Mapping the site of action of the Green Revolution hormone gibberellin

Gibberellins (GA) represent a key class of hormone signals that promote plant growth and development (1). A key part of the Green Revolution, which saw crop yields more than double, was the development of new dwarf varieties, many of which were later found to have mutations in the GA pathway (2⇓–4). Thus, understanding GA’s growth regulation represents a prime target for further increasing crop production. Considerable progress has recently been made dissecting the molecular basis of GA action (reviewed in ref. 1); however, despite these advances, it remains unclear how this key hormone promotes growth at either the cellular, tissue, or organ levels of organization. In PNAS, Shani et al. (5) describe how GA is distributed within root tissues of the model plant Arabidopsis thaliana. By developing a fluorescent-labeled GA, the authors were able to demonstrate that this key growth-promoting hormone signal accumulates within a specific root tissue and developmental zone. The Arabidopsis primary root has a simple structure composed, along the radial axis, of concentric layers of epidermal, cortical, endodermal, pericycle, and stele (vascular) tissues; and along the apical-basal axis of spatially distinct meristem, elongation, and differentiation zones (see schematic in Fig. 1). Cells divide close to the root tip in the meristematic zone; then, after stopping dividing and entering the elongation zone, cells undergo rapid expansion; cells then eventually cease growth upon entering the differentiation zone. Studies have found that mutating components of the GA biosynthesis or signaling pathways results in a significantly shorter root length (6, 7) because of GA promoting cell division in the root meristem (8, 9) and cell expansion in the elongation zone (7, 10). Schematic illustrations of (Left) the tissue organization and zones within the …