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

Dissection and validation of a promising QTL controlling spikelet number on 5B in bread wheat

KEY MESSAGE

Five environmentally stable QTLs for spikelet number per spike and days to heading were identified using a high-genetic map containing 95,444 SNPs, among which QSns.ucas-5B was validated using residual heterozygous line at multiple environments. Spikelet number per spike (SNS) and days to heading (DTH) play pivotal roles in the improvement of wheat yield. In this study, a high-density genetic map for a recombinant inbred lines (RILs) population derived from Zhengnong 17 (ZN17) and Yangbaimai (YBM) was constructed using 95,444 single-nucleotide polymorphism (SNP) markers from the Wheat660K SNP array. Our study identified a total of five environmentally stable QTLs for SNS and DTH, one of which was named QSns.ucas-5B, with a physical interval of approximately 545.4-552.1 Mb on the 5BL chromosome arm. Importantly, the elite haplotype within QSns.ucas-5B showed a consistent and positive effect on SNS, grain number and weight per spike, without extending the days to heading. These findings provide a foundation for future efforts to map and clone the gene(s) responsible for QSns.ucas-5B and further indicate the potential application of the developed and validated InDel marker of QSns.ucas-5B for molecular breeding purposes, aimed at improving wheat grain yield.

Improvement and Re-Evolution of Tetraploid Wheat for Global Environmental Challenge and Diversity Consumption Demand

Allotetraploid durum wheat is the second most widely cultivated wheat, following hexaploid bread wheat, and is one of the major protein and calorie sources of the human diet. However, durum wheat is encountered with a severe grain yield bottleneck due to the erosion of genetic diversity stemming from long-term domestication and especially modern breeding programs. The improvement of yield and grain quality of durum wheat is crucial when confronted with the increasing global population, changing climate environments, and the non-ignorable increasing incidence of wheat-related disorders. This review summarized the domestication and evolution process and discussed the durum wheat re-evolution attempts performed by global researchers using diploid einkorn, tetraploid emmer wheat, hexaploid wheat (particularly the D-subgenome), etc. In addition, the re-evolution of durum wheat would be promoted by the genetic enrichment process, which could diversify allelic combinations through enhancing chromosome recombination (pentaploid hybridization or pairing of homologous chromosomes gene Ph mutant line induced homoeologous recombination) and environmental adaptability via alien introgressive genes (wide cross or distant hybridization followed by embryo rescue), and modifying target genes or traits by molecular approaches, such as CRISPR/Cas9 or RNA interference (RNAi). A brief discussion of the future perspectives for exploring germplasm for the modern improvement and re-evolution of durum wheat is included.

Identification of the Q Gene Playing a Role in Spike Morphology Variation in Wheat Mutants and Its Regulatory Network

The wheat AP2 family gene Q controls domestication traits, including spike morphology and threshability, which are critical for the widespread cultivation and yield improvement of wheat. Although many studies have investigated the molecular mechanisms of the Q gene, its direct target genes, especially those controlling spike morphology, are not clear, and its regulatory pathways are not well established. In this study, we conducted gene mapping of a wheat speltoid spike mutant and found that a new allele of the Q gene with protein truncation played a role in spike morphology variation in the mutant. Dynamic expression levels of the Q gene throughout the spike development process suggested that the transcript abundances of the mutant were decreased at the W6 and W7 scales compared to those of the WT. We identified several mutation sites on the Q gene and showed that mutations in different domains resulted in distinct phenotypes. In addition, we found that the Q gene produced three transcripts via alternative splicing and that they exhibited differential expression patterns in nodes, internodes, flag leaves, and spikes. Finally, we identified several target genes directly downstream of Q, including TaGRF1-2D and TaMGD-6B, and proposed a possible regulatory network. This study uncovered the target genes of Q, and the results can help to clarify the mechanism of wheat spike morphology and thereby improve wheat grain yield.

