Cytokinin activity during early kernel development corresponds positively with yield potential and later stage ABA accumulation in field-grown wheat (Triticum aestivum L.)

Seed yield as a function of cytokinin-regulated gene expression in wild Kentucky bluegrass (Poa pratensis)

BACKGROUND

Kentucky bluegrass (Poa pratensis L.) panicle development is a coordinated process of cell proliferation and differentiation with distinctive phases and architectural changes that are pivotal to determine seed yield. Cytokinin (CK) is a key factor in determining seed yield that might underpin the second "Green Revolution". However, whether there is a difference between endogenous CK content and seed yields of Kentucky bluegrass, and how CK-related genes are expressed to affect enzyme regulation and downstream seed yield in Kentucky bluegrass remains enigmatic.

RESULTS

In order to establish a potential link between CK regulation and seed yield, we dissected and characterized the Kentucky bluegrass young panicle, and determined the changes in nutrients, 6 types of endogenous CKs, and 16 genes involved in biosynthesis, activation, inactivation, re-activation and degradation of CKs during young panicle differentiation of Kentucky bluegrass. We found that high seed yield material had more meristems compared to low seed yield material. Additionally, it was found that seed-setting rate (SSR) and lipase activity at the stage of spikelet and floret primordium differentiation (S3), as well as 1000-grain weight (TGW) and zeatin-riboside (ZR) content at the stages of first bract primordium differentiation (S1) and branch primordium differentiation (S2) showed a significantly positive correlation in the two materials. And zeatin, ZR, dihydrozeatin riboside, isopentenyl adenosine and isopentenyl adenosine riboside contents were higher in seed high yield material than those in seed low yield material at S3 stage. Furthermore, the expressions of PpITP3, PpITP5, PpITP8 and PpLOG1 were positively correlated with seed yield, while the expressions of PpCKX2, PpCKX5 and PpCKX7 were negatively correlated with seed yield in Kentucky bluegrass.

CONCLUSIONS

Overall, our study established a relationship between CK and seed yield in Kentucky bluegrass. Perhaps we can increase SSR and TGW by increasing lipase activity and ZR content. Of course, using modern gene editing techniques to manipulate CK related genes such as PpITP3/5/8, PpLOG1 and PpCKX2/5/7, will be a more direct and effective method in Kentucky bluegrass, which requires further trial validation.

Plant Hormone Pathway Is Involved in Regulating the Embryo Development Mechanism of the Hydrangea macrophylla Hybrid

The research is aimed to elucidate the role of plant hormones in regulating the development of hybrid embryos in Hydrangea macrophylla. Fruits from the intraspecific cross of H. macrophylla 'Otaksa' × 'Coerulea' were selected at the globular, heart, and torpedo stages of embryo development. Transcriptome sequencing and differential gene expression analysis were conducted. The results showed that fruit growth followed a single "S-shaped growth curve, with globular, heart, and torpedo embryos appearing at 30, 40, and 50 d post-pollination, respectively, and the embryo maintaining the torpedo shape from 60 to 90 d. A total of 12,933 genes was quantified across the three developmental stages, with 3359, 3803, and 3106 DEGs in the S1_vs_S2, S1_vs_S3, and S2_vs_S3 comparisons, respectively. Among these, 133 genes related to plant hormone biosynthesis and metabolism were differentially expressed, regulating the synthesis and metabolism of eight types of plant hormones, including cytokinin, auxin, gibberellin, abscisic acid, and jasmonic acid. The pathways with the most differentially expressed genes were cytokinin, auxin, and gibberellin, suggesting these hormones may play crucial roles in embryo development. In the cytokinin pathway, CKX (Hma1.2p1_0579F.1_g182670.gene, Hma1.2p1_1194F.1_g265700.gene, and NewGene_12164) genes were highly expressed during the globular embryo stage, promoting rapid cell division in the embryo. In the auxin pathway, YUC (Hma1.2p1_0271F.1_g109005.gene and Hma1.2p1_0271F.1_g109020.gene) genes were progressively up-regulated during embryo growth; the early response factor AUX/IAA (Hma1.2p1_0760F.1_g214260.gene) was down-regulated, while the later transcriptional activator ARF (NewGene_21460, NewGene_21461, and Hma1.2p1_0209F.1_g089090.gene) was up-regulated, sustaining auxin synthesis and possibly preventing the embryo from transitioning to maturity. In the gibberellin pathway, GA3ox (Hma1.2p1_0129F.1_g060100.gene) expression peaked during the heart embryo stage and then declined, while the negative regulator GA2ox (Hma1.2p1_0020F.1_g013915.gene) showed the opposite trend; and the gibberellin signaling repressor DELLA (Hma1.2p1_1054F.1_g252590.gene) increased over time, potentially inhibiting embryo development and maintaining the torpedo shape until fruit maturity. These findings preliminarily uncover the factors affecting the development of hybrid H. macrophylla embryos, laying a foundation for further research into the regulatory mechanisms of H. macrophylla hybrid embryo development.

