Plant Biology Beatnik Kinase

Wei Wong

Science Signaling, AAAS, Washington, DC 20005, USA

Root hairs, which are tubular outgrowths of the root epidermis in plants such as Arabidopsis, display planar polarity because the hairs are positioned near the basal ends of hair-forming cells, which in turn are oriented toward the maximum end of a concentration gradient of the hormone auxin in the root tip. Several different mechanisms have been proposed to underlie the formation of the auxin concentration gradient. In characterizing planar polarity in Arabidopsis mutants, Ikeda et al. found evidence that generation of this concentration gradient depends on localized suppression of auxin biosynthesis. Plants carrying the recessive beatnik (btk) mutation had root hairs positioned more basally than those in wild-type plants (a phenotype that the authors referred to as hyperpolar). btk was mapped to a point mutation that caused a Glu⁶²⁶Lys substitution in the kinase CTR1 (CONSTITUTIVE TRIPLE RESPONSE1), which reduced (but not did abolish) kinase activity. Root tips of plants carrying the btk mutation in CTR1 (ctr1^btk) exhibited a more extensive auxin response throughout the root tip (as assessed by a fluorescent reporter) and higher overall auxin concentrations compared with wild-type plants. Auxin biosynthesis was enhanced in ctr1^btk root tips due to increased expression of two auxin biosynthesis genes, WEI2 (WEAK ETHYLENE INSENSITIVE2) and WEI7. The authors also showed that mutations in genes encoding the auxin carrier proteins AUX1 (AUXIN-RESISTANT1) or PIN2 (PINFORMED2) partially suppressed the hyperpolar phenotype of ctr1^btk mutants, suggesting that these carriers redistributed auxin from its point of biosynthesis throughout the root tip. Thus, regulation of auxin biosynthesis in the Arabidopsis root tip by CTR1 appears to be a critical factor in dictating the planar polarity of root hairs.

Y. Ikeda, S. Men, U. Fischer, A. N. Stepanova, J. M. Alonso, K. Ljung, M. Grebe, Local auxin biosynthesis modulates gradient-directed planar polarity in Arabidopsis. Nat. Cell Biol. 11, 731–738 (2009). [PubMed]

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