A conditional sterile mutation eliminates surface components from Arabidopsis pollen and disrupts cell signaling during fertilization.

Abstract

Plants distinguish among the pollen grains that land on the stigma, permitting only compatible pollen to fertilize egg cells. To investigate these cell-cell interactions, Arabidopsis mutations that affect pollen-pistil communication were isolated. A male-sterile mutation that disrupts pollen-pistil interactions by eliminating the extracellular pollen coat (tryphine) is described here. Stigma cells that contact the mutant pollen produce callose, a carbohydrate synthesized in response to foreign pollen. The mutant pollen fails to germinate because it does not absorb water from the stigma, yet germinates in vitro, indicating it is viable. The defect is also conditional; high humidity results in pollen hydration and successful fertilization. Analysis of mature, mutant pollen indicated that it is deficient in long-chain lipids and has none of the lipoidic tryphine normally present on its surface. Immature mutant pollen grains have aberrant tryphine that disappears during pollen development. The sterile plants also lack stem waxes, and pollen from other wax-defective (eceriferum) mutants with reduced fertility has few of the lipid droplets normally present in tryphine. These results demonstrate that tryphine is critical for pollen-stigma interactions and suggest that tryphine lipids are required for fertilization, either by directly signaling the stigma or by stabilizing other tryphine components.[Key Words: Pollen; Arabidopsis; compatibility; lipids; tryphine; eceriferum] Received February 22, 1993; revised version accepted April 7, 1993. Fertilization in flowering plants involves many cell-cell interactions, including adhesion of pollen grains to the -stigma surface, growth of pollen tubes through the pistil, migration of sperm cells within the pollen tube, and, ultimately, fusion of the sperm with egg cells; yet few of the molecules required for these interactions have been identified. As a first step in understanding these cellsignaling events, several sterile Arabidopsis mutants blocked in early steps of fertilization were identified. In this paper, one such mutant, defective in the interaction between pollen grains and stigma cells, is described.The signaling events that occur during fertilization presumably require direct contact between cells and rely on molecules present in low abundance. Few of these molecules have been identified through in vitro reconstitution experiments, in part because manipulation of reproductive tissues is technically difficult. In contrast, genetic approaches depend solely on the function of molecules, rather than on their abundance, and allow cellcell communication events to be identified and characterized in living plants. Because most plant species are ~Corresponding author.hermaphrodites (producing both sperm and eggs), mutant plants defective in either the male or the female reproductive process can be obtained and propagated readily.Arabidopsis thaliana is well-suited to a genetic approach to fertilization. The advantages of its short life cycle, small physical size, and sma...

Signaling Events in Pollen Acceptance or Rejection in the Arabidopsis Species

“…Arabidopsis species possesses a dry stigma that is covered with papillae on the stigmatic surface. Once a compatible pollen grain comes in contact with a stigmatic papilla, the early stages of pollen capture and adhesion, pollen hydration, and germination are closely regulated (Elleman et al 1992 ;Preuss et al 1993 ;Kandasamy et al 1994 ;Zinkl et al 1999 ). The ability to conduct genetic screens with relative ease in A. thaliana has aided in identifying a number of factors that regulate pollen-stigma interactions (Preuss et al 1993 ;Hulskamp et al 1995 ;Nishikawa et al 2005 ).…”

“…The pollen coat is then involved in further adhesion of the pollen grain to the stigmatic papilla. At this stage, the pollen coat fl ows out from the pollen grain toward the stigmatic papilla to mix with the lipidic and proteinaceous surface of the stigmatic papilla, creating a more robust connection at the pollen-stigma interface (Elleman et al 1992 ;Preuss et al 1993 ;Kandasamy et al 1994 ;Zinkl et al 1999 ).…”

“…The result is an interface between the pollen grain and the papilla where signaling is hypothesized to occur, and water is transferred from the papilla to the pollen grain for hydration (Elleman and Dickinson 1990 ;Preuss et al 1993 ). Pollen hydration is necessary for the desiccated pollen grain to become metabolically active and proceed to the next stage of pollen germination and formation of a pollen tube, and components of the pollen-stigma interface are important to this process (Preuss et al 1993 ).…”

“…Pollen hydration is necessary for the desiccated pollen grain to become metabolically active and proceed to the next stage of pollen germination and formation of a pollen tube, and components of the pollen-stigma interface are important to this process (Preuss et al 1993 ). On the female side, the specifi c lipid content in the stigmatic cuticle was indirectly implicated in the control of pollen hydration through the study of the A. thaliana fi ddlehead ( fdh ) mutant.…”

“…The importance of pollen coat lipids in the hydration process was determined by the study of impaired pollen hydration in A. thaliana eceriferum ( cer ) mutants (Preuss et al 1993 ;Hulskamp et al 1995 ). The cer mutants have defects in the long-chain lipid synthesis, and there was a reduction or loss of pollen coat on the surface of the cer mutant pollen grains.…”