Figure 5. IBA and naxillin act locally in the root cap.

(a) pIBR10::GUS; pIBR3::GUS; pIBR1::n3GFP; pAIM1::n3GFP and pMFP2::n3GFP expression patterns, showing overlapping expression in root cap cells of the primary root. Inset, sections at the root cap of pIBR3::GUS seedlings. Scale bars, 10 μm. (b) Lateral root density of wild-type plants germinated on control medium for 5 d and transferred to medium supplemented with IBA and/or naxillin at indicated concentrations (in μM) for five more days (NAA was used as a control). before transfer, seedlings were left intact, the apical part of the root was removed (shoot with upper part of root; left) or the shoot and basal part of the root were removed (root tip segment; right) as indicated. Error bars represent means ± s.d. Statistical significance was calculated using a standard two-sided t-test (**P < 0.001) (c) Short-term treatment of Arabidopsis seedlings with naxillin increases the flux of IBA to free IAA in root tips. Five-day-old seedlings were incubated with medium containing either 10 μM [¹³C₁]IBA or 10 μM [¹³C₁]IBA and 25 μM naxillin for 2 h. The percentage enrichment of [¹³C₁]IAA derived from [¹³C₁]IBA in the free IAA pool was determined. Error bars represent s.d. observed in four biological replicates, and the asterisk indicates the statistically significant (standard two-sided t-test, P = 0.028) difference between the untreated and naxillin-treated samples. (d) Hypothetical model for the selective action of IBA and naxillin toward lateral root development.