Chromatin-associated high mobility group (HMG) proteins of the HMGB family are versatile architectural factors assisting various DNA-dependent processes such as transcription and recombination. Here, transgenic tobacco lines were generated that ectopically express the maize HMGB1 protein, as detected by immunoblot analyses. The shoot morphology of HMGB1 expressing plants does not differ from that of control plants. By contrast, tobacco seedlings expressing HMGB1 are impaired in the growth of the primary root, relative to control plants. The reduced primary root length is correlated with the accumulation of small cells in the cell division zone (but not in the elongation zone) of the roots of transgenic plants. This "short-root" phenotype is specific for HMGB1, as is not observed with HMGB4 expressing plants, and it is transient in that it is restricted to young seedlings (less than or equal to28 days), as the effect gradually disappears with older plants. (C) 2004 Elsevier Inc. All rights reserved.