Down-Regulation of Cytokinin Oxidase 2 Expression Increases Tiller Number and Improves Rice Yield

A nitrogen-responsive cytokinin oxidase/dehydrogenase regulates root response to high ammonium in rice

Plant root system is significantly influenced by high soil levels of ammonium nitrogen, leading to reduced root elongation and enhanced lateral root branching. In Arabidopsis, these processes have been reported to be mediated by phytohormones and their downstream signaling pathways, while the controlling mechanisms remain elusive in crops. Through a transcriptome analysis of roots subjected to high/low ammonium treatments, we identified a cytokinin oxidase/dehydrogenase encoding gene, CKX3, whose expression is induced by high ammonium. Knocking out CKX3 and its homologue CKX8 results in shorter seminal roots, fewer lateral roots, and reduced sensitivity to high ammonium. Endogenous cytokinin levels are elevated by high ammonium or in ckx3 mutants. Cytokinin application results in shorter seminal roots and fewer lateral roots in wild-type, mimicking the root responses of ckx3 mutants to high ammonium. Furthermore, CKX3 is transcriptionally activated by type-B RR25 and RR26, and ckx3 mutants have reduced auxin content and signaling in roots under low ammonium. This study identified RR25/26-CKX3-cytokinin as a signal module that mediates root responses to external ammonium by modulating of auxin signaling in the root meristem and lateral root primordium. This highlights the critical role of cytokinin metabolism in regulating rice root development in response to ammonium.

Genomic insights into cytokinin oxidase/dehydrogenase (CKX) gene family, identification, phylogeny and synteny analysis for its possible role in regulating seed number in Pigeonpea (Cajanus cajan (L.) Millsp.)

Cytokinin oxidase/dehydrogenase (CKX) regulates cytokinin levels in plants which are vital for plant growth and development. However, there is a paucity of evidence regarding their role in controlling embryo/seed development in pigeonpea. This comprehensive study provides information on the identification and characterization of CKX genes in pigeonpea. A genome-wide analysis identified 18 CKX genes, each with distinct structure, expression patterns, and possible diverse functions. Domain analysis revealed the presence of the sequences including FAD and CK-Binding domain, and subcellular localization analysis showed that almost 50 % of them reside within the nucleus. They were observed to be located unevenly on chromosome numbers 2, 4, 6, 7, and 11 with a majority of them present on the scaffolds. The 8 homologous pairs and various orthologous gene pairs provided further insights into their evolution pattern. Further, SNP/Indels variation in CKX genes and haplotype groups among contrasting genotypes for SNPP (seed number per pod) were analyzed. Spatiotemporal expression analysis revealed the significant expression pattern of CcCKX15, CcCKX17, and CcCKX2 in genotypes carrying low SNPP reiterating their possible role as negative regulators. These genes can be potential targets to undertake seed and biomass improvement in pigeonpea.

Temporal Gene Expression Profiles From Pollination to Seed Maturity in Sorghum Provide Core Candidates for Engineering Seed Traits

Sorghum (Sorghum bicolor (L.) Moench) is a highly nutritional multipurpose millet crop. However, the genetic and molecular regulatory mechanisms governing sorghum grain development and the associated agronomic traits remain unexplored. In this study, we performed a comprehensive transcriptomic analysis of pistils collected 1-2 days before pollination, and developing seeds collected -2, 10, 20 and 30 days after pollination of S. bicolor variety M35-1. Out of 31 337 genes expressed in these stages, 12 804 were differentially expressed in the consecutive stages of seed development. These exhibited 10 dominant expression patterns correlated with the distinct pathways and gene functions. Functional analysis, based on the pathway mapping, transcription factor enrichment and orthology, delineated the key patterns associated with pollination, fertilization, early seed development, grain filling and seed maturation. Furthermore, colocalization with previously reported quantitative trait loci (QTLs) for grain weight/size revealed 48 differentially expressed genes mapping to these QTL regions. Comprehensive literature mining integrated with QTL mapping and expression data shortlisted 25, 17 and 8 core candidates for engineering grain size, starch and protein content, respectively.

Variation in WIDTH OF LEAF AND GRAIN contributes to grain and leaf size by controlling LARGE2 stability in rice

Grain and flag leaf size are two important agronomic traits that influence grain yield in rice (Oryza sativa). Many quantitative trait loci (QTLs) and genes that regulate these traits individually have been identified, however, few QTLs and genes that simultaneously control these two traits have been identified. In this study, we conducted a genome-wide association analysis in rice and detected a major locus, WIDTH OF LEAF AND GRAIN (WLG), that was associated with both grain and flag leaf width. WLG encodes a RING-domain E3 ubiquitin ligase. WLGhap.B, which possesses five single nucleotide polymophysim (SNP) variations compared to WLGhap.A, encodes a protein with enhanced ubiquitination activity that confers increased rice leaf width and grain size, whereas mutation of WLG leads to narrower leaves and smaller grains. Both WLGhap.A and WLGhap.B interact with LARGE2, a HETC-type E3 ligase, however, WLGhap.B exhibits stronger interaction with LARGE2, thus higher ubiquitination activity toward LARGE2 compared with WLGhap.A. Lysine1021 is crucial for the ubiquitination of LARGE2 by WLG. Loss-of-function of LARGE2 in wlg-1 phenocopies large2-c in grain and leaf width, suggesting that WLG acts upstream of LARGE2. These findings reveal the genetic and molecular mechanism by which the WLG-LARGE2 module mediates grain and leaf size in rice and suggest the potential of WLGhap.B in improving rice yield.

Cytokinin and reproductive shoot architecture: bigger and better?

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

Cataloging the Genetic Response: Unveiling Drought-Responsive Gene Expression in Oil Tea Camellia (Camellia oleifera Abel.) through Transcriptomics

Drought stress is a critical environmental factor that significantly impacts plant growth and productivity. However, the transcriptome analysis of differentially expressed genes in response to drought stress in Camellia oleifera Abel. is still unclear. This study analyzed the transcriptome sequencing data of C. oleifera under drought treatments. A total of 20,674 differentially expressed genes (DEGs) were identified under drought stress, with the number of DEGs increasing with the duration of drought. Specifically, 11,793 and 18,046 DEGs were detected after 8 and 15 days of drought treatment, respectively, including numerous upregulated and downregulated genes. Gene Ontology (GO) enrichment analysis showed that the DEGs were primarily involved in various biological processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that carbon metabolism, glyoxylate and dicarboxylate metabolism, proteasome, glycine, serine, and threonine metabolism were the main affected pathways. Among the DEGs, 376 protein kinases, 42 proteases, 168 transcription factor (TF) genes, and 152 other potential functional genes were identified, which may play significant roles in the drought response of C. oleifera. The expression of relevant functional genes was further validated using quantitative real-time PCR (qRT-PCR). These findings contribute to the comprehension of drought tolerance mechanisms in C. oleifera and bolster the identification of drought-resistant genes for molecular breeding purposes.

Cytokinin oxidase2-deficient mutants improve panicle and grain architecture through cytokinin accumulation and enhance drought tolerance in indica rice

KEY MESSAGE

The Osckx2 mutant accumulates cytokinin thereby enhancing panicle branching, grain yield, and drought tolerance, marked by improved survival rate, membrane integrity, and photosynthetic function. Cytokinins (CKs) are multifaceted hormones that regulate growth, development, and stress responses in plants. Cytokinins have been implicated in improved panicle architecture and grain yield; however, they are inactivated by the enzyme cytokinin oxidase (CKX). In this study, we developed a cytokinin oxidase 2 (Osckx2)-deficient mutant using CRISPR/Cas9 gene editing in indica rice and assessed its function under water-deficit and salinity conditions. Loss of OsCKX2 function increased grain number, secondary panicle branching, and overall grain yield through improved cytokinin content in the panicle tissue. Under drought conditions, the Osckx2 mutant conserved more water and demonstrated improved water-saving traits. Through reduced transpiration, Osckx2 mutants showed an improved survival response than the wild type to unset dehydration stress. Further, Osckx2 maintained chloroplast and membrane integrity and showed significantly improved photosynthetic function under drought conditions through enhanced antioxidant protection systems. The OsCKX2 function negatively affects panicle grain number and drought tolerance, with no discernible impact in response to salinity. The finding suggests the utility of the beneficial Osckx2 allele in breeding to develop climate-resilient, high-yielding cultivars for future food security.

