Genetic Screens Have Identified Homologous and Plant Specific MTBinding Proteins

An approach aimed at identifying mutants with impaired MT organization was conducted using immunofluorescence microscopy [15]. This screen identified a temperature-sensitive Arabidopsis mutant that had shortened and disorganized MTs at restrictive temperatures. This mutant, microtubule organization l (morl), encodes an ortholog of the highly conserved MAP215/Dis1 family of MAPs. At restrictive temperatures, this mutant demonstrated severe phenotypes throughout development, including helical twisting of organs and isotropic cell expansion.

Helical twisting and isotropic cell growth phenotypes, such as those observed in the morl mutant, are common in mutants that affect MT organization. The Arabidopsis mutant, spirall (sprl), showed a distinctive twisted growth phenotype in various organs of the plant [44]. The sprl locus codes for a novel 12-kDa MT-binding protein that labels all four MT arrays and is concentrated at the growing plus ends of cortical MTs [10, 11]. This protein is thought to serve as a stabilizing factor for cortical MTs, possibly by crosslinking proteins at the plus end of the MT.

Another mutant with impaired MT function showed visible defects in cell shape. Leaves of the maize mutant, tanl, possessed cells that divided in abnormal orientations without altering leaf shape [45]. TAN1 is another novel plant MT-binding protein that binds to MTs in vivo, and it is thought to have a role in guiding expanding phragmo-plasts to pre-established division sites on the cell cortex [46]. Several other interesting mutants were shown to have altered MT-binding protein activity. For example, fragile fibre2, a mutant that affects cell wall biosynthesis, encoded a katanin-like MT-severing protein [13], and the spr2/tortifolia1 mutant demonstrates helical growth and codes for a novel MT-binding protein that regulates cortical MT orientation [12, 47]. The Zwichel gene codes for a kinesin-like CaM-binding protein (KCBP), and plants having mutations in this gene have unusual trichome phenotypes [48].

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