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《分子植物育种》 ISSN:1672-416X CN: 46-1068/S 2007年第5卷第2期第156至158页 |
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| Applied Plant Genomics:From Genome to Field |
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| Using Genomic Information for Altering Bolting and Flowering Behaviour of Crop Plants |
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C. Jung1*
W. Qian 1
B. Büttner 1
U. Hohmann 1
E. Mutasa-Gottgens2
T. Chia 2
A. Müller1
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1 Plant Breeding Institute, Christian-Albrechts-University of Kiel, Olshausenstr, 40, Kiel, D-24098, Germany; 2 Broom’s Barn Research Station, Higham, Bury St Edmunds, UK
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摘要
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Altering the phenological development of a plant is a major target of many plant breeding programs and can be exploited to achieve different outcomes. For example, in seed crops, the transition from the vegetative to the reproductive phase (lowering) should be as early as possible if unfavourable environmental conditions (drought, heat, cold) endanger seed harvest. By contrast, delayed lowering favours higher biomass production and increased yield potential by extending the grain illing phase. Flowering time is also important in crops where only vegetative parts of the plant are harvested because shoot elongation and lowering would substantially reduce yields. Floral transition is a major developmental switch that is tightly controlled by a network of proteins that perceive and integrate developmental and environmental signals to promote or inhibit the transition to lowering. In model species like Arabidopsis thaliana and Antirrhinum majus, many of the key genes have been identiied and functionally characterized (Putterill et al., 2004; He and Amasino, 2005; Baurle and Dean, 2006). Plant genome and EST sequencing projects are beginning to unveil the presence and evolutionary conservation of these genes in cultivated species (Albert et al., 2005; Hecht et al., 2005), including ornamental plants and trees (Bohlenius et al., 2006). In plant breeding, these sequences can be used in two different applications, (i) marker assisted selection (MAS) and (ii) genetic modiication. MAS enables rapid identiication of plants with favourable characters and accelerates introgression of exotic plant material into breeding programs, speeding up adaptation to local environmental conditions like temperature and day length. Another example for the potential of MAS application can be seen in biennial crop species where only vegetative organs are harvested. In these crops, lowering should be avoided by the use of cultivars that lack major genes for annuality, but early lowering is still induced under unfavourable environmental conditions due to the presence of minor genes which can be selected against by MAS. Genetic manipulation of key regulators of loral transition has been successfully applied to both speed up and delay lowering in crop plants. For example, different MADS box genes, LFY and FT homologs have been used to accelerate lowering in oilseed rape (Brassica napus) (Chandler et al., 2005), rice (He et al., 2000) and apple (Kotoda et al., 2003 ), while TFL homologs have been used to delay or abolish lowering in the forage grass Festuca rubra (Jensen et al., 2004). The latter provides opportunities for breeding non-lowering forage grasses with superior digestibility. In addition to the naturally occurring genes, new allelic variants for both loral transition and vernalization genes are also expected from TILLING programs which have been launched for a number of crop species (Slade and Knauf, 2005). The full potential for exploiting genomic data emerging from crop species will be realised with improved knowledge of the expression and function of desired target genes in the individual crops. To this end, we are studying loral transition genes in sugar beet and oilseed rape. In oilseed rape, we wish to improve the utility of a hybrid breeding system by selecting for early lowering. Here, distantly-related Chinese pollinators have been employed however, these lines lower late under European growth conditions. A backcrossing program has been initiated to select lines with high combining ability and earliness. We plan to use sequences from different lowering gene homologs as functional markers to select for earliness. One candidate gene was identiied that exhibits sequence variation between spring and winter material, with two variants being found preferentially in one ecotype or the other, respectively. In sugar beet (Beta vulgaris L.), bolting and flowering is undesirable because it drastically reduces root yieldand interferes with harvest operations. Traditionally, sugar beet in moderate climates is sown in spring and harvested in the autumn. An extension of the growing season by sowing in the autumn (i.e., one year before harvest) and cultivation over winter is expected to increase yield substantially assuming adequate disease control (Jaggard and Werker, 1999). However, winter cultivation in central Europe is currently not possible because vernalization would result in bolting and yield loss. To develop