Drought Protective Effects of Exogenous ABA and Kinetin on Lettuce: Sugar Content, Antioxidant Enzyme Activity, and Productivity

Drought is an environmental stressor that significantly impacts plant growth and development. Comprehending the complexity of drought stress and water utilization in the context of plant growth and development holds significant importance for sustainable agriculture. The aim of this study was to evaluate the effect of exogenously applied phytohormones on lettuce (Lactuca sativa L.) sugar content profiles and antioxidant enzyme activity and productivity. Lettuce plants were grown under normal and drought conditions in a growth chamber with a photoperiod of 14/10 h (day/night). Kinetin and abscisic acid were applied separately and in combinations when the second leaf was fully expanded. The results showed that sugar accumulation and productivity of the pretreated plants under drought were significantly higher than the controls. The perspective offered by this work showed that growth-related and stress-related phytohormones significantly influenced plant sugar metabolism, metabolic profiles, and productivity, thus enabling the control of yield and quality.

Are TaNAC Transcription Factors Involved in Promoting Wheat Yield by cis-Regulation of TaCKX Gene Family?

NAC transcription factors (TFs) are one of the largest TF families in plants, and TaNACs have been known to participate in the regulation of the transcription of many yield-regulating genes in bread wheat. The TaCKX gene family members (GFMs) have already been shown to regulate yield-related traits, including grain mass and number, leaf senescence, and root growth. The genes encode cytokinin (CK) degrading enzymes (CKXs) and are specifically expressed in different parts of developing wheat plants. The aim of the study was to identify and characterize TaNACs involved in the cis-regulation of TaCKX GFMs. After analysis of the initial transcription factor data in 1.5 Kb cis-regulatory sequences of a total of 35 homologues of TaCKX GFMs, we selected five of them, namely TaCKX1-3A, TaCKX22.1-3B, TaCKX5-3D, TaCKX9-1B, and TaCKX10, and identified five TaNAC genes: TaNACJ-1, TaNAC13a, TaNAC94, TaNACBr-1, and TaNAC6D, which are potentially involved in the cis-regulation of selected TaCKX genes, respectively. Protein feature analysis revealed that all of the selected TaNACs have a conserved NAC domain and showed a stable tertiary structure model. The expression profile of the selected TaNACs was studied in 5 day-old seedling roots, 5-6 cm inflorescences, 0, 4, 7, and 14 days-after-pollination (DAP) spikes, and the accompanying flag leaves. The expression pattern showed that all of the selected TaNACs were preferentially expressed in seedling roots, 7 and 14 DAP spikes, and flag leaves compared to 5-6 cm inflorescence and 0 and 4 DAP spikes and flag leaves in Kontesa and Ostka spring wheat cultivars (cvs.). In conclusion, the results of this study highlight the potential role of the selected TaNACs in the regulation of grain productivity, leaf senescence, root growth, and response to various stresses.

Differential effects of elevated CO2 on awn and glume metabolism in durum wheat (Triticum durum)

While the effect of CO2 enrichment on wheat (Triticum spp.) photosynthesis, nitrogen content or yield has been well-studied, the impact of elevated CO2 on metabolic pathways in organs other than leaves is poorly documented. In particular, glumes and awns, which may refix CO2 respired by developing grains and be naturally exposed to higher-than-ambient CO2 mole fraction, could show specific responses to elevated CO2 . Here, we took advantage of a free-air CO2 enrichment experiment and performed multilevel analyses, including metabolomics, ionomics, proteomics, major hormones and isotopes in Triticum durum . While in leaves, elevated CO2 tended to accelerate amino acid metabolism with many significantly affected metabolites, the effect on glumes and awns metabolites was modest. There was a lower content in compounds of the polyamine pathway (along with uracile and allantoin) under elevated CO2 , suggesting a change in secondary N metabolism. Also, cytokinin metabolism appeared to be significantly affected under elevated CO2 . Despite this, elevated CO2 did not affect the final composition of awn and glume organic matter, with the same content in carbon, nitrogen and other elements. We conclude that elevated CO2 mostly impacts on leaf metabolism but has little effect in awns and glumes, including their composition at maturity.

