Interactions between SQUAMOSA and SHORT VEGETATIVE PHASE MADS-box proteins regulate meristem transitions during wheat spike development

Inflorescence architecture is an important determinant of crop productivity. The number of spikelets produced by the wheat inflorescence meristem (IM) before its transition to a terminal spikelet (TS) influences the maximum number of grains per spike. Wheat MADS-box genes VERNALIZATION 1 (VRN1) and FRUITFULL 2 (FUL2) (in the SQUAMOSA-clade) are essential to promote the transition from IM to TS and for spikelet development. Here we show that SQUAMOSA genes contribute to spikelet identity by repressing MADS-box genes VEGETATIVE TO REPRODUCTIVE TRANSITION 2 (VRT2), SHORT VEGETATIVE PHASE 1 (SVP1), and SVP3 in the SVP clade. Constitutive expression of VRT2 resulted in leafy glumes and lemmas, reversion of spikelets to spikes, and downregulation of MADS-box genes involved in floret development, whereas the vrt2 mutant reduced vegetative characteristics in spikelets of squamosa mutants. Interestingly, the vrt2 svp1 mutant showed similar phenotypes to squamosa mutants regarding heading time, plant height, and spikelets per spike, but it exhibited unusual axillary inflorescences in the elongating stem. We propose that SQUAMOSA–SVP interactions are important to promote heading, formation of the TS, and stem elongation during the early reproductive phase, and that downregulation of SVP genes is then necessary for normal spikelet and floral development. Manipulating SVP and SQUAMOSA genes can contribute to engineering spike architectures with improved productivity.

Functional characterization of developmental genes reveals ways to modify the wheat spike architecture to increase the number of grains and improve productivity.     

Genes WHEAT FRIZZY PANICLE and SHAM RAMIFICATION 2 independently regulate differentiation of floral meristems in wheat

Background

Inflorescences of wheat species, spikes, are characteristically unbranched and bear one sessile spikelet at a spike rachis node. Development of supernumerary spikelets (SSs) at rachis nodes or on the extended rachillas is abnormal. Various wheat morphotypes with altered spike morphology, associated with the development of SSs, present an important genetic resource for studies on genetic regulation of wheat inflorescence development.

Results

Here we characterized diploid and tetraploid wheat lines of various non-standard spike morphotypes, which allowed for identification of a new mutant allele of the WHEAT FRIZZY PANICLE (WFZP) gene that determines spike branching in diploid wheat Ttiticum monococcum L. Moreover, we found that the development of SSs and spike branching in wheat T. durum Desf. was a result of a wfzp-A/TtBH-A1 mutation that originated from spontaneous hybridization with T. turgidum convar. сompositum (L.f.) Filat. Detailed characterization of the false-true ramification phenotype controlled by the recessive sham ramification 2 (shr2) gene in tetraploid wheat T. turgidum L. allowed us to suggest putative functions of the SHR2 gene that may be involved in the regulation of spikelet meristem fate and in specification of floret meristems. The results of a gene interaction test suggested that genes WFZP and SHR2 function independently in different processes during spikelet development, whereas another spike ramification gene(s) interact(s) with SHR2 and share(s) common functions.

Conclusions

SS mutants represent an important genetic tool for research on the development of the wheat spikelet and for identification of genes that control meristem activities. Further studies on different non-standard SS morphotypes and wheat lines with altered spike morphology will allow researchers to identify new genes that control meristem identity and determinacy, to elucidate the interaction between the genes, and to understand how these genes, acting in concert, regulate the development of the wheat spike.

