A genomic library screen and PCR-based strategies were employed to isolate six genes with sequence similarity to a cotton fibre-specific mRNA encoding an alpha-expansin. alpha-Expansins are cell wall proteins that facilitate cell wall extension by disruption of non-covalent bonds between wall components. The characterisation and expression analysis of these six novel genes ( GhExp1-GhExp6) is described. Four of them ( GhExp3- GhExp6) are expressed within multiple tissues of the cotton plant and two ( GhExp1 and GhExp2) give rise to transcripts that are specific to the developing cotton fibre. GhExp1 transcripts are highly abundant in the fibre, while transcripts for GhExp2 are detected at a low level. Cotton fibres are highly elongated cells of the ovule epidermis, and we envisage that GhExp1 may play an important role in cell wall extension during development.

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Homologies between spinach nuclear (n) DNA and Chloroplast (pt) DNA, have been detected with a clone bank of spinach ptDNA as hybridization probes to restriction fragments of nDNA prepared from purified root nuclei. Every cloned fragment of ptDNA showed homologies to discrete restriction fragments of nDNA, different from those of ptDNA, indicating integration of these homologies into nDNA. While most ptDNA clones were relatively large and probably contained several genes, sequence homologies were also found to the cloned plastid gene for RuBP carboxylase and the β subunit of ptATPase. Many of the homologies in nDNA occur in regions of the genome that are highly methylated and are not digested by the methylation sensitive restriction endonucleases HpaII and MspI. In contrast these enzymes cleave ptDNA into small fragments which allows the nDNA homologies to be distinguished in total root DNA. The sequence homologies observed were not due to contaminating non nuclear sequences as shown by hybridization to mitochondrial (mt) and bacterial DNAs. The total amount of homology to ptDNA in nDNA is equivalent to about five copies of the plastome per haploid nuclear genome. The homologies generally appear to be in individual segments of less than 2 kbp in length, integrated into several different places in the genome.

The role of chloroplast (cp) DNA in plastid and chloroplast function is discussed, particularly in relation to the interaction with nuclear DNA. The evolution of the chloroplast genome and the endosymbiont hypothesis are related to our results and those of others which show the occurrence of cpDNA sequences common to the nuclear and chloroplast genome.

Higher plant cells contain three genetic compartments which separately transcribe DNA and translate messenger (m)RNA into polypeptides. Most plant proteins derive from nuclear genes and their mRNAs are translated on ribosomes in the cytoplasm. Many fewer genes are located within mitochondrial (mt) or plastid (including chloroplast (cp)) DNA, and mRNAs from these genomes are translated on separate populations of ribosomes within the particular organelles. The cytoplasmic genomes are much smaller than that of the nucleus and they are present in multiple copies in each organelle. As there are many chloroplasts and mitochondria in each cell, these genomes may contribute a large proportion of total cellular DNA. Chloroplast DNA, in particular, has been extensively used to study plant phylogeny. The reasons why it is useful and some of the possible problems associated with its use in taxonomy are discussed.