Characterization and expression of a cDNA encoding a tubuliform silk protein of the golden web spider Nephila antipodiana

Spider silks are renowned for their excellent mechanical properties. Although several spider fibroin genes, mainly from dragline and capture silks, have been identified, there are still many members in the spider fibroin gene family remain uncharacterized. In this study, a novel silk cDNA clone from the golden web spider Nephila antipodiana was isolated. It is serine rich and contains two almost identical fragments with one varied gap region and one conserved spider fibroin-like C-terminal domain. Both in situ hybridization and immunoblot analyses have shown that it is specifically expressed in the tubuliform gland. Thus, it likely encodes the silk fibroin from the tubuliform gland, which supplies the main component of the inner egg case. Unlike other silk proteins, the protein encoded by the novel cDNA in water solution exhibits the characteristic of an alpha-helical protein, which implies the distinct property of the egg case silk, though the fiber of tubuliform silk is mainly composed of beta-sheet structure. Its sequence information facilitates elucidation of the evolutionary history of the araneoid fibroin genes.     

New pollen-specific receptor kinases identified in tomato,maize and Arabidopsis: the tomato kinases show overlapping but distinct localization patterns on pollen tubes

We previously characterized LePRK1 and LePRK2, pollen-specific receptor kinases from tomato (Muschietti et al., 1998). Here we identify a similar receptor kinase from maize, ZmPRK1, that is also specifically expressed late in pollen development, and a third pollen receptor kinase from tomato, LePRK3. LePRK3 is less similar to LePRK1 and LePRK2 than either is to each other. We used immunolocalization to show that all three LePRKs localize to the pollen tube wall, in partially overlapping but distinct patterns. We used RT-PCR and degenerate primers to clone homologues of the tomato kinases from other Solanaceae. We deduced features diagnostic of pollen receptor kinases and used these criteria to identify family members in the Arabidopsis database. RT-PCR confirmed pollen expression for five of these Arabidopsis candidates; two of these are clearly homologues of LePRK3. Our results reveal the existence of a distinct pollen-specific receptor kinase gene family whose members are likely to be involved in perceiving extracellular cues during pollen tube growth.     

Role of accelerated style senescence in pathogen defense

Plants, like animals, suffer from a variety of diseases that are transmitted via their sexual organs. In many species, the flowers senesce rapidly after pollination or fertilization. In ongoing studies of the impacts of a transposon insertional mutation in the gene that encodes the most abundant isoform of a major group-1 pollen allergen of maize, we found that pollen tubes with the mutant allele grow significantly slower in vivo than pollen with the wild-type allele. Here, we report that under field conditions, maize silks (styles) pollinated with pollen bearing the slower-growing mutant allele take significantly longer to senesce, and the resulting ears (infructescences) have dramatically higher incidence of "fungal ear rot" disease than silks pollinated with pollen bearing the wild-type allele. Because ear rot fungi gain access to the developing ear by growing on and through the silks, we propose that accelerated senescence of silks after fertilization is a defense against pathogens such as those causing ear rot. In addition, we divided the silks on each ear into two halves and experimentally varied the type of pollen (wild type, mutant, unpollinated) that was placed onto each half of the silks. Senescence of unpollinated silks was accelerated when ovaries on the other half of the ear were fertilized.     

Maize pollen coat xylanase facilitates pollen tube penetration into silk during sexual reproduction

Cell wall hydrolases are well documented to be present on pollen, but their roles on the stigma during sexual reproduction have not been previously demonstrated. We explored the function of the tapetum-synthesized xylanase, ZmXYN1, on maize (Zea mays L.) pollen. Transgenic lines (xyl-less) containing little or no xylanase in the pollen coat were generated with use of an antisense construct of the xylanase gene-coding region driven by the XYN1 gene promoter. Xyl-less and wild-type plants had similar vegetative growth. Electron microscopy revealed no appreciable morphological difference in anther cells and pollen between xyl-less lines and the wild type, whereas immunofluorescence microscopy and biochemical analyses indicated an absence of xylanase on xyl-less pollen. Xyl-less pollen germinated as efficiently as wild-type pollen in vitro in a liquid medium but less so on gel media of increasing solidity or on silk, which is indicative of partial impaired water uptake. Once germinated in vitro or on silk, the xyl-less and wild-type pollen tubes elongated at comparable rates. Tubes of germinated xyl-less pollen on silk did not penetrate into the silk as efficiently as tubes of wild-type pollen, and this lower efficiency could be overcome by the addition of xylanase to the silk. For wild-type pollen, coat xylanase activity on oat spelled xylan in vitro and tube penetration into silk were inhibited by xylose but not glucose. The overall findings indicate that maize pollen coat xylanase facilitates pollen tube penetration into silk via enzymatic xylan hydrolysis.     

植物自交不亲和基因研究进展

自交不亲和性的研究是植物生殖生物学和分子生物学研究的热点之一,对自交不亲和基因和蛋白质的深入研究是解析自交不亲和性机理的关键.对控制孢子体自交不亲和性和配子体自交不亲和性的S基因及其蛋白质产物的分子生物学研究进展进行了综述.孢子体自交不亲和性植物S位点上至少存在3个基因,即SLG、SRK和SCR基因.其中SLG、SRK基因控制雌蕊自交不亲和性,而SCR控制花粉自交不亲和性.配子体自交不亲和植物雌蕊S基因产物为S-RNase,具有核酸酶活性;配子体自交不亲和植物花粉S基因产物尚未找到.     

An independently evolved Dipteran silk with features common to Lepidopteran silks

Male hilarine flies (Diptera: Empididae: Empidinae) present prospective mates with silk-wrapped gifts. The silk is produced by specialised cells located in the foreleg basitarsus of the fly. In this report, we describe 2.3 kbp of the silk gene from a hilarine fly (Hilara spp.) that was identified from highly expressed mRNA extracted from the prothoracic basitarsus of males. Using specific primers, we found that the silk gene is expressed in the basitarsi and not in any other part of the male fly. The silk gene from the basitarsi cDNA library matched an approximately 220 kDa protein from the silk-producing basitarsus. Although the predicted silk protein sequence was unlike any other protein sequence in available databases, the architecture and composition of the predicted protein had features in common with previously described silks. The convergent evolution of these features in the Hilarini silk and other silks emphasises their importance in the functional requirements of silk proteins.