4. Concluding remarks
There certainly is a need for novel selectable markers to minimize the use of antibiotics, herbicides and resistance-causing genes in plant sciences. There is also currently a strong public opinion against using antibiotic markers for producing transgenic crop plants. The selection on d-lactate adds a new marker system to molecular plant- and breeding science providing a good alternative to the traditional markers (especially antibiotics). Furthermore, this new marker extends the range of available alternative selection systems, which is important when multiple constructs should be selected simultaneously in the context of stepwise transformations.
The overexpression of d-LDH may not cause negative effects on the transgenic plants because d-LDH participates in an endogenous detoxification pathway and has high substrate specificity [5]. However, one could argue that the constitutively expressed d-LDH might interfere with plant metabolism during further growth and/or lead to a potential deregulation of other genes. It should also be considered that overexpression of an endogenous gene as a selectable marker could lead to co-suppression of the transgene and/or the endogenous gene in subsequent generations [8], [9] and [10], leading to a negative impact on the yield of transgenic crops lines. To avoid these consequences, d-LDH could be expressed under the control of an inducible promoter or a promoter that would only confer expression during early developmental stages. We expect that this alternative selection system will become a useful new tool in plant sciences, also addressing public concerns against antibiotic-based selection markers at the same time.