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Polar nuclear migration is crucial during the development of diverse eukaryotes. In plants, root hair growth requires polar nuclear migration into the outgrowing hair. However, knowledge about the dynamics and the regulatory mechanisms underlying nuclear movements in root epidermal cells remains limited. Here, we show that both auxin and Rho-of-Plant (ROP) signaling modulate polar nuclear position at the inner epidermal plasma membrane domain oriented to the cortical cells during cell elongation as well as subsequent polar nuclear movement to the outer domain into the emerging hair bulge in Arabidopsis (Arabidopsis thaliana). Auxin signaling via the nuclear AUXIN RESPONSE FACTOR7 (ARF7)/ARF19 and INDOLE ACETIC ACID7 pathway ensures correct nuclear placement toward the inner membrane domain. Moreover, precise inner nuclear placement relies on SPIKE1 Rho-GEF, SUPERCENTIPEDE1 Rho-GDI, and ACTIN7 (ACT7) function and to a lesser extent on VTI11 vacuolar SNARE activity. Strikingly, the directionality and/or velocity of outer polar nuclear migration into the hair outgrowth along actin strands also are ACT7 dependent, auxin sensitive, and regulated by ROP signaling. Thus, our findings provide a founding framework revealing auxin and ROP signaling of inner polar nuclear position with some contribution by vacuolar morphology and of actin-dependent outer polar nuclear migration in root epidermal hair cells.
In a self-paced reading study on German sluicing, Paape (Paape, 2016) found that reading times were shorter at the ellipsis site when the antecedent was a temporarily ambiguous garden-path structure. As a post-hoc explanation of this finding, Paape assumed that the antecedent’s memory representation was reactivated during syntactic reanalysis, making it easier to retrieve. In two eye tracking experiments, we subjected the reactivation hypothesis to further empirical scrutiny. Experiment 1, carried out in French, showed no evidence in favor in the reactivation hypothesis. Instead, results for one out of the three types of garden-path sentences that were tested suggest that subjects sometimes failed to resolve the temporary ambiguity in the antecedent clause, and subsequently failed to resolve the ellipsis. The results of Experiment 2, a conceptual replication of Paape’s (Paape, 2016) original study carried out in German, are compatible with the reactivation hypothesis, but leave open the possibility that the observed speedup for ambiguous antecedents may be due to occasional retrievals of an incorrect structure.