Leaf Senescence In Wheat Research Articles

Wheat NAC transcription factor NAC5-1 is a positive regulator of senescence.

Wheat (Triticum aestivum L.) is an important source of both calories and protein in global diets, but there is a trade-off between grain yield and protein content. The timing of leaf senescence could mediate this trade-off as it is associated with both declines in photosynthesis and nitrogen remobilization from leaves to grain. NAC transcription factors play key roles in regulating senescence timing. In rice, OsNAC5 expression is correlated with increased protein content and upregulated in senescing leaves, but the role of the wheat ortholog in senescence had not been characterized. We verified that NAC5-1 is the ortholog of OsNAC5 and that it is expressed in senescing flag leaves in wheat. To characterize NAC5-1, we combined missense mutations in NAC5-A1 and NAC5-B1 from a TILLING mutant population and overexpressed NAC5-A1 in wheat. Mutation in NAC5-1 was associated with delayed onset of flag leaf senescence, while overexpression of NAC5-A1 was associated with slightly earlier onset of leaf senescence. DAP-seq was performed to locate transcription factor binding sites of NAC5-1. Analysis of DAP-seq and comparison with other studies identified putative downstream target genes of NAC5-1 which could be associated with senescence. This work showed that NAC5-1 is a positive transcriptional regulator of leaf senescence in wheat. Further research is needed to test the effect of NAC5-1 on yield and protein content in field trials, to assess the potential to exploit this senescence regulator to develop high-yielding wheat while maintaining grain protein content.

Water stress effects on stay green and chlorophyll fluorescence with focus on yield characteristics of diverse bread wheats.

The study provided an insight toward better understanding of stay-green mechanisms for drought tolerance improvement and identified that synthetic-derived wheats proved as a promising germplasm for improved tolerance against water stress. Stay-green (SG) trait is considered to be related with the ability of wheat plants to maintain photosynthesis and CO2 assimilation. The present study explored the interaction of water stress with SG expression through physio-biochemical, agronomic and phenotypic responses among diverse wheat germplasm comprising of 200 synthetic hexaploids, 12 synthetic derivatives, 97 landraces and 16 conventional bread wheat varieties, for 2years. The study established that variation of SG trait existed in the studied wheat germplasm and there was positive association between SG trait and tolerance to water stress. The relationship of SG trait with chlorophyll content (r = 0.97), ETR (r = 0.28), GNS (r = 0.44), BMP (r = 0.34) and GYP (r = 0.44) was particularly promising under water stress environment. Regarding chlorophyll fluorescence, the positive correlation of фPSII (r = 0.21), qP (r = 0.27) and ETR (r = 0.44) with grain yield per plant was noted. The improved ΦPSII and Fv/Fm of PSII photochemistry resulted in the high photosynthesis activity in SG wheat genotypes. Regarding relative water content and photochemical quenching coefficient, synthetic-derived wheats were better by maintaining 20.9, 9.8 and 16.1% more RWC and exhibiting 30.2, 13.5 and 17.9% more qP when compared with landraces, varieties and synthetic hexaploids, respectively, under water stress environment. Synthetic derived wheats also exhibited relatively more SG character with good yield and were more tolerant to water stress in terms of grain yield, grain weight per plant, better photosynthetic performance through chlorophyll fluorescence measurement, high leaf chlorophyll and proline content, and hence, may be used as novel sources for breeding drought tolerant materials. The study will further facilitate research on wheat leaf senescence and will add to better understanding of SG mechanisms for drought tolerance improvement.

Increasingly amplified stimulation mediated by TaNAC69-B is crucial for the leaf senescence in wheat.

