Tissue-specific expression of the ethylene biosynthetic machinery regulates root growth in maize

Although the hormonal control of root growth and development has been extensively studied, relatively little is known about the role that ethylene plays in cereal root development. In this work, we have investigated how the ethylene biosynthetic machinery is spatially regulated in maize roots and how changes in its expression alter root growth. ACC synthase (ZmACS) expression was observed in the root cap and in cortical cells whereas ACC oxidase (ZmACO) expression was detected in the root cap, protophloem sieve elements, and the companion cells associated with metaphloem sieve elements. Roots from Zmacs6 mutants exhibited significantly reduced ethylene production, a smaller root cap of increased cell number but smaller cell size, accelerated elongation of metaxylem, cortical, and epidermal cells, and increased vacuolation of cells in the calyptrogen of the root cap, phenotypes that were complemented by exogenous ACC. Zmacs6 mutant roots exhibited increased growth when largely unimpeded, a phenotype complemented by exogenous ACC, whereas loss of ZmACS2 expression had less of an effect. In contrast, Zmacs6 plants exhibited reduced root growth in soil. These results suggest that expression of ZmACS6 is important in regulating growth of maize roots in response to physical resistance.