Yuri T. Yamamoto, Christopher G. Taylor, Gregoria N. Acedo, Chi-Lien Cheng, Mark A. Conkling, Characterization of cis-Acting Sequences Regulating Root-Specific Gene Expression in Tobacco, The Plant Cell, Vol. 3, No. 4 (Apr., 1991), pp. 371-382
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Yamamoto et al. (1991) studied this question.