An Arabidopsis mutant deficient in phosphatidylinositol-4-phosphate kinases ss1 and ss2 displays altered auxin-related responses in roots

Phosphatidylinositol 4-kinases (PI4Ks) are the first enzymes that commit phosphatidylinositol into the phosphoinositide pathway. Here, we show that Arabidopsis thaliana seedlings deficient in PI4K beta 1 and beta 2 have several developmental defects including shorter roots and unfinished cytokinesis. The pi4k beta 1 beta 2 double mutant was insensitive to exogenous auxin concerning inhibition of root length and cell elongation; it also responded more slowly to gravistimulation. The pi4kss1ss2 root transcriptome displayed some similarities to a wild type plant response to auxin. Yet, not all the genes displayed such a constitutive auxin-like response. Besides, most assessed genes did not respond to exogenous auxin. This is consistent with data with the transcriptional reporter DR5-GUS. The content of bioactive auxin in the pi4kss1ss2 roots was similar to that in wild-type ones. Yet, an enhanced auxin-conjugating activity was detected and the auxin level reporter DII-VENUS did not respond to exogenous auxin in pi4kss1ss2 mutant. The mutant exhibited altered subcellular trafficking behavior including the trapping of PIN-FORMED 2 protein in rapidly moving vesicles. Bigger and less fragmented vacuoles were observed in pi4kss1ss2 roots when compared to the wild type. Furthermore, the actin filament web of the pi4kss1ss2 double mutant was less dense than in wild-type seedling roots, and less prone to rebuilding after treatment with latrunculin B. A mechanistic model is proposed in which an altered PI4K activity leads to actin filament disorganization, changes in vesicle trafficking, and altered auxin homeostasis and response resulting in a pleiotropic root phenotypes.

Automated summary

This study reveals that Arabidopsis thaliana seedlings deficient in PI4K beta 1 and beta 2 exhibit several developmental defects, including shorter roots and unfinished cytokinesis. The double mutant shows reduced sensitivity to exogenous auxin and gravistimulation. The root transcriptome of the mutant displays some similarities to the wild type response to auxin, but not all genes show constitutive auxin-like response. Enhanced auxin-conjugating activity and altered subcellular trafficking behavior are observed in the mutant, leading to changes in auxin homeostasis and response and resulting in pleiotropic root phenotypes.