Developmental phases and STM expression during Arabidopsis shoot organogenesis

Abstract

Shoot organogenesis in Arabidopsis thaliana wasstudied with regard to the timing of key developmental phases and expression ofthe SHOOTMERISTEMLESS (STM) gene.Shoot regeneration in the highly organogenic ecotype C24 was affected byexplanttype and age. The percentage of C24 cotyledon explants producing shootsdecreased from 90% to 26% when donor seedlings were more than 6 dold, but 96% of root explants produced shoots regardless of the age of thedonorplant. Using explant transfer experiments, it was shown that C24 cotyledonexplants required about 2 days to become competent and another 8-10 days tobecome determined for shoot organogenesis. A C24 line containing the promoterofthe SHOOTMERISTEMLESS (STM) genelinked to the β-glucuronidase(GUS) gene was used as a tool for determining the timingofde novo shoot apical meristem (SAM) development incotyledon and root explants. Cotyledon and root explants from anSTM:GUS transgenic C24 line were placed on shoot inductionmedium and GUS expression was examined after 6-16 days ofculture. GUS expression could be found in localizedregionsof callus cells on root and cotyledon explants after 12 days indicating thatthese groups of cells were expressing the STM gene, hadreached the key time point of determination, and were producing an organizedSAM. This was consistent with the timing of determination as indicated byexplant transfer experiments. Root explants from anSTM:GUStransgenic Landsberg erecta line and a two-step tissue culture method revealedasimilar pattern of localized GUS expression duringde novo shoot organogenesis. This is the first studydocumenting the timing and pattern of expression of theSTMgene during de novo shoot organogenesis.