Floret Primordia Research Articles

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

BackgroundKentucky 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.ResultsIn 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.ConclusionsOverall, 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.

Differences between two wheat genotypes in the development of floret primordia and contents of pigments and hormones

Promoting more floret primordia within a spike to acquire fertile potential during the differentiation and pre-dimorphism phases is critical for increasing the number of fertile florets per spike (NFFs). However, it is yet unknown the physiological mechanism regulating the complex and dynamic process. This study aimed to clarify how intra-spike hormones, pigments, and assimilates coordinate with each other to regulate spike morphology and then floret primordia development. A two-year field experiment was conducted with two winter wheat genotypes: N50 (big-spike with greater NFFs) and SM22 (medium-spike with fewer NFFs). We monitored high temporal and spatial-resolution changes in the number and morphology of floret primordia within a spike, as well as in intra-spike hormones, pigments, and assimilates. Our results revealed that the big-spike genotype had more NFFs than the medium-spike genotype, not only because they had more spikelets, but also because they had greater NFFs mainly at central spikelets. More floret primordia at central spikelets had sufficient time to develop and acquire fertile potential during the differentiation phase (167–176 d after sowing, DAS) and the pre-dimorphism phase (179 DAS) for the big-spike genotype than the medium-spike genotype. Floret primordia with fertile morphology during the pre-dimorphism phase always developed into fertile florets during the dimorphism phase. Those early-developed floret primordia most proximal and intermediate to the rachis in the big-spike genotype developed faster than the medium-spike genotype. Correspondingly, the spike dry matter and pigments (chlorophyll a, chlorophyll b, carotene, and carotenoids) content during 170–182 DAS, auxin (IAA) and cytokinin (CTK) content on 167 DAS were significantly higher in the big-spike genotype than in the medium-spike genotype, while jasmonic acid (JA) content was significantly lower in the big-spike genotype compared to the medium-spike genotype during 167–182 DAS. Since the significant differences in intra-spike hormone content of the two genotypes appear earlier than those in dry matter and pigments, we propose a possible model that helped the N50 genotype (big-spike) to form more fertile florets, taking the intra-spike hormone content as a signaling molecule induced assimilates and pigments synthesis, which accelerated the development of more floret primordia during the differentiation phase and then acquired fertile potential during the pre-dimorphism phase, finally improved the NFFs. Our high temporal and spatial-resolution analysis provides an accurate time window for precision cultivation and effective physiological breeding to improve the number of fertile florets in wheat.

Dissecting the molecular basis of spike traits by integrating gene regulatory networks and genetic variation in wheat

Spike architecture influences both grain weight and grain number per spike, which are the two major components of grain yield in bread wheat (Triticum aestivum L.). However, the complex wheat genome and the influence of various environmental factors pose challenges in mapping the causal genes that affect spike traits. Here, we systematically identified genes involved in spike trait formation by integrating information on genomic variation and gene regulatory networks controlling young spike development in wheat. We identified 170 loci that are responsible for variations in spike length, spikelet number per spike, and grain number per spike through genome-wide association study and meta-QTL analyses. We constructed gene regulatory networks for young inflorescences at the double ridge stage and the floret primordium stage, in which the spikelet meristem and the floret meristem are predominant, respectively, by integrating transcriptome, histone modification, chromatin accessibility, eQTL, and protein–protein interactome data. From these networks, we identified 169 hub genes located in 76 of the 170 QTL regions whose polymorphisms are significantly associated with variation in spike traits. The functions of TaZF-B1, VRT-B2, and TaSPL15-A/D in establishment of wheat spike architecture were verified. This study provides valuable molecular resources for understanding spike traits and demonstrates that combining genetic analysis and developmental regulatory networks is a robust approach for dissection of complex traits.

Phenotypic and transcriptome profiling of spikes reveals the regulation of light regimens on spike growth and fertile floret number in wheat.

