Plant Physiol. (1993) 103: 979-985 zyxwvutsrq Pollen Tube and Root-Hair Tip Growth 1s Disrupted in a Mutant zyxw of zyx Arabidopsis thaliana’ John Schiefelbein*, Moira Calway, JamesMasucci, and Susan Ford Department zyxwvutsr of Biology, University of Michigan, Ann Arbor, Michigan 481 O9 l h e expansion of both root hairs and pollen tubes occurs by a process known as tip growth. In this report, an Arabidopsis thaliana mutant (tipl) is described that displays defects in both root-hair and pollen-tube growth. l h e root hairs of the zyxwvutsrqp fipl mutant plants are shorter than those of the wild-type plants and branched at their base. l h e tipl pollen-tube growth defect was identified by the aberrant segregation ratio of phenotypically normal to mutant seeds in siliques from self-pollinated, heterozygous plants. Homo- zygous mutant seeds are not randomly distributed in the siliques, comprising only 14.4% of the total seeds, 5.3% of the seeds from the bottom half, and 2.2% of the seeds from the bottom quarter of the heterozygous siliques. Studies of pollen-tube growth in vivo showed that mutant pollen tubes grow more slowly than wild-type pollen through the transmitting tissue of wild-type flowers. Co- segregation studies indicate that the root-hair and pollen-tube defects are caused by the same genetic lesion. Based on these findings, the TlPl gene is likely to encode a product involved in a fundamental aspect of tip growth in plant cells. The morphogenesis of plant cells is largely dependent upon the proper control of the direction and extent of cellular expansion. A well-characterized mode of plant cell expansion is tip growth, whereby new cell growth is limited to a single growing point and leads to the formation of a tubular-shaped cell. Two cell types that are known to display tip growth are pollen tubes and root hairs (Schnepf, 1986; Steer and Steer, 1989; Heath, 1990). The polarized growth of root hairs and pollen tubes is similar in severa1 respects. Ultrastructural studies have re- vealed a characteristic organization of the cytoplasmic con- tents in these cells, with exocytotic Golgi vesicles and ER elements at the apex, the larger organelles (Golgi bodies, ER, and mitochondria) in the subapical zone, and the enlarging central vacuole at the proximal part of the cells (Schnepf, 1986). Also, the growth of each cell is associated with, and dependent upon, a continuous influx of Ca2+ at the apex and an interna1 Caz+gradient (Reiss and Herth, 1979; Clarkson et al., 1988; Schiefelbein et al., 1992). Furthermore, normal root-hair and pollen-tube morphogenesis requires a func- tional actin cytoskeleton (Heath, 1990). ~ Financia1 support was provided by a grant from the National Science Foundation (DCB-9004568), a Natural Sciences and Engi- neering Research Council postdoctoral fellowship (M.G.), and a National Institutes of Health developmental biology training grant (S.F.). * Corresponding author; fax 1-313-747-0884. 979 Because of the similarity in the growth of root hairs and pollen tubes, it is possible that genes exist whose products are required for the expansion of both cell types. Numerous studies have demonstrated that a significant proportion of the genes expressed in pollen are also expressed in vegetative tissue (Ottaviano and Mulcahy, 1989). Using isozyme anal- yses (Tanksley et al., 1981; Sari Gorla et al., 1986) and RNA hybridization studies (Willing et al., 1984; Willing and Mas- carenhas, 1984), the degree of overlap between gametophytic and sporophytic gene expression has been estimated at 40 to 70%. Although these studies provide a general understanding of the leve1 of genetic overlap in the sporophytic and game- tophytic phases of plant development, they do not provide detailed information on the specific sporophytic cells or tis- sues where the pollen genes are expressed. In particular, there has been no report of the identification or characteriza- tion of genes expressed in both pollen tubes and root hairs. One strategy to identify genes essential for the tip-growth process is to identify mutations that simultaneously alter growth of both root-hair and pollen-tube cells. A large col- lection of root-hair morphology mutants in zyx Arabidopsis thal- iana has previously been generated and partially character- ized (Schiefelbein and Somerville, 1990). In this report a mutant that displays defects in both root-hair formation and pollen-tube growth is described from this collection. The product of the affected gene is suggested to play an important role in tip growth in each cell type. MATERIALS AND METHODS Plant Materials and Growth Conditions The mutant lines of Arabidopsis thaliana (L.) Heynh. de- scribed in this report are derived from the Columbia ecotype. The ethyl methanesulfonate mutagenesis that generated the tipl mutant has been described (Estelle and Somerville, 1987; Schiefelbein and Somerville, 1990). A11 comparative studies were conducted with plants that had been backcrossed to the Columbia wild type for at least two generations. For growth of plants in Petri dishes, seeds were surface- sterilized, placed on the surface of agarose-solidified medium, chilled at 4OC, and incubated in a vertical orientation as previously described (Schiefelbein and Somerville, 1990). Root-hair phenotypes were scored by examining the seedling roots after 4 to 6 d of growth under these conditions. The growth of Arabidopsis plants in soil has been described (Schie- felbein and Somerville, 1990). www.plant.org on December 25, 2015 - Published by www.plantphysiol.org Downloaded from Copyright © 1993 American Society of Plant Biologists. All rights reserved.