Genetic interactions involved in the inhibition of heading by heading date QTL, <Emphasis Type="Italic">Hd2</Emphasis> in rice under long-day conditions

Heading date is the one of the most important traits in rice breeding, because it defines where rice can be cultivated and influences the expression of various agronomic traits. To examine the inhibition of heading by Heading date 2 (Hd2), previously detected on the distal end of chromosome 7’s long arm by quantitative trait locus (QTL) analysis, we developed backcross inbred lines (BILs) from Koshihikari, a leading Japanese cultivar, and Hayamasari, an extremely early heading cultivar. The BILs were cultivated under natural field conditions in Tsukuba Japan, and under long-day (14.5 h), extremely long-day (18 h), and short-day (10 h) conditions. Combinations of several QTLs near Hd1, Hd2, Ghd7, Hd5, and Hd16 were detected under these four conditions. Analysis of advanced backcross progenies revealed genetic interactions between Hd2 and Hd16 and between Hd2 and Ghd7. In the homozygous Koshihikari genetic background at Hd16, inhibition of heading by the Koshihikari allele at Hd2 was smaller than that with the Hayamasari Hd16 allele. Similarly, in the homozygous Koshihikari genetic background at Ghd7, the difference in heading date caused by different alleles at Hd2 was smaller than in plants homozygous for the Hayamasari Ghd7 allele. Based on these results, we conclude that Hd2 and its genetic interactions play an important role in controlling heading under long-day conditions. In addition, QTLs near Hd2, Hd16, and Ghd7, which are involved in inhibition of heading under long-day conditions, function in the same pathway that controls heading date.     

Mapping of a thermo-sensitive earliness <Emphasis Type="Italic">per se</Emphasis> gene on <Emphasis Type="Italic">Triticum monococcum</Emphasis> chromosome 1A<Superscript>m</Superscript>

An earliness per se gene, designated Eps-A^m1, was mapped in diploid wheat in F₂ and single-seed descent mapping populations from the cross between cultivated (DV92) and wild (G3116) Triticum monococcum accessions. A QTL with a peak on RFLP loci Xcdo393 and Xwg241, the most distal markers on the long arm of chromosome 1A^m, explained 47% of the variation in heading date (LOD score 8.3). Progeny tests for the two F_2:3 families with critical recombination events between Xcdo393 and Xwg241 showed that the gene was distal to Xcdo393 and linked to Xwg241. Progeny tests and replicated experiments with line #3 suggested that Eps-A^m1 was distal to Xwg241. This gene showed a large effect on heading date in the controlled environment experiments, and a smaller, but significant, effect under natural conditions. Eps-A^m1 showed significant epistatic interactions with photoperiod and vernalization treatments, suggesting that the different classes of genes affecting heading date interact as part of a complex network that controls the timing of flowering induction. Besides its interactions with other genes affecting heading date, Eps-A^m1 showed a significant interaction with temperature. The effect of temperature was larger in plants carrying the DV92 allele for late flowering than in those carrying the G3116 allele for early flowering. Average differences in heading date between the experiments performed at 16 °C and 23 °C were approximately 11 days (P < 0.001) for the lines carrying the Eps-A^m1 allele for early flowering but approximately 50 days (P < 0.0001) for the lines carrying the allele for late flowering. The large differences in heading time (average 80 days) observed between plants carrying the G3116 and DV92 alleles when grown at 16 °C, suggest that it would be possible to produce very detailed maps for this gene to facilitate its future positional cloning.     

Fine mapping of <Emphasis Type="Italic">qSTV11</Emphasis><Superscript><Emphasis Type="Italic">KAS</Emphasis></Superscript>, a major QTL for rice stripe disease resistance

Rice stripe disease, caused by rice stripe virus (RSV), is one of the most serious diseases in temperate rice-growing areas. In the present study, we performed quantitative trait locus (QTL) analysis for RSV resistance using 98 backcross inbred lines derived from the cross between the highly resistant variety, Kasalath, and the highly susceptible variety, Nipponbare. Under artificial inoculation in the greenhouse, two QTLs for RSV resistance, designated qSTV7 and qSTV11 ^KAS , were detected on chromosomes 7 and 11 respectively, whereas only one QTL was detected in the same location of chromosome 11 under natural inoculation in the field. The stability of qSTV11 ^KAS was validated using 39 established chromosome segment substitution lines. Fine mapping of qSTV11 ^KAS was carried out using 372 BC₃F_2:3 recombinants and 399 BC₃F_3:4 lines selected from 7,018 BC₃F₂ plants of the cross SL-234/Koshihikari. The qSTV11 ^KAS was localized to a 39.2 kb region containing seven annotated genes. The most likely candidate gene, LOC_Os11g30910, is predicted to encode a sulfotransferase domain-containing protein. The predicted protein encoded by the Kasalath allele differs from Nipponbare by a single amino acid substitution and the deletion of two amino acids within the sulfotransferase domain. Marker-resistance association analysis revealed that the markers L104-155 bp and R48-194 bp were highly correlated with RSV resistance in the 148 landrace varieties. These results provide a basis for the cloning of qSTV11 ^KAS , and the markers may be used for molecular breeding of RSV resistant rice varieties.     