The spike growth phase is critical for the establishment of fertile floret (grain) numbers in wheat (Triticum aestivum L.). Then, how to shorten the spike growth phase and increase grain number synergistically? Here, we showed high-resolution analyses of floret primordia(FP) number, morphology and spike transcriptomes during the spike growth phase under three light regimens. The development of all FP in a spike could be divided into four distinct stages: differentiation (Stage I), differentiation and morphology development concurrently (Stage II), morphology development (Stage III), and polarization (Stage IV). Compared to the short photoperiod, the long photoperiod shortened spike growth and stimulated early flowering by shortening Stage III; however, this reduced assimilate accumulation, resulting in fertile floret loss. Interestingly, long photoperiod supplemented with red light shortened the time required to complete Stages I-II, then raised assimilates supply in the spike and promoted anther development before polarization initiation, thereby increasing fertile FP number during Stage III, and finally maintained fertile FP development during Stage IV until they became fertile florets via a predicted dynamic gene network. Our findings proposed a light regimen, critical stagesand candidate regulators that achieved a shorter spike growth phase and a higher fertile floret number in wheat.

Tae-miR396b regulates TaGRFs in spikes of three wheat spike mutants.

Tillering and spike differentiation are key agronomic traits for wheat (Triticum aestivum L.) production. Numerous studies have shown that miR396 and growth-regulating factor genes (GRFs) are involved in growth and development of different plant organs. Previously, we have reported that wheat miR396b (tae-miR396b) and their targets TaGRFs (T. aestivum GRFs) play important roles in regulating wheat tillering. This study was to investigate the regulatory roles of tae-miR396b and TaGRFs played during wheat spike development. Wheat cultivar Guomai 301 (wild type, WT) and its three sipke mutants dwarf round spike mutant (drs), apical spikelet sterility mutant (ass) and prematurely terminated spike differentiation mutant (ptsd1) were studied. Three homeologous genes of tae-miR396b on the long arms of chromosomes 6A, 6B, and 6D were identified, and they encoded the same mature miRNA. Complementary sequences of mature tae-miR396b were identified in 23 TaGRFs, indicating they were the target genes of tae-miR396b. Tae-miR396b had different regulatory effects on TaGRFs between Guomai 301 and its mutants. TaGRF2-7A was confirmed to be the target gene of tae-miR396b by molecular interaction assay. The expression levels of tae-miR396b and TaGRFs were different between WT and mutants drs, ass and ptsd1 at the floret primordium visible (S1), the two awns/spikelet reaching apical meristem of the spikelet (S2), and the green anther stage (S3). The expression level of tae-miR396b in WT was significantly higher than that in mutants drs and ass. The most TaGRFs were negatively regulated by tae-miR396b. The abnormal expressions of TaGRF1 (6A, 6D), TaGRF2 (7A, 7B, 7D), TaGRF4 (6A, 6B), TaGRF5 (4A, 7A, 7D), and TaGRF10 (6A, 6B, 6D) were important causes for abnormal spike development in the three mutants. This study laid foundation for further elucidating functions of tae-miR396b and TaGRFs underlying wheat spike development. Regulating tae-miR396b and TaGRFs will be a new approach for wheat high yield breeding.

Low sucrose availability reduces basal spikelet fertility by inducing abscisic acid and jasmonic acid synthesis in wheat.

Within a spike of wheat, the central spikelets usually generate three to four fertile florets, while the basal spikelets generate zero to one fertile floret. The physiological and transcriptional mechanism behind the difference in fertility between the basal and central spikelets is unclear. This study reports a high temporal resolution investigation of transcriptomes, number and morphology of floret primordia, and physiological traits. The W6.5-W7.5 stage was regarded as the boundary to distinguish between fertile and abortive floret primordia; those floret primordia reaching the W6.5-W7.5 stage during the differentiation phase (3-9 d after terminal spikelet stage) usually developed into fertile florets in the next dimorphism phase (12-27 d after terminal spikelet stage), whereas the others aborted. The central spikelets had a greater number of fertile florets than the basal spikelets, which was associated with more floret primordia reaching the W6.5-W7.5 stage. Physiological and transcriptional results demonstrated that the central spikelets had a higher sucrose content and lower abscisic acid (ABA) and jasmonic acid (JA) accumulation than the basal spikelets due to down-regulation of genes involved in ABA and JA synthesis. Collectively, we propose a model in which ABA and JA accumulation is induced under limiting sucrose availability (basal spikelet) through the up-regulation of genes involved in ABA and JA synthesis; this leads to floret primordia in the basal spikelets failing to reach their fertile potential (W6.5-W7.5 stage) during the differentiation phase and then aborting. This fertility repression model may also regulate spikelet fertility in other cereal crops and potentially provides genetic resources to improve spikelet fertility.