Leaf senescence involves massive multidimensional alterations, such as nutrient redistribution, and is closely related to crop yield and quality. No apical meristem, Arabidopsis transcription activation factor, and Cup-shaped cotyledon (NAC)-type transcription factors integrate various signals and modulate an enormous number of target genes to ensure the appropriate progression of leaf senescence. However, few leaf senescence-related NACs have been functionally characterized in wheat. Based on our previous RNA-sequencing (RNA-seq) data, we focused on a NAC family member, TaNAC69-B, which is increasingly expressed during leaf senescence in wheat. Overexpression of TaNAC69-B led to precocious leaf senescence in wheat and Arabidopsis, and affected several agricultural traits in transgenic wheat. Moreover, impaired expression of TaNAC69-B by virus-induced gene silencing retarded the leaf senescence in wheat. By RNA-seq and quantitative real-time polymerase chain reaction analysis, we confirmed that some abscisic acid (ABA) biosynthesis genes, including AAO3 and its ortholog in wheat, TraesCS2B02G270600 (TaAO3-B), were elevated by the overexpression of TaNAC69-B. Consistently, we observed more severe ABA-induced leaf senescence in TaNAC69-B-OE wheat and Arabidopsis plants. Furthermore, we determined that TaNAC69-B bound to the NAC binding site core (CGT) on the promoter regions of AAO3 and TaAO3-B. Moreover, we confirmed elevated ABA levels in TaNAC69-B-OE wheat lines. Although TaNAC69-B shares 39.83% identity (amino acid) with AtNAP, TaNAC69-B did not completely restore the delayed leaf senescence in the atnap mutant. Collectively, our results revealed a positive feedback loop, consisting of TaNAC69-B, ABA biosynthesis and leaf senescence, that is essential for the regulation of leaf senescence in wheat.

Insights into the Bioinformatics and Transcriptional Analysis of the Elongator Complexes (ELPs) Gene Family of Wheat: TaELPs Contribute to Wheat Abiotic Stress Tolerance and Leaf Senescence.

Elongator complexes (ELPs) are the protein complexes that promote transcription through histone acetylation in eukaryotic cells and interact with elongating RNA polymerase II (RNAPII). ELPs' role in plant growth and development, signal transduction, and response to biotic and abiotic stresses have been confirmed in model plants. However, the functions of the wheat ELP genes are not well documented. The present study identified 18 members of the ELPs from the wheat genome with a homology search. Further, bioinformatics and transcription patterns in response to different stress conditions were analyzed to dissect their potential regulatory mechanisms in wheat. Gene duplication analysis showed that 18 pairs of ELP paralogous genes were derived from segmental duplication, which was divided into six clades by protein phylogenetic and cluster analysis. The orthologous analysis of wheat TaELP genes showed that TaELP genes may have evolved from orthologous genes of other plant species or closely related plants. Moreover, a variety of cis-acting regulatory elements (CAREs) related to growth and development, hormone response, and biotic and abiotic stresses were identified in the TaELPs' promoter regions. The qRT-PCR analysis showed that the transcription of TaELPs was induced under hormone, salt, and drought stress and during leaf senescence. The TaELP2 gene was silenced with BSMV-VIGS, and TaELP2 was preliminarily verified to be involved in the regulation of wheat leaf senescence. Overall, TaELP genes might be regulated by hormone signaling pathways and response to abiotic stress and leaf senescence, which could be investigated further as potential candidate genes for wheat abiotic stress tolerance and yield improvement.

From synthesis to the biological effect of isoprenoid 2′-deoxyriboside and 2′,3′-dideoxyriboside cytokinin analogues

Isoprenoid cytokinins are a class of naturally occurring plant signaling molecules. A series of prepared compounds derived from isoprenoid cytokinins (isopentenyladenine, trans-zeatin and cis-zeatin) with attached 2′-deoxy-d-ribose or 2′,3′-dideoxy-d-ribose at the N9 position of the purine were prepared and their biological activities were examined. Different synthetic approaches were employed. The final compounds were characterized with variety of physicochemical methods (TLC, HPLC-MS, and NMR) and their cytokinin activity was determined in classical bioassays such as Amaranthus, tobacco callus, detached wheat leaf senescence and Arabidopsis thaliana root elongation inhibition assay. In addition, compounds were screened for activation of the cytokinin signaling pathway (bacterial receptor, competitive ligand binding and ARR5::GUS assay) to provide a detailed assessment of CK structure-activity relationship. The prepared compounds were found to be non-toxic to human cells and the majority of assays exhibited the highest activity of free bases while 2′,3′-dideoxyribosides had very weak or no activity. In contrast to the free bases, all 2′-deoxyriboside derivatives were not toxic to tobacco callus even at the highest tested concentration (10−4 moL/l) and compound 1 (iPdR) induced betacyanin synthesis at higher concentration even stronger than iP free base in the Amaranthus bioassay. The general cytokinin activity pattern base > riboside >2′-deoxyriboside > 2′,3′-dideoxyriboside was distinguished.

