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). At the molecular scale, GA promotes growth by triggering the degradation of the growth-repressing DELLA proteins. GA could promote Arabidopsis root growth by coordinating the simultaneous degradation of DELLA proteins in every tissue, or alternatively, DELLA degradation may only be required in one or more tissues. Several pieces of evidence suggest that GA is distributed unequally between tissues and growth zones. For example, the root endodermis has been shown to be particularly important in GA regulation of root growth: Ubeda Tomás et al. (8, 10) reported that by tissue specifically expressing a nondegradable form of DELLA, root growth was blocked when GA signaling was prevented within endodermal cells. Recently, direct measurements of GA metabolites using a mass spectrometry (MS)-based approach in the maize leaf (which has equivalent developmental zones to the Arabidopsis root) have revealed that bioactive GA levels are high in the so-called transition zone where cells cease to divide and start to expand (11). Nevertheless, directly determining exactly where the GA signal accumulated at a cellular level of resolution to control organ growth remained to be resolved until now. Shani et al. (5) are able to address exactly where GA accumulated in root tissues by adopting a state-of-the-art imaging (rather than MS-based) solution. By attaching a fluorescent tag using variable lengths of an acyl amide linker to the C6 position of the tetracyclic di-terpenoid structures of GA3 and GA4, the authors are able to recover several fluorescent GA-surrogates (termed GA3-Fl and GA4-Fl) that retained bioactivity. GA3Fl and GA4-Fl were also shown to promote the interaction between the GA receptor GID1 and its DELLA target using in vitro coimmunoprecipitation and yeast two-hybrid assays. Importantly, no breakdown products of GA3-Fl and GA4-Fl were detected after incubation with roots, indicating that these unique GA variants were stable in planta. Hence, wherever GA3-Fl and GA4-Fl accumulated was likely to represent the tissue site of GA. Using these fluorescent compounds, Shani et al. (5) showed that GA accumulates within a specific primary root tissue and developmental zone. After short incubations (∼15 min) with GA3-Fl and GA4-Fl, Arabidopsis roots were observed to accumulate these GA variants specifically in endodermal cells within the elongation zone (see schematic summary in Fig. 1). In contrast, the pattern of GA3-Fl and GA4-Fl accumulation was not observed in the shr-2 radial patterning mutant that lacked an endodermal layer. The striking endodermal accumulation of GA3Fl and GA4-Fl in wild-type roots is in complete agreement with earlier functional studies (7, 8) and the tissue distribution of the GA response reporter, RGApro::RGA-GFP (5). Hence, root endodermal cells in the transition/elongation zones (that accumulated Fig. 1. Schematic illustrations of (Left) the tissue organization and zones within the Arabidopsis primary root, and (Right) the cellular distribution of the hormone signal GA. The concentric layers of epidermal, cortical, endodermal, pericycle, and stele (vascular) tissues are color coded (see key); the GA distribution is denoted in red.