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Recombinant cDNA libraries to poly(A)RNA isolated from mature pollen of Zea mays and Tradescantia paludosa have been constructed. Northern blot analyses indicate that several of the clones are unique to pollen and are not expressed in vegetative tissues. The majority, however, are expressed both in pollen and vegetative tissues. Southern hybridizations show that the pollen specific sequences in corn are present in one or a very few copies in the genome. By using several of the clones as probes, it was found that there are at least two different groups of mRNAs with respect to their synthesis. The mRNAs of the first group represented by the pollen specific clones are synthesized after microspore mitosis and increase in concentration up to maturity. The second group, exemplified by actin mRNA, begins to accumulate soon after meiosis, reaches its maximum by late pollen interphase, and decreases thereafter. Although the actin mRNA and the pollen specific mRNAs studied show very different patterns of initiation of synthesis and accumulation during pollen development, the rates of decline of these mRNAs during the first 60 minutes of germination and pollen tube growth in Tradescantia are similar and reflect the previously observed declines in rates of protein synthesis during this period.The pollen grains of Tradescantia paludosa (16), corn (17), and tobacco (30) contain a store of presynthesized messenger RNAs (mRNAs) at the time of their release from the anther. These mRNAs have been shown to code in cell-free translation systems for proteins that are similar to the proteins synthesized during pollen germination and tube growth (9,17 shown that a large fraction (>64%) of the genes expressed in pollen are also expressed in vegetative tissues, whereas no more than 60% of the genes expressed in shoots are similar to those expressed in pollen (31). Similar hybridizations have been carried out with RNA from corn pollen and the results are similar to those obtained with Tradescantia (RP Willing, JP Mascarenhas, unpublished data). To further our knowledge of pollen development it is essential that the genes that are expressed in pollen, especially those unique to pollen, be isolated and characterized in some detail in order to understand their developmentally specific regulation and functions. We report here the construction of two cDNA libraries made to pollen mRNAs from Tradescantia and corn, and the utilization of some of the clones to answer questions about the nature of the pollen expressed genes and their transcription during pollen development and tube growth. MATERIALS AND METHODSPlant Material. Tradescantia paludosa L. plants were grown in the greenhouse and pollen was collected and stored as previously described (15). Corn (Zea mays L.) pollen was collected from field grown plants of the cultivar 'Gold Cup' (Harris Seeds, Rochester, NY). Pollen was quick frozen in liquid N2 and stored at -70°C. For later experiments the inbred line of corn W-22 (Illinois Foundation Seeds) was used. Various vegetative and ...

Mature ungerminated pollen grains of Zea mays L. contain presynthesized messenger RNAs. This has been demonstrated by the isolation of poly(A)RNA and its translation in the wheat germ and reticulocyte cell free systems into polypeptides many of which are similar to those synthesized in germinating pollen. Each corn pollen grain contains between 352-705 pg of total RNA and 8.9-17.8 pg of poly(A)RNA. During germination of corn pollen at least 260 different polypeptides are synthesized as determined by labeling and 2-dimensional gel electrophoresis. These results are discussed with reference to other plants and the number of different genes expressed during pollen development.

Rates of solute leakage from excised discs of cucumber (Cucumis sativus L. cv Straight Eight) cotyledons were altered by temperature during plasmolysis in the manner of a simple diffusion phenomenon; the log of the leakage rate increased in proportion to the temperature. During deplasmolysis, however, leakage rates responded to temperature with a very different pattern: chilling conditions (below about 20°C) caused large increases in leakage rates, indicating disruption of membrane integrity in the tissues. The time course of restoration of normal leakage rates after deplasmolysis/chilling damage indicated a rapid repair of the lesions. A similar sensitivity to low temperatures was found during rehydration after leaf desiccation, with low temperatures again causing high leakage rates. It is suggested that low temperatures interfere with membrane expansion, possibly by lowering elasticity and hindering the incorporation of lipid material into the expanding membrane. The expansion of tissues at low temperatures may cause lesions in cellular membranes, contributing to chilling injury.Membrane damage in plant cells is known to occur under diverse environmental stresses. Rates of solute leakage have been used to monitor damage resulting from freezing (22), chilling (1 1), desiccation (2, 10), osmotic shock (6), and toluene treatment (25). By studying the dynamics of solute leakage, one may be able to estimate not only the extent of damage, but also the timing of damage and repair, and the conditions that cause damage. In the case of chilling injury, it has been assumed that low temperature per se is a stress condition which causes leakage (12,20). We have studied the interactions of temperature with cell expansion and contraction and find that, under conditions of cell expansion at low temperatures, there is a large increase in membrane damage as assessed by solute leakage rates. MATERIALS AND METHODSCotyledonary leaves of cucumber seedlings (Cucumis sativus L. cv Straight Eight) were harvested 14 d after sowing under greenhouse conditions. Leaf discs I cm in diameter were cut from the cotyledonary leaves; 33 discs (I g fresh weight ± 0.02 g) were used for each treatment. The discs were rinsed for 5 min in deionized H20 to remove electrolytes from cut cells. Plasmolysis was obtained by placing rinsed leaf discs in 20 ml 0.5 M (-12 (0)