Agriculture Reference
In-Depth Information
horticulture occurred in the latter half of the twentieth century following the work
by scientists like Murashige (
1964
) and Nitsch and Nitsch (
1956
). Plant tissue cul-
ture was enthusiastically embraced as a method for clonal propagation in the 1960s;
however, genetic instability following
in vitro
culture of many species limited its
commercial application. The aim of micropropagation is to produce clonal or true-
to-type plants of elite or selected genotypes
in vitro
(Debergh and Read
1991
). New
approaches to micropropagation, including those that are not prone to production
of genetic off-types, have been applied where high levels of heterozygosity exist in
breeding lines, for rapid release of new cultivars and for germplasm storage. A sys-
tem that is genetically stable but has lower multiplication rates is based on produc-
tion of micro-cuttings from axillary buds of nodal cuttings dissected from apically
dominant shoots (Drew
1992
,
1996
). This protocol has been applied successfully to
coffee, papaya, neem, passionfruit (Drew
1991
,
1992
,
1993
,
1997
, respectively) and
to many other temperate and tropical species worldwide.
Meristem culture provided a unique way of producing virus tested plants and
was applied to many horticultural crops following the pioneer work by Morel and
Martin (
1952
) who produced virus-tested
Dahlia
plants. Genetic off-type plants
produced after callus culture were a problem for those concerned with clonal micro-
propagation. However, this variation provided new sources of genetic material for
the horticultural industry following the development of methods for i
n vitro
screen-
ing of cell suspension cultures. Many of the elite cultivars used in commercial hor-
ticultural practice are genetic variants that have occurred naturally in cultivated or
wild populations and have then been clonally or vegetatively propagated. Examples
such as the seedless navel orange, the spineless cayenne pineapple and the sweet
kesington mango have made major contributions to commercial horticulture. Some
in vitro
techniques generate high levels of genetic variants and provided a method
to produce stable, heritable changes and the potential to change one or two charac-
teristics of an elite genotype without changing the remainder of the genome.
In vitro
selection of useful variants has resulted in a number of valuable new commercial
lines of tomatoes (Tomes
1990
). Useful somaclonal variants have been isolated
from cultures of papaya (Sharma and Skidmore
1988
), apple (Utkhede
1986
), peach
(Hammerschlag
1988
), pear (Brisset et al.
1988
), citrus (Ben-Hayyim and Goffer
1989
; Spiegel-Roy et al.
1983
) and many other crop species.
Haploidy, Embryo Culture and Plant Breeding
Haploidy provided a technique for rapid production of doubled haploids for use in
breeding programmes and to facilitate selection of mutations which are often reces-
sive characters. Subsequently,
in vitro
screening for genetic variants in cell cultures
derived from haploid cells has produced novel genotypes, e.g.
Xanthomonas
resis-
tant pepper (Hwang et al.
1998
). One of the original and noteworthy applications of
haploidy was in asparagus breeding. All staminate populations that produce spears
with low fibre content are desirable.
In vitro
androgenesis followed by diploidisa-
tion, produced 50 % homozygous YY “supermales” (Thevinin
1974
). Fertile super-