A dynamic regulation of nitrogen on floret primordia development in wheat

Nitrogen (N) fertilization is critical for spike and floret development, which affects the number of fertile florets per spike (NFFs). However, the physiological regulation of the floret development process by N fertilization is largely unknown. A high temporal-resolution investigation of floret primordia number and morphology, dry matter, and N availability was conducted under three N fertilization levels: 0 (N0), 120 (N1) and 240 (N2) kg ha−1. Interestingly, fertile florets at anthesis stage were determined by those floret primordia with meiotic ability at booting stage: meiotic ability was a threshold that predicted whether a floret primordium became fertile or abortive florets. Because the developmental rate of the 4th floret primordium in the central spikelet was accelerated and then they acquired meiotic ability, the NFFs increased gradually as N application increased, but the increase range decreased under N2. There were no differences in spike N concentration among treatments, but leaf N concentration was increased in the N1 and N2 treatments. Correspondingly, dry matter accumulation and N content of the leaf and spike in the N1 and N2 treatments was increased as compared to N0. Clearly, optimal N fertilization increased leaf N availability and transport of assimilates to spikes, and allowed more floret primordia to acquire meiotic ability and become fertile florets, finally increasing NFFs. There was no difference in leaf N concentration between N1 and N2 treatment, whereas soil N concentration at 0–60 cm soil layers was higher in N2 than in N1 treatment, implying that there was still some N fertilization that remained unused. Therefore, improving the leaf’s ability to further use N fertilizer is vital for greater NFFs.

Dynamic gene regulatory networks improving spike fertility through regulation of floret primordia fate in wheat.

The developmental process of spike is critical for spike fertility through affecting floret primordia fate in wheat; however, the genetic regulation of this dynamic and complex developmental process remains unclear. Here, we conducted a high temporal-resolution analysis of spike transcriptomes and monitored the number and morphology of floret primordia within spike. The development of all floret primordia in a spike was clearly separated into three distinct phases: differentiation, pre-dimorphism and dimorphism. Notably, we identified that floret primordia with meiosis ability at the pre-dimorphism phase usually develop into fertile floret primordia in the next dimorphism phase. Compared to control, increasing plant space treatment achieved the maximum increasement range (i.e., 50%) in number of fertile florets by accelerating spike development. The process of spike fertility improvement was directed by a continuous and dynamic regulatory network involved in transcription factor and genes interaction. This was based on the coordination of genes related to heat shock protein and jasmonic acid biosynthesis during differentiation phase, and genes related to lignin, anthocyanin and chlorophyll biosynthesis during dimorphism phase. The multi-dimensional association with high temporal-resolution approach reported here allows rapid identification of genetic resource for future breeding studies to realise the maximum spike fertility potential in more cereal crops.

Pre-anthesis spike growth dynamics and its association to yield components among elite bread wheat cultivars (Triticum aestivum L. spp.) under Mediterranean climate

Wheat (Triticum spp.) grain yield (GY) is highly associated with grain number per unit area (GN m-2). Biomass accumulation and partitioning are essential to understand pre-anthesis spike growth dynamics which determines spike dry matter at anthesis (SDMa) - a GN determinant. Spike growth takes place during the stem elongation period (SE), from terminal spikelet to anthesis, following leaf and spikelet initiation (LS) from sowing to terminal spikelet. In this study, bread wheat cultivars were examined under Mediterranean semi-arid conditions to determine (i) the varietal differences in pre-anthesis phase duration, (ii) whether this variability influences biomass partitioning and spike-related traits, and (iii) to what extent, the genotypic variations in pre-anthesis phase duration and spike growth are associated with yield components. A panel of Israeli commercial bread wheat cultivars were grown in the field during 2016–17 (three environments) and 2017–18 (two environments) and characterized for pre-anthesis phases, floral conditions and spike fertility via histological measurements; spike traits, dry matter accumulation, partitioning at anthesis and maturity and for yield components. Significant variability in the timing of pre-anthesis phases was detected within the tested panel for both LS and SE phases. LS duration was positively associated with growing degree days (GDD) to anthesis across environments (0.72–0.90) while variation in SE was related with differences in GDD to anthesis but to a lesser extent and not in all environments (0.47–0.79). In addition, LS duration, and occasionally SE, were favourably related with a higher dry matter of fertile florets spike-1 (at anthesis) and SDM (at both anthesis and maturity). Principal component analysis (PCA) clearly separates environments and to a lesser extent cultivars within each environment. Two cultivar pairs 'Zahir-Yuval' and 'Negev-Gedera', which flowered concurrently, showed significant differences in the durations of LS and SE phases across most of the environments. Longer LS, in cultivars Zahir and Negev, exhibited increased spikelets spike-1, whereas longer SE (e.g., in Yuval and Gedera) enhanced spike fertility through improving the survival rate of floret primordia (FSR%) of central spikelets. However, there was a trade-off for FSR at the proximal and distal spike portions, resulting in lower grain set (%) leading to reduction of final GN (or GY) in cultivars with longer SE. In the tested panel and under our environmental conditions characterized by growing cycle and Mediterranean climate, the duration of both LS and SE contribute to spike fertility, but LS duration seemed a stronger driver than SE for GN and yield enhancement. Our results highlight the importance of pre-anthesis phases, especially the role of LS in wheat yield increment during the short growing cycle. The varietal combination with variable LS and SE duration could be implemented in breeding pipelines and used as pre-breeding materials for GN improvement. Furthermore, the findings will encourage pre-anthesis traits adoption in Mediterranean bread wheat future breeding programs.

