An Acidic 39-kDa Protein Secreted from Stigmas of Tobacco Has an Amino-Terminal Motif That is Conserved among Thaumatin-Like Proteins

In Solanaceae, pollen grains germinate on the stigmatic exudate and pollen tubes elongate within the extracellular matrix (ECM) of the transmitting tissue. Therefore, the stigmatic exudate and the ECM of the stylar transmitting tissue are the only sites at which the pistil interacts with the pollen tube before the pollen tube reaches the ovule. Thus', it is necessary to characterize the stigmatic exudate and the ECM of the transmitting tissue if we are clearly to understand the details of pollen-pistil interactions and the environment in which the pollen tube develops. Several cDNAs for proteins that are secreted in the ECM of the transmitting tissue of tobacco have been isolated (Ori et al. 1990, Goldman et al. 1992, Wang et al. 1993). TTS-1 and TTS-2, two proline-rich proteins, have been reported to attract pollen tubes and to stimulate their growth (Cheung et al. 1995). Ori et al. (1990) reported that Sp41a and Sp41b, two /?-l ,3-glucanases, are members of the family of class two pathogenesis-related (PR) proteins (PR-2). Some defense-related proteins, such as proteinase inhibitor, osmotin, chitinase, and flower-specific thionin, are also expressed in the pistil, and the expression of such proteins is developmentally regulated (Atkinson et al. 1993, Neale et al. 1990, Leung 1992, Gu et al. 1992, Karunanndaa et al. 1994). The large spaces between the transmitting cells