Characterization and detection of epistatic interactions of 3 QTLs, Hd1, Hd2, and Hd3, controlling heading date in rice using nearly isogenic lines

To characterize quantitative trait loci (QTLs), we used marker-assisted selection (MAS) to develop three nearly isogenic lines (NILs) differing only for the presence of a single, specific QTL (QTL-NILs) –Hd1, Hd2, and Hd3 – for heading date in rice. The three lines contained the chromosomal region of the target QTL from donor variety Kasalath(indica) in the genetic background of var. Nipponbare (japonica). To analyze epistatic interactions in pairs of these QTLs, we also used MAS to develop four combined QTL-NILs with 2 of the 3 QTLs or with all 3. Different daylength treatment testing of the QTL-NILs revealed that the three QTLs control photoperiod sensitivity. Genetic analysis of F₂ populations derived from crosses between the three QTL-NILs with a single QTL using molecular markers revealed the existence of epistatic interactions between Hd1 and Hd2, and Hd2 and Hd3. These interactions were also confirmed by the analysis of combined QTL-NILs under different daylength conditions. The existence of an epistatic interaction between Hd1 and Hd3 was also clarified. Based on these results, we suggest that the Kasalath allele of Hd3 does not affect photoperiod sensitivity by itself but that it is involved in enhancement of the expression of the Nipponbare alleles of Hd1 and Hd2. Received: 22 October 1999 / Accepted: 21 March 2000     

Synteny between a major heading-date QTL in perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.) and the<Emphasis Type="Italic"> Hd3</Emphasis> heading-date locus in rice

The genetic control of induction to flowering has been studied extensively in both model and crop species because of its fundamental biological and economic significance. An ultimate aim of many of these studies has been the application of the understanding of control of flowering that can be gained from the study of model species, to the improvement of crop species. The present study identifies a region of genetic synteny between rice and Lolium perenne, which contains the Hd3 heading-date QTL in rice and a major QTL, accounting for up to 70% of the variance associated with heading date in L. perenne. The identification of synteny between rice and L. perenne in this region demonstrates the direct applicability of the rice genome to the understanding of biological processes in other species. Specifically, this syntenic relationship will greatly facilitate the genetic dissection of aspects of heading-date induction by enabling the magnitude of the genetic component of the heading-date QTL in L. perenne to be combined with the sequencing and annotation information from the rice genome.Communicated by Q. Zhang     

<Emphasis Type="Italic">Trichoderma harzianum</Emphasis>: a biocontrol agent against <Emphasis Type="Italic">Bipolaris oryzae</Emphasis>

Rice brown spot, caused by Bipolaris oryzae, can be a serious disease causing a considerable yield loss. Trichoderma harzianum is an effective biocontrol agent for a number of plant fungal diseases. Thus, this research was carried out to investigate the mechanisms of action by which T. harzianum antagonizes Bipolaris oryzae in vitro, and the efficacy of spray application of a spore suspension of T. harzianum for control of rice brown spot disease under field conditions. In vitro, the antagonistic behavior of T. harzianum resulted in the overgrowth of B. oryzae by T. harzianum, while the␣antifungal metabolites of T.␣harzianum completely prevented the linear growth of B. oryzae. Light and scanning electron microscope (SEM) observations showed no evidence that mycoparasitism contributed to the aggressive nature of the tested isolate of T. harzianum against B. oryzae. Under field conditions, spraying of a spore suspension of T. harzianum at 10⁸ spore ml⁻¹ significantly reduced the disease severity (DS) and disease incidence (DI) on the plant leaves, and also significantly increased the grain yield, total grain carbohydrate, and protein, and led to a significant increase in the total photosynthetic pigments (chlorophyll a and b and carotenoids) in rice leaves.     