Type I MADS-box transcription factor TaMADS-GS regulates grain size by stabilizing cytokinin signalling during endosperm cellularization in wheat

Grain size is one of the important traits in wheat breeding programs aimed at improving yield, and cytokinins, mainly involved in cell division, have a positive impact on grain size. Here, we identified a novel wheat gene TaMADS-GS encoding type I MADS-box transcription factor, which regulates the cytokinins signalling pathway during early stages of grain development to modulate grain size and weight in wheat. TaMADS-GS is exclusively expressed in grains at early stage of seed development and its knockout leads to delayed endosperm cellularization, smaller grain size and lower grain weight. TaMADS-GS protein interacts with the Polycomb Repressive Complex 2 (PRC2) and leads to repression of genes encoding cytokinin oxidase/dehydrogenases (CKXs) stimulating cytokinins inactivation by mediating accumulation of the histone H3 trimethylation at lysine 27 (H3K27me3). Through the screening of a large wheat germplasm collection, an elite allele of the TaMADS-GS exhibits higher ability to repress expression of genes inactivating cytokinins and a positive correlation with grain size and weight, thus representing a novel marker for breeding programs in wheat. Overall, these findings support the relevance of TaMADS-GS as a key regulator of wheat grain size and weight.

Cytokinin Translocation to, and Biosynthesis and Metabolism within, Cereal and Legume Seeds: Looking Back to Inform the Future

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

Transgenerational Paternal Inheritance of TaCKX GFMs Expression Patterns Indicate a Way to Select Wheat Lines with Better Parameters for Yield-Related Traits

Members of the TaCKX gene family (GFMs) encode the cytokinin oxygenase/dehydrogenase enzyme (CKX), which irreversibly degrades cytokinins in the organs of wheat plants; therefore, these genes perform a key role in the regulation of yield-related traits. The purpose of the investigation was to determine how expression patterns of these genes, together with the transcription factor-encoding gene TaNAC2-5A, and yield-related traits are inherited to apply this knowledge to speed up breeding processes. The traits were tested in 7 days after pollination (DAP) spikes and seedling roots of maternal and paternal parents and their F₂ progeny. The expression levels of most of them and the yield were inherited in F₂ from the paternal parent. Some pairs or groups of genes cooperated, and some showed opposite functions. Models of up- or down-regulation of TaCKX GFMs and TaNAC2-5A in low-yielding maternal plants crossed with higher-yielding paternal plants and their high-yielding F₂ progeny reproduced gene expression and yield of the paternal parent. The correlation coefficients between TaCKX GFMs, TaNAC2-5A, and yield-related traits in high-yielding F₂ progeny indicated which of these genes were specifically correlated with individual yield-related traits. The most common was expressed in 7 DAP spikes TaCKX2.1, which positively correlated with grain number, grain yield, spike number, and spike length, and seedling root mass. The expression levels of TaCKX1 or TaNAC2-5A in the seedling roots were negatively correlated with these traits. In contrast, the thousand grain weight (TGW) was negatively regulated by TaCKX2.2.2, TaCKX2.1, and TaCKX10 in 7 DAP spikes but positively correlated with TaCKX10 and TaNAC2-5A in seedling roots. Transmission of TaCKX GFMs and TaNAC2-5A expression patterns and yield-related traits from parents to the F₂ generation indicate their paternal imprinting. These newly shown data of nonmendelian epigenetic inheritance shed new light on crossing strategies to obtain a high-yielding F₂ generation.

Fine Mapping and Cloning of a qRA2 Affect the Ratooning Ability in Rice (Oryza sativa L.)

