Editors' ChoicePlant biology

Fat Roots Implicate New Plant Hormone

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Science Signaling  25 Nov 2008:
Vol. 1, Issue 47, pp. ec410
DOI: 10.1126/scisignal.147ec410

In plants, the properties of the cell wall are regulated in part by control of deposition of cellulose microfibrils, the orientation of which determines whether a structure grows radially or longitudinally. Xu et al. identified two genes in Arabidopsis that encode receptor-like Ser-Thr kinase (RLK) family members that, when disrupted together, caused a swollen root phenotype consistent with inappropriate radial expansion. The authors named the kinases FEI1 and FEI2. Although FEI1 and FEI2 had kinase activity in vitro, catalytically inactive mutants still retained most, but not all, of their ability to rescue the phenotype of fei1 fei2 mutant plants. Genetic and biochemical studies indicated that the FEI proteins function to regulate biosynthesis of cellulose. The plant hormone ethylene influences root growth, and the authors showed that inhibition of ethylene biosynthesis could restore normal root growth in the fei1 fei2 mutant plants. However, pharmacological inhibitors of ethylene signaling were not effective. The authors thus explored whether a precursor of ethylene might be the active agent. The FEI proteins interacted (in a yeast two-hybrid assay) with ACS5 [a member of the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase family of enzymes]. The product of ACS is ACC, the immediate biochemical precursor of ethylene. The authors favor a model in which the FEI proteins may act as scaffolds to localized ACS proteins in a manner analogous to the function of KSR (kinase suppressor of Ras) proteins in mammals. Furthermore, they propose that ACC may itself act as a signaling molecule, thus explaining why inhibition of ethylene synthesis (during which ACC is produced), but not ethylene signaling itself, was the critical factor in rescue of the fei1 fei2 mutants.

S.-L. Xu, A. Rahman, T. I. Baskin, J. J. Kieber, Two leucine-rich repeat receptor kinases mediate signaling linking cell wall biosynthesis and ACC synthase in Arabidopsis. Plant Cell preview, published 18 November 2008 as 10.1105/tpc.108.063354. [PubMed]

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