Recessive mutations at three loci cause the collapse of mature xylem cells in inflorescence stems of Arabidopsis. These irregular xylem (irx) mutations were identified by screening plants from a mutagenized population by microscopic examination of stem sections. The xylem cell defect was associated with an up to eightfold reduction in the total amount of cellulose in mature inflorescence stems. The amounts of cell wall-associated phenolics and polysaccharides were unaffected by the mutations. Examination of the cell walls by using electron microscopy demonstrated that the decreases in cellulose content of irx lines resulted in an alteration of the spatial organization of cell wall material. This suggests that a normal pattern of cellulose deposition may be required for assembly of lignin or polysaccharides. The reduced cellulose content of the stems also resulted in a decrease in stiffness of the stem material. This is consistent with the irregular xylem phenotype and suggests that the walls of irx plants are not resistant to compressive forces. Because lignin was implicated previously as a major factor in resistance to compressive forces, these results suggest either that cellulose has a direct role in providing resistance to compressive forces or that it is required for the development of normal lignin structure. The irx plants had a slight reduction in growth rate and stature but were otherwise normal in appearance. The mutations should be useful in facilitating the identification of factors that control the synthesis and deposition of cellulose and other cell wall components.

Medical Education 2012: 46: 757–765 Context  Many tests of medical knowledge, from the undergraduate level to the level of certification and licensure, contain multiple‐choice items. Although these are efficient in measuring examinees’ knowledge and skills across diverse content areas, multiple‐choice items are time‐consuming and expensive to create. Changes in student assessment brought about by new forms of computer‐based testing have created the demand for large numbers of multiple‐choice items. Our current approaches to item development cannot meet this demand. Methods  We present a methodology for developing multiple‐choice items based on automatic item generation (AIG) concepts and procedures. We describe a three‐stage approach to AIG and we illustrate this approach by generating multiple‐choice items for a medical licensure test in the content area of surgery. Results  To generate multiple‐choice items, our method requires a three‐stage process. Firstly, a cognitive model is created by content specialists. Secondly, item models are developed using the content from the cognitive model. Thirdly, items are generated from the item models using computer software. Using this methodology, we generated 1248 multiple‐choice items from one item model. Conclusions  Automatic item generation is a process that involves using models to generate items using computer technology. With our method, content specialists identify and structure the content for the test items, and computer technology systematically combines the content to generate new test items. By combining these outcomes, items can be generated automatically.

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