GROWTH-CHARACTERISTICS AND TRANSFORMABILITY OF SOYBEAN EMBRYOGENIC CULTURES

Embryogenic cultures of soybean [Glycine max (L.) Men. cv. Jack and As grow A2872] were established in liquid Finer and Nagasawa medium, main tained by transfer to fresh medium at biweekly intervals, and subjecte d to microprojectile bombardment over time. Cultures were not amenable to transformation until they were at least 6 months old. Over time, d ifferent cell lines of the same genotype acquired very different cultu re phenotypes. Histological analysis of cell lines differing in transf ormation ability showed that the most transformable cultures had cytop lasmic-rich cells in the outermost layers of the tissue. In contrast, the outer layers of less transformable cultures contained cells with p rominent vacuoles. Although fresh weight accumulation of the cultures was curvilinear during the 2-week subculture period, a burst of mitoti c activity was evident shortly after transfer to fresh medium. This ac tivity usually lasted from the 2nd to the 6th day following subculture , and peaked on the 4th day. Tissues at or near this stage always prod uced more transient expression of a reporter gene than did bombardment s at other times. In addition, the cell lines most amenable to transfo rmation also exhibited the highest mitotic index. Thus any treatment t o increase the mitotic index, especially when the cell lines are less than 6 months old, may facilitate the transformation of cell lines fro m which efficient recovery of transgenic plants is still possible.