Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot

Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, lo ss of a ground tissue layer, in the root, Analysis of the shoot phenotype o f scr mutants revealed that both hypocotyl and shoot inflorescence also hav e a radial pattern defect, loss of a normal starch sheath layer, and conseq uently are unable to sense gravity in the shoot. Analogous to its expressio n in the endodermis of the root, SCR is expressed in the starch sheath of t he hypocotyl and inflorescence stem. The SCR expression pattern in leaf bun dle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin- formed mutant background suggested the possible origins of the starch sheat h in the shoot inflorescence. Analysis of SCR expression and the mutant phe notype from the earliest stages of embryogenesis revealed a tight correlati on between defective cell divisions and SCR expression in cells that contri bute to ground tissue radial patterning in both embryonic root and shoot. O ur data provides evidence that the same molecular mechanism regulates the r adial patterning of ground tissue in both root and shoot during embryogenes is as well as postembryonically.