Liquid-based method for producing plant embryos

1. A method for developing plant embryos from proliferative plant cells, comprising

(A) culturing proliferative plant cells for a first period of time in a liquid embryonic development medium that comprises

(i) one or more phytohormones,

(ii) a source of reduced nitrogen, and

(iii) carbohydrate,

wherein the liquid embryonic medium either (a) does not contain a non-permeating osmotic agent, or (b) comprises a low percentage concentration of a non-permeating osmotic agent selected from the group consisting of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, and 3.5%;

(B) separating the liquid embryonic medium from the embryos after expiration of the first period of time; and

(C) conditioning the embryos by storing them in a high relative humidity environment for a second period of time prior to germination;

wherein the conditioned embryos are plant embryos developed from proliferative plant cells.

2. The method of claim 1 , wherein the liquid embryonic development medium does not comprise a non-permeating osmotic agent.

3. The method of claim 2 , wherein the non-permeating osmotic agent is polyethylene glycol.

4. The method of claim 1 , wherein the phytohormone is abscisic acid (ABA) at a concentration from about 1 mg/L to about 100 mg/L.

5. The method of claim 4 , wherein the concentration of ABA is about 21 mg/l.

6. The method of claim 1 , wherein the source of reduced nitrogen is an amino acid.

7. The method of claim 6 , wherein the amino acid is L-glutamine at a concentration from about 0.1 g/l to about 6.0 g/l.

8. The method of claim 1 , wherein the carbohydrate is a sugar selected from the group consisting of maltose, sucrose, and fructose.

9. The method of claim 8 , wherein the liquid embryonic development medium comprises from about 1% to about 10% maltose.

10. The method of claim 1 , wherein the liquid embryonic development medium further comprises activated carbon.

11. The method of claim 10 , wherein the concentration of activated carbon is from about 0.1 g/l to about 2.5 g/l.

12. The method of claim 10 , wherein the concentration of activated carbon is about 1.25 g/l.

13. The method of claim 1 , wherein the liquid embryonic development medium further comprises myo-inositol at a concentration from about 10 mg/l to about 1000 mg/l.

14. The method of claim 13 , wherein the concentration of myo-inositol is about 100 mg/l.

15. The method of claim 1 , wherein the liquid embryonic development medium does not contain a gelling agent.

16. The method of claim 1 , wherein the osmolarity of the liquid embryonic development medium is from about 50 mmol/kg to about 200 mmol/kg.

17. The method of claim 1 , wherein the osmolarity of the liquid embryo development medium is about 82 mmol/kg.

18. The method of claim 1 , wherein the proliferative plant cells are incubated in the liquid embryo development medium for no more than 1 to 9 weeks during the first period of time.

19. The method of claim 1 , wherein the proliferative plant cells are conifer cells.

20. The method of claim 19 , wherein conifer is selected from the group consisting of Eastern white pine, Western white, Sugar pine, Red pine, Pitch pine, Jack pine, Longleaf pine, Shortleaf pine, Loblolly pine, Slash pine, Virginia pine, Ponderosa pine, Jeffrey pine, Pond pine, and Lodgepole pine, Radiata pine and hybrid crosses thereof.

21. The method of claim 1 , wherein the embryos are stored in a high relative humidity environment for 1-7 weeks during the second period of time.

22. The method of claim 1 , wherein step (C) is performed in a different vessel to that in which steps (A) and (B) are performed.

23. The method of claim 1 , wherein the method steps are automated.

24. The method of claim 22 , wherein steps (A) and (B) are performed in a bioreactor.

25. The method of claim 1 , wherein the liquid embryonic development medium comprises 21 mg/L ABA, 1.25 g/L activated carbon, 100 mg/L myo-inositol, 1.45 g/l glutamine, and 2% maltose.

26. The method of claim 1 , further comprising pre-preparing embryogenic tissue in a liquid medium containing more than 1.45 g/l glutamine prior to the culturing step of step (A).

27. The method of claim 26 , wherein the liquid medium comprises 3 g/l of glutamine.

28. The method of claim 27 , wherein the liquid medium is the Mi3 medium of Table 1 that contains no Phytagel.

29. The method of claim 28 , wherein the pre-preparation of embryogenic tissue in the liquid medium prior to the culturing step of step (A) is performed in a bioreactor.