groups but led to earlier decreases in sucrose and total soluble sugar (TSS) contents. Correlation analyses showed that the BA treatments changed the interaction between carbohydrate and endogenous hormone contents during bulblet regeneration. We found that two sucrose degradation enzyme-related genes, cell wall invertase (CWIN) and sucrose synthase, exhibited exactly opposite expression patterns during the competence stage. In addition, the regeneration system that obtained more bulblets showed significantly higher expression of LsCWIN2 than those that obtained fewer bulblets. Our data demonstrate the essential role of BA in accelerating sucrose degradation and the selection of a dominant sucrose cleavage pattern at the competence stage of in vitro bulblet regeneration. We propose that a relatively active CWIN-catalyzed pathway at the competence stage might promote bulblet regeneration, thus influencing bulb yield.

Meta-analysis of transcriptomic studies of cytokinin-treated rice roots defines a core set of cytokinin response genes

Cytokinins regulate diverse aspects of plant growth and development, primarily through modulation of gene expression. The cytokinin-responsive transcriptome has been thoroughly described in dicots, especially Arabidopsis, but much less so in monocots. Here, we present a meta-analysis of five different transcriptomic analyses of rice (Oryza sativa) roots treated with cytokinin, including three previously unpublished experiments. We developed a treatment method in which hormone is added to the media of rice seedlings grown in sterile hydroponic culture under a continuous airflow, which resulted in minimal perturbation of the seedlings, thus greatly reducing changes in gene expression in the absence of exogenous hormone. We defined a core set of 205 upregulated and 86 downregulated genes that were differentially expressed in at least three of the transcriptomic datasets. This core set includes genes encoding the type-A response regulators (RRs) and cytokinin oxidases/dehydrogenases, which have been shown to be primary cytokinin response genes. GO analysis revealed that the upregulated genes were enriched for terms related to cytokinin/hormone signaling and metabolism, while the downregulated genes were significantly enriched for genes encoding transporters. Variations of type-B RR binding motifs were significantly enriched in the promoters of the upregulated genes, as were binding sites for other potential partner transcription factors. The promoters of the downregulated genes were generally enriched for distinct cis-acting motifs and did not include the type-B RR binding motif. This analysis provides insight into the molecular mechanisms underlying cytokinin action in a monocot and provides a useful foundation for future studies of this hormone in rice and other cereals.

Arabidopsis WRKY71 regulates ethylene-mediated leaf senescence by directly activating EIN2, ORE1 and ACS2 genes

Leaf senescence is a pivotal step in the last stage of the plant life cycle and is influenced by various external and endogenous cues. A series of reports have indicated the involvement of the WRKY transcription factors in regulating leaf senescence, but the molecular mechanisms and signaling pathways remain largely unclear. Here we provide evidence demonstrating that WRKY71 acts as a positive regulator of leaf senescence in Arabidopsis. WRKY71-1D, an overexpressor of WRKY71, exhibited early leaf senescence, while wrky71-1, the WRKY71 loss-of-function mutant, displayed delayed leaf senescence. Accordingly, a set of senescence-associated genes (SAGs) were substantially elevated in WRKY71-1D but markedly decreased in wrky71-1. Chromatin immunoprecipitation assays indicated that WRKY71 can bind directly to the promoters of SAG13 and SAG201. Transcriptome analysis suggested that WRKY71 might mediate multiple cues to accelerate leaf senescence, such as abiotic stresses, dark and ethylene. WRKY71 was ethylene inducible, and treatment with the ethylene precursor 1-amino-cyclopropane-1-carboxylic acid enhanced leaf senescence in WRKY71-1D but caused only a marginal delay in leaf senescence in wrky71-1. In vitro and in vivo assays demonstrated that WRKY71 can directly regulate ETHYLENE INSENSITIVE2 (EIN2) and ORESARA1 (ORE1), genes of the ethylene signaling pathway. Consistently, leaf senescence of WRKY71-1D was obviously retarded in the ein2-5 and nac2-1 mutants. Moreover, WRKY71 was also proved to interact with ACS2 in vitro and in vivo. Treatment with AgNO₃ and aminoethoxyvinylglycine and acs2-1 could greatly arrest the leaf senescence of WRKY71-1D. In conclusion, our data revealed that WRKY71 mediates ethylene signaling and synthesis to hasten leaf senescence in Arabidopsis.

