Proceedings of the XLVI Italian
Society of Agricultural Genetics - SIGA Annual Congress
Giardini
Naxos, Italy - 18/21 September, 2002
ISBN 88-900622-3-1
Oral
Communication Abstract - S1a
ADVANCES IN GENETIC TRANSFORMATION FOR
MOLECULAR BREEDING OF CITRUS
PEÑA L., CERVERA M., GHORBEL R.,
DOMÍNGUEZ A., FAGOAGA C., JUÁREZ J., PINA J.A., NAVARRO L.
Department of Plant Protection and
Biotechnology, Instituto Valenciano de Investigaciones Agrarias (I.V.I.A.),
Apartado Oficial, 46113-Moncada, Valencia, Spain
Citrus is the
most economically important fruit tree crop in the world, with a production of
almost 100 million tons in 2001. Citrus species have a complex reproductive biology.
Some important genotypes have total or partial pollen and/or ovule sterility
and cannot be used as parents in breeding programs. There are many cases of
cross- and self-incompatibility. Most species are apomictic, which means that
adventitious embryos initiate directly from maternal nucellar cells precluding
the development of zygotic embryos, and thus the recovery of sexual progeny
populations. They have a long juvenile period and most species need at least 5
years to start flowering in subtropical areas. All these features together with
their large plant size, high heterozigosity, lack of basic knowledge about how
the most important horticultural traits are inherited, and quantitative
inheritance of most characters have greatly impeded genetic improvement of
citrus through conventional breeding methods. Although some breeding programs
for citrus improvement started more than 100 years ago, nowadays most
rootstocks are citrus species without any improvement, and most important
varieties have been originated by budsport mutations and chance seedlings. Only
a few hybrid varieties are economically relevant in certain local markets.
Many different
citrus genotypes are commercially grown in a wide diversity of soil and
climatic conditions, implicating that trees are subjected to important abiotic
and biotic stresses that limit the production and, in some instances, the use
of certain rootstocks and varieties. The main abiotic stresses are acid,
alkaline, and salty soils, flooding and drought, freezing and high
temperatures. Citrus trees are also affected by many pests and diseases caused
by nematodes, fungi, bacteria, spiroplasmas, phytoplasmas, viruses and viroids.
Some diseases are spread throughout the world, as those produced by the
oomycete Phytophthora sp., or by the Citrus tristeza virus (CTV), that
preclude the use of certain excellent rootstocks, and severely restrict fruit
production and quality of important varieties in some countries.
At the same time
that citrus industry is threatened by important biotic and abiotic stresses,
markets of developed countries demand fruit of increasing quality. In this
situation, genetic improvement of citrus is a major priority. Genetic
transformation can be used as a powerful tool to improve citrus rootstocks and
varieties opening the way for
the introduction of specific traits into known genotypes without altering their
genetic elite background. We have established genetic transformation systems
for citrus species and hybrids, which are allowing us now to incorporate
transgenes of potential agricultural interest into them. Some of our research
lines are the following: 1, we are introducing meristem-identity transgenes
into citrus with the aim to shorten the juvenile period and consequently to
accelerate genetic improvement programs; 2, we are introducing transgenes into
citrus to get higher tolerance or resistance against diseases caused by
oomycetes as Phytophthora sp., and viruses as Citrus tristeza virus; and 3, we are
introducing transgenes involved in hormonal biosynthetic pathways into citrus
to try to modulate tree size. Furthermore, we are conducting a field trial with
transgenic citrus plants to investigate phenology of trees, transgene
stability, frequency of transgene dispersion through the pollen, and transgene
inheritance to the progeny. Recent developments from all these projects will be
presented.