     

The petunia AGL6 gene has a SEPALLATA-like function in floral patterning

SEPALLATA ( SEP ) MADS-box genes are required for the regulation of floral meristem determinacy and the specification of sepals, petals, stamens, carpels and ovules, specifically in angiosperms. The SEP subfamily is closely related to the AGAMOUS LIKE6 ( AGL6 ) and SQUAMOSA ( SQUA ) subfamilies. So far, of these three groups only AGL6 -like genes have been found in extant gymnosperms. AGL6 genes are more similar to SEP than to SQUA genes, both in sequence and in expression pattern. Despite the ancestry and wide distribution of AGL6 -like MADS-box genes, not a single loss-of-function mutant exhibiting a clear phenotype has yet been reported; consequently the function of AGL6 -like genes has remained elusive. Here, we characterize the Petunia hybrida AGL6 ( PhAGL6 , formerly called PETUNIA MADS BOX GENE4 / pMADS4 ) gene, and show that it functions redundantly with the SEP genes FLORAL BINDING PROTEIN2 ( FBP2 ) and FBP5 in petal and anther development. Moreover, expression analysis suggests a function for PhAGL6 in ovary and ovule development. The PhAGL6 and FBP2 proteins interact in in vitro experiments overall with the same partners, indicating that the two proteins are biochemically quite similar. It will be interesting to determine the functions of AGL6 -like genes of other species, especially those of gymnosperms.     

A intervarietal genetic map and QTL analysis for yield traits in wheat

A new genetic linkage map was constructed based on recombinant inbred lines (RILs) derived from the cross between the Chinese winter wheat (Triticum aestivum L.) varieties, Chuang 35050 and Shannong 483 (ChSh). The map included 381 loci on all the wheat chromosomes, which were composed of 167 SSR, 94 EST-SSR, 76 ISSR, 26 SRAP, 15 TRAP, and 3 Glu loci. This map covered 3636.7 cM with 1327.7 cM (36.5%), 1485.5 cM (40.9%), and 823.5 cM (22.6%) for A, B, and D genome, respectively, and contained 13 linkage gaps. Using the RILs and the map, we detected 46 putative QTLs on 12 chromosomes for grain yield (GY) per m², thousand-kernel weight (TKW), spike number (SN) per m², kernel number per spike (KNS), sterile spikelet number per spike (SSS), fertile spikelet number per spike (FSS), and total spikelet number per spike (TSS) in four environments. Each QTL explained 4.42–70.25% phenotypic variation. Four QTL cluster regions were detected on chromosomes 1D, 2A, 6B, and 7D. The most important QTL cluster was located on chromosome 7D near the markers of Xwmc31, Xgdm67, and Xgwm428, in which 8 QTLs for TKW, SN, SSS and FSS were observed with very high contributions (27.53–67.63%).     

Zebularine treatment is associated with deletion of FT‐B1 leading to an increase in spikelet number in bread wheat

The number of rachis nodes (spikelets) on a wheat spike is a component of grain yield that correlates with flowering time. The genetic basis regulating flowering in cereals is well understood, but there are reports that flowering time can be modified at a high frequency by selective breeding, suggesting that it may be regulated by both epigenetic and genetic mechanisms. We investigated the role of DNA methylation in regulating spikelet number and flowering time by treating a semi‐spring wheat with the demethylating agent, Zebularine. Three lines with a heritable increase in spikelet number were identified. The molecular basis for increased spikelet number was not determined in 2 lines, but the phenotype showed non‐Mendelian inheritance, suggesting that it could have an epigenetic basis. In the remaining line, the increased spikelet phenotype behaved as a Mendelian recessive trait and late flowering was associated with a deletion encompassing the floral promoter, FT‐B1. Deletion of FT‐B1 delayed the transition to reproductive growth, extended the duration of spike development, and increased spikelet number under different temperature regimes and photoperiod. Transiently disrupting DNA methylation can generate novel flowering behaviour in wheat, but these changes may not be sufficiently stable for use in breeding programs.     

Chromosomal Localization of Genes Conferring Desirable Agronomic Traits from Wheat-Agropyron cristatum Disomic Addition Line 5113

Creation of wheat-alien disomic addition lines and localization of desirable genes on alien chromosomes are important for utilization of these genes in genetic improvement of common wheat. In this study, wheat-Agropyron cristatum derivative line 5113 was characterized by genomic in situ hybridization (GISH) and specific-locus amplified fragment sequencing (SLAF-seq), and was demonstrated to be a novel wheat-A. cristatum disomic 6P addition line. Compared with its parent Fukuhokomugi (Fukuho), 5113 displayed multiple elite agronomic traits, including higher uppermost internode/plant height ratio, larger flag leaf, longer spike length, elevated grain number per spike and spikelet number per spike, more kernel number in the middle spikelet, more fertile tiller number per plant, and enhanced resistance to powdery mildew and leaf rust. Genes conferring these elite traits were localized on the A. cristatum 6P chromosome by using SLAF-seq markers and biparental populations (F₁, BC₁F₁ and BC₁F₂ populations) produced from the crosses between Fukuho and 5113. Taken together, chromosomal localization of these desirable genes will facilitate transferring of high-yield and high-resistance genes from A. cristatum into common wheat, and serve as the foundation for the utilization of 5113 in wheat breeding.     

