Wheat bread-making quality can be improved by use of high-molecular-weight glutenin subunits (HMW-GSs) from wild relatives. Aegilops longissima is a close relative of wheat that contains a number of HMW-GS-encoding genes including 1S^lx2.3*. In this study, transgenic wheat lines overexpressing 1S^lx2.3* were obtained by Agrobacterium-mediated transformation and used to investigate the genetic contribution of 1S^lx2.3* to wheat flour-processing quality. The 1S^lx2.3* transgene was stably inherited and expressed over generations. Expression of 1S^lx2.3* increased the relative expression of 1Dx2 and 1Dy12 and reduced that of 1By18 during grain development. In general, integration of 1S^lx2.3* stimulated the accumulation of endogenous HMW-GSs and low-molecular-weight glutenin subunits in wheat kernels, greatly increasing the glutenin: gliadin ratio and resulting in faster formation of protein bodies in the endosperm during grain development. A wheat material with improved flour-making quality was developed in which 1S^lx2.3* improved wheat bread-making quality.