PopulusPtERF85 Balances Xylem Cell Expansion and Secondary Cell Wall Formation in Hybrid Aspen

Secondary growth relies on precise and specialized transcriptional networks that determine cell division, differentiation, and maturation of xylem cells. We identified a novel role for the ethylene-induced Populus Ethylene Response Factor PtERF85 (Potri.015G023200) in balancing xylem cell expansion and secondary cell wall (SCW) formation in hybrid aspen (Populus tremula x tremuloides). Expression of PtERF85 is high in phloem and cambium cells and during the expansion of xylem cells, while it is low in maturing xylem tissue. Extending PtERF85 expression into SCW forming zones of woody tissues through ectopic expression reduced wood density and SCW thickness of xylem fibers but increased fiber diameter. Xylem transcriptomes from the transgenic trees revealed transcriptional induction of genes involved in cell expansion, translation, and growth. The expression of genes associated with plant vascular development and the biosynthesis of SCW chemical components such as xylan and lignin, was down-regulated in the transgenic trees. Our results suggest that PtERF85 activates genes related to xylem cell expansion, while preventing transcriptional activation of genes related to SCW formation. The importance of precise spatial expression of PtERF85 during wood development together with the observed phenotypes in response to ectopic PtERF85 expression suggests that PtERF85 contributes to the transition of fiber cells from elongation to secondary cell wall deposition.

CYTOKININ-RESPONSIVE GROWTH REGULATOR regulates cell expansion and cytokinin-mediated cell cycle progression

The cytokinin (CK) phytohormones have long been known to activate cell proliferation in plants. However, how CKs regulate cell division and cell expansion remains unclear. Here, we reveal that a basic helix-loop-helix transcription factor, CYTOKININ-RESPONSIVE GROWTH REGULATOR (CKG), mediates CK-dependent regulation of cell expansion and cell cycle progression in Arabidopsis thaliana. The overexpression of CKG increased cell size in a ploidy-independent manner and promoted entry into the S phase of the cell cycle, especially at the seedling stage. Furthermore, CKG enhanced organ growth in a pleiotropic fashion, from embryogenesis to reproductive stages, particularly of cotyledons. In contrast, ckg loss-of-function mutants exhibited smaller cotyledons. CKG mainly regulates the expression of genes involved in the regulation of the cell cycle including WEE1. We propose that CKG provides a regulatory module that connects cell cycle progression and organ growth to CK responses.

The diverse roles of cytokinins in regulating leaf development

Leaves provide energy for plants, and consequently for animals, through photosynthesis. Despite their important functions, plant leaf developmental processes and their underlying mechanisms have not been well characterized. Here, we provide a holistic description of leaf developmental processes that is centered on cytokinins and their signaling functions. Cytokinins maintain the growth potential (pluripotency) of shoot apical meristems, which provide stem cells for the generation of leaf primordia during the initial stage of leaf formation; cytokinins and auxins, as well as their interaction, determine the phyllotaxis pattern. The activities of cytokinins in various regions of the leaf, especially at the margins, collectively determine the final leaf morphology (e.g., simple or compound). The area of a leaf is generally determined by the number and size of the cells in the leaf. Cytokinins promote cell division and increase cell expansion during the proliferation and expansion stages of leaf cell development, respectively. During leaf senescence, cytokinins reduce sugar accumulation, increase chlorophyll synthesis, and prolong the leaf photosynthetic period. We also briefly describe the roles of other hormones, including auxin and ethylene, during the whole leaf developmental process. In this study, we review the regulatory roles of cytokinins in various leaf developmental stages, with a focus on cytokinin metabolism and signal transduction processes, in order to shed light on the molecular mechanisms underlying leaf development.

Metabolic Control of Gametophore Shoot Formation through Arginine in the Moss Physcomitrium patens

Shoot formation is accompanied by active cell proliferation and expansion, requiring that metabolic state adapts to developmental control. Despite the importance of such metabolic reprogramming, it remains unclear how development and metabolism are integrated. Here, we show that disruption of ANGUSTIFOLIA3 orthologs (PpAN3s) compromises gametophore shoot formation in the moss Physcomitrium patens due to defective cell proliferation and expansion. Trans-omics analysis reveals that the downstream activity of PpAN3 is linked to arginine metabolism. Elevating arginine level by chemical treatment leads to stunted gametophores and causes Ppan3 mutant-like transcriptional changes in the wild-type plant. Furthermore, ectopic expression of AtAN3 from Arabidopsis thaliana ameliorates the defective arginine metabolism and promotes gametophore formation in Ppan3 mutants. Together, these findings indicate that arginine metabolism is a key pathway associated with gametophore formation and provide evolutionary insights into the establishment of the shoot system in land plants through the integration of developmental and metabolic processes.