Genomic insights into CKX genes: key players in cotton fibre development and abiotic stress responses

Cytokinin oxidase/dehydrogenase (CKX), responsible for irreversible cytokinin degradation, also controls plant growth and development and response to abiotic stress. While the CKX gene has been studied in other plants extensively, its function in cotton is still unknown. Therefore, a genome-wide study to identify the CKX gene family in the four cotton species was conducted using transcriptomics, quantitative real-time PCR (qRT-PCR) and bioinformatics. As a result, in G. hirsutum and G. barbadense (the tetraploid cotton species), 87 and 96 CKX genes respectively and 62 genes each in G. arboreum and G. raimondii, were identified. Based on the evolutionary studies, the cotton CKX gene family has been divided into five distinct subfamilies. It was observed that CKX genes in cotton have conserved sequence logos and gene family expansion was due to segmental duplication or whole genome duplication (WGD). Collinearity and multiple synteny studies showed an expansion of gene families during evolution and purifying selection pressure has been exerted. G. hirsutum CKX genes displayed multiple exons/introns, uneven chromosomal distribution, conserved protein motifs, and cis-elements related to growth and stress in their promoter regions. Cis-elements related to resistance, physiological metabolism and hormonal regulation were identified within the promoter regions of the CKX genes. Expression analysis under different stress conditions (cold, heat, drought and salt) revealed different expression patterns in the different tissues. Through virus-induced gene silencing (VIGS), the GhCKX34A gene was found to improve cold resistance by modulating antioxidant-related activity. Since GhCKX29A is highly expressed during fibre development, we hypothesize that the increased expression of GhCKX29A in fibres has significant effects on fibre elongation. Consequently, these results contribute to our understanding of the involvement of GhCKXs in both fibre development and response to abiotic stress.

Natural variations of HvSRN1 modulate the spike rachis node number in barley

Grain number, one of the major determinants of yield in Triticeae crops, is largely determined by spikelet number and spike rachis node number (SRN). Here, we identified three quantitative trait loci (QTLs) for SRN using 145 recombinant inbred lines derived from a barley R90/1815D cross. qSRN1, the major-effect QTL, was mapped to chromosome 2H and explained up to 38.77% of SRN variation. Map-based cloning revealed that qSRN1 encodes the RAWUL domain-containing protein HvSRN1. Further analysis revealed that two key SNPs in the HvSRN1 promoter region (∼2 kb upstream of the transcription start site) affect the transcript level of HvSRN1 and contribute to variation in SRN. Similar to its orthologous proteins OsLAX2 and ZmBA2, HvSRN1 showed protein-protein interactions with HvLAX1, suggesting that the LAX2-LAX1 model for spike morphology regulation may be conserved in Poaceae crops. CRISPR-Cas9-induced HvSRN1 mutants showed reduced SRN but increased grain size and weight, demonstrating a trade-off effect. Our results shed light on the role of HvSRN1 variation in regulating the balance between grain number and weight in barley.

FRIZZLE PANICLE (FZP) regulates rice spikelets development through modulating cytokinin metabolism

BACKGROUND

The number of grains per panicle is an important factor in determining rice yield. The DST-OsCKX2 module has been demonstrated to regulate panicle development in rice by controlling cytokinin content. However, to date, how the function of DST-OsCKX2 module is regulated during panicle development remains obscure.

RESULT

In this study, the ABNORMAL PANICLE 1 (ABP1), a severely allele of FRIZZY PANICLE (FZP), exhibits abnormal spikelets morphology. We show that FZP can repress the expression of DST via directly binding to its promotor. Consistently, the expression level of OsCKX2 increased and the cytokinin content decreased in the fzp mutant, suggesting that the FZP acts upstream of the DST-OsCKX2 to maintain cytokinin homeostasis in the inflorescence meristem.

CONCLUSIONS

Our results indicate that FZP plays an important role in regulating spikelet development and grain number through mediating cytokinin metabolism.

Genetic Basis of Grain Size and Weight in Rice, Wheat, and Barley

Grain size is a key component of grain yield in cereals. It is a complex quantitative trait controlled by multiple genes. Grain size is determined via several factors in different plant development stages, beginning with early tillering, spikelet formation, and assimilates accumulation during the pre-anthesis phase, up to grain filling and maturation. Understanding the genetic and molecular mechanisms that control grain size is a prerequisite for improving grain yield potential. The last decade has brought significant progress in genomic studies of grain size control. Several genes underlying grain size and weight were identified and characterized in rice, which is a model plant for cereal crops. A molecular function analysis revealed most genes are involved in different cell signaling pathways, including phytohormone signaling, transcriptional regulation, ubiquitin-proteasome pathway, and other physiological processes. Compared to rice, the genetic background of grain size in other important cereal crops, such as wheat and barley, remains largely unexplored. However, the high level of conservation of genomic structure and sequences between closely related cereal crops should facilitate the identification of functional orthologs in other species. This review provides a comprehensive overview of the genetic and molecular bases of grain size and weight in wheat, barley, and rice, focusing on the latest discoveries in the field. We also present possibly the most updated list of experimentally validated genes that have a strong effect on grain size and discuss their molecular function.

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.

GhCKX14 responding to drought stress by modulating antioxi-dative enzyme activity in Gossypium hirsutum compared to CKX family genes

BACKGROUND

Cytokinin oxidase/dehydrogenase (CKX) plays a vital role in response to abiotic stress through modulating the antioxidant enzyme activities. Nevertheless, the biological function of the CKX gene family has yet to be reported in cotton.

RESULT

In this study, a total of 27 GhCKXs were identified by the genome-wide investigation and distributed across 18 chromosomes. Phylogenetic tree analysis revealed that CKX genes were clustered into four clades, and most gene expansions originated from segmental duplications. The CKXs gene structure and motif analysis displayed remarkably well conserved among the four groups. Moreover, the cis-acting elements related to the abiotic stress, hormones, and light response were identified within the promoter regions of GhCKXs. Transcriptome data and RT-qPCR showed that GhCKX genes demonstrated higher expression levels in various tissues and were involved in cotton's abiotic stress and phytohormone response. The protein-protein interaction network indicates that the CKX family probably participated in redox regulation, including oxidoreduction or ATP levels, to mediate plant growth and development. Functionally identified via virus-induced gene silencing (VIGS) found that the GhCKX14 gene improved drought resistance by modulating the antioxidant-related activitie.

CONCLUSIONS

In this study, the CKX gene family members were analyzed by bioinformatics, and validates the response of GhCKX gene to various phytohormone treatment and abiotic stresses. Our findings established the foundation of GhCKXs in responding to abiotic stress and GhCKX14 in regulating drought resistance in cotton.

GmJAZ3 interacts with GmRR18a and GmMYC2a to regulate seed traits in soybean

Seed weight is usually associated with seed size and is one of the important agronomic traits that determine yield. Understanding of seed weight control is limited, especially in soybean plants. Here we show that Glycine max JASMONATE-ZIM DOMAIN 3 (GmJAZ3), a gene identified through gene co-expression network analysis, regulates seed-related traits in soybean. Overexpression of GmJAZ3 promotes seed size/weight and other organ sizes in stable transgenic soybean plants likely by increasing cell proliferation. GmJAZ3 interacted with both G. max RESPONSE REGULATOR 18a (GmRR18a) and GmMYC2a to inhibit their transcriptional activation of cytokinin oxidase gene G. max CYTOKININ OXIDASE 3-4 (GmCKX3-4), which usually affects seed traits. Meanwhile, the GmRR18a binds to the promoter of GmMYC2a and activates GmMYC2a gene expression. In GmJAZ3-overexpressing soybean seeds, the protein contents were increased while the fatty acid contents were reduced compared to those in the control seeds, indicating that the GmJAZ3 affects seed size/weight and compositions. Natural variation in JAZ3 promoter region was further analyzed and Hap3 promoter correlates with higher promoter activity, higher gene expression and higher seed weight. The Hap3 promoter may be selected and fixed during soybean domestication. JAZ3 orthologs from other plants/crops may also control seed size and weight. Taken together, our study reveals a novel molecular module GmJAZ3-GmRR18a/GmMYC2a-GmCKXs for seed size and weight control, providing promising targets during soybean molecular breeding for better seed traits.