A genetic playground for enhancing grain number in cereals

Improving the yield stability of cereal crops with a view to bolstering global food security is an important priority. The components of final grain number per plant at harvest are determined by fertile spikes per plant, number of fertile spikelets per spike and number of grains per spikelet. In this review article, we focus on the genetic factors of floral development and inflorescence architecture known to influence grain number and provide a broad overview of genes and genetic pathways that potentially can be manipulated to increase the yield of cereal crops, in particular wheat (Triticum aestivum) and barley (Hordeum vulgare). In addition, we discuss the outcome of multidisciplinary genomics knowledge to identify potential gene targets to develop conceptual ideotypes to meet the future demand.     

MOSAIC FLORAL ORGANS1, an AGL6-Like MADS Box Gene,Regulates Floral Organ Identity and Meristem Fate in Rice

Floral organ identity and meristem determinacy in plants are controlled by combinations of activities mediated by MADS box genes. AGAMOUS-LIKE6 (AGL6)-like genes are MADS box genes expressed in floral tissues, but their biological functions are mostly unknown. Here, we describe an AGL6-like gene in rice (Oryza sativa), MOSAIC FLORAL ORGANS1 (MFO1/MADS6), that regulates floral organ identity and floral meristem determinacy. In the flower of mfo1 mutants, the identities of palea and lodicule are disturbed, and mosaic organs were observed. Furthermore, the determinacy of the floral meristem was lost, and extra carpels or spikelets developed in mfo1 florets. The expression patterns of floral MADS box genes were disturbed in the mutant florets. Suppression of another rice AGL6-like gene, MADS17, caused no morphological abnormalities in the wild-type background, but it enhanced the phenotype in the mfo1 background, indicating that MADS17 has a minor but redundant function with that of MFO1. Whereas single mutants in either MFO1 or the SEPALLATA-like gene LHS1 showed moderate phenotypes, the mfo1 lhs1 double mutant showed a severe phenotype, including the loss of spikelet meristem determinacy. We propose that rice AGL6-like genes help to control floral organ identity and the establishment and determinacy of the floral meristem redundantly with LHS1.     

Effects of exogenous hormones on floret development and grain setin wheat

At specific stages during floret development, solutions of IAA,GA₃, zeatin and ABA were injected into the leaf sheath around theyoung spike of wheat (Triticum aestivum L.) to study theregulating effects of exogenous hormones on floret development. Zeatin promotedfloret development and significantly increased the number of fertile florets aswell as grain set, especially at the stage of anther-lobe formation. Zeatinalsoincreased the sugar concentrations in spikes at anthesis. In contrast, IAA,GA₃ and ABA inhibited floret development, with different patternsforeach of the hormones. IAA inhibited the development of the whole spike and allflorets in the spikelets such that grain loss occurred in all positions in thespikelets. GA₃ increased the number of fertile florets per spike,butdecreased grain set of the third floret in each spikelet, especially whenapplied at terminal spikelet formation. ABA inhibited floret development, anddecreased the number of fertile florets and grain set at almost all developmentstages, except at anther-lobe formation. The inhibitory effect of ABA wasmainlyon the first and third florets in each spikelet.     