Transcriptome Changes Reveal the Molecular Mechanisms of Humic Acid-Induced Salt Stress Tolerance in Arabidopsis

Humic acid (HA) is a principal component of humic substances, which make up the complex organic matter that broadly exists in soil environments. HA promotes plant development as well as stress tolerance, however the precise molecular mechanism for these is little known. Here we conducted transcriptome analysis to elucidate the molecular mechanisms by which HA enhances salt stress tolerance. Gene Ontology Enrichment Analysis pointed to the involvement of diverse abiotic stress-related genes encoding HEAT-SHOCK PROTEINs and redox proteins, which were up-regulated by HA regardless of salt stress. Genes related to biotic stress and secondary metabolic process were mainly down-regulated by HA. In addition, HA up-regulated genes encoding transcription factors (TFs) involved in plant development as well as abiotic stress tolerance, and down-regulated TF genes involved in secondary metabolic processes. Our transcriptome information provided here provides molecular evidences and improves our understanding of how HA confers tolerance to salinity stress in plants.

Convergence and Divergence of Sugar and Cytokinin Signaling in Plant Development

Plants adjust their growth and development through a sophisticated regulatory system integrating endogenous and exogenous cues. Many of them rely on intricate crosstalk between nutrients and hormones, an effective way of coupling nutritional and developmental information and ensuring plant survival. Sugars in their different forms such as sucrose, glucose, fructose and trehalose-6-P and the hormone family of cytokinins (CKs) are major regulators of the shoot and root functioning throughout the plant life cycle. While their individual roles have been extensively investigated, their combined effects have unexpectedly received little attention, resulting in many gaps in current knowledge. The present review provides an overview of the relationship between sugars and CKs signaling in the main developmental transition during the plant lifecycle, including seed development, germination, seedling establishment, root and shoot branching, leaf senescence, and flowering. These new insights highlight the diversity and the complexity of the crosstalk between sugars and CKs and raise several questions that will open onto further investigations of these regulation networks orchestrating plant growth and development.

Transcriptomics Reveals Fast Changes in Salicylate and Jasmonate Signaling Pathways in Shoots of Carbonate-Tolerant Arabidopsis thaliana under Bicarbonate Exposure

High bicarbonate concentrations of calcareous soils with high pH can affect crop performance due to different constraints. Among these, Fe deficiency has mostly been studied. The ability to mobilize sparingly soluble Fe is a key factor for tolerance. Here, a comparative transcriptomic analysis was performed with two naturally selected Arabidopsis thaliana demes, the carbonate-tolerant A1(c+) and the sensitive T6(c-). Analyses of plants exposed to either pH stress alone (pH 5.9 vs. pH 8.3) or to alkalinity caused by 10 mM NaHCO3 (pH 8.3) confirmed better growth and nutrient homeostasis of A1(c+) under alkaline conditions. RNA-sequencing (RNA-seq) revealed that bicarbonate quickly (3 h) induced Fe deficiency-related genes in T6(c-) leaves. Contrastingly, in A1(c+), initial changes concerned receptor-like proteins (RLP), jasmonate (JA) and salicylate (SA) pathways, methionine-derived glucosinolates (GS), sulfur starvation, starch degradation, and cell cycle. Our results suggest that leaves of carbonate-tolerant plants do not sense iron deficiency as fast as sensitive ones. This is in line with a more efficient Fe translocation to aerial parts. In A1(c+) leaves, the activation of other genes related to stress perception, signal transduction, GS, sulfur acquisition, and cell cycle precedes the induction of iron homeostasis mechanisms yielding an efficient response to bicarbonate stress.

