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Abstract
Plant cells are able to elongate anisotropically due to a constraining network of cellulose microfibrils in their cell walls and cortical microtubules underneath the plasma membrane. As it is believed that the cortical microtubules (CMTs) somehow direct the organized deposition of cellulose microfibrils (CMFs) identification of proteins involved in controlling microtubule (MT) dynamics or organization will help us better understand the cellular processes of anisotropic growth. Mutation of the Arabidopsis katanin-like MT-severing protein, FRA2, hereafter called AtKTN1, an ortholog of the animal p60 subunit of katanin, caused a dwarf phenotype with reductions in the anisotropic growth of cells in all organs. Transmission electron microscopic examination of cells in the interfascicular region of wild-type and AtKTN1 mutant stems showed a reduction in the thickness of secondary wall in mutant cells and revealed differences in secondary wall structure. Defects in AtKTN1 function caused a defect in MT dynamics in cells as determined by confocal microscopy and appeared to involve a delay in the disassembly of the perinuclear MT array after cytokinesis. This defect results in a disorganized CMT array in cells of elongating roots, stems, and petioles. Visualization of CMFs in the primary and secondary cell walls of these cell types using field emission scanning electron microscopy showed a correlation between disruption of the CMTs and CMFs. CMFs in wild-type cells were typically arranged transverse to the axis of elongation while those of AtKTN1 mutants were disorganized and appear to form bands of microfibrils that changed their orientation over short distances. Overexpression of AtKTN1 cDNA in transgenic Arabidopsis did not result in the fragmentation of CMTs but affected CMT organization, the deposition pattern of CMFs, and anisotropic growth. Analysis of the AtKTN1 mutant revealed the importance of the katanin-like MTsevering protein in the proper formation of the CMT array in elongating Arabidopsis cells. Lack of katanin function is implicated in the delay of the disassembly of the perinuclear MT array which leads to aberrantly organized CMTs and a reduction of anisotropic growth.