冬小麦产量性状的遗传效应分析

以农大139等7个冬小麦品种进行4×3不完全双列杂交,采用加性-显性及其与环境互作的遗传模型对亲本、F_1和F_2的株高、主穗穗长、单株穗数、主穗小穗数、千粒重和单株粒重6个性状进行遗传分析．结果表明,株高和主穗长主要受加性效应控制,而单株穗数、主穗小穗数、千粒重和单株粒重主要受显性×环境互作效应影响．矮2杂交后代易选到矮株,奥里生杂交后代易得到大穗材料,农林10号和奥里生的杂交后代分蘖力将加强,814的杂交后代小穗数增多,814杂交后代千粒重增高．所有产量性状未表现明显的杂种优势．     

Phenotypic and genetic analysis of spike and kernel characteristics in wheat reveals long-term genetic trends of grain yield components

Key message

Phenotypic and genetic analysis of six spike and kernel characteristics in wheat revealed geographic patterns as well as long-term trends arising from breeding progress, particularly in regard to spikelet fertility, i.e. the number of kernels per spikelet, a grain yield component that appears to underlie the increase in the number of kernels per spike.

Abstract

Wheat is a staple crop of global relevance that faces continuous demands for improved grain yield. In this study, we evaluated a panel of 407 winter wheat cultivars for six characteristics of spike and kernel development. All traits showed a large genotypic variation and had high heritabilities. We observed geographic patterns for some traits in addition to long-term trends showing a continuous increase in the number of kernels per spike. This breeding progress is likely due to the increase in spikelet fertility, i.e. the number of kernels per spikelet. While the number of kernels per spike and spikelet fertility were significantly positively correlated, both traits showed a significant negative correlation with thousand-kernel weight. Genome-wide association mapping identified only small- and moderate-effect QTL and an effect of the phenology loci Rht-D1 and Ppd-D1 on some of the traits. The allele frequencies of some QTL matched the observed geographic patterns. The quantitative inheritance of all traits with contributions of additional small-effect QTL was substantiated by genomic prediction. Taken together, our results suggest that some of the examined traits were already the basis of grain yield progress in wheat in the past decades. A more targeted exploitation of the available variation, potentially coupled with genomic approaches, may assist wheat breeding in continuing to increase yield levels globally.

     

花同源异型MADS-box基因在被子植物中的功能保守性和多样性

MADS-box基因家族成员作为转录调控因子在被子植物花发育调控中发挥关键作用。本文以模式植物拟南芥（Arabidopsis thaliana）和水稻（Oryza sativa）为例,综述了近10年来对被子植物（又称有花植物）两大主要类群——核心真双子叶植物和单子叶植物花同源异型MADS-box基因的研究成果,分析MADS-box基因在被子植物中的功能保守性和多样性,同时探讨双子叶植物花发育的ABCDE模型在多大程度上适用于单子叶植物。     

Nutritional regulation of <Emphasis Type="Italic">ANR1</Emphasis> and other root-expressed MADS-box genes in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The ANR1 MADS-box gene in Arabidopsis thaliana (L.) Heynh. has previously been identified as a key regulator of lateral root growth in response to signals from external nitrate (NO₃⁻). We have used quantitative real-time PCR to investigate the responsiveness of ANR1 and 11 other root-expressed MADS-box genes to fluctuations in the supply of N, P and S. ANR1 expression in roots of hydroponically grown Arabidopsis plants was specifically regulated by changes in the N supply, being induced by N deprivation and rapidly repressed by N re-supply. This pattern of N responsiveness differs from the NO₃⁻ -inducibility of ANR1 previously observed in Arabidopsis root cultures [H.M. Zhang and B.G. Forde (1998) Science 279:407–409]. Seven of the other MADS-box genes responded to N in a manner similar to ANR1, but less strongly, while four (AGL12, AGL17, AGL18 and AGL79) were unaffected. Six of the N-regulated genes (ANR1, AGL14, AGL16, AGL19, SOC1 and AGL21) belong to just two clades within the type II MADS-box lineage, while the other two (AGL26 and AGL56) belong to the poorly characterized type I lineage. Only SOC1 was additionally found to respond to changes in the P and S supply, suggesting a possible role in a general response to nutrient stress. Studies with an ANR1 transposon-insertion mutant provided no evidence for regulatory interactions between ANR1 and the other root-expressed MADS-box genes. The implications of the current data for our understanding of the role of ANR1 and other MADS box genes in the nutritional regulation of lateral root growth are discussed.