Unique cell wall abnormalities in the putative phosphoinositide phosphatase mutant AtSAC9

Abstract

SAC9 is a putative phosphoinositide phosphatase in Arabidopsis thaliana involved in phosphoinositide signaling. sac9-1 plants have a constitutively stressed phenotype with shorter roots which notably accumulate phosphatidylinositol 4,5-bisphosphate and its hydrolysis product inositol trisphosphate. We investigated the primary roots of sac9-1 seedlings at the cytological and ultrastructural level to determine the structural basis for this altered growth. Despite the normal appearance of organelles and cytoplasmic elements, our studies reveal extreme abnormalities of cell wall and membrane structures in sac9-1 primary root cells, regardless of cell type, position within the meristematic area, and plane of section. Cell wall material was deposited locally and in a range of abnormal shapes, sometimes completely fragmenting the cell. Simple protuberances, broad flanges, diffuse patches, elaborate folds, irregular loops and other complex three-dimensional structures were found to extend randomly from the pre-existing cell wall. Abundant vesicles and excessive membrane material were associated with these irregular wall structures. We argue that a perturbed phosphoinositide metabolism most likely induces these observed abnormalities and hypothesize that a disorganized cytoskeleton and excessive membrane trafficking mediate the cell wall defects.

Increasing Phosphatidylinositol (4,5)-Bisphosphate Biosynthesis Affects Basal Signaling and Chloroplast Metabolism in Arabidopsis thaliana

“…The sac9 mutants were reported to have constitutively closed stomata [ 46 ]. These differences in the phenotype of the sac9 mutant and Hs PIPKIα plants may reflect differences in the levels of PtdInsP 2 in the leaves of the sac9 and Hs PIPKIα or other effects of the sac9 mutation that may have more direct effects on membrane biogenesis and cell wall deposition [ 74 ].…”

“…Supplementary Figure 4B shows a heat map of changes detailed in Table 3 . Some of the transcript changes may reflect systemic changes in vascular transport and cell wall structure associated with up-regulation of the PI pathway [ 51 , 74 , 83 , 84 , 85 ]. Transcripts of PIPKs were first reported associated with vasculature [ 51 , 86 ] and these transcript changes may reflect tissue specific sensitivity to the expression of the Hs PIPKIα or a reflection of effects on long distance signaling by InsP 3 -mediated events [ 87 ].…”

Precision targeting by phosphoinositides: how PIs direct endomembrane trafficking in plants

“…Finally, a plant specific pathway heavily relying on polarized membrane trafficking and not treated in this review is the establishment of the cell plate during cytokinesis. Several mutants defective in phosphoinositide metabolism harbor aberrant cell division phenotypes [72,74] and . This paper shows that multiple EXO70 subunits localize to various subcellular localizations in growing tobacco pollen tubes, suggesting that different EXO70 subunits might target exocytosis at various locations within the same cell.…”

SAC phosphoinositide phosphatases at the tonoplast mediate vacuolar function in Arabidopsis

“…A group of phosphatases, designated suppressor of actin (SAC) domain phosphatases, has been identified in lower eukaryotes, mammals, and plants (21). Whereas yeast and humans have only five genes harboring the SAC domain, the genome of the model plant Arabidopsis thaliana contains nine genes, of which some have been functionally characterized and demonstrated to be involved in the regulation of stress responses (22)(23)(24), polarized root hair expansion (3), or cell wall formation (25).…”