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.

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(mediumspike 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.

LATERAL FLORET 1 induced the three-florets spikeletin rice

With the support by the National Natural Science Foundation of China,a study by the research group led by Prof.He Guanghua(何光华)from the Rice Research Institute,Southwest University demonstrates that LATERAL FLORET 1induces the'three-florets spikelet'in rice,which was published in PNAS(2017,114(37):9984-9989).

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(2018/2019and 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.

Changes in Levels of Endogenous Plant Hormones During Floret Development in Wheat Genotypes of Different Spike Sizes

The levels of endogenous plant hormones regulate floret development and degeneration, and thus grain set in flower crops. This study was undertaken to characterize the changes of endogenous hormone levels during floret development in three wheat ( Triticum aestivum L.) genotypes: “97J1' with the highest grain set and fertile florets per spike, “H8679' with the lowest grain set and fertile florets per spike, and a medium, “YM158'. The results showed that the peak level of ABA appeared between stamen and pistil differentiation and antherlobe formation of floret development, and the timing delayed with the size of spike (earliest in “H8679” and latest in “97J1”). From antherlobe formation to meiosis, the levels of ABA and GA 1+3 decreased sharply in the ears of “97J1”, while in the ears of “H8679” there was only a slight decrease in ABA, and even an increase in GA 1+3 . The ratio of isopentenyladenosine (iPA)/ABA and IAA/ABA in the ears of “97J1” increased sharply from antherlobe formation to meiosis, but changed only slightly in the ears of “H8679”. At antherlobe formation, IAA and GA 1+3 levels were higher in the ears of “97J1”, but lower in the ears of “H8679” than in the leaves. At meiosis, ABA, GA 1+3 and IAA levels in the “97J1” ears were much lower than in the leaves, but similar in “H8679”. These results indicated that the sharp decreases of ABA and GA 1+3 in ears from antherlobe formation to meiosis and the lowest maintenance at meiosis may be favorable for development of fertile florets and enhancement of grain set in wheat.

Genetic Analysis of Streaked and Abnormal Floret Mutant st-fon

A double mutant with streaked leaf and abnormal floret was found and temporarily named streaked leaf and floral organ number mutant （st-fon）. For this mutant, besides white streak appeared on culm, leaves and panicles, the number of floral organs increased and florets cracked. The extreme phenotype was that several small florets grew from one floret or branch rachis in small florets extended and developed into panicles. By using transmission electron microscope to observe the ultrastructure of white histocytes of leaves at the seedling stage, the white tissues which showed abnormal plastids, lamellas and thylakoids could not develop into normal chloroplast, and the development of chloroplast was blocked at the early growth stage of plastid. Scanning electron microscope and paraffin section were also used to observe the development of floral organs, and the results indicated that the development of floral meristem was out of order and unlimited, whereas in the twisty leaves, vascular bundle sheath cells grew excessively, or some bubbly cells increased. Genetic analyses carried out by means of cross and backcross with four normal-leaf-color materials revealed that the mutant is of cytoplasm inheritance.

The soft glumes of common wheat are sterile-lemmas as determined by the domestication gene Q

The Q gene in common wheat encodes an APETALA2(AP2) transcription factor that causes the free threshing attribute. Wheat spikelets bearing several florets are subtended by a pair of soft glumes that allow free liberation of seeds. In wild species, the glumes are tough and rigid,making threshing difficult. However, the nature of these "soft glumes", caused by the domestication allele Q is not clear. Here, we found that over expression of Q in common wheat leads to homeotic florets at glume positions. We provide phenotypic, microscopy, and marker genes evidence to demonstrate that the soft glumes of common wheat are in fact lemma-like organs, or so-called sterile-lemmas. By comparing the structures subtending spikelets in wheat and other crops such as rice and maize, we found that AP2 genes may play conserved functions in grasses by manipulating vestigial structures, such as floret-derived soft glumes in wheat and empty glumes in rice. Conversion of these seemingly vegetative organs to reproductive organs may be useful in yield improvement of crop species.

Effects of Nitrogen Application in Different Wheat Growth Stages on the Floret Development and GrainYield of Winter Wheat

The study was carried out on the effect of nitrogen application in different wheat growth stage on the floret development, the photosynthetic rate, the yield and its components of winter wheat. The result indicated that nitrogen application in the pistil-stamen primordium formation stage and the tetrad formation stage of wheat growth prolonged the duration of floret development, promoted the balance growth of floret and reduced the floret decadence number, thus increased the grain number per spike. Nitrogen application in the middle and in the late stages of wheat development increased the photosynthetic ability of the plant leaves in the later stage, and also lengthened the peak of grain filling stage, thus enhanced the grain weight and yield of wheat significantly.