Ratooning ability is a key factor that influences the ratoon rice yield in areas where light and temperature are not sufficient for second-season rice. Near-isogenic lines (NILs) are the most powerful tools for the detection and precise mapping of quantitative trait loci (QTLs). In this study, using 176 NILs, we identified a novel QTL for ratooning ability in NIL128. First, we mapped the QTL between the markers Indel12-29 and Indel12-31, which encompass a region of 233 kb. The rice genome annotation indicated the existence of three candidate genes in this region that may be related to ratooning ability. Through gene prediction and cDNA sequencing, we speculated that the target gene of ratooning ability is LOC_Os02g51930 which encodes cytokinin glucosyl transferases (CGTs), hereafter named qRA2. Further analysis showed that qra2 was a 1-bp substitution in the first exon in NIL128, which resulted in the premature termination of qRA2. The results of the knockdown experiment showed that the Jiafuzhan knockdown mutants exhibited the ratooning ability phenotype of NIL128. Interestingly, the qRA2 gene was found to improve ratooning ability without affecting major agronomic traits. These results will help us better understand the genetic basis of rice ratooning ability and provide a valuable gene resource for breeding strong ratoon rice varieties.

Cis-Regulation by NACs: A Promising Frontier in Wheat Crop Improvement

Crop traits are controlled by multiple genes; however, the complex spatio-temporal transcriptional behavior of genes cannot be fully understood without comprehending the role of transcription factors (TFs) and the underlying mechanisms of the binding interactions of their cis-regulatory elements. NAC belongs to one of the largest families of plant-specific TFs and has been associated with the regulation of many traits. This review provides insight into the cis-regulation of genes by wheat NACs (TaNACs) for the improvement in yield-related traits, including phytohormonal homeostasis, leaf senescence, seed traits improvement, root modulation, and biotic and abiotic stresses in wheat and other cereals. We also discussed the current potential, knowledge gaps, and prospects of TaNACs.

Role of cytokinins in seed development in pulses and oilseed crops: Current status and future perspective

Cytokinins constitutes a vital group of plant hormones regulating several developmental processes, including growth and cell division, and have a strong influence on grain yield. Chemically, they are the derivatives of adenine and are the most complex and diverse group of hormones affecting plant physiology. In this review, we have provided a molecular understanding of the role of cytokinins in developing seeds, with special emphasis on pulses and oilseed crops. The importance of cytokinin-responsive genes including cytokinin oxidases and dehydrogenases (CKX), isopentenyl transferase (IPT), and cytokinin-mediated genetic regulation of seed size are described in detail. In addition, cytokinin expression in germinating seeds, its biosynthesis, source-sink dynamics, cytokinin signaling, and spatial expression of cytokinin family genes in oilseeds and pulses have been discussed in context to its impact on increasing economy yields. Recently, it has been shown that manipulation of the cytokinin-responsive genes by mutation, RNA interference, or genome editing has a significant effect on seed number and/or weight in several crops. Nevertheless, the usage of cytokinins in improving crop quality and yield remains significantly underutilized. This is primarily due to the multigene control of cytokinin expression. The information summarized in this review will help the researchers in innovating newer and more efficient ways of manipulating cytokinin expression including CKX genes with the aim to improve crop production, specifically of pulses and oilseed crops.

Combining QTL mapping and gene co-expression network analysis for prediction of candidate genes and molecular network related to yield in wheat

BACKGROUND

Wheat (Triticum aestivum L.) is an important cereal crop. Increasing grain yield for wheat is always a priority. Due to the complex genome of hexaploid wheat with 21 chromosomes, it is difficult to identify underlying genes by traditional genetic approach. The combination of genetics and omics analysis has displayed the powerful capability to identify candidate genes for major quantitative trait loci (QTLs), but such studies have rarely been carried out in wheat. In this study, candidate genes related to yield were predicted by a combined use of linkage mapping and weighted gene co-expression network analysis (WGCNA) in a recombinant inbred line population.

RESULTS

QTL mapping was performed for plant height (PH), spike length (SL) and seed traits. A total of 68 QTLs were identified for them, among which, 12 QTLs were stably identified across different environments. Using RNA sequencing, we scanned the 99,168 genes expression patterns of the whole spike for the recombinant inbred line population. By the combined use of QTL mapping and WGCNA, 29, 47, 20, 26, 54, 46 and 22 candidate genes were predicted for PH, SL, kernel length (KL), kernel width, thousand kernel weight, seed dormancy, and seed vigor, respectively. Candidate genes for different traits had distinct preferences. The known PH regulation genes Rht-B and Rht-D, and the known seed dormancy regulation genes TaMFT can be selected as candidate gene. Moreover, further experiment revealed that there was a SL regulatory QTL located in an interval of about 7 Mbp on chromosome 7A, named TaSL1, which also involved in the regulation of KL.

