Breeding Cycles Expedited by FT‐mediated Reduction in Generation Time

Abstract

A significant fraction of the total time required to develop new cultivars is dedicated to inbreeding, a process where alleles become fixed at the majority of loci across the plant genome to result in inbred lines exhibiting a high degree of phenotypic uniformity. After an initial cross is made between two or more lines, at least three generations of inbreeding are typically conducted in segregating populations to develop new lines that are subsequently evaluated for economically important traits (such as yield) in replicated field testing. Opportunity for selection, which increases the frequency of desirable alleles in succeeding generations, is dependent on the traits of interest and the particular breeding methodology being utilized.The time required to develop new inbred lines and to complete a breeding cycle is dependent on generation time (duration from seed to seed) and the potential to carry out multiple generations of inbreeding per year. When multiple breeding cycles are taken into consideration, this duration affects the potential rate of genetic gain per year (Fehr, 1987). A number of techniques have been described to reduce the time needed to develop inbred lines. For example, the single-seed descent (SSD) method of breeding (Brim, 1966) is used by breeders of some crops to advance multiple generations per year through utilization of greenhouse or off-season nursery facilities. Doubled-haploid procedures that can generate completely inbred lines in only a few steps have also been developed for some ABSTRACT Methods for rapid inbreeding are desirable for reducing the time required to complete a breeding cycle and to increase genetic gain per year. Constitutive expression of FLOWERING LOCUS T (FT) derived from Arabidopsis has previously been shown to reduce generation time in tobacco (Nicotiana tabacum L.) and other crop plants. Here, we used tobacco as a model system to investigate the utility of a modified single-seed descent (SSD) breeding method where transgenic expression of 35S:FT was used to reduce generation time during inbreeding. The transgene was maintained in hemizygous condition during inbreeding, and null segregants were isolated in the F 4:5 generation to create nontransgenic F 5:6 lines that were produced in nearly half the time that would have been required using conventional SSD. opportunities for selection among 35S:FT plants during inbreeding were demonstrated by selecting for quantitative levels of resistance to the soil-borne pathogen Phytophthora nicotianae. populations of selected lines exhibited significantly higher levels of resistance relative to random lines, greater frequencies of highly resistant genotypes, and lower frequencies of highly susceptible genotypes. The system outlined here could lead to more rapid commercialization of improved cultivars in crop species amenable to manipulation of flowering time via transgenic expression of FT-like genes. Strategies to enhance the frequency of favorable alleles in resulting populations can be incorporated into the method.

Rapid generation cycling of an F2 population derived from a cross between Lens culinaris Medik. and Lens ervoides (Brign.) Grande after aphanomyces root rot selection

“…Since then, numerous authors have published methods to shorten the generation cycles for, for example, Brassica (Williams & Hill, ), pea (Mobini & Warkentin, ; Ribalta et al., ), wheat (De Pauw & Clarke, ) or rice (Janwan, Sreewongchai, & Sripichitt, ). Eickholt and Lewis () reduced regeneration times through the introduction of a constitutively expressed flowering locus. Other protocols use an in vitro step in which immature seeds are dissected out and cultured on artificial media such as for field pea (Ochatt et al., ), tomato (Bhattarai, de la Pena, Midmore, & Palchamy, ) or sunflower (Jambhulkar, ).…”