Pollination dynamics, grain weight and grain cell number within the inflorescence and spikelet in oat and wheat

Oat (Avena sativa L.) and wheat (Triticum aestivum L.) vary in the structure of their inflores-cences and also in how pollination proceeds within the inflorescence. In both species the grain position in the spikelet determines grain weight potential. Primary grains in oat and proximal grains in wheat weigh more than secondary and distal grains. This variation in grain weight can potentially result from differences in post-pollination cell division in the grain. In this study pollination duration and dynamics were analyzed from head samples collected at two-day intervals, starting from the pollination of the most advanced floret. The number of grain cells was determined for individual grains throughout the inflorescence, starting from the pollination event. When mature, grain position in the spikelet and spike was noted and grain weight assessed. Pollination advance in oat proceeded from the uppermost primary floret towards the basal spikelets in ten to eleven days. Within the spikelet, the primary floret was pollinated on average one day earlier than the secondary floret. In wheat, pollination duration was four to five days, starting from the proximal florets in the mid-section of the inflorescence progressing towards the apical and basal spikelets. Proximal florets were pollinated one to two days earlier than distal florets. Maximum cell number in primary grains exceeded that of secondary grains in two oat cultivars. Similarly, primary grains were heavier than secondary grains. Cell number and single grain weight were correlated in terms of grain position in the spikelet (primary – secondary) and cultivar. Oat cultivar Belinda had a higher single grain weight than Fiia, which was also expressed as larger grain cell number. In wheat, proximal grains had higher maximum cell numbers and were also heavier than distal grains. This grain weight gradient was apparent throughout the inflorescence. Consequently, grain cell number is one of the possible regulators of grain-filling capacity in both cereal crops.

Nuclear phylogenomics of Asteraceae with increased sampling provides new insights into convergent morphological and molecular evolution

Convergent morphological evolution is widespread in flowering plants,and understanding this phenomenon relies on well-resolved phylogenies.Nuclear phylogenetic reconstruction using transcriptome datasets has been successful in various angiosperm groups,but it is limited to taxa with available fresh materials.Asteraceae,which are one of the two largest angiosperm families and are important for both ecosystems and human livelihood,show multiple examples of convergent evolution.Nuclear Asteraceae phylogenies have resolved relationships among most subfamilies and many tribes,but many phylogenetic and evolutionary questions regarding subtribes and genera remain,owing to limited sampling.Here,we increased the sampling for Asteraceae phylogenetic reconstruction using transcriptomes and genome-skimming datasets and produced nuclear phylogenetic trees with 706 species representing two-thirds of recognized subtribes.Ancestral character reconstruction supports multiple convergent evolutionary events in Asteraceae,with gains and losses of bilateral floral symmetry correlated with diversification of some subfamilies and smaller groups,respectively.Presence of the calyx-related pappus may have been especially important for the success of some subtribes and genera.Molecular evolutionary analyses support the likely contribution of duplications of MADS-box and TCP floral regulatory genes to innovations in floral morphology,including capitulum inflorescences and bilaterally symmetric flowers,potentially promoting the diversification of Asteraceae.Subsequent divergences and reductions in CYC2 gene expression are related to the gain and loss of zygomorphic flowers.This phylogenomic work with greater taxon sampling through inclusion of genome-skimming datasets reveals the feasibility of expanded evolutionary analyses using DNA samples for understanding convergent evolution.

Molecular mechanisms underlying the negative effects of transient heatwaves on crop fertility

Transient heatwaves occurring more frequently as the climate warms,yet their impacts on crop yield are severely underestimated and even overlooked.Heatwaves lasting only a few days or even hours during sensitive stages,such as microgametogenesis and flowering,can significantly reduce crop yield by disrupting plant reproduction.Recent advances in multi-omics and GWAS analysis have shed light on the specific organs(e.g.,pollen,lodicule,style),key metabolic pathways(sugar and reactive oxygen species metabolism,Ca2+homeostasis),and essential genes that are involved in crop responses to transient heatwaves during sensitive stages.This review therefore places particular emphasis on heat-sensitive stages,with pollen development,floret opening,pollination,and fertilization as the central narrative thread.The multifaceted effects of transient heatwaves and their molecular basis are systematically reviewed,with a focus on key structures such as the lodicule and tapetum.A number of heat-tolerance genes associated with these processes have been identified in major crops like maize and rice.The mechanisms and key heat-tolerance genes shared among different stages may facilitate the more precise improvement of heat-tolerant crops.

From the floret to the canopy:High temperature tolerance during flowering

Heat waves induced by climate warming have become common in food-producing regions worldwide,frequently coinciding with high temperature(HT)-sensitive stages of many crops and thus threatening global food security.Understanding the HT sensitivity of reproductive organs is currently of great interest for increasing seed set.The responses of seed set to HT involve multiple processes in both male and female reproductive organs,but we currently lack an integrated and systematic summary of these responses for the world’s three leading food crops(rice,wheat,and maize).In the present work,we define the critical high temperature thresholds for seed set in rice(37.2℃±0.2℃),wheat(27.3℃±0.5℃),and maize(37.9℃±0.4℃)during flowering.We assess the HT sensitivity of these three cereals from the microspore stage to the lag period,including effects of HT on flowering dynamics,floret growth and development,pollination,and fertilization.Our review synthesizes existing knowledge about the effects of HT stress on spikelet opening,anther dehiscence,pollen shedding number,pollen viability,pistil and stigma function,pollen germination on the stigma,and pollen tube elongation.HT-induced spikelet closure and arrest of pollen tube elongation have a catastrophic effect on pollination and fertilization in maize.Rice benefits from pollination under HT stress owing to bottom anther dehiscence and cleistogamy.Cleistogamy and secondary spikelet opening increase the probability of pollination success in wheat under HT stress.However,cereal crops themselves also have protective measures under HT stress.Lower canopy/tissue temperatures compared with air temperatures indicate that cereal crops,especially rice,can partly protect themselves from heat damage.In maize,husk leaves reduce inner ear temperature by about 5℃compared with outer ear temperature,thereby protecting the later phases of pollen tube growth and fertilization processes.These findings have important implications for accurate modeling,optimized crop management,and breeding of new varieties to cope with HT stress in the most important staple crops.