Comparative transcriptome analysis revealed differential gene expression involved in wheat leaf senescence between stay-green and non-stay-green cultivars.

Breeders agree that leaf senescence is a favorable process for wheat seed yield improvement due to the remobilization of leaf nutrients. However, several studies have suggested that staying green may be an important strategy for further increasing wheat yields. In this study, we performed a comparative transcriptome analysis between wheat cultivars CN17 and CN19 after heading and also measured photosynthetic parameters, chlorophyll (Chl) contents, and antioxidant enzyme activities at various time points after heading. The physiological and biochemical indexes revealed that CN17 exhibited a functionally stay-green phenotype while CN19 did not. We identified a total of 24,585 and 34,410 differential expression genes between genotypes at two time-points and between time-points in two genotypes, respectively, and we also found that 3 (37.5%) genes for leaf senescence, 46 (100%) for photosynthesis – antenna protein, 33 (70.21%) for Chl metabolism and 34 (68%) for antioxidative enzyme activity were upregulated in CN17 compared with CN19 during leaf senescence, which could be regulated by the differential expression of SAG39 (senescence-associated gene 39), while 22 (100%) genes for photosynthesis – antenna proteins, 6 (46.15%) for Chl metabolism and 12 (80%) for antioxidative enzyme activity were upregulated in CN17 compared with CN19 before the onset of leaf senescence. Here, we further clarified the expression profiles of genes associated with a functional stay-green phenotype. This information provides new insight into the mechanism underlying delayed leaf senescence and a new strategy for breeders to improve wheat yields.

Raised bed planting promotes grain number per spike in wheat grown after rice by improving spike differentiation and enhancing photosynthetic capacity

The yield of wheat in wheat–rice rotation cropping systems in the Yangtze River Plain, China, is adversely impacted by waterlogging. A raised bed planting (RBP) pattern may reduce waterlogging and increase the wheat yield after rice cultivation by improving the grain number per spike. However, the physiological basis for grain formation under RBP conditions remains poorly understood. The present study was performed over two growing seasons (2018/2019 and 2019/2020) to examine the effects of the planting pattern (i.e., RBP and flat planting (FP)) on the floret and grain formation features and leaf photosynthetic source characteristics of wheat. The results indicated that implementation of the RBP pattern improved the soil–plant nitrogen (N) supply during floret development, which facilitated balanced floret development, resulting in a 9.5% increase in the number of fertile florets per spike. Moreover, the RBP pattern delayed wheat leaf senescence and increased the photosynthetic source capacity by 13.9%, which produced more assimilates for grain filling. Delayed leaf senescence was attributed to the resultant high leaf N content and enhanced antioxidant metabolism. Correspondingly, under RBP conditions, 7.6–8.6% more grains per spike were recorded, and the grain yield was ultimately enhanced by 10.4–12.7%. These results demonstrate that the improvement of the spike differentiation process and the enhancement of the leaf photosynthetic capacity were the main reasons for the increased grain number per spike of wheat under the RBP pattern, and additional improvements in this technique should be achievable through further investigation.