High responsiveness of maize grain yield to nitrogen supply is explained by high ear growth rate and efficient ear nitrogen allocation

Increasing the response of maize yield to nitrogen (N) application is essential for food security and economic benefits of the farmers. However, the physiological mechanism underlining N responsiveness is unclear. Three maize cultivars with different N-responsiveness, BMY (the low), ZD2 (the medium) and ZD958 (the high) were grown in the field under a wide range of N supply (from 0 to 240 kg N per ha) in two years. Plant growth rate, plant N uptake rate, ear growth rate and ear N allocation rate, and their relationship to grain yield formation were investigated during the early critical period (14 days before silking). The direct path coefficient of grain number to grain yield was 0.82, which explained 96% of the variation in grain yield (R 2 = 0.91). There was not a trade-off between grain number and other yield components. ZD958 got more grains per ear than the other cultivars in response to N supplies. The variation in grain number was mostly explained by the number of floret primordia (57% for BMY, 93% for ZD2 and ZD958). When N supply increased from 0 to 240 kg N ha -1 , ZD2 and ZD958 did not differ in nitrogen nutrition index, and were 7.2–32.8% higher than BMY. The number of grains and floret primordia was independent of plant growth rate and plant N uptake rate, although these rates increased with N supply. Ear growth rate and ear N allocation rate of ZD958 were higher compared with the other cultivars, especially under high N supply. Except BMY, the number of floret primordium and grain per ear depended on ear growth rate and ear N allocation rate. There was a positive correlation between ear growth rate and ear N allocation rate in ZD2 and ZD958. However, ear N allocation rate of BMY did not increase when its ear growth rate was greater than 19.2 mg o C -1 . It is concluded that, with increasing N supply, N-responsive maize cultivars had high ear growth rate and allocated more N into the ear during the early critical period, which enhanced the differentiation of floret primordia, hence increasing grain number and final grain yield. • High N-responsiveness of grain yield depends on grain number per ear. • Genotypic variation in grains number is explained by floret primordium development. • Floret primordium number in response to N depends on ear growth and ear N allocation.

PIN and PILS family genes analyses in Chrysanthemum seticuspe reveal their potential functions in flower bud development and drought stress

Auxin affects almost all plant growth and developmental processes. The PIN-FORMED (PIN) and PIN-LIKES (PILS) family genes determine the direction and distribution gradient of auxin flow by polar localization on the cell membrane. However, there are no systematic studies on PIN and PILS family genes in chrysanthemum. Here, 18 PIN and 13 PILS genes were identified in Chrysanthemum seticuspe. The evolutionary relationships, physicochemical properties, conserved motifs, cis-acting elements, chromosome localization, collinearity, and expression characteristics of these genes were analyzed. CsPIN10a, CsPIN10b, and CsPIN10c are unique PIN genes in C. seticuspe. Expression pattern analysis showed that these genes had different tissue specificities, and the expression levels of CsPIN8, CsPINS1, CsPILS6, and CsPILS10 were linearly related to the developmental period of the flower buds. In situ hybridization assay showed that CsPIN1a, CsPIN1b, and CsPILS8 were expressed in floret primordia and petal tips, and CsPIN1a was specifically expressed in the middle of the bract primordia, which might regulate lateral expansion of the bracts. CsPIN and CsPILS family genes are also involved in drought stress responses. This study provides theoretical support for the cultivation of new varieties with attractive flower forms and high drought tolerance.