CONCLUSIONS

A combination of QTL mapping and WGCNA was applied to predicted wheat candidate genes for PH, SL and seed traits. This strategy will facilitate the identification of candidate genes for related QTLs in wheat. In addition, the QTL TaSL1 that had multi-effect regulation of KL and SL was identified, which can be used for wheat improvement. These results provided valuable molecular marker and gene information for fine mapping and cloning of the yield-related trait loci in the future.

The LONELY GUY gene family: from mosses to wheat, the key to the formation of active cytokinins in plants

LONELY GUY (LOG) was first identified in a screen of rice mutants with defects in meristem maintenance. In plants, LOG codes for cytokinin riboside 5'-monophosphate phosphoribohydrolase, which converts inactive cytokinin nucleotides directly to the active free bases. Many enzymes with the PGGxGTxxE motif have been misannotated as lysine decarboxylases; conversely not all enzymes containing this motif are cytokinin-specific LOGs. As LOG mutants clearly impact yield in rice, we investigated the LOG gene family in bread wheat. By interrogating the wheat (Triticum aestivum) genome database, we show that wheat has multiple LOGs. The close alignment of TaLOG1, TaLOG2 and TaLOG6 with the X-ray structures of two functional Arabidopsis thaliana LOGs allows us to infer that the wheat LOGs 1-11 are functional LOGs. Using RNA-seq data sets, we assessed TaLOG expression across 70 tissue types, their responses to various stressors, the pattern of cis-regulatory elements (CREs) and intron/exon patterns. TaLOG gene family members are expressed variously across tissue types. When the TaLOG CREs are compared with those of the cytokinin dehydrogenases (CKX) and glucosyltransferases (CGT), there is close alignment of CREs between TaLOGs and TaCKXs reflecting the key role of CKX in maintaining cytokinin homeostasis. However, we suggest that the main homeostatic mechanism controlling cytokinin levels in response to biotic and abiotic challenge resides in the CGTs, rather than LOG or CKX. However, LOG transgenics and identified mutants in rice variously impact yield, providing interesting avenues for investigation in wheat.

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.

Genotype-Dependent Effect of Silencing of TaCKX1 and TaCKX2 on Phytohormone Crosstalk and Yield-Related Traits in Wheat

The influence of silenced TaCKX1 and TaCKX2 on coexpression of other TaCKX gene family members (GFMs), phytohormone regulation and yield-related traits was tested in awned-spike cultivar. We documented a strong feedback mechanism of regulation of TaCKX GFM expression in which silencing of TaCKX1 upregulated expression of TaCKX2 genes and vice versa. Additionally, downregulation of TaCKX2 highly upregulated the expression of TaCKX5 and TaNAC2-5A. In contrast, expression of these genes in silenced TaCKX1 was downregulated. Silenced TaCKX1 T₂ lines with expression decreased by 47% had significantly higher thousand grain weight (TGW) and seedling root mass. Silenced TaCKX2 T₂ lines with expression of TaCKX2.2.1 and TaCKX2.2.2 decreased by 33% and 30%, respectively, had significantly higher chlorophyll content in flag leaves. TaCKX GFM expression, phytohormone metabolism and phenotype were additionally modified by Agrobacterium-mediated transformation. Two novel phytohormones, phenylacetic acid (PAA) and topolins, lack of gibberellic acid (GA) and changed phytohormone contents in the 7 days after pollination (DAP) spikes of the awned-spike cultivar compared to a previously tested, awnless one, were detected. We documented that major mechanisms of coregulation of the expression of TaCKX GFMs were similar in different spring wheat cultivars, but, depending on content and composition of phytohormones, regulation of yield-related traits was variously impacted.