Molecular Network for Regulation of Seed Size in Plants

The size of seeds is particularly important for agricultural development, as it is a key trait that determines yield. It is controlled by the coordinated development of the integument, endosperm, and embryo. Large seeds are an important way of improving the ultimate "sink strength" of crops, providing more nutrients for early plant growth and showing certain tolerance to abiotic stresses. There are several pathways for regulating plant seed size, including the HAIKU (IKU) pathway, ubiquitin-proteasome pathway, G (Guanosine triphosphate) protein regulatory pathway, mitogen-activated protein kinase (MAPK) pathway, transcriptional regulators pathway, and phytohormone regulatory pathways including the auxin, brassinosteroid (BR), gibberellin (GA), jasmonic acid (JA), cytokinin (CK), Abscisic acid (ABA), and microRNA (miRNA) regulatory pathways. This article summarizes the seed size regulatory network and prospective ways of improving yield. We expect that it will provide a valuable reference to researchers in related fields.

Genome-wide association analysis for emergence of deeply sown rice (Oryza sativa) reveals novel aus-specific phytohormone candidate genes for adaptation to dry-direct seeding in the field

Dry direct-seeded rice (dry-DSR) is typically sown deeply to circumvent the need for irrigation, and thus seedling emergence is a crucial trait affecting plant stand and yield. To breed elite cultivars that use less water and are climate-resilient, an understanding of the genomic regions and underlying genes that confer emergence in deeply sown dry-DSR would be highly advantageous. A combined diversity panel of 470 rice accessions (RDP1 plus aus subset of 3K RGP) was evaluated with 2.9 million single nucleotide polymorphisms (SNPs) to identify associations with dry-DSR traits in the field and component traits in a controlled-environment experiment. Using genome-wide association study (GWAS) analyses, we identified 18 unique QTLs on chromosomes 1, 2, 4, 5, 6, 7, 9, 10, and 11, explaining phenotypic variance ranging from 2.6% to 17.8%. Three QTLs, namely, qSOE-1.1, qEMERG-AUS-1.2, and qEMERG-AUS-7.1, were co-located with previously reported QTLs for mesocotyl length. Among the identified QTLs, half were associated with the emergence of aus, and six were unique to the aus genetic group. Based on functional annotation, we identified eleven compelling candidate genes that primarily regulate phytohormone pathways such as cytokinin, auxin, gibberellic acid, and jasmonic acid. Prior studies indicated that these phytohormones play a critical role in mesocotyl length under deep sowing. This study provides new insight into the importance of aus and indica as desirable genetic resources to mine favorable alleles for deep-sowing tolerance in rice. The candidate genes and marker-tagged desirable alleles identified in this study should benefit rice breeding programs directly.

Cold-upregulated glycosyltransferase gene 1 (OsCUGT1) plays important roles in rice height and spikelet fertility

Glycosyltransferases (GTs) regulate many physiological processes and stress responses in plants. However, little is known about the function of GT in rice development. In this study, molecular analyses revealed that the expression of a rice GT gene (Cold-Upregulated Glycosyltransferase Gene 1, CUGT1) is developmentally controlled and stress-induced. OsCUGT1 was knocked out by using the clustered regularly interspaced short palindromic repeats (CRISPR) system to obtain the mutant oscugt1, which showed a severe dwarf and sterility phenotype. Further cytological analyses indicated that the dwarfism seen in the oscugt1 mutant might be caused by fewer and smaller cells. Histological pollen analysis suggests that the spikelet sterility in oscugt1 mutants may be caused by abnormal microsporogenesis. Moreover, multiple transgenic plants with knockdown of OsCUGT1 expression through RNA interference were obtained, which also showed obvious defects in plant height and fertility. RNA sequencing revealed that multiple biological processes associated with phenylpropanoid biosynthesis, cytokinin metabolism and pollen development are affected in the oscugt1 mutant. Overall, these results suggest that rice OsCUGT1 plays an essential role in rice development.

Cloning of Three Cytokinin Oxidase/Dehydrogenase Genes in Bambusa oldhamii

Cytokinin oxidase/dehydrogenase (CKX) catalyzes the irreversible breakdown of active cytokinins, which are a class of plant hormones that regulate cell division. According to conserved sequences of CKX genes from monocotyledons, PCR primers were designed to synthesize a probe for screening a bamboo genomic library. Cloned results of three genes encoding cytokinin oxidase were named as follows: BoCKX1, BoCKX2, and BoCKX3. In comparing the exon-intron structures among the above three genes, there are three exons and two introns in BoCKX1 and BoCKX3 genes, whereas BoCKX2 contains four exons and three introns. The amino acid sequence of BoCKX2 protein shares 78% and 79% identity with BoCKX1 and BoCKX3 proteins, respectively. BoCKX1 and BoCKX3 genes are particularly closely related given that the amino acid and nucleotide sequence identities are more than 90%. These three BoCKX proteins carried putative signal peptide sequences typical of secretion pathway, and a GHS-motif was found at N-terminal flavin adenine dinucleotide (FAD) binding domain, suggesting that BoCKX proteins might covalently conjugate with an FAD cofactor through a predicted histidine residue.

Genome-wide identification of the soybean cytokinin oxidase/dehydrogenase gene family and its diverse roles in response to multiple abiotic stress

Cytokinin oxidase/dehydrogenase (CKX) irreversibly degrades cytokinin, regulates growth and development, and helps plants to respond to environmental stress. Although the CKX gene has been well characterized in various plants, its role in soybean remains elusive. Therefore, in this study, the evolutionary relationship, chromosomal location, gene structure, motifs, cis-regulatory elements, collinearity, and gene expression patterns of GmCKXs were analyzed using RNA-seq, quantitative real-time PCR (qRT-PCR), and bioinformatics. We identified 18 GmCKX genes from the soybean genome and grouped them into five clades, each comprising members with similar gene structures and motifs. Cis-acting elements involved in hormones, resistance, and physiological metabolism were detected in the promoter regions of GmCKXs. Synteny analysis indicated that segmental duplication events contributed to the expansion of the soybean CKX family. The expression profiling of the GmCKXs genes using qRT-PCR showed tissue-specific expression patterns. The RNA-seq analysis also indicated that GmCKXs play an important role in response to salt and drought stresses at the seedling stage. The responses of the genes to salt, drought, synthetic cytokinin 6-benzyl aminopurine (6-BA), and the auxin indole-3-acetic acid (IAA) at the germination stage were further evaluated by qRT-PCR. Specifically, the GmCKX14 gene was downregulated in the roots and the radicles at the germination stage. The hormones 6-BA and IAA repressed the expression levels of GmCKX1, GmCKX6, and GmCKX9 genes but upregulated the expression levels of GmCKX10 and GmCKX18 genes. The three abiotic stresses also decreased the zeatin content in soybean radicle but enhanced the activity of the CKX enzymes. Conversely, the 6-BA and IAA treatments enhanced the CKX enzymes' activity but reduced the zeatin content in the radicles. This study, therefore, provides a reference for the functional analysis of GmCKXs in soybean in response to abiotic stresses.

Modifications of Phytohormone Metabolism Aimed at Stimulation of Plant Growth, Improving Their Productivity and Tolerance to Abiotic and Biotic Stress Factors

Due to the growing human population, the increase in crop yield is an important challenge for modern agriculture. As abiotic and biotic stresses cause severe losses in agriculture, it is also crucial to obtain varieties that are more tolerant to these factors. In the past, traditional breeding methods were used to obtain new varieties displaying demanded traits. Nowadays, genetic engineering is another available tool. An important direction of the research on genetically modified plants concerns the modification of phytohormone metabolism. This review summarizes the state-of-the-art research concerning the modulation of phytohormone content aimed at the stimulation of plant growth and the improvement of stress tolerance. It aims to provide a useful basis for developing new strategies for crop yield improvement by genetic engineering of phytohormone metabolism.