Brassinosteroid (BR) biosynthetic gene <Emphasis Type="Italic">lhdd10</Emphasis> controls late heading and plant height in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Key message

A Brd2 allele suppresses heading date by altering the expression of heading date regulators such as OsMADS50 , and also negatively regulates chlorophyll biosynthesis.

Abstract

Heading date and plant height are important determinants of yield in rice (Oryza sativa L.). In this study, we characterized a late heading, dwarf mutant known as lhdd10 selected following ethyl methane sulfonate (EMS)-treatment of ssp. indica cultivar 93-11. lhdd10 showed late heading, dwarfness and slightly darker-green leaves than wild-type 93-11 under long-day and short-day conditions. We isolated lhdd10 by map-based cloning; it encoded a putative FAD-linked oxidoreductase protein (a brassinosteroid biosynthetic gene) that localized to the nucleus. LHDD10 was constitutively expressed in various tissues, but more so in shoot apices and panicles. Our data showed that lhdd10 influences heading date by controlling the expression of heading date regulators, such as OsMADS50 in both LD and SD conditions. lhdd10 also negatively regulated expression of chlorophyll biosynthetic genes to reduce the chlorophyll content. Our data indicated that BRs play important roles in regulating heading date and chlorophyll biosynthesis. This work provides material that will allow study of how BRs regulate heading date in rice.

     

Overexpression of <Emphasis Type="Italic">AP1-</Emphasis>like genes from <Emphasis Type="Italic">Asteraceae</Emphasis> induces early-flowering in transgenic <Emphasis Type="Italic">Chrysanthemum</Emphasis> plants

Chrysanthemum is one of the most important commercial cut flowers in the world. Early-flowering cultivars are required to produce quality chrysanthemum flowers with a lower cost of production. To shorten the vegetative growth phase of chrysanthemum, three AP1-like genes from Asteraceae were constitutively overexpressed in 80 independent transgenic chrysanthemum lines. All lines were characterized by PCR and RT-PCR and demonstrated that overexpression of compositae AP1-homologs in transgenic chrysanthemum under long-day conditions had no effect on plant development compared to non-transgenic controls. Conversely, under short-day conditions, transgenic plants commenced bud initiation 2 wk earlier than non-transgenic chrysanthemum plants. Subsequently, transgenic chrysanthemum flowers showed color earlier and resulted in full opening of inflorescences 3 wk prior to non-transgenic control plants. These results open new possibilities for genetic improvement and breeding of chrysanthemum cultivars.     

A new <Emphasis Type="Italic">Em</Emphasis>-like protein from <Emphasis Type="Italic">Lactuca sativa</Emphasis>, <Emphasis Type="Italic">LsEm1</Emphasis>, enhances drought and salt stress tolerance in <Emphasis Type="Italic">Escherichia coli</Emphasis> and rice

Late embryogenesis abundant (LEA) proteins are closely related to abiotic stress tolerance of plants. In the present study, we identified a novel Em-like gene from lettuce, termed LsEm1, which could be classified into group 1 LEA proteins, and shared high homology with Cynara cardunculus Em protein. The LsEm1 protein contained three different 20-mer conserved elements (C-element, N-element, and M-element) in the C-termini, N-termini, and middle-region, respectively. The LsEm1 mRNAs were accumulated in all examined tissues during the flowering and mature stages, with a little accumulation in the roots and leaves during the seedling stage. Furthermore, the LsEm1 gene was also expressed in response to salt, dehydration, abscisic acid (ABA), and cold stresses in young seedlings. The LsEm1 protein could effectively reduce damage to the lactate dehydrogenase (LDH) and protect LDH activity under desiccation and salt treatments. The Escherichia coli cells overexpressing the LsEm1 gene showed a growth advantage over the control under drought and salt stresses. Moreover, LsEm1-overexpressing rice seeds were relatively sensitive to exogenously applied ABA, suggesting that the LsEm1 gene might depend on an ABA signaling pathway in response to environmental stresses. The transgenic rice plants overexpressing the LsEm1 gene showed higher tolerance to drought and salt stresses than did wild-type (WT) plants on the basis of the germination performances, higher survival rates, higher chlorophyll content, more accumulation of soluble sugar, lower relative electrolyte leakage, and higher superoxide dismutase activity under stress conditions. The LsEm1-overexpressing rice lines also showed less yield loss compared with WT rice under stress conditions. Furthermore, the LsEm1 gene had a positive effect on the expression of the OsCDPK9, OsCDPK13, OsCDPK15, OsCDPK25, and rab21 (rab16a) genes in transgenic rice under drought and salt stress conditions, implying that overexpression of these genes may be involved in the enhanced drought and salt tolerance of transgenic rice. Thus, this work paves the way for improvement in tolerance of crops by genetic engineering breeding.     