Molecular Insights into Inflorescence Meristem Specification for Yield Potential in Cereal Crops

Flowering plants develop new organs throughout their life cycle. The vegetative shoot apical meristem (SAM) generates leaf whorls, branches and stems, whereas the reproductive SAM, called the inflorescence meristem (IM), forms florets arranged on a stem or an axis. In cereal crops, the inflorescence producing grains from fertilized florets makes the major yield contribution, which is determined by the numbers and structures of branches, spikelets and florets within the inflorescence. The developmental progression largely depends on the activity of IM. The proper regulations of IM size, specification and termination are outcomes of complex interactions between promoting and restricting factors/signals. Here, we focus on recent advances in molecular mechanisms underlying potential pathways of IM identification, maintenance and differentiation in cereal crops, including rice (Oryza sativa), maize (Zea mays), wheat (Triticum aestivum), and barley (Hordeum vulgare), highlighting the researches that have facilitated grain yield by, for example, modifying the number of inflorescence branches. Combinatorial functions of key regulators and crosstalk in IM determinacy and specification are summarized. This review delivers the knowledge to crop breeding applications aiming to the improvements in yield performance and productivity.

APETALA2 functions as a temporal factor together with BLADE-ON-PETIOLE2 and MADS29 to control flower and grain development in barley

Cereal grain develops from fertilised florets. Alterations in floret and grain development greatly influence grain yield and quality. Despite this, little is known about the underlying genetic control of these processes, especially in key temperate cereals such as barley and wheat. Using a combination of near-isogenic mutant comparisons, gene editing and genetic analyses, we reveal that HvAPETALA2 (HvAP2) controls floret organ identity, floret boundaries, and maternal tissue differentiation and elimination during grain development. These new roles of HvAP2 correlate with changes in grain size and HvAP2-dependent expression of specific HvMADS-box genes, including the B-sister gene, HvMADS29. Consistent with this, gene editing demonstrates that HvMADS29 shares roles with HvAP2 in maternal tissue differentiation. We also discovered that a gain-of-function HvAP2 allele masks changes in floret organ identity and grain size due to loss of barley LAXATUM.A/BLADE-ON-PETIOLE2 (HvBOP2) gene function. Taken together, we reveal novel pleiotropic roles and regulatory interactions for an AP2-like gene controlling floret and grain development in a temperate cereal.

CRISPR/Cas9 editing of wheat TaQ genes alters spike morphogenesis and grain threshability

The TaQ alleles as one of the AP2-like transcription factors in common wheat (Triticum aestivum) play an important role in the evolution of spike characteristics from wild and domesticated emmer to modern wheat cultivars. Its loss-of-function mutant not only changed threshability and spike architecture but also affected plant height, flowering time, and floret structure. However, the comprehensive functions of TaAQ and TaDq genes in wheat have not been fully elucidated yet. Here, CRISPR/SpCas9 was used to edit wheat TaAQ and TaDq. We obtained homozygous plants in the T₁ generation with loss of function of only TaAQ or TaDq and simultaneous loss of function of TaAQ and TaDq to analyze the effect of these genes on wheat spikes and floret shapes. The results demonstrated that the TaAQ-edited plants and the TaAQ and TaDq simultaneously-edited plants were nearly similar in spike architecture, whereas the TaDq-edited plants were different from the wild-type ones only in plant height. Moreover, the TaAQ-edited plants or the TaAQ and TaDq simultaneously-edited plants were more brittle than the wild-type and the TaDq-edited plants. Based on the expression profiling, we postulated that the VRN1, FUL2, SEP2, SEP5, and SEP6 genes might affect the number of spikelets and florets per spike in wheat by regulating the expression of TaQ. Combining the results of this report and previous reports, we conceived a regulatory network of wheat traits, including plant height, spike shape, and floral organs, which were influenced by AP2-like family genes. The results achieved in this study will help us to understand the regulating mechanisms of TaAQ and TaDq alleles on wheat floral organs and inflorescence development.