Multiple Layers of Regulation on Leaf Senescence: New Advances and Perspectives

Leaf senescence is the last stage of leaf development and is an orderly biological process accompanied by degradation of macromolecules and nutrient recycling, which contributes to plant fitness. Forward genetic mutant screening and reverse genetic studies of senescence-associated genes (SAGs) have revealed that leaf senescence is a genetically regulated process, and the initiation and progression of leaf senescence are influenced by an array of internal and external factors. Recently, multi-omics techniques have revealed that leaf senescence is subjected to multiple layers of regulation, including chromatin, transcriptional and post-transcriptional, as well as translational and post-translational levels. Although impressive progress has been made in plant senescence research, especially the identification and functional analysis of a large number of SAGs in crop plants, we still have not unraveled the mystery of plant senescence, and there are some urgent scientific questions in this field, such as when plant senescence is initiated and how senescence signals are transmitted. This paper reviews recent advances in the multiple layers of regulation on leaf senescence, especially in post-transcriptional regulation such as alternative splicing.

Interplay between Hormones and Several Abiotic Stress Conditions on Arabidopsis thaliana Primary Root Development

As sessile organisms, plants must adjust their growth to withstand several environmental conditions. The root is a crucial organ for plant survival as it is responsible for water and nutrient acquisition from the soil and has high phenotypic plasticity in response to a lack or excess of them. How plants sense and transduce their external conditions to achieve development, is still a matter of investigation and hormones play fundamental roles. Hormones are small molecules essential for plant growth and their function is modulated in response to stress environmental conditions and internal cues to adjust plant development. This review was motivated by the need to explore how Arabidopsis thaliana primary root differentially sense and transduce external conditions to modify its development and how hormone-mediated pathways contribute to achieve it. To accomplish this, we discuss available data of primary root growth phenotype under several hormone loss or gain of function mutants or exogenous application of compounds that affect hormone concentration in several abiotic stress conditions. This review shows how different hormones could promote or inhibit primary root development in A. thaliana depending on their growth in several environmental conditions. Interestingly, the only hormone that always acts as a promoter of primary root development is gibberellins.

Gene expression changes occurring at bolting time are associated with leaf senescence in Arabidopsis

In plants, the vegetative to reproductive phase transition (termed bolting in Arabidopsis) generally precedes age-dependent leaf senescence (LS). Many studies describe a temporal link between bolting time and LS, as plants that bolt early, senesce early, and plants that bolt late, senesce late. The molecular mechanisms underlying this relationship are unknown and are potentially agriculturally important, as they may allow for the development of crops that can overcome early LS caused by stress-related early-phase transition. We hypothesized that leaf gene expression changes occurring in synchrony with bolting were regulating LS. ARABIDOPSIS TRITHORAX (ATX) enzymes are general methyltransferases that regulate the adult vegetative to reproductive phase transition. We generated an atx1, atx3, and atx4 (atx1,3,4) triple T-DNA insertion mutant that displays both early bolting and early LS. This mutant was used in an RNA-seq time-series experiment to identify gene expression changes in rosette leaves that are likely associated with bolting. By comparing the early bolting mutant to vegetative WT plants of the same age, we were able to generate a list of differentially expressed genes (DEGs) that change expression with bolting as the plants age. We trimmed the list by intersection with publicly available WT datasets, which removed genes from our DEG list that were atx1,3,4 specific. The resulting 398 bolting-associated genes (BAGs) are differentially expressed in a mature rosette leaf at bolting. The BAG list contains many well-characterized LS regulators (ORE1, WRKY45, NAP, WRKY28), and GO analysis revealed enrichment for LS and LS-related processes. These bolting-associated LS regulators may contribute to the temporal coupling of bolting time to LS.

Maleic hydrazide elicits global transcriptomic changes in chemically topped tobacco to influence shoot bud development

MAIN CONCLUSION

Transcriptomic analysis revealed maleic hydrazide suppresses apical and axillary bud development by altering the expression of genes related to meristem development, cell division, DNA replication, DNA damage and recombination, and phytohormone signaling. Topping (removal of apical buds) is a common agricultural practice for some crop plants including cotton, cannabis, and tobacco. Maleic hydrazide (MH) is a systemic suckercide, a chemical that inhibits shoot bud growth, used to control the growth of apical (ApB) and axillary buds (AxB) following topping. However, the influence of MH on gene expression and the underlying molecular mechanism of controlling meristem development are not well studied. Our RNA sequencing analysis showed that MH significantly influences the transcriptomic landscape in ApB and AxB of chemically topped tobacco. Gene ontology (GO) enrichment analysis revealed that upregulated genes in ApB were enriched for phosphorelay signal transduction, and the regulation of transition timing from vegetative to reproductive phase, whereas downregulated genes were largely associated with meristem maintenance, cytokinin metabolism, cell wall synthesis, photosynthesis, and DNA metabolism. In MH-treated AxB, GO terms related to defense response and oxylipin metabolism were overrepresented in upregulated genes. GO terms associated with cell cycle, DNA metabolism, and cytokinin metabolism were enriched in downregulated genes. Expression of KNOX and MADS transcription factor (TF) family genes, known to be involved in meristem development, were affected in ApB and AxB by MH treatment. The promoters of MH-responsive genes are enriched for several known cis-acting elements, suggesting the involvement of a subset of TF families. Our findings suggest that MH affects shoot bud development in chemically topped tobacco by altering the expression of genes related to meristem development, DNA repair and recombination, cell division, and phytohormone signaling.

