水稻幼穗-颖花发育的研究进展

水稻是世界上最重要的粮食作物之一,也是单子叶植物发育生物学研究的理想模式植物,水稻幼穗和颖花的发育还是最终产量赖以依托的基础。对水稻幼穗和颖花发育的研究已成为植物分子遗传学和作物科学共同的研究焦点。近年来,有关水稻幼穗和颖花发育的研究取得了长足的进展,本文对幼穗和颖花的发育过程、栽培措施和环境因子对幼穗和颖花发育的影响以及幼穗和颖花发育的相关基因等方面的国内外进展进行了综述,同时指出了目前研究中存在的问题及相应的研究对策。     

Four rice QTL controlling number of spikelets per panicle expressed the characteristics of single Mendelian gene in near isogenic backgrounds

Development of quantitative trait loci (QTL) near isogenic lines is a crucial step to QTL isolation using the strategy of map-based cloning. In this study, a recombinant inbred line (RIL) population derived from two indica rice varieties, Zhenshan 97 and HR5, was employed to map QTL for spikelets per panicle (SPP). One major QTL (qSPP7) and three minor QTL (qSPP1, qSPP2 and qSPP3) were identified on chromosomes 7, 1, 2 and 3, respectively. Four sets of near isogenic lines (NILs) BC₄F₂ targeted for the four QTL were developed by following a standard procedure of consecutive backcross, respectively. These QTL were not only validated in corresponding NILs, but also explained amounts of phenotypic variation with much larger LOD scores compared with those identified in RILs. SPP in the four QTL-NILs expressed bimodal or discontinuous distributions and followed the expected segregation ratio of single Mendelian factor by progeny test. Finally, qSPP1, qSPP2, qSPP3 and qSPP7 were respectively mapped to a locus, 0.5 cM from MRG2746, 0.6 cM from MRG2762, 0.8 cM from RM49 and 0.7 cM from MRG4436, as co-dominant markers on the basis of progeny tests. These results indicate no matter how small effect minor QTL is, QTL may still express the characteristics of single Mendelian factor in NILs and isolation of minor QTL will be possible using high quality NILs. Pyramiding these QTL into a variety will largely enhance rice grain yield.     

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.     

Induction of pollen plantlets in rice by spikelet culture

Induction of pollen callus and subsequent regeneration of plantlets from the callus have been achieved from rice spikelets cultured in a liquid medium containing sucrose, -naphthalene-acetic acid and kinetin. When spikelets are cultured in a medium containing 6% sucrose, calluses are released into the medium where they continue to increase in size without undergoing organogenesis. On the other hand, in a medium containing 2% sucrose, calluses are retained within the anther locule where they differentiate into plantlets. Cytological studies have shown that calluses have their origin either in the vegetative cell of asymmetrically dividing pollen grains or in both cells of pollen grains which divide more or less equally.     

Characterization and fine mapping of NGP4c(t), a novel gene controlling the number of grains per panicle in rice

Journal of Genetics -     

Fine mapping SPP1, a QTL controlling the number of spikelets per panicle, to a BAC clone in rice (Oryza sativa)

Near isogenic lines (NILs) can be used to efficiently handle a target quantitative trait locus (QTL) by blocking genetic background noise. One QTL, SPP1, which controls the number of spikelets per panicle (SPP), was located on chromosome 1, near Gn1a, a cloned gene for rice production in a recombinant inbred line population. NILs of the SPP1 regions were quickly obtained by self-crossing recombinant inbred line 30 which is heterozygous around SPP1. Using a random NIL-F₂ population of 210 individuals, we mapped SPP1 to a 2.2-cM interval between RM1195 and RM490, which explained 51.1% of SPP variation. The difference in SPP between the two homozygotes was 44. F₂-1456, one NIL-F₂ plant, was heterozygous in the SPP1 region but was fixed in the region of Gn1a gene. This plant F₃ family showed a very wide variation in SPP, which suggested that it was SPP1 but Gn1a affected the variation of SPP in this population. In a word, SPP1 is a novel gene distinct from Gn1a. Four newly developed InDel markers were used for high-resolution mapping of SPP1 with a large NIL-F₂ population. Finally, it was narrowed down to a bacterial artificial chromosome clone spanning 107 kb; 17 open reading frames have been identified in the region. Of them, LOC_Os01g12160, which encodes an IAA synthetase, is the most interesting candidate gene.     

Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

Background and Aims

The inflorescence of grass species such as wheat, rice and maize consists of a unique reproductive structure called the spikelet, which is comprised of one, a few, or several florets (individual flowers). When reproductive growth is initiated, the inflorescence meristem differentiates a spikelet meristem as a lateral branch; the spikelet meristem then produces a floret meristem as a lateral branch. Interestingly, in wheat, the number of fertile florets per spikelet is associated with ploidy level: one or two florets in diploid, two or three in tetraploid, and more than three in hexaploid wheats. The objective of this study was to identify the mechanisms that regulate the architecture of the inflorescence in wheat and its relationship to ploidy level.

Methods

The floral anatomy of diploid (Triticum monococcum), tetraploid (T. turgidum ssp. durum) and hexaploid (T. aestivum) wheat species were investigated by light and scanning electron microscopy to describe floret development and to clarify the timing of the initiation of the floret primordia. In situ hybridization analysis using Wknox1, a wheat knotted1 orthologue, was performed to determine the patterning of meristem formation in the inflorescence.

Key Results

The recessive natural mutation of tetraploid (T. turgidum ssp. turgidum) wheat, branching head (bh), which produces branched inflorescences, was used to demonstrate the utility of Wknox1 as a molecular marker for meristematic tissue. Then an analysis of Wknox1 expression was performed in diploid, tetraploid and hexaploid wheats and heterochronic development of the floret meristems was found among these wheat species.

Conclusions

It is shown that the difference in the number of floret primordia in diploid, tetraploid and hexaploid wheats is caused by the heterochronic initiation of floret meristem development from the spikelet meristem.Key words: Triticum, wheat, inflorescence, spikelet, floret, meristem, heterochrony, heterochronic development, knotted1, polyploidy     

Fine mapping of a major quantitative trait loci, qSSP7, controlling the number of spikelets per panicle as a single Mendelian factor in rice

In our previous studies, one putative QTL affecting number of spikelets per panicle (SPP) was identified in the pericentromeric region of rice chromosome 7 using a recombinant inbred population. In order to define the QTL (qSPP7), RI50, a recombinant inbred line with 70% of genetic background same as the female parent of Zhenshan 97, was selected to produce near-isogenic lines for the target region in the present study. In a BC₂F₂ population consisting of 190 plants, the frequency distribution of SPP was shown to be discontinuous and followed the expected Mendelian ratios (1:2:1 by progeny test) for single locus segregation. qSPP7 was mapped to a 0.4 cM region between SSR marker RM3859 and RFLP marker C39 based on tests of the BC₂F₂ population and its progeny. Its additive and dominant effects on SPP were 51.1 and 24.9 spikelets, respectively. Of great interest, the QTL region also had effects on grain yield per plant (YD), 1,000 grain weight (GW), tillers per plant (TPP) and seed setting ratio (SR). Significant correlations were observed between SPP and YD (r = 0.66) and between SPP and SR (r = −0.29) in the progeny test. 1082 extremely small panicle plants of a BC₃F₂ population containing 8,400 individuals were further used to fine map the QTL. It turns out that qSPP7 co-segregated with two markers, RM5436 and RM5499 spanning a physical distance of 912.4 kb. Overall results suggested that recombination suppression occurred in the region and positional cloning strategy is infeasible for qSPP7 isolation. The higher grain yield of Minghui 63 homozygote as compared to the heterozygote suggested that Minghui 63 homozygote at qSPP7 in hybrid rice could further improve its yield. Y. Z. Xing and W. J. Tang contributed equally to this work.     