Clearance rates in the Arctic bivalves <Emphasis Type="Italic">Hiatella arctica</Emphasis> and <Emphasis Type="Italic">Mya </Emphasis>sp.

Filtration was studied in two Arctic clams, Hiatella arctica and Mya sp., collected in Young Sound, Northeast Greenland. Clearance rates were determined as a function of ambient temperature and algal cell concentration, using the clearance method and feeding with a unicellular flagellate. For both species, clearance rates increased with increasing temperature from <у up to 4-8°C. At higher temperatures, filtration ceased and the clams closed their valves. Clearance rates were also determined in temperate specimens of H. arctica collected on the west coast of Sweden. For these specimens, clearance rates increased with increasing temperature from 0 to 18-20°C. When weight-specific clearance rates were compared between the two populations and between species, there were no differences at 1°C. Clearance rates in Arctic H. arctica were maximal at algal cell concentrations corresponding to 2.5-8 µg chlorophyll a l^-1. Temperature compensation in Arctic bivalves is discussed and it is concluded that adaptations to constant low temperatures consist of a lower minimum temperature, for active filtration. Low clearance rates due to low temperatures did not seem to limit growth, under the prevailing conditions in Young Sound.     

Hd1, Ghd7, and DTH8 synergistically determine the rice heading date and yield-related agronomic traits

Heading date determines the seasonal and regional adaptation of rice(Oryza sativa L.) varieties and is mainly controlled by photoperiod sensitivity(PS). The core heading date genes Hd1, Ghd7, DTH8, and PRR37 act synergistically in regulating the PS. In this study, we systematically analyze the heading date,PS, and agronomic traits of eight homozygous lines with various combinations of Hd1, Ghd7, and DTH8 alleles in the prr37 background under long-day(LD) and short-day(SD) conditions, respectivel...     

Complete sequence of <Emphasis Type="Italic">Tvv1</Emphasis>, a family of Ty<Emphasis Type="Italic">1 copia</Emphasis>-like retrotransposons of <Emphasis Type="Italic">Vitis vinifera</Emphasis> L., reconstituted by chromosome walking

A chromosome-walking strategy was used to sequence and characterize retrotransposons in the grapevine genome. The reconstitution of a family of retroelements, named Tvv1, was achieved by six successive steps. These elements share a single, highly conserved open reading frame 4,153 nucleotides-long, putatively encoding the gag, pro, int, rt and rh proteins. Comparison of the Tvv1 open reading frame coding potential with those of drosophila copia and tobacco Tnt1, revealed that Tvv1 is closely related to Ty1 copia-like retrotransposons. A highly variable untranslated leader region, upstream of the open reading frame, allowed us to differentiate Tvv1 variants, which represent a family of at least 28 copies, in varying sizes. This internal region is flanked by two long terminal repeats in direct orientation, sized between 149 and 157 bp. Among elements theoretically sized from 4,970 to 5,550 bp, we describe the full-length sequence of a reference element Tvv1-1, 5,343 nucleotides-long. The full-length sequence of Tvv1-1 compared to pea PDR1 shows a 53.3% identity. In addition, both elements contain long terminal repeats of nearly the same size in which the U5 region could be entirely absent. Therefore, we assume that Tvv1 and PDR1 could constitute a particular class of short LTRs retroelements.     

Multiple paternity in <Emphasis Type="Italic">Rhipicephalus</Emphasis> (<Emphasis Type="Italic">Boophilus</Emphasis>) <Emphasis Type="Italic">microplus</Emphasis> confirmed by microsatellite analysis

The aim of this study was to determine if individual ticks among the progeny of a single female Rhipicephalus (Boophilus) microplus tick removed from cattle under natural conditions are the result of mating with one or several males. To this end, simulations were run using an existing dataset of genotypes from 8 microsatellite loci to predict the number of samples required and the best locus. Subsequently, 14–22 progeny from each of 15 engorged female ticks removed from three cows, and the engorged females themselves, were genotyped for the BmM1 locus and the minimum number of potential male parents was determined for each progeny group. Of the 15 progeny groups, 10 must have been sired by more than one male, as indicated by the presence of five unique alleles among the progeny or three unique alleles that could not have been contributed by the female. This finding demonstrates multiple paternity in R. microplus.