Comprehensive analysis of Q gene near-isogenic lines reveals key molecular pathways for wheat domestication and improvement

The wheat AP2-like transcription factor gene Q has played a major role in domestication by conferring the free-threshing character and pleiotropically affecting numerous other traits. However, little information is known regarding the molecular mechanisms associated with the regulation of these traits by Q, especially for the structural determination of threshability. Here, transcriptome analysis of immature spike tissues in three lines nearly isogenic for Q revealed over 3000 differentially expressed genes (DEGs) involved in a number of pathways. Using phenotypic, microscopic, transcriptomic, and tissue-specific gene expression analyses, we demonstrated that Q governs threshability through extensive modification of wheat glumes including their structure, cell wall thickness, and chemical composition. Critical DEGs and pathways involved in secondary cell wall synthesis and regulation of the chemical composition of glumes were identified. We also showed that the mutation giving rise to the Q allele synchronized the expression of genes for micro-sporogenesis that affected pollen fertility, and may determine the final grain number for wheat spikes. Transcriptome dissection of genes and genetic pathways regulated by Q should further our understanding of wheat domestication and improvement.

Multifloret spikelet improves rice yield

The typical rice (Oryza sativa) spikelet contains a single fertile floret and produces only one grain; by contrast, Brachypodium distachyon spikelets contain multiple fertile florets and produce several grains. To increase yield, rice breeders have traditionally focused on panicle morphology (branch number and length, spikelet density), but have not considered the number of florets in each spikelet. Production of rice spikelets with more florets could further increase the number of grains per panicle. Here, we describe two novel approaches - altering meristem determinacy and restoring lateral floret formation - for breeding rice cultivars with a multifloret spikelet, thereby increasing the number of grains per panicle and potentially improving yield.

APETALA 2-like genes AP2L2 and Q specify lemma identity and axillary floral meristem development in wheat

The spikelet is the basic unit of the grass inflorescence. In tetraploid (Triticum turgidum) and hexaploid wheat (Triticum aestivum), the spikelet is a short indeterminate branch with two proximal sterile bracts (glumes) followed by a variable number of florets, each including a bract (lemma) with an axillary flower. Varying levels of miR172 and/or its target gene Q (AP2L5) result in gradual transitions of glumes to lemmas, and vice versa. Here, we show that AP2L5 and its related paralog AP2L2 play critical and redundant roles in the specification of axillary floral meristems and lemma identity. AP2L2, also targeted by miR172, displayed similar expression profiles to AP2L5 during spikelet development. Loss-of-function mutants in both homeologs of AP2L2 (henceforth ap2l2) developed normal spikelets, but ap2l2 ap2l5 double mutants generated spikelets with multiple empty bracts before transitioning to florets. The coordinated nature of these changes suggest an early role of these genes in floret development. Moreover, the flowers of ap2l2 ap2l5 mutants showed organ defects in paleas and lodicules, including the homeotic conversion of lodicules into carpels. Mutations in the miR172 target site of AP2L2 were associated with reduced plant height, more compact spikes, promotion of lemma-like characters in glumes and smaller lodicules. Taken together, our results show that the balance in the expression of miR172 and AP2-like genes is crucial for the correct development of spikelets and florets, and that this balance has been altered during the process of wheat and barley (Hordeum vulgare) domestication. The manipulation of this regulatory module provides an opportunity to modify spikelet architecture and improve grain yield.

Architecture of Wheat Inflorescence: Insights from Rice

The inflorescence architecture of grass crops affects the number of kernels and final grain yield. Great progress has been made in genetic analysis of rice inflorescence development in the past decades. However, the advances in wheat largely lag behind those in rice due to the repetitive and polyploid genomes of wheat. In view of the similar branching patterns and developmental characteristics between rice and wheat, the studies on inflorescence architecture in rice will facilitate related studies in wheat in the future. Here, we review the developmental regulation of inflorescences in rice and wheat and highlight several pathways that potentially regulate the inflorescence architecture of wheat.