The rice FON1 gene controls vegetative and reproductive development by regulating shoot apical meristem size

Most plant organs develop from meristems. Rice FON1, which is an ortholog of Clv1, regulates stem cell proliferation and organ initiation. The point muta-tions, fon1-1 and fon1-2, disrupt meristem balance, resulting in alteration of floral organ numbers and the architecture of primary rachis branches. In this study, we identified two knockout alleles, fon1-3 and fon1-4, generated by T-DNA and Tos17 insertion, respectively. Unlike the previously isolated point mutants, the null mutants have alterations not only of the reproductive organs but also of vegetative tissues, producing fewer tillers and secondary rachis branches. The mutant plants are semi-dwarfs due to delayed leaf emergence, and leaf senescence is delayed. SEM analysis showed that the shoot apical meristems of fon1-3 mutants are enlarged. These results indicate that FON1 controls vegetative as well as reproductive development by regulating meristem size.     

Mutations associated with floral organ number in rice

How floral organ number is specified is an interesting subject and has been intensively studied in Arabidopsis thaliana. In rice (Oryza sativa L.), mutations associated with floral organ number have been identified. In three mutants of rice, floral organ number 1 (fon1) and the two alleles, floral organ number 2-1 (fon2-1) and floral organ number 2-2 (fon2-2), the floral organs were increased in number centripetally. Lodicules, homologous to petals, were rarely affected, and stamens were frequently increased from six to seven or eight. Of all the floral organs the number of pistils was the most frequently increased. Among the mutants, fon1 showed a different spectrum of organ number from fon2 -1 and fon2 -2. Lodicules were the most frequently affected in fon1, but pistils of more than half of fon1 flowers were unaffected; in contrast, the pistils of most flowers were increased in fon2 -1 and fon2-2. Homeotic conversion of organ identity was also detected at a low frequency in ectopically formed lodicules and stamens. Lodicules and stamens were partially converted into anthers and stigmas, respectively. Concomitant with the increased number of floral organs, each mutant had an enlarged apical meristem. Although meristem size was comparable among the three mutants and wild type in the early phase of flower development, a significant difference became apparent after the lemma primordium had differentiated. In these mutants, the size of the shoot apical meristem in the embryo and in the vegetative phase was not affected, and no phenotypic abnormalities were detected. These results do not coincide with those for Arabidopsis in which clavatal affects the sizes of both shoot and floral meristems, leading to abnormal phyllotaxis, inflorescence fasciation and increased floral organs. Accordingly, it is considered that FON1 and FON2 function exclusively in the regulation of the floral meristem, not of the vegetative meristem.Abbreviation DIC differential interference contrast This work was supported in part by Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture of Japan.     

Fine mapping of a quantitative trait locus for grain number per panicle from wild rice (Oryza rufipogon Griff.)

SIL040, an introgression line (IL) developed by introgressing chromosomal segments from an accession of Oryza rufipogon into an indica cultivar Guichao 2, showed significantly less grains per panicle than the recurrent parent Guichao 2. Quantitative trait locus (QTL) analysis in F₂ and F₃ generations derived from the cross between SIL040 and Guichao 2 revealed that gpa7, a QTL located on the short arm of chromosome 7, was responsible of this variation. Alleles from O. rufipogon decreased grains per panicle. To fine mapping of gpa7, a high-resolution map with 1,966 F₂ plants derived from the cross between SIL040 and Guichao 2 using markers flanking gpa7 was constructed, and detailed quantitative evaluation of the structure of main panicle of each of F₃ families derived from recombinants screened was performed. By two-step substitution mapping, gpa7 was finally narrowed down to a 35-kb region that contains five predicted genes in cultivated rice. The fact that QTLs for five panicle traits (length of panicle, primary branches per panicle, secondary branches per panicle, grains on primary branches and grains on secondary branches) were all mapped in the same interval as that for gpa7 suggested that this locus was associated with panicle structure, showing pleiotropic effects. The characterizing of panicle structure of IL SIL040 further revealed that, during the domestication from common wild allele to cultivated rice one at gpa7, not only the number of branches and grains per panicle increased significantly, more importantly, but also the ratio of secondary branches per panicle to total branches per panicle and the ratio of grains on secondary branches per panicle to total grains per panicle increased significantly. All these results reinforced the idea that gpa7 might play an important role in the regulation of grain number per panicle and the ratio of secondary branches per panicle during the domestication of rice panicle.Feng Tian and Zuo Feng Zhu contributed equally to this work.     