The chrysanthemum lavandulifolium genome and the molecular mechanism underlying diverse capitulum types.

Cultivated chrysanthemum (Chrysanthemum × morifolium Ramat.) is a beloved ornamental crop due to the diverse capitula types among varieties, but the molecular mechanism of capitulum development remains unclear. Here, we report a 2.60 Gb chromosome-scale reference genome of C. lavandulifolium, a wild Chrysanthemum species found in China, Korea and Japan. The evolutionary analysis of the genome revealed that only recent tandem duplications occurred in the C. lavandulifolium genome after the shared whole genome triplication (WGT) in Asteraceae. Based on the transcriptomic profiling of six important developmental stages of the radiate capitulum in C. lavandulifolium, we found genes in the MADS-box, TCP, NAC and LOB gene families that were involved in disc and ray floret primordia differentiation. Notably, NAM and LOB30 homologs were specifically expressed in the radiate capitulum, suggesting their pivotal roles in the genetic network of disc and ray floret primordia differentiation in chrysanthemum. The present study not only provides a high-quality reference genome of chrysanthemum but also provides insight into the molecular mechanism underlying the diverse capitulum types in chrysanthemum.

Developmental patterns and rates of organogenesis across modern and well-adapted wheat cultivars

Genetic gains in wheat yield are required to meet future demand. In order to design strategic crosses, it is essential to quantify the genetic variation among elite cultivars on traits responsible for the adaptation and the generation of yield components. We explored the degree of variation within well-adapted commercial cultivars on developmental phases (vegetative, early reproductive and late reproductive), final leaf number and phyllochron, leaf and spikelet plastochron, number of spikelets, florets, and dynamics of floret primordia development. Variation in flowering time among cultivars was minor but they differed in the partitioning of that time into particular phases. Main differences were found for the length of the late reproductive phase, partly linked to those in final leaf number. Variations on number of fertile florets at anthesis were mostly explained by floret survival, due to differential ability of labile florets to maintain normal development between cultivars. These differences were related to the duration of the floret development, but not to the synchrony of development among florets. These findings provide useful information for breeders when selecting genotypes for strategic crosses, as considering floret survival through duration of floret development in one of the parents (for which these processes shall be characterised in the elite germplasm) might increase the likelihood of transgressive segregation towards high-yielding offspring.

Wheat Developmental Traits as Affected by the Interaction between Eps-7D and Temperature under Contrasting Photoperiods with Insensitive Ppd-D1 Background

Earliness per se (Eps) genes are important to fine tune adaptation, and studying their probable pleiotropic effect on wheat yield traits is worthwhile. In addition, it has been shown that some Eps genes interact with temperature and therefore determining the likely Eps × temperature interaction is needed for each newly identified Eps gene. We studied two NILs differing in the newly identified Eps-7D (carrying insensitive Ppd-D1 in the background) under three temperature regimes (9, 15 and 18 °C) and two photoperiods (12 and 24 h). Eps-7D affected time to anthesis as expected and the Eps-7D-late allele extended both the period before and after terminal spikelet. The interaction effect of Eps-7D × temperature was significant but not cross-over: the magnitude and level of significance of the difference between NILs with the late or early allele was affected by the growing temperature (i.e., difference was least at 18 °C and largest at 9 °C), and the differences caused due to temperature sensitivity were influenced by photoperiod. The rate of leaf initiation was faster in NIL with Eps-7D-early than with the late allele which compensated for the shorter duration of leaf initiation resulting in similar final leaf number between two NILs. Eps-7D-late consistently increased spike fertility through improving floret primordia survival as a consequence of extending the late reproductive phase.