Carpel-specific down-regulation of GhCKXs in cotton significantly enhances seed and fiber yield

Cytokinin is considered to be an important driver of seed yield. To increase the yield of cotton while avoiding the negative consequences caused by constitutive overproduction of cytokinin, we down-regulated specifically the carpel genes for cytokinin oxidase/dehydrogenase (CKX), a key negative regulator of cytokinin levels, in transgenic cotton. The carpel-specific down-regulation of CKXs significantly enhanced cytokinin levels in the carpels. The elevated cytokinin promoted the expression of carpel- and ovule-development-associated genes, GhSTK2, GhAG1, and GhSHP, boosting ovule formation and thus producing more seeds in the ovary. Field experiments showed that the carpel-specific increase of cytokinin significantly increased both seed yield and fiber yield of cotton, without resulting in detrimental phenotypes. Our study details the regulatory mechanism of cytokinin signaling for seed development, and provides an effective and feasible strategy for yield improvement of seed crops.

Evolutionary expansion and expression dynamics of cytokinin-catabolizing CKX gene family in the modern amphidiploid mustard (Brassica sp.)

Plant cytokinins (CKs) promote development and physiological processes, drought tolerance, root architecture, and ultimately crop productivity. Biologically active CKs (iP, tZ, and cZ) are precisely maintained in the vegetative and floral tissues through their irreversible degradation by developmentally regulated CK-catabolizing cytokinin oxidase/dehydrogenase (CKX) enzyme. A meta-analysis of CKX proteins was performed through an exhaustive exploration of multiple genome databases of cyanobacteria, bryophyte, monocot and eudicot plants to reveal the intricate evolutionary profiles of CKX enzymes specific to the family Brassicaceae. At least 175 unique paralogous/orthologous CKX sequences were successfully retrieved and phylogenetically clustered into distinct groups. Observations of structural divergences among paralogous sequences compared to their orthologs indicated that the progenitor CKX sequence had been subjected to massive structural modifications, possibly as a result of the evolutionary split between monocots and eudicots. An analysis of dN/dS comparisons of orthologous genes revealed that segmental CKX gene duplications have evolved primarily under purifying selection. Further, 24 CKX genes with conserved signature domain were identified in the amphidiploid Brassica juncea genome (AABB; 2n = 36). Genetic evolution of paralogous and orthologous genes was largely responsible for the expansion of CKX homoeologs in the amphidiploid Brassica genomes. Also, comparative analyses of 1.5 kb-long upstream regulatory regions of BjCKX genes identified various development- and stress-responsive elements. Spatial and temporal expression profiles of CKX genes were primarily attributed to their structural diversity observed in the 5'-regulatory regions along with species evolution. This data suggested that CKX duplicate genes had partitioned their spatial expression (= function) during evolution. These findings illustrated the evolutionary importance of CKX genes during plant development, and also suggested their deployment for future crop improvement programs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03294-0.

Genome-Wide Analysis and Evolutionary Perspective of the Cytokinin Dehydrogenase Gene Family in Wheat (Triticum aestivum L.)

Cytokinin dehydrogenase (CKX; EC.1.5.99.12) regulates the level of cytokinin (CK) in plants and is involved in CK regulatory activities. In different plants, a small gene family encodes CKX proteins with varied numbers of members. These genes are expanded in the genome mainly due to segmental duplication events. Despite their biological importance, CKX genes in Triticum aestivum have yet to be studied in depth. A total of 11 CKX subfamilies were identified with similar gene structures, motifs, domains, cis-acting elements, and an average signal peptide of 25 amino acid length was found. Introns, ranging from one to four, were present in the coding regions at a similar interval in major CKX genes. Putative cis-elements such as abscisic acid, auxin, salicylic acid, and low-temperature-, drought-, and light-responsive cis-regulatory elements were found in the promoter region of majority CKX genes. Variation in the expression pattern of CKX genes were identified across different tissues in Triticum. Phylogenetic analysis shows that the same subfamily of CKX clustered into a similar clade that reflects their evolutionary relationship. We performed a genome-wide identification of CKX family members in the Triticum aestivum genome to get their chromosomal location, gene structure, cis-element, phylogeny, synteny, and tissue- and stage-specific expression along with gene ontology. This study has also elaborately described the tissue- and stage-specific expression and is the resource for further analysis of CKX in the regulation of biotic and abiotic stress resistance, growth, and development in Triticum and other cereals to endeavor for higher production and proper management.

Multifaceted roles of zinc finger proteins in regulating various agronomic traits in rice

Rice is an important cereal crop, which provides staple food for more than half of the world's population. To meet the demand of the ever-growing population in the next few decades, an extra increase in rice yield is an urgent need. Given that various agronomic traits contribute to the yield of rice, deciphering the key regulators involved in multiple agronomic trait formation is particularly important. As a superfamily of transcription factors, zinc finger proteins participate in regulating multiple genes in almost every stage of rice growth and development. Therefore, understanding zinc finger proteins underlying regulatory network would provide insights into the regulation of agronomic traits in rice. To this end, we intend to summarize the current advances in zinc finger proteins, with emphasis on C2H2 and CCCH proteins, and then discuss their potential in improving rice yield.

Killing two birds with a single stone-genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

The development of high-yielding, bio-fortified, stress-tolerant crop cultivars is the need of the hour in the wake of increasing global food insecurity, abrupt climate change, and continuous shrinking of resources and landmass suitable for agriculture. The cytokinin group of phytohormones positively regulates seed yield by simultaneous regulation of source capacity (leaf senescence) and sink strength (grain number and size). Cytokinins also regulate root-shoot architecture by promoting shoot growth and inhibiting root growth. Cytokinin oxidase/dehydrogenase (CKX) are the only enzymes that catalyze the irreversible degradation of active cytokinins and thus negatively regulate the endogenous cytokinin levels. Genetic manipulation of CKX genes is the key to improve seed yield and root-shoot architecture through direct manipulation of endogenous cytokinin levels. Downregulation of CKX genes expressed in sink tissues such as inflorescence meristem and developing seeds, through reverse genetics approaches such as RNAi and CRISPR/Cas9 resulted in increased yield marked by increased number and size of grains. On the other hand, root-specific expression of CKX genes resulted in decreased endogenous cytokinin levels in roots which in turn resulted in increased root growth indicated by increased root branching, root biomass, and root-shoot biomass ratio. Enhanced root growth provided enhanced tolerance to drought stress and improved micronutrient uptake efficiency. In this review, we have emphasized the role of CKX as a genetic factor determining yield, micronutrient uptake efficiency, and response to drought stress. We have summarised the efforts made to increase crop productivity and drought stress tolerance in different crop species through genetic manipulation of CKX family genes.

Melatonin Mediates Axillary Bud Outgrowth by Improving Nitrogen Assimilation and Transport in Rice

Melatonin plays an important role in plant resistance to biotic and abiotic stresses. However, whether melatonin is involved in the regulation of plant architecture, such as the formation of axillary bud outgrowth or tillering, in rice remains unknown. Here, we found that different concentrations of melatonin influenced axillary bud outgrowth in rice, and moderate melatonin concentrations also alleviated the inhibition of axillary bud outgrowth in the presence of high concentrations of basic amino acids lysine and arginine. Furthermore, transcriptome analysis demonstrated that genes involved in nitrogen metabolism and phytohormone signal transduction pathways may affect axillary bud outgrowth, which is regulated by melatonin. We determined that the differentially expressed genes glutamine synthetase OsGS2 and amino acid transporter OsAAP14, which are involved in nitrogen metabolism and are regulated by melatonin and basic amino acids, were the key regulators of axillary bud outgrowth in rice. In addition, we validated the functions of OsGS2 and OsAAP14 using rice transgenic plants with altered axillary bud outgrowth and tillers. Taken together, these results suggest that melatonin mediates axillary bud outgrowth by improving nitrogen assimilation and transport in rice.