Tillering and panicle branching genes in rice

Rice (Oryza sativa L.) is one of the most important staple food crops in the world, and rice tillering and panicle branching are important traits determining grain yield. Since the gene MONOCULM 1 (MOC 1) was first characterized as a key regulator in controlling rice tillering and branching, great progress has been achieved in identifying important genes associated with grain yield, elucidating the genetic basis of yield-related traits. Some of these important genes were shown to be applicable for molecular breeding of high-yielding rice. This review focuses on recent advances, with emphasis on rice tillering and panicle branching genes, and their regulatory networks.     

Brassinosteroids function in spikelet differentiation and degeneration in rice

Brassinosteroids(BRs) play crucial roles in many aspects of plant development. However, their function in spikelet differentiation and degeneration in rice(Oryza sativa L.) remains unclear. Here, we investigated the roles of these phytohormones in spikelet development in fieldgrown rice subjected to five different nitrogen(N)fertilization treatments during panicle differentiation. BR levels and expression of genes involved in BR biosynthesis and signal transduction were measured in spikelets. Pollen fertility and the number of differentiated spikelets were closely associated with 24-epicastasterone(24-epiCS) and28-homobrassinolide(28-homoBL) levels in spikelets.Enhanced BR biosynthesis and signal transduction, in response to N treatment, enhanced spikelet differentiation, reduced spikelet degeneration, and increased grain yield. Increases in proton-pumping ATPase activity, ATPconcentration, energy charge, and antioxidant system(AOS) levels were consistent with 24-epiCS and28-homoBL concentrations. Exogenous application of24-epiCS or 28-homoBL on young panicles induced a marked increase in endogenous 24-epiCS or 28-homoBL levels, energy charge, AOS levels, spikelet differentiation, and panicle weight. The opposite effects were observed following treatment with a BR biosynthesis inhibitor. Our findings indicate that, in rice, BRs mediate the effects of N fertilization on spikelet development and play a role in promoting spikelet development through increasing AOS levels and energy charge during panicle development.     

Identification and characterization of OsEBS,a gene involved in enhanced plant biomass and spikelet number in rice

Common wild rice (Oryza rufipogon Griff.) is an important genetic reservoir for rice improvement. We investigated a quantitative trait locus (QTL), qGP5‐1, which is related to plant height, leaf size and panicle architecture, using a set of introgression lines of O. rufipogon in the background of the Indica cultivar Guichao2 (Oryza sativa L.). We cloned and characterized qGP5‐1 and confirmed that the newly identified gene OsEBS (enhancing biomass and spikelet number) increased plant height, leaf size and spikelet number per panicle, leading to an increase in total grain yield per plant. Our results showed that the increased size of vegetative organs in OsEBS‐expressed plants was enormously caused by increasing cell number. Sequence alignment showed that OsEBS protein contains a region with high similarity to the N‐terminal conserved ATPase domain of Hsp70, but it lacks the C‐terminal regions of the peptide‐binding domain and the C‐terminal lid. More results indicated that OsEBS gene did not have typical characteristics of Hsp70 in this study. Furthermore, Arabidopsis (Arabidopsis thaliana) transformed with OsEBS showed a similar phenotype to OsEBS‐transgenic rice, indicating a conserved function of OsEBS among plant species. Together, we report the cloning and characterization of OsEBS, a new QTL that controls rice biomass and spikelet number, through map‐based cloning, and it may have utility in improving grain yield in rice.     

Studies on the nature of relationships between grain size,spikelet number,grain yield and spikelet filling in late duration varieties of rice (Oryza sativa L.)

Summary Studies on the nature of relationship between grain size (in terms of thousand grain weight), grain number, rate of filling and yield have been worked out with late duration (140–150 days) rice varieties. Grain number per m², grains per panicle and yield showed a negative relationship with grain size. There was no compensatory balance between grain size and grain number within a range of thousand grain weight of 15 to 30 g. Based on the peaks in grain filling activity, the varieties were classified into four groups. There was no relationship between grain size and rate of grain filling thereby suggesting that grain filling is determined by inherent endogenous factors.