Author: Ekatpure Sachin Chandrakant
Introduction:
In so many crops, use of conventional breeding practices are difficult to complete because the crops having long juvenile periods that requires long breeding cycles, Due to photoperiod and vernalization requirements it is very difficult to match the flowering of all plants at a time. Also plants those are propagated asexually have limited flowering potential. Due to these limitations, and there is need to develop the crop varieties resistant to various factors in the environment, like climate change, decreasing water resources and invasive biotic stresses. For these reasons virus induced flowering (VIF) method could be used to accelerate the transition to reproductive growth and thus smooth the progress of research and breeding (McGarry et al., 2016).
Regulated flowering delays the breeding programme:
Production of necessary gametes for fertile flower is the ultimate goal of the plant breeding and hence, obtaining such flowers in a synchronized manner and in a practical period of time can be challenging. For initiation of flowering requires the coordination of developmental and environmental signals. There are number of plants experience an extended juvenile phase, before being capable of undergoing to reproductive growth, and this can drastically delay development of new lines with favored traits. The most classic example of flowering after the juvenile phase in fruit and nut trees are, almond, cherry, and peach requiring 3 to 5 years, in citrus trees takes 5 to 10 years, and apples and pear takes 6 to 12 years; so developing new cultivars can span decades. Due to breeding program for fruit and nut trees with superior fruit traits, disease resistance and post-harvest physiology are adopting advanced techniques such as marker assisted breeding and genome wide association mapping to make these long breeding cycles as efficient as possible (Hardner et al., 2016). With rapid flowering we can accelerate the conventional breeding and helps to realize benefits from genomic selection methods. Vernalization, is the process of prolonged chilling to stimulate flowering, can also constrain when and where flowering occurs, so breeder cannot relied on this method. Photoperiodism also poses a challenge for breeders.
How breeding can be accelerated?
During breeding programme, the transition to reproductive growth is being accelerated through the ectopic expression of FLOWERING LOCUS T (FT), which codes for the long distance flowering signal toflorigen. The FT signal is well studied in Arabidopsis thaliana (AtFT) and enormous research across diverse plant species show that FT and FT-like genes coordinate the environmental and endogenous signals governing the transition to reproductive growth. TERMINAL FLOWER1 (TFL1) is in the same gene family like FT, but TFL1 (and homologs) acts as a competitive inhibitor for FT. Therefore, either FT gain of function or TFL1 loss of function results in the more determinate growth and faster transition towards reproductive growth. Many other genes in the flowering pathway are similarly used as breeding tools to accelerate flowering; for example, apple transformed with a flowering-associated MADS-box gene from silver birch, BdMADS4, demonstrates precocious flowering and permits one breeding cycle per year. The use of transgenic lines with precocious flowering is generally called rapid cycling or “FasTrack” breeding, and is usually together with marker assisted selection (MAS) for desired traits.
However, accelerated flowering through transgene overexpression can be problematic for a large number of reasons. Standard transformation methods are time consuming and require skilled labor; they are inefficient in many species and are usually applicable in a limited number of genetic backgrounds; and many important crop varieties and species remain recalcitrant. The restriction of transformation to a small number of genetic backgrounds can have large impacts on breeding progress. In addition, though Somaclonal variation is a clear breeding method in many plant species, it can introduce unwanted genetic and epigenetic changes. Somaclonal variation is commonly seen in conventional as well as transgenic in vitro regeneration methods. In addition, graft transmission of FT-induced transgenic flowering is difficult.
Virus-induced flowering method:
As a best alternative method to stable transformation is use of viral vectors to deliver FT orthologs to different crop plants to bring determinate growth patterns and precocious flowering. This approach makes sense in the FT gene product, florigen, is phloem-mobile compound and naturally identify its way into apices to influence meristem, and viruses use the phloem as a pathway to establish systemic infections. Therefore, the coupling an FT orthologs with a virus-based vector that can amplify the inserted sequence and move it systemically will promote flowering. This method is an allusion to the popular technique of virus-induced gene silencing (VIGS), the use of a virus vectors to deliver sequences that promote flowering was termed virus-induced flowering (VIF). Because virus infection progresses through whole plants, techniques for transformation in sterile tissue culture do not need to be developed, and there is no risk of somaclonal variation.
The first demonstration of VIF was used in Zucchini yellow mosaic virus to deliver FT to cucurbits, stimulating flowering in short-day melon under non-inductive long days. This early work demonstrated that FT encoded florigenic properties, and this laid down the foundation for VIF as a breeding tool. In the citrus breeding program, Citrus leaf blotch virus (CLBV) was used to deliver AtFT and CiFT (the FT ortholog from Valencia oranges, Citrus sinesis), and precocious flowering occurred within 4-6 months instead of the typical > 6 years.
Conclusion:
Many breeding programs already based on the genome assisted approaches, like genome wide selection and marker assisted selection, to increase breeding effectiveness VIF is a promising new tool for increase research and breeding in crop species that show late flowering or are difficult to breed. This technique has potential to overcome the number of problems of floral induction through horticultural and transgenic way. However there is need to additional basic research on strain specificity and sexual transmission, also there is need to fully understand its potential and limitations.
References:
- McGarry, R. C., Klocko, A. L., Pang, M., Strauss, S. H. and Ayre, B. G. 2016. Virus induced flowering: An application of reproductive biology to benefit plant research and breeding. Plant Physiol. Preview. 10.1104. pp. 16.
- Hardner, C. M., Evans, K., Brien, C., Bliss, F. and Peace, C. 2016. Genetic architecture of apple fruit quality traits following storage and implications for genetic improvement. Tree Genet. Genomes 12: 1-21.
About Author / Additional Info:
PhD research scholar (Plant biotechnology), interested in genetic engineering and molecular biology studies