Cytokinin oxidase/dehydrogenase family genes exhibit functional divergence and overlap in rice growth and development, especially in control of tillering

Cytokinins play key roles in plant growth and development, and hence their biosynthesis and degradation have been extensively studied. Cytokinin oxidase/dehydrogenases (CKXs) are a group of enzymes that regulate oxidative cleavage to maintain cytokinin homeostasis. In rice, 11 CKX genes have been identified to date; however, most of their functions remain unknown. In this study, we comprehensively examined the expression patterns and functions of the CKXs in rice by using CRISPR/Cas9 technology to construct mutants of all 11 genes. The results revealed that the ckx single-mutants and higher-order ckx4 ckx9 mutant lines showed functional overlaps and sub-functionalization. Notably, the ckx1 ckx2 and ckx4 ckx9 double-mutants displayed contrasting phenotypic changes in tiller number and panicle size compared to the wild-type. In addition, we identified several genes with significantly altered expression in both the ckx4 and ckx9 single-mutant and double-mutant plants. Many of the differentially expressed genes were found to be associated with auxin and cytokinin pathways, and cytokinins in the ckx4 ckx9 double-mutant were increased compared to the wild-type. Taken together, our findings provide new insights into the functions of CKX genes in rice growth and may provide the foundations for future studies aimed at improving rice yield.

OsCKX2 regulates phosphate deficiency tolerance by modulating cytokinin in rice

Cytokinin oxidase/dehydrogenases (CKXs) are key enzymes that degrade cytokinins (CTKs) and play an essential role in plant growth and development. The present study analyzed the phenotypic and physiological characteristics of OsCKX2 overexpressing (OE) and knockout (KO) rice plants after exposure to phosphate (Pi) deficiency and the transcriptome and metabolome to investigate the function of OsCKX2 in response to Pi deficiency. OsCKX2 KO plants demonstrated higher endogenous CTK levels than wild-type (WT) under Pi deficiency. Further analysis indicated more robust tolerance of OsCKX2 KO plants to Pi deficiency, which exhibited higher phosphorus concentration, larger shoot biomass, and lesser leaf yellowing under Pi deficiency; whereas the opposite was observed for OsCKX2 OE plants. Transcriptome and metabolome analyses revealed that overexpression of OsCKX2 downregulated the transcriptional levels of genes related to Pi transporters, membrane lipid metabolism, and glycolysis, and reduced the consumption of metabolites in membrane lipid metabolism and glycolysis. On the contrary, knockout of OsCKX2 upregulated the expression of Pi transporters, and increased the consumption of metabolites in membrane lipid metabolism and glycolysis. These results indicated that OsCKX2 impacted Pi uptake, recycling, and plant growth via Pi transporters, phospholipid hydrolysis, and glycolysis under Pi deficiency. Overall, OsCKX2 negatively regulated Pi deficiency tolerance by modulating CTKs in rice.

Over-Expression of Dehydroascorbate Reductase Improves Salt Tolerance, Environmental Adaptability and Productivity in Oryza sativa

Abiotic stress induces reactive oxygen species (ROS) generation in plants, and high ROS levels can cause partial or severe oxidative damage to cellular components that regulate the redox status. Here, we developed salt-tolerant transgenic rice plants that overexpressed the dehydroascorbate reductase gene (OsDHAR1) under the control of a stress-inducible sweet potato promoter (SWPA2). OsDHAR1-expressing transgenic plants exhibited improved environmental adaptability compared to wild-type plants, owing to enhanced ascorbate levels, redox homeostasis, photosynthetic ability, and membrane stability through cross-activation of ascorbate-glutathione cycle enzymes under paddy-field conditions, which enhanced various agronomic traits, including root development, panicle number, spikelet number per panicle, and total grain yield. dhar2-knockdown plants were susceptible to salt stress, and owing to poor seed maturation, exhibited reduced biomass (root growth) and grain yield under paddy field conditions. Microarray revealed that transgenic plants highly expressed genes associated with cell growth, plant growth, leaf senescence, root development, ROS and heavy metal detoxification systems, lipid metabolism, isoflavone and ascorbate recycling, and photosynthesis. We identified the genetic source of functional genomics‒based molecular breeding in crop plants and provided new insights into the physiological processes underlying environmental adaptability, which will enable improvement of stress tolerance and crop species productivity in response to climate change.

Cytokinins: A Genetic Target for Increasing Yield Potential in the CRISPR Era

Over the last decade, remarkable progress has been made in our understanding the phytohormones, cytokinin's (CKs) biosynthesis, perception, and signalling pathways. Additionally, it became apparent that interfering with any of these steps has a significant effect on all stages of plant growth and development. As a result of their complex regulatory and cross-talk interactions with other hormones and signalling networks, they influence and control a wide range of biological activities, from cellular to organismal levels. In agriculture, CKs are extensively used for yield improvement and management because of their wide-ranging effects on plant growth, development and physiology. One of the primary targets in this regard is cytokinin oxidase/dehydrogenase (CKO/CKX), which is encoded by CKX gene, which catalyses the irreversible degradation of cytokinin. The previous studies on various agronomically important crops indicated that plant breeders have targeted CKX directly. In recent years, prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system has been increasingly used in editing the CKO/CKX gene and phenomenal results have been achieved. This review provides an updated information on the applications of CRISPR-based gene-editing tools in manipulating cytokinin metabolism at the genetic level for yield improvement. Furthermore, we summarized the current developments of RNP-mediated DNA/transgene-free genomic editing of plants which would broaden the application of this technology. The current review will advance our understanding of cytokinins and their role in sustainably increase crop production through CRISPR/Cas genome editing tool.

Genome-Wide Identification and Characterisation of Cytokinin-O-Glucosyltransferase (CGT) Genes of Rice Specific to Potential Pathogens

Cytokinin glucosyltransferases (CGTs) are key enzymes of plants for regulating the level and function of cytokinins. In a genomic identification of rice CGTs, 41 genes with the plant secondary product glycosyltransferases (PSPG) motif of 44-amino-acid consensus sequence characteristic of plant uridine diphosphate (UDP)-glycosyltransferases (UGTs) were identified. In-silico physicochemical characterisation revealed that, though the CGTs belong to the same subfamily, they display varying molecular weights, ranging from 19.6 kDa to 59.7 kDa. The proteins were primarily acidic (87.8%) and hydrophilic (58.6%) and were observed to be distributed in the plastids (16), plasma membrane (13), mitochondria (5), and cytosol (4). Phylogenetic analysis of the CGTs revealed that their evolutionary relatedness ranged from 70-100%, and they aligned themselves into two major clusters. In a comprehensive analysis of the available transcriptomics data of rice samples representing different growth stages only the CGT, Os04g25440.1 was significantly expressed at the vegetative stage, whereas 16 other genes were highly expressed only at the reproductive growth stage. On the contrary, six genes, LOC_Os07g30610.1, LOC_Os04g25440.1, LOC_Os07g30620.1, LOC_Os04g25490.1, LOC_Os04g37820.1, and LOC_Os04g25800.1, were significantly upregulated in rice plants inoculated with Rhizoctonia solani (RS), Xoo (Xanthomonas oryzae pv. oryzae) and Mor (Magnaporthe oryzae). In a qRT-PCR analysis of rice sheath tissue susceptible to Rhizoctonia solani, Mor, and Xoo pathogens, compared to the sterile distilled water control, at 24 h post-infection only two genes displayed significant upregulation in response to all the three pathogens: LOC_Os07g30620.1 and LOC_Os04g25820.1. On the other hand, the expression of genes LOC_Os07g30610.1, LOC_Os04g25440, LOC_Os04g25490, and LOC_Os04g25800 were observed to be pathogen-specific. These genes were identified as the candidate-responsive CGT genes and could serve as potential susceptibility genes for facilitating pathogen infection.