Two Cyc2CL transcripts (Cyc2CL-1 and Cyc2CL-2) may play key roles in the petal and stamen development of ray florets in chrysanthemum

BackgroundChrysanthemum morifolium is one of the most popular ornamental crops. The capitulum, which is the main ornamental part of chrysanthemum plants, consists of ligulate marginal ray florets, an attractive corolla (petals), and radially hermaphroditic disc florets, but no stamens. In Asteraceae species, the zygomorphic ray florets evolved from the actinomorphic disc florets. During this process, the zygomorphic ligulate corolla arose and the stamens were aborted. Although molecular genetic research has clarified ray floret development to some extent, the precise molecular mechanism underlying ray floret development in chrysanthemum remained unclear.ResultsA CYC2-like gene, Cyc2CL, was cloned from C. morifolium ‘Fenditan’. Subsequent analyses revealed that the alternative splicing of Cyc2CL, which occurred in the flower differentiation stage, resulted in the production of Cyc2CL-1 and Cyc2CL-2 in the apical buds. Prior to this stage, only Cyc2CL-1 was produced in the apical buds. A fluorescence in situ hybridization analysis of labeled Cyc2CL-1 and Cyc2CL-2 RNA indicated that Cyc2CL-2 was first expressed in the involucre tissue during the final involucre differentiation stage, but was subsequently expressed in the receptacle and floret primordia as the floral bud differentiation stage progressed. Moreover, Cyc2CL-2 was highly expressed in the inflorescence tissue during the corolla formation stage, and the expression remained high until the end of the floral bud differentiation stage. Furthermore, the overexpression of Cyc2CL-1 and Cyc2CL-2 in transgenic Arabidopsis inhibited stamen and petal development. Therefore, both Cyc2CL-1 and Cyc2CL-2 encode candidate regulators of petal development and stamen abortion and are important for the ray floret development in chrysanthemum.ConclusionIn this study, we characterized the alternatively spliced transcripts of the CYC2-like gene that differ subtly regarding expression and function. The data presented herein will be useful for clarifying the regulatory mechanisms associated with the CYC2-like gene and may also be important for identifying the key genes and molecular mechanisms controlling the development of ray florets in chrysanthemum.

Nitrogen allocation and remobilization contributing to low-nitrogen tolerance in stay-green maize

Nitrogen (N) efficient cultivars can achieve relatively high grain yield under reduced N input. Normally modern stay-green maize cultivars are high-yielding and more N efficient than the old senescent cultivars. However, less is known about the genotypic difference in N use efficiency (NUE) among stay-green high-yielding cultivars. This information is crucial for further genetic improvement in NUE of the modern stay-green cultivars. In the present study, two stay-green hybrids with different NUE, Zhengdan958 (ZD958, N-efficient) and Liangyu99 (LY99, N-inefficient), were grown under 60 (LN) and 180 kg N ha−1 (HN) conditions in a 4-year field experiment with split-design. The two hybrids did not differ in grain yield under HN. Under LN, grain yield and grain N content of LY99 was 23.6 % and 30.3 % lower compared to that of ZD958, respectively. LY99 had few grain number per row than ZD958. Correspondingly, floret primordia number decreased by 17.7 %, and grain abortion increased by 12.7 % in LY99 compared to ZD958. During critical period around silking, N concentration in the young ear of LY99 is lower than that in ZD958. Also, the maximum absolute ear growth rate and exponential duration of LY99 were 22.2 % and 6.0 % lower than that of ZD958 under LN, respectively. In LN conditions, the two hybrids did not differ in post-silking N accumulation. However, N remobilization from vegetative organs to grains in LY99 was 41.3 % lower than that in ZD935. In conclusion, NUE in stay-green cultivars could be improved by efficient N allocation into the young ear and vegetative-N remobilization during post-silking period under insufficient N supply.

Identification and analysis of a differentially expressed wheat RING-type E3 ligase in spike primordia development during post-vernalization.

