**ALSV Vector Substantially Shortens Generation Time of Horticultural Plants Horticultural Plants**

**ALSV Vector Substantially Shortens Generation Time of** 

DOI: 10.5772/intechopen.70317

Ichiro Kasajima, Chunjiang Li, Noriko Yamagishi and Nobuyuki Yoshikawa Noriko Yamagishi and Nobuyuki Yoshikawa Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70317

Ichiro Kasajima, Chunjiang Li,

#### **Abstract**

Flowering of plants is tightly regulated by both plant maturity and seasons in the year. Now that the *Flowering Locus T* (*FT*) gene has been revealed to encode the flowering hormone florigen, researchers are seeking to regulate and modify flowering behaviours by using florigen as a genetic tool. In place of transgenic approaches, *Apple latent spherical virus* (ALSV) vector was successful in promoting flowering of both model plants (Arabidopsis and tobacco), and fruit trees (e.g. apple, pear, and loquat), vegetables (e.g. tomato and cucumber), legumes (e.g. soybean), and ornamental flowers (e.g. petunia, Japanese gentian and Eustoma). In so doing, *FT* was expressed and/or *TFL1* was suppressed by the ALSV vector. ALSV is a latent (non-pathogenic) virus isolated from an apple tree. After induction of flowering and seed production in crops, ALSV is not transferred to most of the next-generation seedlings, or it can be artificially removed from the infected plant by incubation at high temperature. Thus, the generation times of horticultural plants are approximately halved, and the generation time of apple plants is substantially shortened to within one year. Hence, ALSV technology is expected to be useful as a part of New Plant Breeding Techniques (NPBT) for agricultural application.

**Keywords:** early flowering, florigen, FT, generation time, virus vector

#### **1. Introduction**

Breeding of horticultural plants can take several years or more. Plant cultivars are most frequently generated by crossing between different cultivars to combine various advantageous traits together, such as fruit/flower quality, pest tolerance and vigorous growth habit. Cultivars are often crossed with each other repeatedly to generate and fix such favourable

traits. One generation time (from germination, flowering, seed set and germination of the next-generation seedling) is usually several months in herbaceous plants and several years in trees under field conditions [1]. Thus, crossing of herbaceous vegetables and ornamental flowers can be performed every year, while breeding of fruit trees may only be performed once every several years. If flowering could be accelerated in horticultural plants, we would be able to cross vegetables and flowers many times each year and cross fruit trees every year.

In order to cross vegetables several times in a year, problems related to the seasonality and day-length sensitivity of plant flowering also need to be addressed. Plants are either shortday, long-day or day-neutral (an aspect of the photoperiodism, the response to day length). Short-day plants flower in autumn and long-day plants flower in spring. Day-neutral plants flower in any growing season. Day length is now controlled by artificial lighting, but biotechnology to accelerate flowering will also solve the problem of day-length sensitivity without the need for regulating day length using lighting equipment. Alternatively, daylength sensitivity is also one of the important traits of horticultural plants. Ornamental flowers are harvested only when the plants set flowers. Fruits can be harvested only after the flowering seasons. Because of stable responses of horticultural plants to day length, farmers make great efforts to control time of flowering. Such artificial controls are possible only with lighting equipment, but the majority of crops are produced using sunlight alone. Gene modification by biotechnology such that a series of cultivars with different daylength responses are prepared for major horticultural cultivars will thus benefit agricultural production.

Basic studies using model plants have revealed the genes controlling flowering time in higher plants. After a functional gene is isolated in model plants, that gene and its homologs are often found to exert the same function in other plant species. This means that the flowering of vegetables, ornamental flowers and fruit trees can be controlled by this gene [2]. The name of the gene controlling plant flowering is '*Flowering locus T*' (*FT*). Initially, plant biologists aimed to express the *FT* gene in crops by transformation, which actually accelerated flowering [3–7]. This suggested the functional use of *FT* in controlling flowering time, but transgenic plants are not typically applicable to crop production. Plant biologists then attempted nontransgenic expression of *FT* in crops. To our knowledge, there have been no scientific papers demonstrating the control of flowering time by application of FT protein, *FT* DNA or *FT* RNA without genetic transformations or spontaneous mutations in plant genomic DNA. We used a virus vector derived from *Apple latent spherical virus* (ALSV) to deriver *FT* RNA and express FT protein in plant tissues. This system turned out to be very successful in promoting flowering in horticultural plants. This chapter explains in detail the characteristics of the ALSV vector, activities of *FT* and its related genes, and how the ALSV vector is used to promote flowering.
