A bifunctional selectable marker for wheat transformation contributes to the characterization of male-sterile phenotype induced by a synthetic Ms2 gene.

key messageAn engineered selectable marker combining herbicide resistance and yellow fluorescence contributes to the characterization of male-sterile phenotype in wheat, the severity of which correlates with expression levels of a synthetic Ms2 gene. Genetic transformation of wheat is conducted using selectable markers, such as herbicide and antibiotic resistance genes. Despite their proven effectiveness, they do not provide visual control of the transformation process and transgene status in progeny, which creates uncertainty and prolongs screening procedures. To overcome this limitation, this study developed a fusion protein by combining gene sequences encoding phosphinothricin acetyltransferase and mCitrine fluorescent protein. The fusion gene, introduced into wheat cells by particle bombardment, enabled herbicide selection, and visual identification of primary transformants along with their progeny. This marker was then used to select transgenic plants containing a synthetic Ms2 gene. Ms2 is a dominant gene whose activation in wheat anthers leads to male sterility, but the relationship between the expression levels and the male-sterile phenotype is unknown. The Ms2 gene was driven either by a truncated Ms2 promoter containing a TRIM element or a rice promoter OsLTP6. The expression of these synthetic genes resulted in complete male sterility or partial fertility, respectively. The low-fertility phenotype was characterized by smaller anthers than the wild type, many defective pollen grains, and low seed sets. The reduction in the size of anthers was observed at earlier and later stages of their development. Consistently, Ms2 transcripts were detected in these organs, but their levels were significantly lower than those in completely sterile Ms2TRIM::Ms2 plants. These results suggested that the severity of the male-sterile phenotype was modulated by Ms2 expression levels and that higher levels may be key to activating total male sterility.