SUMMARYSerration found along leaf margins shows species-specific characters. Whereas compound leaf development is well studied, the process of serration formation is largely unknown. To understand mechanisms of serration development, we investigated distinctive features of cells that could give rise to tooth protrusion in the simpleleaf plant Arabidopsis. After the emergence of a tooth, marginal cells, except for cells at the sinuses and tips, started to elongate rapidly. Localized cell division seemed to keep cells at the sinus smaller, rather than halt cell elongation. As leaves matured, the marginal cell number between teeth became similar in any given tooth. These results suggest that teeth are formed by repetition of an unknown mechanism that spatially monitors cell number and regulates cell division. We then examined the role of CUP-SHAPED COTYLEDON 2 (CUC2) in serration development. cuc2-3 forms fewer hydathodes and auxin maxima, visualized by DR5rev::GFP, at the leaf margin, suggesting that CUC2 patterns serration through the regulation of auxin. In contrast to a previous interpretation, comparison of leaf outlines revealed that CUC2 promotes outgrowth of teeth rather than suppression of growth at the sinuses. We found that mutants with increased CUC2 expression form ectopic tissues and mis-express SHOOT MERISTEMLESS (STM) at the sinus between the enhanced teeth. Similar but infrequent STM expression was found in the wild type, indicating STM involvement in the serration of simple leaves. Our study provides insights into the morphological and molecular mechanisms for leaf development and tooth formation, and highlights similarities between serration and compound leaf development.

MICROTUBULE ORGANIZATION 1 (MOR1) is a plant member of the highly conserved MAP215/Dis1 family of microtubuleassociated proteins. Prior studies with the temperature-sensitive mor1 mutants of Arabidopsis (Arabidopsis thaliana), which harbor single amino acid substitutions in an N-terminal HEAT repeat, proved that MOR1 regulates cortical microtubule organization and function. Here we demonstrate by use of live cell imaging and immunolabeling that the mor1-1 mutation generates specific defects in the microtubule arrays of dividing vegetative cells. Unlike the universal cortical microtubule disorganization in elongating mor1-1 cells, disruption of mitotic and cytokinetic microtubule arrays was not detected in all dividing cells. Nevertheless, quantitative analysis identified distinct defects in preprophase bands (PPBs), spindles, and phragmoplasts. In nearly one-half of dividing cells at the restrictive temperature of 30°C, PPBs were not detected prior to spindle formation, and those that did form were often disrupted. mor1-1 spindles and phragmoplasts were short and abnormally organized and persisted for longer times than in wild-type cells. The reduced length of these arrays predicts that the component microtubule lengths are also reduced, suggesting that microtubule length is a critical determinant of spindle and phragmoplast structure, orientation, and function. Microtubule organizational defects led to aberrant chromosomal arrangements, misaligned or incomplete cell plates, and multinucleate cells. Antiserum raised against an N-terminal MOR1 sequence labeled the full length of microtubules in interphase arrays, PPBs, spindles, and phragmoplasts. Continued immunolabeling of the disorganized and short microtubules of mor1-1 at the restrictive temperature demonstrated that the mutant mor1-1 L174F protein loses function without dissociating from microtubules, providing important insight into the mechanism by which MOR1 may regulate microtubule length.

MOR1, the Arabidopsis thaliana homologue of the Xenopus microtubule-associated protein MAP215, is required for spatial organization of the acentrosomal microtubule arrays of plant cells. To determine how loss of MOR1 function affects microtubule dynamics, we compared various parameters of microtubule dynamics in the temperature-sensitive mor1-1 mutant at its permissive and restrictive temperatures, 21°C and 31°C, respectively. Dynamic events were tracked in live cells expressing either GFP-tagged β-tubulin or the plus end tracking EB1. Microtubule growth and shrinkage velocities were both dramatically reduced in mor1-1 at 31°C and the incidence and duration of pause events increased. Interestingly, the association of EB1 with microtubule plus ends was reduced in mor1-1 whereas side wall binding increased, suggesting that MOR1 influences the association of EB1 with microtubules either by modulating microtubule plus end structure or by interacting with EB1. Although mor1-1 microtubules grew and shrank more slowly than wild-type microtubules at 21°C, the incidence of pause was not altered, suggesting that pause events, which occur more frequently at 31°C, have a major detrimental role in the spatial organization of cortical microtubules. Extensive increases in microtubule dynamics in wild-type cells when shifted from 21°C to 31°C underline the importance of careful temperature control in live cell imaging.