Fibers. A model for studying cell differentiation, cell elongation, and cell wall biosynthesis.

A prominent anatomical feature in the inflorescence stems of Arabidopsis is the presence of fiber cells in the interfascicular regions (Fig. 1). The feasibility of using interfascicular fibers in the inflorescence stems of Arabidopsis as a model for studying cell differentiation, cell elongation, and cell wall biosynthesis has increased significantly since the completion of the Arabidopsis genome sequencing project. Because fibers are not essential for plant survival under greenhouse conditions, it is conceivable that mutants disrupting fiber cell differentiation would not be lethal and thus can be isolated. Considering the possibility that fiber and xylem cells evolved via activation of the same mechanisms for secondary wall formation (Mauseth, 1988), the study of fiber cell differentiation may also help us understand the molecular mechanisms regulating xylem cell differentiation. Recent studies on several Arabidopsis mutants have already demonstrated the feasibility of studying fiber differentiation in this model organism (Turner and Somerville, 1997; Zhong et al., 1997; Turner and Hall, 2000; Burk et al., 2001). The findings in these studies indicate that the molecular mechanisms underlying fiber differentiation have broad implications in our understanding of cell differentiation, cell elongation, and cell wall biosynthesis. In this essay, we show that the sclerenchyma cells present in the interfascicular regions of Arabidopsis inflorescence stems are fiber cells. We also present examples of mutants with defects in the development of interfascicular fiber cells. Interfascicular fiber cells with thick secondary cell wall (Fig. 1, A and B) are formed when internodes of Arabidopsis inflorescence stems cease elongation. These fibers provide mechanical support to the heavy plant body as evidenced by the ifl1 mutant in which lack of interfascicular fibers causes a pendent shoot phenotype (Zhong et al., 1997). Anatomical examination shows that in wild-type Arabidopsis inflorescence stems, three or four layers of interfascicular cells located next to the endodermis differentiate into fiber cells (Fig. 1A; Zhong and Ye, 1999). These developing fiber cells are easily recognized in elongating internodes by their tapered ends (Fig. 2A). They undergo remarkable elongation and appear to reach their maximum length before massive secondary wall thickening occurs (G. Freshour, M.G. Hahn, and Z.-H. Ye, unpublished data). Based on their morphology and elongation pattern, these interfascicular sclerenchyma cells are apparently fiber cells (Fig. 2, B and C). Because of their thick cell wall at maturity, which can be easily recognized by histological staining (Fig. 2B), fiber cells have traditionally been used for studying cell differentiation (Aloni, 1987). Early studies by Aloni (1976, 1978) and Sachs (1972) have convincingly shown that auxin polar transport regulates fiber differentiation, and auxin together with gibberellin and cytokinin is required for normal development of fiber cells (Aloni, 1987). Inspired by these early pioneering works, we screened the inflorescence stems of Arabidopsis for mutants with defects in the differentiation of interfascicular fibers. The ifl1 mutant thus isolated completely disrupts normal differentiation of interfascicular fiber cells (Zhong et al., 1997). The interfascicular cells next to the endodermis remain parenchymatous with rectangular shapes (Fig. 2D), indicating that the mutation blocks the initiation of fiber cell differentiation. It is interesting that some interfascicular cells that are not destined to become fibers are ectopically induced to form short fiber-like cells in the ifl1 mutant (Fig. 2D). The IFL1/ REV gene has been shown to encode a homeodomain Leu-zipper protein (Zhong and Ye, 1999; Ratcliffe et al., 2000). We have found recently that the ifl1 mutations dramatically reduce the auxin polar transport activity in both inflorescence stems and hypocotyls, and auxin polar transport inhibitors alter the normal differentiation of interfascicular fibers in the inflorescence stems of wild-type Arabidopsis (Zhong and Ye, 2001). These findings directly link the IFL1/REV functions to the early physiological studies regarding the role of auxin flow in fiber differentiation. After initiation of cell differentiation, fiber precursor cells undergo significant elongation at both ends, a phenomenon that is referred as intrusive growth (Mauseth, 1988). A recorded extreme example is Boehmeria nivea in which fiber precursor cells are about 1 This work was supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture. * Corresponding author; e-mail [email protected]; fax 706 –542–1805.