Advances in AP2/ERF super-family transcription factors in plant

In the whole life process, many factors including external and internal factors affect plant growth and development. The morphogenesis, growth, and development of plants are controlled by genetic elements and are influenced by environmental stress. Transcription factors contain one or more specific DNA-binding domains, which are essential in the whole life cycle of higher plants. The AP2/ERF (APETALA2/ethylene-responsive element binding factors) transcription factors are a large group of factors that are mainly found in plants. The transcription factors of this family serve as important regulators in many biological and physiological processes, such as plant morphogenesis, responsive mechanisms to various stresses, hormone signal transduction, and metabolite regulation. In this review, we summarized the advances in identification, classification, function, regulatory mechanisms, and the evolution of AP2/ERF transcription factors in plants. AP2/ERF family factors are mainly classified into four major subfamilies: DREB (Dehydration Responsive Element-Binding), ERF (Ethylene-Responsive-Element-Binding protein), AP2 (APETALA2) and RAV (Related to ABI3/VP), and Soloists (few unclassified factors). The review summarized the reports about multiple regulatory functions of AP2/ERF transcription factors in plants. In addition to growth regulation and stress responses, the regulatory functions of AP2/ERF in plant metabolite biosynthesis have been described. We also discussed the roles of AP2/ERF transcription factors in different phytohormone-mediated signaling pathways in plants. Genomic-wide analysis indicated that AP2/ERF transcription factors were highly conserved during plant evolution. Some public databases containing the information of AP2/ERF have been introduced. The studies of AP2/ERF factors will provide important bases for plant regulatory mechanisms and molecular breeding.

Nitrate Signaling, Functions, and Regulation of Root System Architecture: Insights from Arabidopsis thaliana

Root system architecture (RSA) is required for the acquisition of water and mineral nutrients from the soil. One of the essential nutrients, nitrate (NO3-), is sensed and transported by nitrate transporters NRT1.1 and NRT2.1 in the plants. Nitrate transporter 1.1 (NRT1.1) is a dual-affinity nitrate transporter phosphorylated at the T101 residue by calcineurin B-like interacting protein kinase (CIPKs); it also regulates the expression of other key nitrate assimilatory genes. The differential phosphorylation (phosphorylation and dephosphorylation) strategies and underlying Ca2+ signaling mechanism of NRT1.1 stimulate lateral root growth by activating the auxin transport activity and Ca2+-ANR1 signaling at the plasma membrane and the endosomes, respectively. NO3- additionally functions as a signal molecule that forms a signaling system, which consists of a vast array of transcription factors that control root system architecture that either stimulate or inhibit lateral and primary root development in response to localized and high nitrate (NO3-), respectively. This review elucidates the so-far identified nitrate transporters, nitrate sensing, signal transduction, and the key roles of nitrate transporters and its downstream transcriptional regulatory network in the primary and lateral root development in Arabidopsis thaliana under stress conditions.

Transcription factor CDF4 promotes leaf senescence and floral organ abscission by regulating abscisic acid and reactive oxygen species pathways in Arabidopsis

Leaf senescence is a highly complex developmental process that is tightly controlled by multiple layers of regulation. Abscisic acid (ABA) and reactive oxygen species (ROS) are two well-known factors that promote leaf senescence. We show here that the transcription factor CDF4 positively regulates leaf senescence. Constitutive and inducible overexpression of CDF4 accelerates leaf senescence, while knockdown of CDF4 delays it. CDF4 increases endogenous ABA levels by upregulating the transcription of the ABA biosynthesis genes 9-cis-epoxycarotenoid dioxygenase 2, 3 (NCED2, 3) and suppresses H₂ O₂ scavenging by repressing expression of the catalase2 (CAT2) gene. NCED2, 3 knockout and CAT2 overexpression partially rescue premature leaf senescence caused by CDF4 overexpression. We also show that CDF4 promotes floral organ abscission by activating the polygalacturonase PGAZAT gene. Based on these results, we propose that the levels of CDF4, ABA, and ROS undergo a gradual increase driven by their interlinking positive feedback loops during the leaf senescence and floral organ abscission processes.

