The role of auxin‐binding protein 1 in the expansion of tobacco leaf cells

Abstract

SummaryTobacco leaf was used to investigate the mechanism of action of auxin-binding protein 1 (ABP1). The distributions of free auxin, ABP1, percentage of leaf nuclei in G2 and the amount of auxin-inducible growth were each determined in control tobacco leaves and leaves over-expressing Arabidopsis ABP1. These parameters were compared with growth of tobacco leaves, measured both spatially and temporally throughout the entire expansion phase. Within a de®ned window of leaf development, juvenile leaf cells that inducibly expressed Arabidopsis ABP1 prematurely advanced nuclei to the G2 phase. The ABP1-induced increase in cell expansion occured before the advance to the G2 phase, indicating that the ABP1-induced G2 phase advance is an indirect effect of cell expansion. The level of ABP1 was highest at the position of maximum cell expansion, maximum auxin-inducible growth and where the free auxin level was the lowest. In contrast, the position of maximum cell division correlated with higher auxin levels and lower ABP1 levels. Consistent with the correlations observed in leaves, tobacco cells (BY-2) in culture displayed two dose-dependent responses to auxin. At a low auxin concentration, cells expanded, while at a relatively higher concentration, cells divided and incorporated [ 3 H]-thymidine. Antisense suppression of ABP1 in these cells dramatically reduced cell expansion with negligible effect on cell division. Taken together, the data suggest that ABP1 acts at a relatively low level of auxin to mediate cell expansion, whereas high auxin levels stimulate cell division via an unidenti®ed receptor.

Yokonolide B, a Novel Inhibitor of Auxin Action, Blocks Degradation of AUX/IAA Factors

“…32 with slight modification. The leaf strips were prepared from the sixth leaf of the seven-leaf staged transgenic tobacco MJ10B (tetracycline-inducible ABP1) or R7 (empty vector control) lines.…”

“…The tip region of young tobacco leaves expressing ABP1 display a developmentally acquired, auxin-specific, epinastic-growth response, whereas the base of young leaves does not respond to auxin (24,47). Young leaves lack expression of ABP1 in the base, and the developmental acquisition of auxin-inducible growth in leaves strongly correlates with the accumulating levels of ABP1 (32). If ABP1 is prematurely expressed in cells at the base of young leaves, expansion occurs in an auxin-dependent manner.…”

Cellular Responses to Auxin: Division versus Expansion

“…Overexpression of ABP1 in tobacco leaves enhances the sensitivity of guard cell Kþ channels to auxin (Bauly et al 2000) and shifts the plasma membrane hyperpolarization of protoplasts to lower auxin concentrations (BarbierBrygoo H., personal communication). Inducible overexpression of ABP1 leads to an increased expansion of some leaf tissues in response to auxin (Jones et al 1998;Chen et al 2001a). The null abp1 mutant in Arabidopsis, in addition to cell division defects, shows altered cell expansion (Chen et al 2001b).…”

The evolutionary development of plant body plans

“…Transgenic addition of a single functional copy of ABP1 rescues them. Examination of aborted embryos reveals anomalous cell wall patterns, although cell elongation is normal (Chen et al 2001). The failure to mediate sustained IAA-induced cell expansion may cause embryo lethality in Arabidopsis knockout plants.…”