Gibberellin Signaling: GRASs Growing Roots Dispatch

Gibberellins are a class of terpenoid molecules that can act as plant growth regulators. This class was discovered through the investigation of sick rice plants that were infested by the fungus Gibberella fujikuroi. In 1926, Eiichi Kurosawa determined that a substance, dubbed gibberellic acid, secreted by the fungus was causing the disease. Subsequently, it was realized that gibberellic acid is an essential endogenous regulator of plant growth. A large number of different gibberellins have been identified to date, but only few of them are biologically active. The involvement of gibberellins in plant development is manifold (Figure 1). Gibberellic acid is essential for seed germination, promotes stem elongation and, in some plants, helps control the shoot transition from vegetative to reproductive development [1]. In recent years, tremendous progress has been made in deciphering the genetic basis of gibberellic acid action by analysis of Arabidopsis mutants with defects in gibberellin biosynthesis or signaling. For example, ga1 mutant plants have severely decreased gibberellin levels because of a defect in the enzyme for the first committed step of gibberellin biosynthesis, and have a pleiotropic phenotype that can be rescued by external application of gibberellic acid [2]. Adult ga1 plants display severe dwarfism due to a lack of stem elongation. This dwarfism can be partially rescued by recessive mutations in the gibberellin-response gene Repressor of GA1 (RGA). Adding another recessive null mutation in the synergistically acting GA Insensitive (GAI) gene — which only weakly suppresses ga1 by itself — restores wild-type stem growth [3,4]. RGA and GAI encode two highly similar transcription factors of the GRAS family [5]. The inhibitory effect of gibberellic acid on the activity of RGA and GAI requires the DELLA motif, an amino-terminal domain found in a subfamily of GRAS proteins. How gibberellic acid regulates the activity of GAI through this domain remains unclear, but the DELLA domain is essential for the strong decrease in nuclear abundance of RGA in response to gibberellic acid treatment [6,7]. In summary, ga1/gai/rga triple mutants display a wild-type stem elongation rate, indicating that, in wild-type plants, gibberellic acid is necessary to release the inhibitory effect of RGA and GAI on this process. In a series of elegant experiments, Fu and Harberd [8] have now shown that this interplay between gibberellic acid and the GAI and RGA factors has a pivotal role, not only in the growth of stems, but also in the growth of plant roots. Primary roots are shortened in ga1 seedlings, but because of the pleiotropic nature of the ga1 mutation it was unclear whether this is a direct consequence of the decrease in gibberellin content. Fu and Harberd [8], however, followed up on the finding that GA1 is highly expressed in rapidly growing tissues, including root tips [9]. They found that the short root phenotype of ga1 Current Biology, Vol. 13, R366–R367, April 29, 2003, ©2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S0960-9822(03)00279-3