Genome-Wide Dissection of the CRF Gene Family in Brassica napus Indicates that BnaCRF8s Specifically Regulate Root Architecture and Phosphate Homeostasis against Phosphate Fluctuation in Plants

Phosphorus (P) is an essential macronutrient required for plant growth and development. The involvement of cytokinin response factors (CRFs) in phosphate (Pi) homeostasis and lateral root (LR) initiation in Arabidopsis has been revealed. However, little is known in oil crops. Here, we performed genome-wide dissection of the CRF family in Brassica napus to identify 44 members, which were evolutionally classified into 6 subgroups. Among them, four BnaCRF8 genes were strongly upregulated by P deprivation, and were selected to be further investigated. Time course qRT-PCR analyses showed that four BnaCRF8 genes were enhanced dramatically after 12 h of P stress. Analyses of the subcellular localization in tobacco leaves indicated that BnaA7.CRF8 and BnaC2.CRF8 were localized in the nucleus. The expression of BnaCRF8 genes had constant negative effects on primary root growth and LR initiation and growth, and it reduced Pi acquisition and plant growth in Arabidopsis. Moreover, the expression of Pi homeostasis-related genes was modulated in BnaA7.CRF8 overexpression plants. These results suggest that BnaCRF8 genes might negatively regulate root architecture and plant growth through transcriptional modification of Pi homeostasis-related components. Overall, this study suggests that upregulation of BnaCRF8 genes might be a smart adaptive strategy to cope with continuous Pi deficiency in the environment.

A Self-Organized PLT/Auxin/ARR-B Network Controls the Dynamics of Root Zonation Development in Arabidopsis thaliana

During organogenesis, coherent organ growth arises from spatiotemporally coordinated decisions of individual cells. In the root of Arabidopsis thaliana, this coordination results in the establishment of a division and a differentiation zone. Cells continuously move through these zones; thus, a major question is how the boundary between these domains, the transition zone, is formed and maintained. By combining molecular genetics with computational modeling, we reveal how an auxin/PLETHORA/ARR-B network controls these dynamic patterning processes. We show that after germination, cell division causes a drop in distal PLT2 levels that enables transition zone formation and ARR12 activation. The resulting PLT2-ARR12 antagonism controls expansion of the division zone (the meristem). The successive ARR1 activation antagonizes PLT2 through inducing the cell-cycle repressor KRP2, thus setting final meristem size. Our work indicates a key role for the interplay between cell division dynamics and regulatory networks in root zonation and transition zone patterning.

Genome-wide identification of EMBRYO-DEFECTIVE (EMB) genes required for growth and development in Arabidopsis

With the emergence of high-throughput methods in plant biology, the importance of long-term projects characterized by incremental advances involving multiple laboratories can sometimes be overlooked. Here, I highlight my 40-year effort to isolate and characterize the most common class of mutants encountered in Arabidopsis (Arabidopsis thaliana): those defective in embryo development. I present an updated dataset of 510 EMBRYO-DEFECTIVE (EMB) genes identified throughout the Arabidopsis community; include important details on 2200 emb mutants and 241 pigment-defective embryo (pde) mutants analyzed in my laboratory; provide curated datasets with key features and publication links for each EMB gene identified; revisit past estimates of 500-1000 total EMB genes in Arabidopsis; document 83 double mutant combinations reported to disrupt embryo development; emphasize the importance of following established nomenclature guidelines and acknowledging allele history in research publications; and consider how best to extend community-based curation and screening efforts to approach saturation for this diverse class of mutants in the future. Continued advances in identifying EMB genes and characterizing their loss-of-function mutant alleles are needed to understand genotype-to-phenotype relationships in Arabidopsis on a broad scale, and to document the contributions of large numbers of essential genes to plant growth and development.