Isopentenyltransferases as master regulators of crop performance: their function, manipulation, and genetic potential for stress adaptation and yield improvement

Isopentenyltransferase (IPT) in plants regulates a rate-limiting step of cytokinin (CTK) biosynthesis. IPTs are recognized as key regulators of CTK homeostasis and phytohormone crosstalk in both biotic and abiotic stress responses. Recent research has revealed the regulatory function of IPTs in gene expression and metabolite profiles including source-sink modifications, energy metabolism, nutrient allocation and storage, stress defence and signalling pathways, protein synthesis and transport, and membrane transport. This suggests that IPTs play a crucial role in plant growth and adaptation. In planta studies of IPT-driven modifications indicate that, at a physiological level, IPTs improve stay-green characteristics, delay senescence, reduce stress-induced oxidative damage and protect photosynthetic machinery. Subsequently, these improvements often manifest as enhanced or stabilized crop yields and this is especially apparent under environmental stress. These mechanisms merit consideration of the IPTs as 'master regulators' of core cellular metabolic pathways, thus adjusting plant homeostasis/adaptive responses to altered environmental stresses, to maximize yield potential. If their expression can be adequately controlled, both spatially and temporally, IPTs can be a key driver for seed yield. In this review, we give a comprehensive overview of recent findings on how IPTs influence plant stress physiology and yield, and we highlight areas for future research.

Cytokinin glucosyl transferases, key regulators of cytokinin homeostasis, have potential value for wheat improvement

The cytokinins, which are N⁶ -substituted adenine derivatives, control key aspects of crop productivity. Cytokinin levels are controlled via biosynthesis by isopentenyl transferase (IPT), destruction by cytokinin oxidase/dehydrogenase (CKX), and inactivation via glucosylation by cytokinin glucosyl transferases (CGTs). While both yield components and tolerance to drought and related abiotic stressors have been positively addressed via manipulation of IPT and/or CKX expression, much less attention has been paid to the CGTs. As naming of the CGTs has been unclear, we suggest COGT, CNGT, CONGT and CNOGT to describe the O-, N- and dual function CGTs. As specific CGT mutants of both rice and arabidopsis showed impacts on yield components, we interrogated the wheat genome database, IWGSC RefSeq v1.0 & v2.0, to investigate wheat CGTs. Besides providing unambiguous names for the 53 wheat CGTs, we show their expression patterns in 70 developmental tissues and their response characteristics to various stress conditions by reviewing more than 1000 RNA-seq data sets. These revealed various patterns of responses and showed expression generally being more limited in reproductive tissues than in vegetative tissues. Multiple cis-regulatory elements are present in the 3 kb upstream of the start codons of the 53 CGTs. Elements associated with abscisic acid, light and methyl jasmonate are particularly over-represented, indicative of the responsiveness of CGTs to the environment. These data sets indicate that CGTs have potential value for wheat improvement and that these could be targeted in TILLING or gene editing wheat breeding programmes.

TaCKX2.2 Genes Coordinate Expression of Other TaCKX Family Members, Regulate Phytohormone Content and Yield-Related Traits of Wheat

TaCKX gene family members (GFMs) play essential roles in the regulation of cytokinin during wheat development and significantly influence yield-related traits. However, detailed function of most of them is not known. To characterize the role of TaCKX2.2 genes we silenced all homoeologous copies of both TaCKX2.2.1 and TaCKX2.2.2 by RNAi technology and observed the effect of silencing in 7 DAP spikes of T₁ and T₂ generations. The levels of gene silencing of these developmentally regulated genes were different in both generations, which variously determined particular phenotypes. High silencing of TaCKX2.2.2 in T₂ was accompanied by slight down-regulation of TaCKX2.2.1 and strong up-regulation of TaCKX5 and TaCKX11, and expression of TaCKX1, TaCKX2.1, and TaCKX9 was comparable to the non-silenced control. Co-ordinated expression of TaCKX2.2.2 with other TaCKX GFMs influenced phytohormonal homeostasis. Contents of isoprenoid, active cytokinins, their conjugates, and auxin in seven DAP spikes of silenced T₂ plants increased from 1.27 to 2.51 times. However, benzyladenine (BA) and abscisic acid (ABA) contents were significantly reduced and GA₃ was not detected. We documented a significant role of TaCKX2.2.2 in the regulation of thousand grain weight (TGW), grain number, and chlorophyll content, and demonstrated the formation of a homeostatic feedback loop between the transcription of tested genes and phytohormones. We also discuss the mechanism of regulation of yield-related traits.