Genome‐wide expression quantitative trait locus studies facilitate isolation of causal genes controlling panicle structure

The morphology of rice (Oryza sativa L.) panicles is an important determinant of grain yield, and elucidation of the genetic control of panicle structure is very important for fulfilling the demand for high yield in breeding programs. In a quantitative trait locus (QTL) study using 82 backcross inbred lines (BILs) derived from Koshihikari and Habataki, 68 QTLs for 25 panicle morphological traits were identified. Gene expression profiling from inflorescence meristems of BILs was obtained. A combination of phenotypic QTL (pQTL) and expression QTL (eQTL) analysis revealed co‐localization between pQTLs and eQTLs, consistent with significant correlations between phenotypic traits and gene expression levels. By combining pQTL and eQTL data, two genes were identified as controlling panicle structure: OsMADS18 modulates the average length of the primary rachis and OsFTL1 has pleiotropic effects on the total number of secondary rachides, number of grains per panicle, plant height and the length of flag leaves. Phenotypes were confirmed in RNA interference knocked‐down plants and overexpressor lines. The combination of pQTL and eQTL analysis could facilitate identification of genes involved in rice panicle formation.     

A developmentally based categorization of branching in Trifolium repens L.: influence of nodal roots

This study describes the successive stages of development of branches from axillary buds in fully rooted plants of Trifolium repens grown in near optimal conditions, and the way in which this developmental pathway differs when nodal root formation is prevented as plants grow out from a rooted base. Cuttings of a single genotype were established in a glasshouse with nodal root systems on the two basal phytomers and grown on so that nodal rooting was either permitted (+R) or prevented (-R). In +R plants, axillary tissues could be assigned to one of four developmental categories: unemerged buds, emerged buds, unbranched lateral branches or secondarily branched lateral branches. In -R plants, branch development was retarded, with the retardation becoming increasingly pronounced as the number of -R phytomers on the primary stolon increased. Retarded elongation of the internodes of lateral shoots on -R plants resulted in the formation of a distinct fifth developmental category: short shoots (defined as branches with two or more leaves but with mean internode length equal to, or less than, 10% of that of the immediately proximal internode on the parent stolon) which had reduced phytomer appearance rates but retained the potential to develop into lateral branches. Transfer of +R plants to -R conditions, and vice versa, after 66 d demonstrated that subsequent branch development was wholly under the control of the youngest nodal root present, regardless of the age and number of root systems proximal to it.     

Genetic and Phenotypic Analysis of <Emphasis Type="Italic">lax1-6</Emphasis>, a Mutant Allele of <Emphasis Type="Italic">LAX PANICLE1</Emphasis> in Rice

Proper function of the LAX1 gene is required for the development of axillary meristem in rice. Here, we report genetic and phenotypic characters of a novel recessive mutant allele of rice LAX1 gene, lax1-6, which showed abnormal panicle phenotypes with few numbers of elongated primary rachis branches. Beside typical lax mutant phenotype, abnormalities of lax1-6 mutant allele were observed with defect lemma and palea primordial in floral organs. The lax1-6 mutant locus was linked between SSR markers RM7594 and RM5389 on chromosome 1 with 1.02% and 1.0% recombination frequencies, respectively. Molecular analysis revealed that the lax1-6 mutant allele was caused by a transversion mutation of nucleotide T to G substitution that resulted in an amino acid substitution from serine (S) to alanine (A) at the 117^th position from amino terminus of a basic helix-loop-helix protein coded by LAX1 gene. Furthermore, we found that the Oryza sativa indica type cv. IRRI347 contained 24 nucleotide deletion in the upstream sequence in the LAX1 gene, but this deletion did not influence panicle morphology, which demonstrated that the deletion is a polymorphism in rice. All together, the lax1-6 mutant is a newly identified allele of LAX1 gene displaying the abnormal axillary meristems and inflorescences in rice.     