Exogenous Application of the Polyamine Spermine Delays Natural Leaf Senescence in Wheat Through Protecting Chlorophyll from Degradation and Preventing Oxidative Stress

Senescence is a genetically regulated process that involves decomposition of cellular structures and the mobilization of the products to other plant parts. The progression of developmental senescence in wheat plant under the effect of foliar application with 100 µM of the polyamine spermine (Spm) was studied. Biochemical changes were recorded in the fourth leaf starting from the onset of senescence at 30 day after sowing (DAS) until the end of leaf senescence at 50 DAS. Leaf senescence was delayed in Spm-treated plants through maintaining water content, preventing chlorophyll degradation and enhancement of total soluble sugars, starch accumulation and total protein compared to controls. In addition, reactive oxygen species and lipid peroxidation were lower in Spm-treated plants than control values. Catalase (CAT) activity was obviously lowered due to Spm-treatment; however, guaiacol peroxidase activity was slightly decreased below the control

A WRKY transcription factor, TaWRKY42-B, facilitates initiation of leaf senescence by promoting jasmonic acid biosynthesis

BackgroundLeaf senescence comprises numerous cooperative events, integrates environmental signals with age-dependent developmental cues, and coordinates the multifaceted deterioration and source-to-sink allocation of nutrients. In crops, leaf senescence has long been regarded as an essential developmental stage for productivity and quality, whereas functional characterization of candidate genes involved in the regulation of leaf senescence has, thus far, been limited in wheat.ResultsIn this study, we analyzed the expression profiles of 97 WRKY transcription factors (TFs) throughout the progression of leaf senescence in wheat and subsequently isolated a potential regulator of leaf senescence, TaWRKY42-B, for further functional investigation. By phenotypic and physiological analyses in TaWRKY42-B-overexpressing Arabidopsis plants and TaWRKY42-B-silenced wheat plants, we confirmed the positive role of TaWRKY42-B in the initiation of developmental and dark-induced leaf senescence. Furthermore, our results revealed that TaWRKY42-B promotes leaf senescence mainly by interacting with a JA biosynthesis gene, AtLOX3, and its ortholog, TaLOX3, which consequently contributes to the accumulation of JA content. In the present study, we also demonstrated that TaWRKY42-B was functionally conserved with AtWRKY53 in the initiation of age-dependent leaf senescence.ConclusionOur results revealed a novel positive regulator of leaf senescence, TaWRKY42-B, which mediates JA-related leaf senescence via activation of JA biosynthesis and has the potential to be a target gene for molecular breeding in wheat.

High-Resolution Mapping of the Novel Early Leaf Senescence Gene Els2 in Common Wheat.

Early leaf senescence negatively impacts the grain yield in wheat (Triticum aestivum L.). Induced mutants provide an important resource for mapping and cloning of genes for early leaf senescence. In our previous study, Els2, a single incomplete dominance gene, that caused early leaf senescence phenotype in the wheat mutant LF2099, had been mapped on the long arm of chromosome 2B. The objective of this study was to develop molecular markers tightly linked to the Els2 gene and construct a high-resolution map surrounding the Els2 gene. Three tightly linked single-nucleotide polymorphism (SNP) markers were obtained from the Illumina Wheat 90K iSelect SNP genotyping array and converted to Kompetitive allele-specific polymerase chain reaction (KASP) markers. To saturate the Els2 region, the Axiom® Wheat 660K SNP array was used to screen bulked extreme phenotype DNA pools, and 9 KASP markers were developed. For fine mapping of the Els2 gene, these KASP markers and previously identified polymorphic markers were analyzed in a large F2 population of the LF2099 × Chinese Spring cross. The Els2 gene was located in a 0.24-cM genetic region flanked by the KASP markers AX-111643885 and AX-111128667, which corresponded to a physical interval of 1.61 Mb in the Chinese Spring chromosome 2BL containing 27 predicted genes with high confidence. The study laid a foundation for a map-based clone of the Els2 gene controlling the mutation phenotype and revealing the molecular regulatory mechanism of wheat leaf senescence.

