A Novel Conversion of Roots into Organs with Shoot Stem Characteristics by Inducing Two Transcription Factors

Somatic plant cells can regenerate shoots and/or roots or adventitious embryonic calluses, which may induce organ formation under certain conditions. Such regenerations occur via dedifferentiation of somatic cells, induction of organs, and their subsequent outgrowth. Despite recent advances in understanding of plant regeneration, many details of shoot induction remain unclear. Here, we artificially induced shoot stem-like green organs (SSOs) in Arabidopsis thaliana roots via simultaneous induction of two transcription factors (TFs), ARABIDOPSIS THALIANA HOMEOBOX PROTEIN 25 (ATHB25, At5g65410) and the B3 family transcription factor REPRODUCTIVE MERISTEM 7 (REM7, At3g18960), which are expressed in shoot apical meristems under normal growth conditions. The SSOs exhibited negative gravitropism and differentiated vascular bundle phenotypes, strongly suggesting the ectopic appearance of shoot stem characteristics in roots. In the SSOs, shoot-specific and root-specific genes were up- and down-regulated, respectively. These TFs can thus induce the expression of genes controlling shoot stem characteristics and disrupt the maintenance of root meristems by ectopic expression in roots. SSO induction appears to occur without development of an embryonic callus in the main root tip or the primordia of lateral roots, suggesting that these TFs alter the fate of root initiation to induce shoot stem characteristics. Our findings thus provide new insights in the development and regeneration of plant shoot stems.