Population differentiation in the tiller architecture of Microstegium vimineum (Poaceae) in relation to habitat

Grass tillers grow by addition of modular units known as phytomers. Differences in phytomer organ size produce subindividual variation with potential adaptive value. Here, patterns in organ mass along tillers in the invasive annual Microstegium vimineum are related to habitat and tiller architecture. In an earlier study, seed families were collected from two populations: one from a sunny, woodland edge and the other from a shady understory in New Jersey, USA. Plants from these seeds were grown in a greenhouse. Phytomers along primary tillers were divided into culms, leaves, and cleistogamous or chasmogamous spikelets and seeds, dried and weighed. These data were used to examine the quantitative genetics of subindividual variation among families and populations, and the relationship of tiller fitness (based on total seed mass) to the mean or subindividual variance of phytomer traits. Phytomer position along a tiller was the major determinant of organ mass. Leaf mass increased from basal to upper nodes; cleistogamous reproductive mass decreased from upper to lower nodes. Phytomer organs were heaviest in the population from the sunny habitat. Family explained < 18% of variation in organ mass. Tiller fitness was positively correlated with mean culm, and leaf mass, but negatively correlated with coefficients of variation. Field‐collected tillers showed evidence of selection for increased leaf mass. Subindividual variation in M. vimineum is mostly due to phytomer position along a tiller and the prevailing light environment. Differentiation between sunny and shady populations suggests selection favors heavier phytomer leaves and culms, especially in the shady understory where this species is most invasive.     

<Emphasis Type="Italic">OsREL2</Emphasis>, a rice TOPLESS homolog functions in axillary meristem development in rice inflorescence

The morphology of the rice inflorescence, called the panicle, is determined mainly by the activities of axillary meristems including primary, secondary, and spikelet meristems. Recently, in maize, the RAMOSA1 ENHANCER LOCUS2 (REL2) gene, orthologous to the Arabidopsis shoot apical meristem fate-determining TOPLESS, was shown to be involved in the regulation of axillary meristem determinacy. In order to investigate the function of the rice REL2 homolog, we identified and characterized the rice REL2 gene (OsREL2). Compared to other rice TPL homologs, OsREL2 gene expression stayed relatively low throughout panicle development. We characterized a T-DNA insertion osrel2 mutant that showed pleiotropic phenotypic defects, such as defects in panicle heading, sterile lemma elongation, and panicle development, suggesting the OsREL2 functions in multiple developmental processes. In particular, osrel2 developed shorter axillary branches and reduced numbers of lateral organs on axillary branches in comparison to the wild-type, indicating that OsREL2 is important in axillary meristem maintenance. Interestingly, osrel2 produced more primary branches and fewer secondary branches than the wild-type. These results suggest that OsREL2 is involved in branch formation regulation, presumably by suppressing primary branch formation and promoting secondary branch formation.     

FON1, an Arabidopsis gene that terminates floral meristem activity and controls flower organ number.

A novel gene that regulates floral meristem activity and controls floral organ number was identified in Arabidopsis and is designated FON1 (for FLORAL ORGAN NUMBER1). The fon1 mutants exhibit normal vegetative development and produce normal inflorescence meristems and immature flowers before stage 6. fon1 flowers become visibly different from wild-type flowers at stage 6, when the third-whorl stamen primordia have formed. The fon1 floral meristem functions longer than does that of the wild type: after the outer three-whorl organ primordia have initiated, the remaining central floral meristem continues to produce additional stamen primordia interior to the third whorl. Prolonged fon1 floral meristem activity also results in an increased number of carpels. The clavata (clv) mutations are known to affect floral meristem activity. We have analyzed the clv1 fon1, clv2 fon1, and clv3 fon1 double mutants. These double mutants all have similar phenotypes, with more stamens and carpels than either fon1 or clv single mutants. This indicates that FON1 and CLV genes function in different pathways to control the number of third- and fourth-whorl floral organs. In addition, to test for possible interactions between FON1 and other floral regulatory genes, we have constructed and analyzed the relevant double mutants. Our results suggest that FON1 does not interact with TERMINAL FLOWER1, APETALA1, APETALA2, or UNUSUAL FLORAL ORGAN. In contrast, normal LEAFY function is required for the expression of fon1 phenotypes. In addition, FON1 and AGAMOUS both seem to affect the domain of APETALA3 function, which also affects the formation of stamen-carpel chimera due to fon1 mutations. Finally, genetic analysis suggests that FON1 interacts with SUPERMAN, which also regulates floral meristem activity.     