non-bolting winter beets, but also allow for induction of flowering for seed production, full control over the timing of floral transition is mandatory. This is difficult or impossible to achieve by traditional breeding, but may be accomplished by targeted genetic modification of flowering time genes and their induction requirements. We aim to transfer, through comparative genetic and genomic approaches, the extensive knowledge of flowering time control in model species to sugar beet, and to exploit this knowledge for the development of novel, high-yielding cultivars. In B. vulgaris, the genes and pathways that regulate floral transition are largely unknown. The tendency forearly bolting (without a requirement for vernalization) is under the control of a single dominant gene termed B and requires appropriate photoperiodic and developmental conditions. The B gene is currently being cloned from its position on chromosome 2 (El-Mezawy et al., 2002; Hohmann et al., 2003; Gaafar et al., 2005). Commercial sugar beet cultivars do not contain a functional B allele and behave as biennials. In the absence of B, induction and timing of flowering depends on vernalization in addition to photoperiod and physiological development. To date, there are no published reports on the identification and characterization of the regulatory genes involved, with one recent exception (Reeves et al., 2006). Despite this, these genes are prime candidates for targeted genetic approaches to suppress, or induce, flowering under controlled conditions. To identify floral transition gene candidates in sugar beet, we performed TBLASTN-based sequence similarity searches of the public sugar beet EST database (BvGI 1.0, www.tigr.org/tigr-scripts/tgi/T_index.cgi?species=beet) with more than 20 flowering time control genes from A. thaliana (Putterill et al., 2004; He and Amasino, 2005). Several ESTs were identified with high levels of homology that is not restricted to known conserved domains, and included homologs of vernalization, photoperiod, and autonomous pathway genes (e.g., VIN3, GI, CO, FVE, FLK). Subsequent identification of corresponding genomic sequences by BAC library screening, restriction fragment fingerprinting and partial sequencing of the genic regionof- interest in representative BACs confirmed sequence homology to the respective flowering time genes. Exon-intron structure was found to be largely conserved between homologs. Importantly, for a subset of genes, sequence and genomic DNA gel blot analysis also revealed the presence of multiple gene family members, thus highlighting the need for genome-wide approaches to enable informed selection of candidate genes for targeted genetic modification of flowering time. To identify the most suitable target genes for the development of winter cultivars, we aim to complement the homology-based approach by a) genome-wide expression profiling, b) functional characterization by RNA interference and overexpression in transgenic plants, c) systematic screening for new allelic variants by TILLING, and d) phenotyping for altered flowering time and bolting resistance. This strategy has so far revealed at least seven CO-like genes in sugar beet (BvCOL-1 to BvCOL-7) amongst which BvCOL-1 is currently the best ortholog of AtCO and has been demonstrated to partially complement the A. thaliana co-2 mutation and to restore AtFT transcription to wild-type levels. In sugar beet rosette leaves, BvCOL-1 transcription is under diurnal regulation in both annual and biennial types although unlike AtCO, which peaks at the end of the light period, BvCOL-1 peaks at the beginning of the light period. Interestingly, transcription is repressed by vernalization and maintained in this state post vernalization until flowering where it remains low in floral tissues. The search for TILLING mutations has not yet revealed functional allelic variants of BvCOL-1 but we are also using transgenic manipulation in sugar beet to further characterise the function of BvCOL-1 in both annual and biennial types In conclusion, our research is intended to provide, in the mid- to longer term, a new genetic tool kit for adaptation of lowering time to speciic environmental conditions, with particular regard to winter cultivation of sugar beet. Genetic modiication of lowering time control and vernalization requirement are also desirable in other cultivated species, e.g. cereals and grain legumes (for drought escape), fodder grasses (for suppression of lowering), and tree species (for acceleration of breeding and research), and more generally for the regional climatic adaptation of elite germplasm. Finally, in the long term, knowledge of loral transition in cultivated species may help to devise strategies for adaptation of crops to the increasingly unpredictable future climate, and to mitigate the adverse effects of climatic change on lowering time.
关键词：
Flowering time control
Beta vulgaris
Brassica napus
Bolting
Altering lowering time
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| DOI:10.3969/mpb.5.2.0156.158 |
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English |
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通讯作者：C. Jung c.jung plantbreeding.uni-kiel.de
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