Molecular mapping of a novel early leaf-senescence gene Els2 in common wheat by SNP genotyping arrays

Early leaf senescence in wheat (Triticum aestivum L.) is one of the limiting factors for developing high yield potential. In this study, a stably inherited, early leaf-senescence mutant LF2099 was initially identified in an M2 population of the common wheat accession H261 after ethyl methanesulfonate (EMS) mutagenesis. Early leaf senescence was observed in the LF2099 mutant during the three-leaf-stage, and then the etiolated area of the wheat leaf increased gradually from the bottom to the top throughout development. Compared with H261, the chlorophyll (Chl a, Chl b) and carotenoid contents and photosynthetic capacity of the mutant were significantly decreased. All of its yield-related traits except for spike length were also significantly reduced. Dissolved cytoplasm, abnormal chloroplast structure, dissolved chloroplast membrane, abnormal thylakoid development, and more plastoglobules were observed in the senescent leaf region of the mutant by transmission electronic microscope. Genetic analysis indicated that the early leaf-senescence phenotype is controlled by an incomplete-dominance nuclear gene, here designated Els2. Using single nucleotide polymorphisms and bulked segregant analysis, the els2 gene was anchored in a region on chromosome 2BL between simple sequence repeat (SSR) markers gpw4043 and wmc149. Six new polymorphic SSR markers were developed from the Chinese Spring 2BL shotgun survey sequence contigs. By means of comparative genomics analyses, the collinearity genomic regions of the els2 locus on wheat 2BL were identified in Brachypodium distachyon chromosome 5, rice (Oryza sativa) chromosome 4 and sorghum (Sorghum bicolor) chromosome 6. Five intron polymorphism (IP) markers were further developed from this collinearity genomic region. Ultimately, Els2 was mapped in a genetic interval of 0.95 cM flanked by IP markers 2BIP09 and 2BIP14. The co-segregating IP markers 2BIP12 and 2BIP17 provide a starting point for the fine mapping and map-based cloning of Els2.

Long-term high light stress induces leaf senescence in wheat (Triticum aestivum L.)

In order to explore the association of long-term high light (HL) with leaf senescence in wheat, two wheat genotypes Xiaoyan 54 (HL-tolerant) and Jing 411 (HL-sensitive) grown in low light were subjected to HL for 2-8 d. The results showed that 6-8 d of HL led to dramatic decline of chlorophyll content, photochemical efficiency, and activity of Rubisco and antioxidant enzymes. Meanwhile, the content of malondialdehyde (MDA), soluble proteins, soluble sugars, superoxide anion, and ascorbic acid increased markedly. Additionally, the expression of senescence-associated genes (SAGs) was induced by HL. Xiaoyan 54 accumulated less MDA, soluble sugars, and superoxide anions but showed a higher ratio of reduced to oxidized ascorbic acid than that in Jing 411 under long-term HL stress. Moreover, most of the assayed SAGs were less induced in Xiaoyan 54 than that in Jing 411. Altogether, the reduction of antioxidant enzyme activities and accumulation of soluble sugars may result in long-term HL-induced leaf senescence.

The involvement of cytokinin and nitrogen metabolism in delayed flag leaf senescence in a wheat stay-green mutant, tasg1

In the present study on a wheat stay-green mutant, tasg1, we found that its delayed senescence at the late filling stage was related to the high cytokinin (CK) and N contents. RNA sequencing suggested that several genes may be responsible for the different senescence processes between wild-type (WT) and tasg1 plants. WT and tasg1 seedlings were treated with NH4NO3, lovastatin, and 6-benzylaminopurine (BAP), and the results suggested that the feedback of CK with N content regulated the leaf senescence in the tasg1 plants. Furthermore, a knock-out of the candidate gene cisZOGT1 (catalytic O-glucosylation in cis-zeatin) in the wheat mutant pool ‘Kronos’ exhibited delayed senescence at the late grain filling stage. Overall, our results suggested the cisZOGT1 gene has an important role in regulating wheat leaf senescence by regulating CK and N metabolism. At the same time, CK and N metabolism involved in delayed flag leaf senescence of tasg1 may be by a feedback pattern.

New Urea Derivatives Are Effective Anti-senescence Compounds Acting Most Likely via a Cytokinin-Independent Mechanism.

