Disruption of Interfascicular Fiber Differentiation in an Arabidopsis Mutant

Abstract

Arabidopsis develops interfascicular fibers in stems for needed support of shoots. To study the molecular mechanisms controlling fiber differentiation, we isolated an interfascicular fiber mutant (ifl1) by screening ethyl methanesulfonate-mutagenized Arabidopsis populations. This mutant lacks normal interfascicular fibers in stems. Interestingly, some interfascicular cells were sclerified in the upper parts but not in the basal parts of the ifl1 stems. These sclerified cells were differentiated at a position different from that of interfascicular fibers in the wild type. Lack of interfascicular fibers correlated with a dramatic change of stem strength. Stems of the mutant could not stand erect and were easily broken by bending. Quantitative measurement showed that it took approximately six times less force to break basal stems of the mutant than of the wild type. In addition, noticeable morphological changes were associated with the mutant, including long stems, dark green leaves with delayed senescence, and reduced numbers of cauline leaves and branches. Genetic analysis showed that the ifl1 mutation was monogenic and recessive. The ifl1 locus was mapped to a region between the 17C2 and 7H9L markers on chromosome 5. Isolation of the ifl1 mutant provides a novel means to study the genetic control of fiber differentiation.

A Kinesin-Like Protein Is Essential for Oriented Deposition of Cellulose Microfibrils and Cell Wall Strength

“…The formation of interfascicular fibers correlates directly with an increase in the mechanical strength of mature stems. This has been demonstrated in the ifl1 mutant, in which disruption in the formation of interfascicular fibers in mature stems dramatically reduces their mechanical strength (Zhong et al, 1997). This finding indicates that the high mechanical strength of mature stems is conferred mainly by the presence of interfascicular fibers.…”

“…Because the high mechanical strength of mature stems is conferred largely by the presence of interfascicular fibers (Zhong et al, 1997), we expected the mechanical strength of fibers to be reduced dramatically in the fra1 mutant. Because no changes in fiber cell morphology and cell wall composition were seen in the fra1 mutant, the reduced mechanical strength of these fibers most likely is caused by the altered deposition of cellulose microfibrils in the secondary walls of fibers.…”

“…Arabidopsis inflorescence stems develop three to four layers of interfascicular fibers next to the endodermis to provide mechanical support to the stems (Zhong et al, 1997. Fibers in Arabidopsis have been shown to be an excellent model in which to study cell differentiation, cell elongation, and cell wall formation.…”

Auxin Acts in Xylem-Associated or Medullary Cells to Mediate Apical Dominance

“…The role of auxin in determining the number of cauline nodes is unclear. Other mutants with modified patterns of vascular development also have been shown to influence the number of cauline nodes (Zhong et al, 1997(Zhong et al, , 1999. However, the increase in the number of nodes observed in axr1-12 may be attributable to indirect auxin effects, such as the modulation of floral transition or the elongation of the primary inflorescence.…”

“…Expression in these cells has not been reported previously in tobacco, in which the 4CL1 promoter has been studied. In Arabidopsis, these cells are known to undergo sclerification, whereas they do not in tobacco (Hauffe et al, 1991;Zhong et al, 1997). Therefore, expression of a 4CL, which is involved in phenylpropanoid metabolism and hence lignification, is predicted in these tissues (Hauffe et al, 1991).…”