A unique chromosome translocation disrupting ClWIP1 leads to gynoecy in watermelon

To understand sex determination in watermelon (Citrullus lanatus), a spontaneous gynoecious watermelon mutant, XHBGM, was selected from the monoecious wild type XHB. Using map-based cloning, resequencing and fluorescence in situ hybridization analysis, a unique chromosome translocation between chromosome 2 and chromosome 3 was found in XHBGM. Based on the breakpoint location in chromosome 2, a putative C2H2 zinc finger transcription factor gene, ClWIP1 (gene ID Cla008537), an orthologue of the melon gynoecy gene CmWIP1, was disrupted. Using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system 9 to edit ClWIP1, we obtained gynoecious watermelon lines. Functional studies showed that ClWIP1 is expressed specifically in carpel primordia and is related to the abortion of carpel primordia in early floral development. To identify the cellular and metabolic processes associated with ClWIP1, we compared the shoot apex transcriptomes of two gynoecious mutants and their corresponding wild types. Transcriptome analysis showed that differentially expressed genes related to the ethylene and cytokinin pathways were upregulated in the gynoecious mutants. This study explores the molecular mechanism of sex determination in watermelon and provides a theoretical and technical basis for breeding elite gynoecious watermelon lines.

Review - Cytokinin Response Factors: Responding to more than cytokinin

Cytokinin Response Factors (CRFs) are a family of transcription factors which make up a side branch of the classical cytokinin two-component signaling pathway. CRFs were originally identified and have been primarily studied in Arabidopsis thaliana, although orthologs have be found throughout all land plants. Research into the evolution of CRFs as sub-group members of the larger APETALA2/Ethylene Response Factor (AP2/ERF) family has yielded interesting and useful insights related to the functional roles of CRFs in plants. Recent studies of CRFs suggest that these transcription factors are a lot more than just a group of cytokinin related genes and play important roles in both plant development and environmental stress response. This review focuses on recent advances in understanding the roles of CRFs beyond cytokinin, in reproductive development and abiotic stress response, as well as to other environmental cues.

Lineage specific conservation of cis-regulatory elements in Cytokinin Response Factors

Expression patterns of genes are controlled by short regions of DNA in promoter regions known as cis-regulatory elements. How expression patterns change due to alterations in cis-regulatory elements in the context of gene duplication are not well studied in plants. Over 300 promoter sequences from a small, well-conserved family of plant transcription factors known as Cytokinin Response Factors (CRFs) were examined for conserved motifs across several known clades present in Angiosperms. General CRF and lineage specific motifs were identified. Once identified, significantly enriched motifs were then compared to known transcription factor binding sites to elucidate potential functional roles. Additionally, presence of similar motifs shows that levels of conservation exist between different CRFs across land plants, likely occurring through processes of neo- or sub-functionalization. Furthermore, significant patterns of motif conservation are seen within and between CRF clades suggesting cis-regulatory regions have been conserved throughout CRF evolution.

Tackling Plant Phosphate Starvation by the Roots

Plant responses to phosphate deprivation encompass a wide range of strategies, varying from altering root system architecture, entering symbiotic interactions to excreting root exudates for phosphorous release, and recycling of internal phosphate. These processes are tightly controlled by a complex network of proteins that are specifically upregulated upon phosphate starvation. Although the different effects of phosphate starvation have been intensely studied, the full extent of its contribution to altered root system architecture remains unclear. In this review, we focus on the effect of phosphate starvation on the developmental processes that shape the plant root system and their underlying molecular pathways.

Cytokinin functions as an asymmetric and anti-gravitropic signal in lateral roots

Directional organ growth allows the plant root system to strategically cover its surroundings. Intercellular auxin transport is aligned with the gravity vector in the primary root tips, facilitating downward organ bending at the lower root flank. Here we show that cytokinin signaling functions as a lateral root specific anti-gravitropic component, promoting the radial distribution of the root system. We performed a genome-wide association study and reveal that signal peptide processing of Cytokinin Oxidase 2 (CKX2) affects its enzymatic activity and, thereby, determines the degradation of cytokinins in natural Arabidopsis thaliana accessions. Cytokinin signaling interferes with growth at the upper lateral root flank and thereby prevents downward bending. Our interdisciplinary approach proposes that two phytohormonal cues at opposite organ flanks counterbalance each other's negative impact on growth, suppressing organ growth towards gravity and allow for radial expansion of the root system.