MM31/EIR1 promotes lateral root formation in Arabidopsis

Abstract

Lateral root formation in Arabidopsis provides a model for the study of auxin function. Tryptophan (Trp) is a precursor of the auxin indoleacetic acid (IAA). To study the physiological function of Trp in auxin-related phenotypes, we examined the effect of Trp on lateral root formation. We found that Trp treatment enhanced lateral root formation and, by screening for mutants in which the effect of Trp on lateral root formation was enhanced, we isolated the mm31 mutant. Based on genetic and physiological analyses, we propose that MM31/EIR1 modulates lateral root formation by regulating the IAA polar transport system, and that auxin transport from the shoot to the root regulates lateral root formation.

Figure 1

Predicted Trp-dependent IAA biosynthetic pathway.

Figure 2

Trp promotes lateral root formation. Wild-type plants were grown for 21 days on agar plates in the absence (A) or presence (B) of 50 µM Trp.

Figure 3

The effect of Trp on lateral root formation was epistatic to AXR1, AXR4 and AUX1. axr1-3 axr4-2 (A and B) and aux1-7 axr4-2 (C and D) double mutants were grown for 21 days on agar plates in the absence (A and C) or presence (B and D) of 50 µM Trp.

Figure 4

Lateral root formation was enhanced in the mm31 mutant in the presence of Trp. Wild-type plants (A and B) and mm31 mutants (C and D) were grown for 14 days on agar plates lacking (A and C) or containing (B and D) 50 µM Trp.

Figure 5

The mm31 mutant responds specifically to Trp. The mm31 mutant was grown for 14 days on agar plates lacking (A) or containing 50 µM Trp (B), 50 µM Gly (C), 50 µM His (D), 50 µM Leu (E), 50 µM Met (F), 50 µM Phe (G) or 50 µM Tyr (H).

Figure 6

PCIB inhibits lateral root formation in both the wild type and the mm31 mutant. Wild-type plants (A and B) and mm31 mutants (C and D) were grown for 14 days on agar plates lacking (A and C) or containing (B and D) 1 µM PCIB.

Figure 7

Fine mapping of the mm31 locus. mm31 was located on the BAC clone MUL3, between molecular markers ILL2 and MSF19 of chromosome 5. Numbers below the molecular markers shown in white boxes indicate the recombination frequency between the markers and the mm31 locus (Number of recombinant chromosomes/Number of chromosomes examined).

Figure 8

Auxin transport inhibitors block lateral root formation in both the wild type and the mm31 mutant. Wild-type plants (A–C) and mm31 mutants (D–F) were grown for 14 days on agar plates lacking (A and D) or containing 1 µM NPA (B and E) or TIBA (C and F).