Cytokinin Oxygenase/Dehydrogenase Inhibitors: An Emerging Tool in Stress Biotechnology Employed for Crop Improvement

In order to meet the global challenges of food security, one of the foremost solutions lies in enhancing the crop productivity. This can be attained by considering key plant hormones such as cytokinins as agrochemicals as cytokinins in particular are known to control the essential processes of the plants. Even though, it has already been established since 1980s that the enzyme, cytokinin oxidase/dehydrogenase (CKO/CKX) deactivates cytokinins; the potential applications of manipulating these enzymes have mostly been speculated to have a high potential in the biotechnology industry and spreads to agriculture, horticulture and agroforestry. The enzyme is critical in maintaining a balanced level of cytokinins in plants. However, it is yet to be fully established that inhibiting this enzyme can be the constant source of improvement in the productivity of plants, even though success has been obtained in some economically important plant species. Furthermore, the impact efficiency of this enzyme may vary from plant to plant, which needs to be evaluated employing tissue culture and other extrinsic applications. This review intends to cover the relevant studies addressing any biological activity of this enzyme in the current context and any associated biotechnological applications specific to enhanced grain yield, abiotic stress tolerance, delayed senescence and in vitro organogenesis among various plants and not only cereals. Moreover, our study will identify the present gaps in research with respect to many important food crops, which will be useful for researchers who are actively involved in providing a foundation for a variety of genetically improved plants achieved through this manner. In addition to this, other ways of engineering the amount of cytokinin levels appropriate for signaling also needs to be analyzed in order to extend the benefits of cytokinin biology to other crops too. The application of these inhibitors can be considered among the best alternates as well as addition to genetically modified plants for overcoming the gaps in crop demand.

A cryptic inhibitor of cytokinin phosphorelay controls rice grain size

Plant hormone cytokinin signals through histidine-aspartic acid (H-D) phosphorelay to regulate plant growth and development. While it is well known that the phosphorelay involves histidine kinases, histidine phosphotransfer proteins (HPs), and response regulators (RRs), how this process is regulated by external components remains unknown. Here we demonstrate that protein phosphatase with kelch-like domains (PPKL1), known as a signaling component of steroid hormone brassinosteroid, is actually a cryptic inhibitor of cytokinin phosphorelay in rice (Oryza sativa). Mutation at a specific amino acid D364 of PPKL1 activates cytokinin response and thus enlarges grain size in a semi-dominant mutant named s48. Overexpression of PPKL1 containing D364, either with the deletion of the phosphatase domain or not, rescues the s48 mutant phenotype. PPKL1 interacts with OsAHP2, one of authentic HPs, and D364 resides in a region resembling the receiver domain of RRs. Accordingly, PPKL1 can utilize D364 to suppress OsAHP2-to-RR phosphorelay, whereas mutation of D364 abolishes the effect. This function of PPKL1 is independent of the phosphatase domain that is required for brassinosteroid signaling. Importantly, editing of the D364-residential region produces a diversity of semi-dominant mutations associated with variously increased grain sizes. Further screening of the edited plants enables the identification of two genotypes that confer significantly improved grain yield. Collectively, our study uncovers a noncanonical cytokinin signaling suppressor and provides a robust tool for seed rational design.

Maize Golden2-like transcription factors boost rice chloroplast development, photosynthesis, and grain yield

Chloroplasts are the sites for photosynthesis, and two Golden2-like factors act as transcriptional activators of chloroplast development in rice (Oryza sativa L.) and maize (Zea mays L.). Rice OsGLK1 and OsGLK2 are orthologous to maize ZmGLK1 (ZmG1) and ZmGLK2 (ZmG2), respectively. However, while rice OsGLK1 and OsGLK2 act redundantly to regulate chloroplast development in mesophyll cells, maize ZmG1 and ZmG2 are functionally specialized and expressed in different cell-specific manners. To boost rice chloroplast development and photosynthesis, we generated transgenic rice plants overexpressing ZmG1 and ZmG2, individually or simultaneously, with constitutive promoters (pZmUbi::ZmG1 and p35S::ZmG2) or maize promoters (pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2). Both ZmG1 and ZmG2 genes were highly expressed in transgenic rice leaves. Moreover, ZmG1 and ZmG2 showed coordinated expression in pZmG1::ZmG1/pZmG2::ZmG2 plants. All Golden2-like (GLK) transgenic plants had higher chlorophyll and protein contents, Rubisco activities and photosynthetic rates per unit leaf area in flag leaves. However, the highest grain yields occurred when maize promoters were used; pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2 transgenic plants showed increases in grain yield by 51%, 47%, and 70%, respectively. In contrast, the pZmUbi::ZmG1 plant produced smaller seeds without yield increases. Transcriptome analysis indicated that maize GLKs act as master regulators promoting the expression of both photosynthesis-related and stress-responsive regulatory genes in both rice shoot and root. Thus, by promoting these important functions under the control of their own promoters, maize GLK1 and GLK2 genes together dramatically improved rice photosynthetic performance and productivity. A similar approach can potentially improve the productivity of many other crops.

Loss of Gn1a/OsCKX2 confers heavy-panicle rice with excellent lodging resistance

Significant achievements have been made in breeding programs for the heavy-panicle-type (HPT) rice (Oryza sativa) in Southwest China. The HPT varieties now exhibit excellent lodging resistance, allowing them to overcome the greater pressures caused by heavy panicles. However, the genetic mechanism of this lodging resistance remains elusive. Here, we isolated a major quantitative trait locus, Panicle Neck Diameter 1 (PND1), and identified the causal gene as GRAIN NUMBER 1A/CYTOKININ OXIDASE 2 (Gn1A/OsCKX2). The null gn1a allele from rice line R498 (gn1a^R498 ) improved lodging resistance through increasing the culm diameter and promoting crown root development. Loss-of-function of Gn1a/OsCKX2 led to cytokinin accumulation in the crown root tip and accelerated the development of adventitious roots. Gene pyramiding between the null gn1a^R498 allele with two gain-of-function alleles, STRONG CULM 2 (SCM2) and SCM3, further improved lodging resistance. Moreover, Gn1a/OsCKX2 had minimal influence on overall rice quality. Our research thus highlights the distinct genetic components of lodging resistance of HPT varieties and provides a strategy for tailor-made crop improvement of both yield and lodging resistance in rice.

A VIN3-like Protein OsVIL1 Is Involved in Grain Yield and Biomass in Rice

In chromatin remodeling, the post-translational modification of histone proteins is mediated by multimeric protein complexes. VERNALIZATION INSENSITIVE3 (VIN3) forms a complex with Polycomb Repressive Complex 2 (PRC2), which mediates the trimethylation of H3K27 to repress target gene expression. In rice, four genes (OsVIL1-OsVIL4) encoding the VIN3-like proteins are expressed ubiquitously in various tissues. Null mutants of osvil2 display pleiotropic phenotypes such as altered flowering time, floral organ defects, and reduced tiller size. In contrast, osvil1 mutants did not show significant phenotypes except in fertilization compared with the wild type. However, transgenic plants overexpressing OsVIL1 showed phenotypes of increased biomass and grain yield. Cross-sections of the basal region of elongating stems revealed that the increased biomass was mediated by inducing cell proliferation in the meristem. Chromatin immunoprecipitation assay indicated that OsVIL1 repressed expression of cytokinin oxidase/dehydrogenase gene (OsCKX2) by binding to the promoter and genic regions of OsCKX2. We also observed that OsVIL1 modified the levels of H3K27me3 in the OsCKX2 chromatin. Because OsCKX2 encodes an enzyme that degrades active cytokinin, we conclude that OsVIL1 functions in the regulation of endogenous active cytokinin levels, thereby increasing plant height and productivity.

Effects of nitrogen topdressing and paclobutrazol at different stages on spike differentiation and yield of winter wheat

Background

Optimal nitrogen (N) application and plant growth regulators can improve wheat productivity. This can help to improve yield level and ensure food security with limited resources in the Huang-Huai-Hai Plain of China (HPC).

Methods

A 2-year field experiment was conducted using a randomized block design with four treatments (TS-N topdressing at pseudostem erection stage ; TPS-N topdressing combined with paclobutrazol application at pseudostem erection stage; TJ-N topdressing at jointing stage; TPJ-N topdressing at combined with paclobutrazol application at jointing stage) in 2011-2013.

