1 Int. J. Plant Sci. 163(1):1–16. 2002.  2002 by The University of Chicago. All rights reserved. 1058-5893/2002/16301-0001$15.00 THE STIGMA SURFACE AND POLLEN-STIGMA INTERACTIONS IN SENECIO SQUALIDUS L. (ASTERACEAE) FOLLOWING CROSS (COMPATIBLE) AND SELF (INCOMPATIBLE) POLLINATIONS Simon J. Hiscock, 1 Karin Hoedemaekers, William E. Friedman, and Hugh G. Dickinson School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, United Kingdom; Department of Experimental Botany, Katholieke Universiteit Nijmegen, Toernooiveld 1, 6525 Nijmegen, Netherlands; Department of Environmental, Population and Organismal Biology, University of Colorado, Boulder, Colorado 80309, U.S.A.; and Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, United Kingdom Senecio squalidus (Asteraceae) has been shown to possess a stigma with characteristics of both “dry” and “wet” types of stigma. The “semidry” stigma of Senecio is characterized by the presence of a surface cuticle overlaid by a proteinaceous pellicle and a small constitutive surface secretion consisting of lipid, carbohydrate, and protein. We anticipate that this semidry stigma may be a general feature of the Asteraceae. Secretion by the Senecio stigma is enhanced after both compatible and incompatible pollinations, when material secreted by the stigma combines with pollenkitt extruded from the alveolar exine of the pollen to form a heterogeneous “attachment foot” between pollen and stigmatic papillae. During this period, discrete inclusions, “wall bodies,” can be seen within cell walls of papillae in contact with pollen grains, apparently exporting their contents across the cell wall and onto the surface of the stigma. Following compatible pollination, the emergent pollen tube grows through the attachment foot and between the tightly packed stigmatic papillae before penetrating the stigma at the base of the papilla cells, where the cuticle is absent. The pollen tube then grows intercellularly, within the middle lamella, through the stigma toward the style. Following incompatible pollinations, devel- opment of pollen is highly variable. Most incompatible pollen grains fail to germinate, but many do germinate to produce pollen tubes, some of which penetrate the stigma before they are inhibited. Such extensive devel- opment of incompatible pollen tubes is unusual for a species with homomorphic sporophytic self-incompat- ibility. These observations are discussed as a comparison with events at the dry stigma surface of Brassica following compatible and incompatible pollinations and in relation to current theories on the evolution of wet and dry stigmas. Keywords: Asteraceae, pollen-stigma interactions, pollination responses, self-incompatibility, Senecio, stigma surface. Introduction The stigma surfaces of flowering plants have been classified as “wet” or “dry” based on the presence or absence, respec- tively, of a copious stigmatic secretion (Heslop-Harrison et al. 1975; Heslop-Harrison and Shivanna 1977). The secretions of wet stigmas, which can be primarily lipid rich, as in the So- lanaceae, or primarily carbohydrate rich, as in the Liliaceae, are required for correct pollen hydration, germination, and penetration of the stigma by pollen tubes (Goldman et al. 1994). Recently, lipidic components of stigmatic secretions, particularly cis-unsaturated triacylglycerides, have been shown to be essential for pollen tube penetration of the stigma and probably for directional growth of the pollen tube on the stigma as well (Lush et al. 1998; Wolters-Arts et al. 1998). Indeed, in the presence of such lipids, pollen tubes will even penetrate leaves, albeit with the cuticle removed (Wolters-Arts et al. 1998). It has been proposed that the role of the lipids is 1 Author for correspondence; telephone 44-1179546835; fax 44- 1179257374; e-mail simon.hiscock@bristol.ac.uk. Manuscript received June 2001; revised manuscript received August 2001. to facilitate the establishment of a gradient of water within the stigmatic secretion that acts as a guidance cue for pollen tubes on the stigma (Lush et al. 1998, 2000). Upon germi- nation, therefore, pollen tubes are presented with a path of increasing water concentration into the more aqueous envi- ronment of the conducting tissue of stigma and style. Dry stigmas, which lack a copious surface secretion, are covered by a continuous cuticle that must be penetrated en- zymatically by pollen tubes, using a cutinase, in order to effect successful fertilization (Heslop-Harrison et al. 1975; Maiti et al. 1979; Hiscock et al. 1994). Covering the cuticle is a thin proteinaceous surface layer, the pellicle, which can be detected, indirectly, by its strong nonspecific esterase activity (Mattson et al. 1974; Heslop-Harrison et al. 1975). The function of the pellicle is unknown, but it has been predicted to play an es- sential role in pollen-stigma recognition, because, in a variety of species, removal of the pellicle with dilute detergents pre- vents pollen tubes from penetrating the stigma (Mattson et al. 1974; Heslop-Harrison and Heslop-Harrison 1975; Heslop- Harrison et al. 1975; Hiscock et al. 1998). Interestingly, despite the dry nature of cuticularized stigmas, a lipidic surface en- vironment is still essential for successful pollen hydration, ger- This content downloaded from 128.103.149.052 on April 11, 2016 12:38:04 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).