The Arabidopsis ENHANCER OF SHOOT REGENERATION (ESR)1 and ESR2 genes are thought to play critical roles in in vitro shoot regeneration. In this study, we investigated the functions and expression patterns of ESR1 and ESR2 during shoot regeneration by using their mutants and promoter-reporter systems. Shoot regeneration efficiencies of esr1 esr2 double mutants from hypocotyl explants decreased drastically; although the effects on shoot regeneration of the esr1 single mutation were much less marked than those of the esr2 single mutation, especially from root explants, their effects were additive. We found that ESR1 was initially expressed 1 day after transfer onto shoot-inducing medium (SIM), compared with 4 days for ESR2 expression. These results suggest that the functions of ESR1 and ESR2 in shoot regeneration are not redundant, even though they encode similar transcription factors and the ESR2 gene substituted with an ESR1 coding region complements the esr2 mutation. We also found that ESR1 expression was localized to a small number of cells in the lateral root meristem (LRM)-like structures, and the ESR1-expressing cells appeared to proliferate to form shoot apical meristem (SAM)-like structures. Thus, ESR1 may be involved in the change of LRM to SAM in tissue culture.
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