KNOX homeobox genes are sufficient in maintaining cultured cells in an undifferentiated state in rice

We produced transgenic rice calli, which constitutively express each of fou r KNOX family class 1 homeobox genes of rice, OSH1, OSH16, OSH15, and OSH71 , and found that constitutive and ectopic expression of such genes inhibits normal regeneration from transformed calli, which showed continuous growth around their shoot-regenerating stages. Transgenic call! transferred onto regeneration medium began to display green spots, a sign of regeneration, b ut most of the transformants continued to propagate green spots at given st ages. In the normal shoot-regeneration process of calli, expression of endo genous OSH1 was restricted in presumptive shoot-regenerating regions of cal li and not observed in other areas. This restricted expression pattern shou ld be required for further differentiation of the regenerating shoots. Thus our present results support the proposed function that KNOX family class 1 homeobox genes play a role in the formation and maintenance of the undeter mined meristematic state of cells. (C) 2001 Wiley-Liss, Inc.