Results

The grain number per ear, thousand kernel weight and yield for the TJ and TPJ treatments were higher than those of the TS and TPS treatments. Grain number per ear, yield, and thousands kernel weigh for the TPJ treatment were significantly higher than for the TS and TPS in 2011-2012 (9.82% and 7.27%, 10.23% and 8.99%, 6.12% and 5.58%) and in 2012-2013 (10.21% and 11.55%, 8.00% and 6.58%, 0.00 and 0.00), respectively. Thousands kernel weight under TJ were significantly higher than those under TS and TPS by 13.21% and 14.03%, respectively in 2012-2013. The floret number, significantly correlated with cytokinin content, was also significantly increased under TJ and TPJ at connectivum differentiation stage. For TPJ treatment, the floret number was significantly higher than for the TS, TPS, and TJ by 19.92%, 10.21%, 6.10% in 2011-2012; it was higher than for the TS and TPS by 28.06% and 29.61% in 2012-2013, respectively. The relative expression level of cytokinin oxidase/dehydrogenase gene (TaCKX2.2) was improved during flowering, when cytokinin content was at high level and was also inhibited by paclobutrazol with different degrees.

Conclusions

Therefore, nitrogen topdressing at jointing stage had increased grain number per ear, thousand kernel weight, and grain yield of wheat. Paclobutrazol could delay spike differentiation and promote cytokinin accumulation that induced expression of TaCKX2.2, maintaining hormonal balance and affecting wheat spike morphogenesis.

Mapping novel QTLs for yield related traits from a popular rice hybrid KRH-2 derived doubled haploid (DH) population

A doubled haploid (DH) population consisting of 125 DHLs derived from the popular rice hybrid, KRH-2 (IR58025A/KMR3R) was utilized for Quantitative Trait Loci (QTL) mapping to identify novel genomic regions associated with yield related traits. A genetic map was constructed with 126 polymorphic SSR and EST derived markers, which were distributed across rice genome. QTL analysis using inclusive composite interval mapping (ICIM) method identified a total of 24 major and minor effect QTLs. Among them, twelve major effect QTLs were identified for days to fifty percent flowering (qDFF12-1), total grain yield/plant (qYLD3-1 and qYLD6-1), test (1,000) grain weight (qTGW6-1 and qTGW7-1), panicle weight (qPW9-1), plant height (qPH12-1), flag leaf length (qFLL6-1), flag leaf width (qFLW4-1), panicle length (qPL3-1 and qPL6-1) and biomass (qBM4-1), explaining 29.95-56.75% of the phenotypic variability with LOD scores range of 2.72-16.51. Chromosomal regions with gene clusters were identified on chromosome 3 for total grain yield/plant (qYLD3-1) and panicle length (qPL3-1) and on chromosome 6 for total grain yield/plant (qYLD6-1), flag leaf length (qFLL6-1) and panicle length (qPL6-1). Majority of the QTLs identified were observed to be co-localized with the previously reported QTL regions. Five novel, major effect QTLs associated with panicle weight (qPW9-1), plant height (qPH12-1), flag leaf width (qFLW4-1), panicle length (qPL3-1) and biomass (qBM4-1) and three novel minor effect QTLs for panicle weight (qPW3-1 and qPW8-1) and fertile grains per panicle (qFGP5-1) were identified. These QTLs can be used in breeding programs aimed to yield improvement after their validation in alternative populations.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03045-7.

Plant Growth Regulators INCYDE and TD-K Underperform in Cereal Field Trials

Using plant growth regulators to alter cytokinin homeostasis with the aim of enhancing endogenous cytokinin levels has been proposed as a strategy to increase yields in wheat and barley. The plant growth regulators INCYDE and CPPU inhibit the cytokinin degrading enzyme cytokinin oxidase/dehydrogenase (CKX), while TD-K inhibits the process of senescence. We report that the application of these plant growth regulators in wheat and barley field trials failed to enhance yields, or change the components of yields. Analyses of the endogenous cytokinin content showed a high concentration of trans-zeatin (tZ) in both wheat and barley grains at four days after anthesis, and statistically significant, but probably biologically insignificant, increases in cisZ-O-glucoside, along with small decreases in cZ riboside (cZR), dihydro Z (DHZ), and DHZR and DHZOG cytokinins, following INCYDE application to barley at anthesis. We discuss possible reasons for the lack of efficacy of the three plant growth regulators under field conditions and comment on future approaches to manipulating yield in the light of the strong homeostatic mechanisms controlling endogenous cytokinin levels.

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.

Function of the pseudo phosphotransfer proteins has diverged between rice and Arabidopsis

The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type-B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type-A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the rice PHP genes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three rice PHPs fail to complement an Arabidopsis php mutant (aphp1/ahp6). Disruption of the three rice PHPs results in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type-A RR and cytokinin oxidase genes. The triple php mutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the rice PHPs does not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

Novel cytokinin oxidase/dehydrogenase inhibitors for enhancing grain yield in crop plants and potential applications in the biotechnology industry

This article comments on:

Nisler J, Kopečný D, Pěkná Z, Končitíková R, Koprna R, Murvanidze N, Werbrouck SPO, Havlíček L, De Diego N, Wimmer Z, Briozzo P, Moréra S, Zalabák D, Spíchal L, Strnad M. 2021. Diphenylurea-derived cytokinin oxidase/dehydrogenase inhibitors for biotechnology and agriculture. Journal of Experimental Botany 72, 355–370.

The soybean (Glycine max L.) cytokinin oxidase/dehydrogenase multigene family; Identification of natural variations for altered cytokinin content and seed yield

Cytokinins (CKs) play a fundamental role in regulating dynamics of organ source/sink relationships during plant development, including flowering and seed formation stages. As a result, CKs are key drivers of seed yield. The cytokinin oxidase/dehydrogenase (CKX) is one of the critical enzymes responsible for regulating plant CK levels by causing their irreversible degradation. Variation of CKX activity is significantly correlated with seed yield in many crop species while in soybean (Glycine max L.), the possible associations between CKX gene family members (GFMs) and yield parameters have not yet been assessed. In this study, 17 GmCKX GFMs were identified, and natural variations among GmCKX genes were probed among soybean cultivars with varying yield characteristics. The key CKX genes responsible for regulating CK content during seed filling stages of reproductive development were highlighted using comparative phylogenetics, gene expression analysis and CK metabolite profiling. Five of the seventeen identified GmCKX GFMs, showed natural variations in the form of single nucleotide polymorphisms (SNPs). The gene GmCKX7-1, with high expression during critical seed filling stages, was found to have a non-synonymous mutation (H105Q), on one of the active site residues, Histidine 105, previously reported to be essential for co-factor binding to maintain structural integrity of the enzyme. Soybean lines with this mutation had higher CK content and desired yield characteristics. The potential for marker-assisted selection based on the identified natural variation within GmCKX7-1, is discussed in the context of hormonal control that can result in higher soybean yield.

Systematic identification and characterization of circular RNAs involved in flag leaf senescence of rice

Circular RNAs (circRNAs) identification, expression profiles, and construction of circRNA-parental gene relationships and circRNA-miRNA-mRNA ceRNA networks indicate that circRNAs are involved in flag leaf senescence of rice. Circular RNAs (circRNAs) are a class of 3'-5' head-to-tail covalently closed non-coding RNAs which have been proved to play important roles in various biological processes. However, no systematic identification of circRNAs associated with leaf senescence in rice has been studied. In this study, a genome-wide high-throughput sequencing analysis was performed using rice flag leaves developing from normal to senescence. Here, a total of 6612 circRNAs were identified, among which, 113 circRNAs were differentially expressed (DE) during the leaf senescence process. Moreover, 4601 (69.59%) circRNAs were derived from the exons or introns of their parental genes, while 2110 (71%) of the parental genes produced only one circRNA. The sequence alignment analysis showed that hundreds of rice circRNAs were conserved among different plant species. Gene Ontology (GO) enrichment analysis revealed that parental genes of DE circRNAs were enriched in many biological processes closely related to leaf senescence. Through weighted gene co-expression network analysis (WGCNA), six continuously down-expressed circRNAs, 18 continuously up-expressed circRNAs and 15 turn-point high-expressed circRNAs were considered to be highly associated with leaf senescence. Additionally, a total of 17 senescence-associated circRNAs were predicted to have parental genes, in which, regulations of three circRNAs to their parental genes were validated by qRT-PCR. The competing endogenous RNA (ceRNA) networks were also constructed. And a total of 11 senescence-associated circRNAs were predicted to act as miRNA sponges to regulate mRNAs, in which, regulation of two circRNAs to eight mRNAs was validated by qRT-PCR. It is discussed that senescence-associated circRNAs were involved in flag leaf senescence probably through mediating their parental genes and ceRNA networks, to participate in several well-studied senescence-associated processes, mainly including the processes of transcription, translation, and posttranslational modification (especially protein glycosylation), oxidation-reduction process, involvement of senescence-associated genes, hormone signaling pathway, proteolysis, and DNA damage repair. This study not only showed the systematic identification of circRNAs involved in leaf senescence of rice, but also laid a foundation for functional research on candidate circRNAs.