Stress-induced senescence is a global agro-economic problem. Cytokinins are considered to be key plant anti-senescence hormones, but despite this practical function their use in agriculture is limited because cytokinins also inhibit root growth and development. We explored new cytokinin analogs by synthesizing a series of 1,2,3-thiadiazol-5-yl urea derivatives. The most potent compound, 1-(2-methoxy-ethyl)-3-1,2,3-thiadiazol-5-yl urea (ASES - Anti-Senescence Substance), strongly inhibited dark-induced senescence in leaves of wheat (Triticum aestivum L.) and Arabidopsis thaliana. The inhibitory effect of ASES on wheat leaf senescence was, to the best of our knowledge, the strongest of any known natural or synthetic compound. In vivo, ASES also improved the salt tolerance of young wheat plants. Interestingly, ASES did not affect root development of wheat and Arabidopsis, and molecular and classical cytokinin bioassays demonstrated that ASES exhibits very low cytokinin activity. A proteomic analysis of the ASES-treated leaves further revealed that the senescence-induced degradation of photosystem II had been very effectively blocked. Taken together, our results including data from cytokinin content analysis demonstrate that ASES delays leaf senescence by mechanism(s) different from those of cytokinins and, more effectively. No such substance has yet been described in the literature, which makes ASES an interesting tool for research of photosynthesis regulation. Its simple synthesis and high efficiency predetermine ASES to become also a potent plant stress protectant in biotechnology and agricultural industries.

Validation of QTL for the Flag Leaf Senescence in Wheat Under Drought

Validation of QTL for the Flag Leaf Senescence in Wheat Under Drought

Transcriptome Analysis of a Premature Leaf Senescence Mutant of Common Wheat (Triticum aestivum L.).

Leaf senescence is an important agronomic trait that affects both crop yield and quality. In this study, we characterized a premature leaf senescence mutant of wheat (Triticum aestivum L.) obtained by ethylmethane sulfonate (EMS) mutagenesis, named m68. Genetic analysis showed that the leaf senescence phenotype of m68 is controlled by a single recessive nuclear gene. We compared the transcriptome of wheat leaves between the wild type (WT) and the m68 mutant at four time points. Differentially expressed gene (DEG) analysis revealed many genes that were closely related to senescence genes. Gene Ontology (GO) enrichment analysis suggested that transcription factors and protein transport genes might function in the beginning of leaf senescence, while genes that were associated with chlorophyll and carbon metabolism might function in the later stage. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that the genes that are involved in plant hormone signal transduction were significantly enriched. Through expression pattern clustering of DEGs, we identified 1012 genes that were induced during senescence, and we found that the WRKY family and zinc finger transcription factors might be more important than other transcription factors in the early stage of leaf senescence. These results will not only support further gene cloning and functional analysis of m68, but also facilitate the study of leaf senescence in wheat.

Preparation, characterization and biological activity of C8-substituted cytokinins

Naturally occurring cytokinins are adenine-based plant hormones. Although, the effect of various substituents at positions N1, C2, N3, N6, N7, or N9 on the biological activity of cytokinins has been studied, the C8-substituted compounds have received little attention. Here, we report the synthesis and in vitro biological testing of thirty-one cytokinin derivatives substituted at the C8 position of the adenine skeleton and twenty-seven compounds which served as their N9-tetrahydropyranyl protected precursors. The cytokinin activity of all the compounds was determined in classical cytokinin biotests (wheat leaf senescence, Amaranthus and tobacco callus assays). With some exceptions, the compounds with a N9-tetrahydropyranyl group were generally less active than their de-protected analogs. The latter were further tested for their ability to activate the Arabidopsis cytokinin receptors AHK3 and CRE1/AHK4 in bacterial receptor activation assays. Using this approach, we identified derivatives bearing short aliphatic chains and retaining high cytokinin activity. Such compounds are suitable candidates for fluorescence labeling or as protein-affinity ligands. We further found that some C8-substituted cytokinins exhibited no or lower cytotoxicity toward tobacco cells when compared to their parent compound. Therefore, we also present and discuss the cytotoxicity of all the compounds against three normal human cell lines.

