Liesbeth De Grauwe

A comparative analysis of the Arabidopsis mutant amp1-1 and a novel weak amp1 allele reveals new functions of the AMP1 protein

Ethylene and gibberellins have a synergistic stimulatory effect on hypocotyl elongation of light-grown Arabidopsis thaliana (L.) Heynh. seedlings. A screen for mutants with decreased response to these hormones led to the isolation of a novel allele (amp1-7) of the ALTERED MERISTEM PROGRAM (AMP) 1 locus. The amp1-7 allele contains a missense mutation causing a phenotype, which is weaker than that of the amp1-1 mutant that carries a nonsense mutation. The mutant phenotype prompted the hypothesis that AMP1 is involved in ethylene and GA signalling pathways or in a parallel pathway-controlling cell and hypocotyl elongation and cellular organization. Amp1 mutants contain higher zeatin concentrations causing enlargement of the apical meristem, which was confirmed by cytokinin application to wild type seedlings. Light grown amp1 seedlings have shorter hypocotyls than wild type; however, application of cytokinins promotes hypocotyl elongation of both Col-0 and amp1. We suggest that in amp1 mutants either zeatin overproduction or its action is strictly localized.

Reciprocal influence of ethylene and gibberellins on response-gene expression in Arabidopsis thaliana

The complexity of hormonal responses and their functional overlap support the presence of an intensive cross-talk between hormone signalling pathways. A detailed analysis of responses induced by ethylene and gibberellin (GA) in a GA-insensitive mutant (gai), an ethylene-resistant mutant (etr1-3), the gai etr1-3 double-mutant, and in wild-type Arabidopsis thaliana plants, revealed multiple interactions between ethylene and GA signal transduction pathways. Ethylene insensitive mutants and wild-type plants treated with 1-methylcyclopropene (1-MCP), an ethylene perception inhibitor, displayed a stronger responsiveness of genes differentially regulated by GA. In addition, microarray-analysis showed that the GA-response in an ethylene-insensitive background is different from that in the wild-type, confirming the importance of ethylene in a plant’s response towards GA. In this paper, we present a number of genes with an altered response-pattern as a direct consequence of cross-talk between ethylene and GA.

The Role of Brassinosteroids in Shoot Gravitropism

In the current model of gravitropism, negative gravitropic (upward) growth of the shoot of a dicotyledonous plant involves sedimentation of starch-containing plastids (statoliths) in the endodermis (starch sheath). Here we show that high levels of brassinosteroids (BRs) counteract gravitropic growth, while BR deficiency enhances gravitropism irrespective of the presence of a functional starch sheath. These data support a role for BRs as negative regulators of the shoot graviresponse.

Novel mechanisms of ethylene-gibberellin crosstalk revealed by the gai eto2-1 double mutant.

DELLA proteins have been shown to act as integrators of the signaling network controlling plant growth. In the January issue of New Phytologist (2008), we analyzed the gai eto2-1 double mutant and corresponding single mutants, with defects in the ethylene-biosynthesis and/or in the gibberellin (GA)-signaling cascade. This research revealed yet unknown modes of cross-talk between the ethylene and GA pathways. Two hypotheses have been put forward. Both essentially suggest the existence of reciprocal post-translational control of ethylene-GA crosstalk. Addendum to: De Grauwe L, Chaerle L, Dugardeyn J, Decat J, Rieu I, Vriezen WH, Baghour M, Moritz T, Beemster GT, Phillips AL, Harberd NP, Hedden P, Van Der Straeten D. Reduced gibberellin response affects ethylene biosynthesis and responsiveness in the Arabidopsis gai eto2-1 double mutant. New Phytologist 2008; 177:128-41.

Signal Crosstalk in the Control of Hypocotyl Elongation in Arabidopsis

Since plants are sessile organisms, environmental factors have a strong influence on their development. Hence, there is a need for adaptive mechanisms which integrate the multiple endogenous and environmental signals resulting in the most adequate biological response. Because of its morphological simplicity, the hypocotyl is often used as a model to study signal integration. Hypocotyl extension is a process regulated by a network of interacting signals, including light and hormones. Both are subject to reciprocal regulation, with hormonal factors influencing the biosynthesis and/or signalling of other hormones. In the following chapter, the current state of knowledge on the regulation of hypocotyl growth will be reviewed.