Cytokinin regulates the activity of the inflorescence meristem and components of seed yield in oilseed rape

The number of flowers and seed-bearing structures formed by the inflorescence meristem and the formation of ovules in the female reproductive part of the flowers are important yield-related traits of crop plants. It has been shown that cytokinin is a pivotal factor regulating these traits. Here, we explore the impact of mutation of CYTOKININ OXIDASE/DEHYDROGENASE (CKX) genes encoding cytokinin-degrading enzymes on these yield-related traits in oilseed rape (Brassica napus L.). We describe the identification of four BnCKX3 and two BnCKX5 genes as regulators of reproductive development in the allotetraploid B. napus. RNA-seq analysis and in situ hybridization showed expression of these genes in reproductive organs. Loss-of-function mutants for each of these CKX gene copies were identified by targeting induced local lesions in genomes (TILLING) and combined by crossing. Sextuple ckx3 ckx5 mutants showed an increased cytokinin concentration and larger and more active inflorescence meristems. They also produced up to 72% more flowers with gynoecia containing 32% more ovules and up to 54% more pods on the main stem. The weight of seeds harvested from the main stem of plants grown in the greenhouse or in the field was increased by 20-32%. Our results show that cytokinin regulates inflorescence meristem and placenta activity in oilseed rape. The work demonstrates the potential to achieve yield enhancement in a dicot crop plant by modulating the cytokinin status through mutagenesis of specific CKX genes.

Natural Variation and Domestication Selection of ZmCKX5 with Root Morphological Traits at the Seedling Stage in Maize

Root system architecture plays a crucial role in water and nutrient acquisition in maize. Cytokinins, which can be irreversibly degraded by the cytokinin oxidase/dehydrogenase (CKX), are important hormones that regulate root development in plants. In this study, ZmCKX5 was resequenced in 285 inbred lines, 68 landraces, and 32 teosintes to identify the significant variants associated with root traits in maize. Sequence polymorphisms and nucleotide diversity revealed that ZmCKX5 might be selected during domestication and improvement processes. Marker–trait association analysis in inbred lines identified 12 variants of ZmCKX5 that were significantly associated with six root traits, including seed root number (SRN), lateral root length (LRL), total root area (RA), root length in 0 to 0.5 mm diameter class (RL005), total root volume (RV), and total root length (TRL). SNP-1195 explained the most (6.01%) phenotypic variation of SRN, and the frequency of this allele G increased from 6.25% and 1.47% in teosintes and landraces, respectively, to 17.39% in inbred lines. Another significant variant, SNP-1406, with a pleiotropic effect, is strongly associated with five root traits, with the frequency of T allele increased from 25.00% and 23.73% in teosintes and landraces, respectively, to 35.00% in inbred lines. These results indicate that ZmCKX5 may be involved in the development of the maize root system and that the significant variants can be used to develop functional markers to accelerate the improvement in the maize root system.

Endoplasmic Reticulum-Localized PURINE PERMEASE1 Regulates Plant Height and Grain Weight by Modulating Cytokinin Distribution in Rice

Cytokinins (CKs) are a class of phytohormones playing essential roles in various biological processes. However, the mechanisms underlying CK transport as well as its function in plant growth and development are far from being fully elucidated. Here, we characterize the function of PURINE PERMEASE1 (OsPUP1) in rice (Oryza sativa L.). OsPUP1 was predominantly expressed in the root, particularly in vascular cells, and CK treatment can induce its expression. Subcellular localization analysis showed that OsPUP1 was predominantly localized to the endoplasmic reticulum (ER). Overexpression of OsPUP1 resulted in growth defect of various aerial tissues, including decreased leaf length, plant height, grain weight, panicle length, and grain number. Hormone profiling revealed that the CK content was decreased in the shoot of OsPUP1-overexpressing seedling, but increased in the root, compared with the wild type. The CK content in the panicle was also decreased. Quantitative reverse transcription-PCR (qRT-PCR) analysis using several CK type-A response regulators (OsRRs) as the marker genes suggested that the CK response in the shoot of OsPUP1-overexpressing seedling is decreased compared to the wild type when CKs are applied to the root. Genetic analysis revealed that BG3/OsPUP4, a putative plasma membrane-localized CK transporter, overcomes the function of OsPUP1. We hypothesize that OsPUP1 might be involved in importing CKs into ER to unload CKs from the vascular tissues by cell-to-cell transport.

Genetic Basis of Tiller Dynamics of Rice Revealed by Genome-Wide Association Studies

A tiller number is the key determinant of rice plant architecture and panicle number and consequently controls grain yield. Thus, it is necessary to optimize the tiller number to achieve the maximum yield in rice. However, comprehensive analyses of the genetic basis of the tiller number, considering the development stage, tiller type, and related traits, are lacking. In this study, we sequence 219 Korean rice accessions and construct a high-quality single nucleotide polymorphism (SNP) dataset. We also evaluate the tiller number at different development stages and heading traits involved in phase transitions. By genome-wide association studies (GWASs), we detected 20 significant association signals for all traits. Five signals were detected in genomic regions near known candidate genes. Most of the candidate genes were involved in the phase transition from vegetative to reproductive growth. In particular, HD1 was simultaneously associated with the productive tiller ratio and heading date, indicating that the photoperiodic heading gene directly controls the productive tiller ratio. Multiple linear regression models of lead SNPs showed coefficients of determination (R²) of 0.49, 0.22, and 0.41 for the tiller number at the maximum tillering stage, productive tiller number, and productive tiller ratio, respectively. Furthermore, the model was validated using independent japonica rice collections, implying that the lead SNPs included in the linear regression model were generally applicable to the tiller number prediction. We revealed the genetic basis of the tiller number in rice plants during growth, By GWASs, and formulated a prediction model by linear regression. Our results improve our understanding of tillering in rice plants and provide a basis for breeding high-yield rice varieties with the optimum the tiller number.

Genetic Basis of Tiller Dynamics of Rice Revealed by Genome-Wide Association Studies

A tiller number is the key determinant of rice plant architecture and panicle number and consequently controls grain yield. Thus, it is necessary to optimize the tiller number to achieve the maximum yield in rice. However, comprehensive analyses of the genetic basis of the tiller number, considering the development stage, tiller type, and related traits, are lacking. In this study, we sequence 219 Korean rice accessions and construct a high-quality single nucleotide polymorphism (SNP) dataset. We also evaluate the tiller number at different development stages and heading traits involved in phase transitions. By genome-wide association studies (GWASs), we detected 20 significant association signals for all traits. Five signals were detected in genomic regions near known candidate genes. Most of the candidate genes were involved in the phase transition from vegetative to reproductive growth. In particular, HD1 was simultaneously associated with the productive tiller ratio and heading date, indicating that the photoperiodic heading gene directly controls the productive tiller ratio. Multiple linear regression models of lead SNPs showed coefficients of determination (R²) of 0.49, 0.22, and 0.41 for the tiller number at the maximum tillering stage, productive tiller number, and productive tiller ratio, respectively. Furthermore, the model was validated using independent japonica rice collections, implying that the lead SNPs included in the linear regression model were generally applicable to the tiller number prediction. We revealed the genetic basis of the tiller number in rice plants during growth, By GWASs, and formulated a prediction model by linear regression. Our results improve our understanding of tillering in rice plants and provide a basis for breeding high-yield rice varieties with the optimum the tiller number.