Identification and function analyses of senescence-associated WRKYs in wheat

Leaf senescence is a positive, highly regulated, complex process, and transcription factors play important roles in the regulation of this process. We identified and characterized 116 WRKYs from the wheat genome database. Thirteen TaWRKYs were confirmed as senescence-associated genes. We focused on TaWRKY7, which is up-regulated in the natural leaf senescence process. TaWRKY7 is expressed in different tissues of wheat and is localized in the nucleus. It shows transcriptional activation activity in yeast cells. The ectopic over-expression of TaWRKY7 in Arabidopsis (Arabidopsis thaliana) significantly promoted early leaf senescence under darkness treatment and prevented leaf moisture losses. TaWRKY7 played important roles in the senescence process and was involved in abiotic stress responses. Our transcriptomic and genetic studies on WRKYs suggest that WRKY transcription factors are a type of vital regulator in leaf senescence in wheat (Triticum aestivum L.).

Physicochemical analyses of plant biostimulant formulations and characterisation of commercial products by instrumental techniques

The objective of this study was to develop instrumental protocols for evaluating physicochemical characteristics of plant biostimulant/biofertiliser formulations. Six formulations (Rygex, Algavyt, Ryzoset, Manek, Ecoryg and Algavyt Zn/Mn) containing algal/plant extracts, humic and amino acids, lipids and inorganic components were assessed for particle size distribution and zeta potential (ZP) by dynamic light scattering (DLS) and gross compositional differences by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The six commercial formulations were assessed by mung bean root and wheat leaf senescence bioassays to determine auxin- and cytokinin-like activities. Among them, Rygex, Manek and Ecoryg showed significant auxin-like activities compared to the other formulations. Only Manek showed very low cytokinin-like activity. In combination, the four instrumental techniques highlighted gross differences in the particle sizes, ZP and compositions of the products, and provided key results on the physicochemical characteristics of the formulations including potential stability of the products. The three techniques (TGA, FTIR and Py-GC/MS) require no sample preparation beyond drying of the materials. The activities of the plant growth hormones have been compared to show advantages of the growth hormone bioassay methods for selecting better formulations.

C2-substituted aromatic cytokinin sugar conjugates delay the onset of senescence by maintaining the activity of the photosynthetic apparatus

Cytokinins are plant hormones with biological functions ranging from coordination of plant growth and development to the regulation of senescence. A series of 2-chloro-N6-(halogenobenzylamino)purine ribosides was prepared and tested for cytokinin activity in detached wheat leaf senescence, tobacco callus and Amaranthus bioassays. The synthetic compounds showed significant activity, especially in delaying senescence in detached wheat leaves. They were also tested in bacterial receptor bioassays using both monocot and dicot members of the cytokinin receptor family. Most of the derivatives did not trigger cytokinin signaling via the AHK3 and AHK4 receptors from Arabidopsis thaliana in the bacterial assay, but some of them specifically activated the ZmHK1 receptor from Zea mays and were also more active than the aromatic cytokinin BAP in an ARR5::GUS cytokinin bioassay using transgenic Arabidopsis plants. Whole transcript expression analysis was performed using an Arabidopsis model to gather information about the reprogramming of gene transcription when senescent leaves were treated with selected C2-substituted aromatic cytokinin ribosides. Genome-wide expression profiling revealed that the synthetic halogenated derivatives induced the expression of genes related to cytokinin signaling and metabolism. They also prompted both up- and down-regulation of a unique combination of genes coding for components of the photosystem II (PSII) reaction center, light-harvesting complex II (LHCII), and the oxygen-evolving complex, as well as several stress factors responsible for regulating photosynthesis and chlorophyll degradation. Chlorophyll content and fluorescence analyses demonstrated that treatment with the halogenated derivatives increased the efficiency of PSII photochemistry and the abundance of LHCII relative to DMSO- and BAP-treated controls. These findings demonstrate that it is possible to manipulate and fine-tune leaf longevity using synthetic aromatic cytokinin analogs.