Characterization of the Rice Floral Organ Number Mutant fon3

A spontaneous rice mutant named floral organ number 3 (fon3) had major mutations in floral organ numbers. Genetic analysis indicated that fort3 acted as a single recessive gene. Microscopic observation showed that the number of floral organs infon3 increased centripetally. For example, the number of pistils was the more frequently increased than organs in the outer whorls. Homeotic conversion of lodicules and glumes into palea/lemma-like organs was observed in some flowers. Scanning electron microscopy observation showed that the size of flower meristems was maintained the same or similar until the lemma primordium started to differentiate, at which time the floral meristem became enlarged, suggesting abnormal development of the inner whorls of rice florets. The relationship of fort3 with other similar rice mutants is discussed.     

The viviparous8 mutation delays vegetative phase change and accelerates the rate of seedling growth in maize

Post-embryonic shoot development in plants can be divided into a juvenile vegetative, an adult vegetative, and a reproductive phase, which are expressed in different domains on the shoot axis. The number and position of the phytomers in each phase are determined by the time at which a plant begins and ceases making phytomers of a particular phase and the rate at which phytomers are made during that phase. The viviparous8 (vp8) mutation of maize increases the number of juvenile vegetative phytomers and decreases the number of adult vegetative phytomers by affecting both of these processes. vp8 increases the number of juvenile vegetative phytomers by increasing the rate of leaf initiation early in shoot development and delaying the juvenile-to-adult transition (vegetative maturation). It reduces the number of adult phytomers because the delay in vegetative maturation is not matched by a corresponding delay in flowering time; vp8 plants produce a tassel at the same time as wild-type plants. Thus, Vp8 normally controls the production of a factor that functions both to repress the rate of growth early in shoot development and to promote vegetative maturation, but which has no major role in floral induction. vp8 dramatically enhances the phenotypes of the dwarf and Teopod mutants and requires a functional Glossy15 gene to prolong the expression of juvenile epidermal traits. Evidence suggesting that vp8 does not affect phase change by reducing the level of abscisic acid is discussed.     

Flowering time genes Heading date 1 and Early heading date 1 together control panicle development in rice

Although flowering time is often associated with plant size, little is known about how flowering time genes affect plant architecture. We grew four rice lines having different flowering time genotypes (hd1 ehd1, hd1 Ehd1, Hd1 ehd1 and Hd1 Ehd1) under distinct photoperiod conditions. By using genotype-treatment combinations that resulted in similar flowering times, we were able to compare the effects of flowering time genes on traits related to plant architecture. The results revealed that the combination of Heading-date 1 (Hd1) and Early heading date 1 (Ehd1) can reduce the number of primary branches in a panicle, resulting in smaller spikelet numbers per panicle; this occurs independently of the control of flowering time. In addition, expression of the Hd3a and Rice Flowering-locus T 1 (RFT1) florigen genes was up-regulated in leaves of the Hd1 Ehd1 line at the time of the floral transition. We further revealed that Hd1 and/or Ehd1 caused up-regulation of Terminal Flower 1-like genes and precocious expression of panicle formation-related genes at shoot apical meristems during panicle development. Therefore, two key flowering time genes, Hd1 and Ehd1, can control panicle development in rice; this may affect crop yields in the field through florigen expression in leaf.     

Morphological characteristics and gene mapping of a dense panicle (dp2) mutant in rice (Oryza sativa L.)

A dense panicle mutant (dp2) derived from the Oryza sativa ssp. japonica cultivar Nipponbare through ethyl methane sulfonate mutagenesis was used in present study. Compared to the wild type, the panicle of dp2 mutant exhibited more branches and denser grains. Further more, the number of spikelets per panicle, number of primary branches and secondary branches of dp2 mutant were significantly increased while the panicle length, and 1,000-grain weight were significantly decreased. The results from the genetic analysis indicated that the dense panicle phenotype was controlled by a single dominance nuclear gene. Polymorphic analysis of SSR and InDel markers demonstrated that the DP2 gene was located at the long arm of chromosome 2, which was further mapped between SSR markers RM341 and RM13356 in a physical region of 398 kb. Within this region, the RCN2 (LOC_Os02g32950) gene which was annotated relating to the development of rice panicle was found. Compared to the wild type, the sequence of RCN2 gene in the dp2 mutant showed that two SNPs replacement had taken place in the promoter region (G–A) and the intron region (A–T), respectively. The dp2 mutant could be a novel mutant of RCN2 gene and this novel mutant might be useful for further studies on this gene.     

