Desiccation-tolerant gymnosperm embryos

Details Desiccation-tolerant gymnosperm embryos Desiccation-tolerant gymnosperm embryos

1997-12-05

2002-04-16

Lankford, Jr., Leon B. (Department: 1651)

Chemistry: molecular biology and microbiology

Plant cell or cell line, per se ; composition thereof;...

C435S422000, C047S057600

Reexamination Certificate

active

06372496

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to mature viable desiccation-tolerant gymnosperm embryos, optionally encapsulated.
BACKGROUND OF THE INVENTION
Somatic embryogenesis offers the potential to produce clonally large numbers of low cost plants of many species. Somatic embryos develop without the surrounding nutritive tissues and protective seed coat, so considerable research has been devoted to causing somatic embryos to functionally mimic seeds with regard to efficient storage and handling qualities. The development of techniques for somatic embryogenesis in conifers has greatly improved the ability to culture conifer tissues in vitro and now offers the means to propagate clonally commercially valuable conifers of a number of species. However, all conifer species suffer from poor plantlet vigor.
It has been suggested to use abscisic acid (ABA) or osmoticum for enhancing storage levels in plant cells. For example, it was shown that somatic embryos of
Theobroma cacao
could be induced to accumulate fatty acids approaching the composition of commercial cocoa butter by increasing the sucrose concentration of the culture medium. Modifying the culture conditions by varying osmotic concentration and/or ABA content similarly improved lipid accumulation in
Brassica napus
L. somatic and microspore derived embryos as well as somatic embryos of carrot and celery. Also, the level of storage lipids in
P. abies
somatic embryos was improved by optimizing the ABA level to between 10-20 &mgr;m, but the somatic embryos contained about 4% of the lipid level obtained by zygotic embryos.
Also, Japanese laid-open patent publication No. 1-218520 describes a process for producing plant body regenerative tissue. The process includes a step of cultivating a plant body regenerative tissue in a medium containing ABA and having an osmotic pressure of 180 to 2500. In order to control the osmotic pressure within the specific range, a non-toxic substance such as sugar, alcohol, an amino acid or glycol is added.
Water stress plays an important role in maintaining embryos in a maturation state (Kermode 1990, Crit. Res. Plant Sci. 9, 155-194). Low water content rather than ABA prevents precocious germination during later stages of development. This is important because precocious germination of embryos during development in seeds would be lethal during desiccation.
A conventional way to water stress plant cells grown in vitro is to increase the osmotic concentration of the culture medium through the use of plasmolysing osmotica. For example, increased concentrations of plasmolysing osmotica such as sucrose have been used to promote somatic embryo maturation of many plant species. Sucrose at levels up to 6% was found to improve somatic embryo development of some conifers but a smaller increase in sucrose from 1 to 3% was previously considered optimal for the maturation of white and Norway spruce somatic embryos. It seems that a higher concentration generally led to repressed embryo development. 3% sucrose is the concentration most commonly used for conifer somatic embryo maturation. Mannitol had a similar effect on maturation of conifer somatic embryos (Roberts 1991). Low levels of mannitol (2-6%) led to a doubling of the number of mature embryos recovered at the end of the maturation period; however, the treatment could only be applied as a short pulse (1 week) as prolonged maturation treatment with mannitol became detrimental to further embryo maturation.
Poor response using sucrose and mannitol or other simple sugars and salts may be because such plasmolysing osmotica are absorbed by the symplast of plant cells. Such absorption facilitates adjustment of tissue osmotic potential (osmotic recovery) without lowering the tissue water content. Additionally, direct or indirect metabolic effects on specific plant metabolites can occur, due to utilization of the solute or its toxic effects.
Alternatives to plasmolysing osmotica are non-permeating high molecular weight compounds such as polyethylene glycol (PEG) or dextran. These compounds are usually available in a wide range of molecular weights. For example, PEG is available in molecular weights ranging from 200 to 35,000. However, only those with a molecular weight above 1000 to 3000 would be non-permeating (Carpita et al, 1979). This is because the large molecular size of these solutes excludes their passage through plant cell walls, so preventing entry into cells and plasmolysis, while still removing water. Consequently, their non-plasmolysing effect reduces tissue water content in a manner similar to the effects of water stress observed in cells of plants subjected to drought conditions. The effect is constant and cell turgor can only be restored by cells actively increasing their cellular solute concentrations. PEG has been most commonly used to apply water stress to whole plants, to osmotically prime whole seeds to synchronize germination and improve seedling vigor.
Embryo drying occurs naturally in most seeds, and has a role to play in the developmental transition between maturation and germination. Thus, desiccation led to enhanced germination of both zygotic and somatic embryos. Desiccation of whole somatic embryos is also an alternative method of germplasm storage. Somatic embryos produced continuously year-round could therefore be dried and stored until the appropriate planting season, or shipped to new locations.
A number of prior publications describe methods for the desiccation of angiosperm somatic embryos. Senaratna et al., in EP application 0300730, describe a method through which in vitro formed plant embryos are desiccated following the application of ABA or other types of environmental stress inducing desiccation tolerance. The angiosperm embryos are induced at the torpedo shaped stage with the source of ABA for a sufficient period of time to cause expression of desiccation tolerance. The induced embryos are then dried to provide stable, viable artificial seeds. In EP 0300730, Senaratna et al. emphasize on the importance of stimulating the embryo at the appropriate stage by the use of signals to develop tolerance to desiccation. It is stressed that if the signals are applied at the incorrect stage of development, the tissue will not respond properly. Angiosperm embryos can undergo maturation in the absence of ABA and it is suggested that ABA be supplied as late as possible during the maturation protocol and applied for a relatively short period of time. Hence, the timing and duration of ABA application seem to be critical.
Japanese laid-open patent publication No. 2-31624 discloses the use of ABA in plant cultures. ABA is used as part of a process for drying embryos prior to storage.
In published PCT specification No. WO 89/05575, a method for the production of synthetic seeds comprising dehydrated somatic embryos is described. The method, which is applicable to monocotyledonous and dicotyledonous embryos, comprises maintaining the somatic embryos in an atmosphere having a relative humidity (r.h.) of from about 30 to about 85% for a period of time sufficient to reduce the moisture content of the embryos from about 85 to 65% to about 4 to 15%. The use of osmotically active materials, once the embryos are matured, is suggested.
Senaratna et al., in 1989, Plant Science, 65, pp. 253-259, describe the induction of desiccation tolerance in alfalfa somatic embryos by exogenous application of ABA in the form of a short pulse. Embryos are then dried to 10 to 15% of their moisture content and stored for at least 3 weeks in the dry state. Senaratna et al. also describe a method by which tolerance to desiccation is induced by exposing the somatic embryos to sub-lethal levels of low temperature, water, nutrient or heat stress prior to desiccation. However, it was demonstrated that some of these stress pre-treatments had deleterious effects on embryo maturation and seedling vigor.
Hence, the prior literature on somatic embryos and artificial seeds shows that desiccation tolerance has been achieved in some angiosperm plant species such as alfalfa

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