ABSTRACT
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.
ABSTRACT
The mRNAs of the mature pollen grain of Tradescantia paludosa at anesthesia and of vegetative shoots have been compared by analyzing the kinetics of hybridization between homologous and heterologous reactions of cDNA to poly(A)RNA in excess. The mRNAs in pollen can be divided into three abundance classes with complexities of 5.2 x 10(4), 1.6 x 10(6), and 2.1 x 10(7) nucleotides. The three classes are made up of sequences that constitute 15, 60, and 24% of the mRNAs and each sequence is present on an average at 26,000, 3,400, and 100 copies, respectively, per pollen grain. About 20,000 different genes are expressed in pollen as compared to about 30,000 in vegetative shoots. Estimates have been made of pollen mRNA sequences shared with those of shoot tissue and of shoot sequences common to those in pollen.
ABSTRACT
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.
ABSTRACT
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 degrees 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.
