Some insights into initiation rate of somatic embryogenesis in maritime pine using Litvay basal formulations containing CPPU
Résumé
Initiation of somatic embryogenesis (SE) from immature excised zygotic embryos (IZE) in maritime pine was originally developed at FCBA using a modified DCR basal formulation (mDCR) with the 2,4-D auxin (13.5 μM) and the BA cytokinin (2.2 μM) as plant growth regulators (PGRs). In the frame of an international collaboration (initiation campaigns 2003-2004) we discovered that a modified Litvay basal medium (mLV) supplemented with the phenylurea cytokinin CPPU (4.04 μM) produced a significant increase in mean initiation rate (77 vs. 34%, Park et al. 2006, PCTOC 86: 87-101). This result was confirmed and strengthened during subsequent 2005-2008 initiation campaigns. Up to 92% mean initiation rate (2005 results) were obtained using the mLV/CPPU (4.04 μM) medium with quite moderate variation between years (62-92%, 4-5 families investigated per year, 200-300 IZE/family). The optimal CPPU concentration was studied and we concluded that up to a fourfold reduction of the original dose (1 μM) could be used without significant effect on initiation rate. Similar conclusions were drawn by reducing the initiation culture time of IZE on mLV/CPPU (4 μM) from 8 weeks to only one week. Interestingly initiation rates remained high (47-55%) when initiation media were deprived of CPPU, thus confirming the high interest of mLV for SE initiation in maritime pine. Substituting CPPU (4.04 μM) for the more conventional 2,4-D (9.04 μM) and BA (4.44 μM) PGRs in combination with 10 or 30 g/l sucrose significantly reduced initiation rates. mLV/CPPU (1 μM) was proposed as the optimal initiation medium at FCBA laboratory.