We identified a RING-type E3 ligase (TaBAH1) protein in winter wheat that targets TaSAHH1 for degradation and might be involved in primordia development by regulating targeted protein degradation. Grain yield per spike in wheat (Triticum aestivum), is mainly determined prior to flowering during mature primordia development; however, the genes involved in primordia development have yet to be characterized. In this study, we demonstrated that, after vernalization for 50days at 4°C, there was a rapid acceleration in primordia development to the mature stages in the winter wheat cultivars Keumgang and Yeongkwang compared with the Chinese Spring cultivar. Although Yeongkwang flowers later than Keumgang under normal condition, it has the same heading time and reaches the WS9 stage of floral development after vernalization for 50days. Using RNA sequencing, we identified candidate genes associated with primordia development in cvs. Keumgang and Yeongkwang, that are differentially expressed during wheat reproductive stages. Among these, the RING-type E3 ligase TaBAH1 (TraesCS5B01G373000) was transcriptionally upregulated between the double-ridge (WS2.5) stage and later stages of floret primordia development (WS10) after vernalization. Transient expression analysis indicated that TaBAH1 was localized to the plasma membrane and nucleus and was characterized by self-ubiquitination activity. Furthermore, we found that TaBAH1 interacts with TaSAHH1 to mediate its polyubiquitination and degradation through a 26S proteasomal pathway. Collectively, the findings of this study indicate that TaBAH1 might play a prominent role in post-vernalization floret primordia development.

Spike growth affects spike fertility through the number of florets with green anthers before floret abortion in wheat

The dynamics of floret initiation/abortion has been explored widely; however, the mechanisms of increasing floret fertility at anthesis in wheat production remain unclear. In this study, morphological and physiological responses of florets per spikelet and per spike to nitrogen and irrigation treatments in the field were investigated. Each spikelet produced the maximum number of living floret primordia (MFP) and showed no difference between treatments at the W7.5 stage; this ability was hardly affected by treatments. We firstly found that gradually abortive florets during W8.5-W10 stages were delayed due to improving assimilates availability in the spike instead of the 4th-5th inthernode; in fact, these florets would be aborted completely and never become fertile florets. Notably, we found that those florets in which the anther turned green (value ≥ 97) at W7.5 always became fertile, and their number showed a significant positive relationship with number of fertile florets (NFF) at W10. Collectively, more available assimilates in spike accelerated anther turning green during period of early floret development, maintained their development normally and becoming fertile florets, but did not make florets that initiate abortion become fertile florets and increase the MFP.

Floret development and spike fertility in wheat: Differences between cultivars of contrasting yield potential and their sensitivity to photoperiod and soil N

Abstract In a previous paper (Field Crops Res. 203, 114–127), we showed that the difference in yield potential between a contemporary and a traditional cultivar was due to differences in fruiting efficiency, likely derived from differences in spike fertility (fertile florets per spike) while having similar spike dry weights at anthesis. In this study, we determined the mechanistic bases of these genotypic differences in spike fertility analysing the initiation of all floret primordia per spike (up to 8), the maximum number of florets initiated per spikelet, and the associated floret developmental rates and their fate to become fertile florets under contrasting photoperiod (natural vs extended) and nitrogen availability (50 or 200 kg N ha−1) during the stem elongation phase. Under potential growing conditions (natural photoperiod, high nitrogen availability), the contemporary cultivar owed its higher spike fertility to the improved rate of floret development, which mainly determined an improved level of floret primordia survival to produce fertile florets. The sensitivity of the floret developmental patterns to faster development due to exposure to an extended photoperiod and, to a larger extent, a reduction in N availability was similar for both cultivars, providing a basis for the consistent differences in spike fertility across a range of environments. The response again determined a main effect through increasing floret mortality reducing therefore the level of fertile florets per spikelet in these conditions.

Earliness per se×temperature interaction: consequences on leaf, spikelet, and floret development in wheat.

Wheat adaptation can be fine-tuned by earliness per se (Eps) genes. Although the effects of Eps genes are often assumed to act independently of the environment, previous studies have shown that they exhibit temperature sensitivity. The number of leaves and phyllochron are considered determinants of flowering time and the numerical components of yield include spikelets per spike and fertile floret number within spikelets. We studied the dynamics of leaf, spikelet, and floret development in near isogenic lines with either late or early alleles of Eps-D1 under seven temperature regimes. Leaf appearance dynamics were modulated by temperature, and Eps alleles had a greater effect on the period from flag leaf to heading than phyllochron. In addition, the effects of the Eps alleles on spikelets per spike were minor, and more related to spikelet plastochron than the duration of the early reproductive phase. However, fertile floret number was affected by the interaction between Eps alleles and temperature. So, at 9 °C, Eps-early alleles had more fertile florets than Eps-late alleles, at intermediate temperatures there was no significant difference, and at 18 °C (the highest temperature) the effect was reversed, with lines carrying the late allele producing more fertile florets. These effects were mediated through changes in floret survival; there were no clear effects on the maximum number of floret primordia.