A Mutation Altering Auxin Homeostasis and Plant Morphology in Arabidopsis

Abstract

Many aspects of plant development are associated with changing concentrations of the phytohormone auxin. Several stages of root formation exhibit extreme sensitivities to exogenous auxin and are correlated with shifts in endogenous auxin concentration. In an effort to elucidate mechanisms regulating development of adventitious roots, an ethyl methanesulfonate-mutagenized M2 population of Arabidopsis was screened for mutants altered in this process. A recessive nuclear mutant, rooty (rty), displayed extreme proliferation of roots, inhibition of shoot growth, and other alterations suggesting elevated responses to auxin or ethylene. Wild-type Arabidopsis seedlings grown on auxin-containing media phenocopied rty, whereas rty seedlings were partially rescued on cytokinin-containing media. Analysis by gas chromatography-selected ion monitoring-mass spectrometry showed endogenous indole-3-acetic acid concentrations to be two to 17 times higher in rty than in the wild type. Dose-response assays with exogenous indole-3-acetic acid indicated equal sensitivities to auxin in tissues of the wild type and rty. Combining rty with mutations conferring resistance to auxin (axr1-3) or ethylene (etr1-1) suggested that root proliferation and restricted shoot growth are auxin effects, whereas other phenotypic alterations are due to ethylene. Four mutant alleles from independently mutagenized populations were identified, and the locus was mapped using morphological and restriction fragment length polymorphism markers to 3.9 centimorgans distal to marker m605 on chromosome 2. The wild-type RTY gene product may serve a critical role in regulating auxin concentrations and thereby facilitating normal plant growth and development.

Sur2 mutations of Arabidopsis thaliana define a new locus involved in the control of auxin homeostasis

“…In this assay, measurements of root and hypocotyl length indicated that the response of sur2 to auxin was not altered (data not shown) as described for the rty mutant (sur1) (King et al, 1995). The sensitivity to cytokinin was also tested by use of the same assay with increasing concentrations of benzyladenine (BA).…”

“…In this case, a mutation would lead to increased levels of free IAA; however, in several transgenic lines overproducing auxin, an increase in the level of free IAA was correlated with an accumulation of conjugated auxin, indicating that de novo IAA biosynthesis is partially controlled by the level of conjugation (Klee et al, 1987;Romano et al, 1993;Sitbon et al, 1991Sitbon et al, , 1992Sitbon et al, , 1993. Similarly, all forms of auxin are higher in the sur1 mutant and it has been postulated that this mutation lies in the auxin biosynthetic pathway or a regulator of that pathway (Boerjan et al, 1995;King et al, 1995). Therefore, the hypothesis that changes in the level of auxin in the sur2 mutant result in deregulation of IAA biosynthesis appears less likely.…”

“…Free IAA levels are elevated from two-to 100-fold, depending on the developmental stage of the plant. More recently, several Arabidopsis mutants that overproliferate lateral roots, namely sur1 (Boerjan et al, 1995), rty (King et al, 1995), hls3 (Lehman et al, 1996), and alf1 (Celenza et al, 1995) have been isolated independently. All are alleles of a single locus on chromosome 2 from the results of allelism tests (Celenza et al, 1995;Lehman et al, 1996;M.…”

“…Delarue, unpublished results). These mutants display an abnormally copious proliferation of roots and contain elevated amounts of free and conjugated IAA (Boerjan et al, 1995;King et al, 1995). Recently, the RTY gene has been cloned by T-DNA tagging and shown to encode a protein similar to tyrosine amino-transferase (Gopalraj et al, 1996).…”

EXORDIUM– a gene expressed in proliferating cells and with a role in meristem function, identified by promoter trapping in Arabidopsis

“…In addition to subjecting plants to exogenous auxin and inhibitor treatments, the EXO-GUS gene was introduced into mutant backgrounds compromised in auxin transport (aux1-7; Bennett et al, 1996;Maher and Martindale, 1980), auxin signalling (axr1-12; Estelle and Somerville, 1987) and auxin levels (rooty, King et al, 1995). Results presented in Figure 6(c,d) show that EXO-GUS activity at the root tip in both aux1-7 and axr1-12 mutant backgrounds, respectively, is much reduced compared to that in AtEM201.…”