Dynamics of the Elongation of Internodes in Maize (Zea mays L.): Analysis of Phases of Elongation and their Relationships to Phytomer Development

The kinetics of elongation of individual internodes of maizestems were studied under field conditions. Thermal time coursesof internode length were recorded using non-destructive methods,based on direct measurement of X-ray photographs or on indirectestimates using heights of leaf collars. These data were complementedby serial dissections of maize stems, and by precise observationof the process of sheath emergence, to specify its role in thekinetics of internode elongation. The kinetics of elongationwere found to be composed of four phases. The rate of elongationrises exponentially during phase I, and then increases sharplyduring a short period (phase II), which is followed by a majorperiod of constant growth rate (phase III) and a shorter periodin which the rate declines (phase IV). During phase I, elongationappears to be integrated at the level of the whole apical cone.From phase II onwards, elongation becomes determined at thelevel of the phytomer. The emergence of the sheath attachedto the internode appears to be a possible trigger for the transitionbetween phase I and phase II, and it may also be involved invariation in final length among phytomers. Copyright 2000 Annalsof Botany Company Zea mays L., internode, elongation, modelling, dynamics, X-rays, collar, phytomer, stem, thermal time, phasic development     

云南元江普通野生稻穗颈维管束和穗部性状的QTL分析

以云南元江普通野生稻为供体亲本,籼稻品种特青为轮回亲本构建高代回交群体,用SSR标记构建连锁图谱,在第1、2、3、4、7和10染色体上定位到7个控制穗颈大维管束数的QTL,在第1、2、3、4和8染色体上定位到5个控制穗颈小维管束数的QTL,在第11和12以外的10条染色体上,共定位到15个控制穗一、二次枝梗数和穗颖花数QTL。来自野生稻的等位基因大多表现负效,能显著减少群体的穗颈维管束数、枝梗数和颖花数,说明从野生稻演化成栽培稻的过程中,可能淘汰了一些对产量不利的QTL,保留了有利的QTL。相当一部分控制穗颈维管束数、枝梗数及颖花数的QTL在染色体上成簇分布或紧密连锁,且加性效应的方向一致,从理论上解释了这些性状表型显著相关的遗传基础,同时也说明在人工选择或自然选择下,这些性状可能存在平行进化或协同进化的关系。     

A gene controlling the number of primary rachis branches also controls the vascular bundle formation and hence is responsible to increase the harvest index and grain yield in rice

The quantitative trait locus controlling the number of primary rachis branches (PRBs) in rice was identified using backcrossed inbred lines of Sasanishiki/Habataki//Sasanishiki///Sasanishiki. The resultant gene was ABERRANT PANICLE ORGANIZATION 1 (APO1). Habataki-genotype segregated reciprocal recombinant lines for the APO1 locus increased both the number of PRB (12–13%) and the number of grains per panicle (9–12%), which increased the grain yield per plant (5–7%). Further recombination dividing this region revealed that different alleles regulated the number of PRB and the number of grains per panicle. The PRB1 allele, which includes the APO1 open reading frame (ORF) and the proximal promoter region, controlled only the number of PRB but not the number of grains per panicle. In contrast, the HI1 allele, which includes only the distal promoter region, increased the grain yield and harvest index in Habataki-genotype plants, nevertheless, the ORF expressed was Sasanishiki type. It also increased the number of large vascular bundles in the peduncle. APO1 expression occurred not only in developing panicles but also in the developing vascular bundle systems. In addition, Habataki plants displayed increased APO1 expression in comparison to Sasanishiki plants. It suggests that APO1 enhances the formation of vascular bundle systems which, consequently, promote carbohydrate translocation to panicles. The HI1 allele is suggested to regulate the amount of APO1 expression, and thereby control the development of vascular bundle systems. These findings may be useful to improve grain yield as well as quality through the improvement of translocation efficiency.