No abstract

A molecular approach has been used to study the role of polyamines in plant development by manipulating the expression of the S‐adenosylmethionine decarboxylase (SAMDC) gene. SAMDC is a key enzyme involved in the biosynthesis of the polyamines spermidine and spermine and is also known to influence the rate of biosynthesis of ethylene. Previously, a cDNA clone of the SAMDC gene of potato has been isolated and characterized. This cDNA clone has been used to make antisense and sense SAMDC constructs under the control of the 35S CaMV or tetracycline‐inducible promoters. Agrobacterium‐mediated transformation has been used to produce transgenic potato plants with the engineered antisense and sense SAMDC genes in order to downregulate or overexpress the SAMDC transcript, respectively. Decreases or increases in the level of SAMDC transcript in the antisense and sense transgenic plants were observed, respectively. Antisense transgenic plants which expressed the engineered SAMDC gene constitutively under the control of the 35S CaMV promoter with a duplicated enhancer region showed a range of stunted phenotypes with highly branched stems, short internodes, small leaves and inhibited root growth. The abnormal characteristics of the antisense plants correlated with the altered levels of SAMDC transcript, SAMDC activity, polyamine content and rate of ethylene evolution. Attempts to produce sense transgenic plants with the 35S SAMDC sense construct were unsuccessful indicating that a constitutive overexpression of the engineered SAMDC is lethal to the plants. When antisense and sense SAMDC transgenes were expressed under the control of a tetracycline‐inducible promoter a number of transgenic plants were produced. In these antisense and sense plants, significant changes in the level of SAMDC transcript, SAMDC activity and free polyamine content were observed on tetracycline‐induction of detached leaves.

cDNA clones of two genes (TUB8 and TUB13) which show a 25-30-fold increase in transcript in the stolon tip during the early stages of tuberisation, have been isolated by differential screening. These genes are also expressed in leaves, stems and roots and the expression pattern in these organs changes on tuberisation. Southern analysis shows homologous sequences in the non-tuberising wild type potato species Solanum brevidens and in Lycopersicon esculentum (tomato). Sequence analysis reveals a high degree of similarity between the TUB13 cDNA, and a human S-adenosylmethionine decarboxylase gene. The predicted TUB8 peptide sequence shows several repeats of alanine, glutamate and proline which suggests a structural role for the encoded protein.

The early-methionine-labelled (Em) polypeptide is the single most abundant cytosolic protein of dry wheat embryos. It is encoded by messenger RNA which accumulates during the later (maturation) stages of embryogenesis. The accumulation of Em mRNA can be induced in isolated developing embryos, in culture, by the application of the plant growth regulator, abscisic acid, which prevents precocious germination.Precocious germination is also inhibited by the culture of embryos under conditions of osmotic stress when accumulation of Em mRNA is induced. This induction occurs in the absence of any significant increase in the endogenous levels of embryonic abscisic acid although there is a requirement for the continued presence of the growth regulator. Additionally, expression of Em genes can be repeated during early germination, if imbibing embryos are subjected to osmotic stress. Induction of Em-gene expression by osmotic stress is consistent with the proposed role of the Em polypeptide in mediating the remarkable tolerance of cereal embryos to the programmed desiccation undergone during their maturation Embryonic development in cereals produces a differentiated embryo held within the developing grain in a developmentally arrested condition, whilst reserves within the endosperm continue to accumulate. The arrested embryo undergoes a period of maturation, during which characteristic changes in the pattern of gene expression occur within it. The maintenance of embryonic arrest and the associated accumulation of a maturation-specific subset of gene products is believed to be controlled by the plant growth regulator, abscisic acid: embryos which are removed from immature grains prior to the onset of maturation and cultured in the presence of abscisic acid do not germinate precociously, but accumulate maturation-specific proteins and mRNA [l -31.In the final stages of maturation, dehydration causes the cessation of metabolic activity within the embryo. Additionally, the developmental programme of the grain becomes radically altered, so that upon subsequent imbibition of water, the previously arrested mature embryo germinates. The maturation-specific gene products within the embryo disappear and the expression of a new subset of germination-specific genes occurs [4, 51. It is thought that the developmental switch effected by dehydration is due in part to the decline of endogenous abscisic acid during the last period of maturation. However, dehydration to a moisture content of z 5% fresh mass also appears to reduce the sensitivity of the embryo to inhibition of germination by exogenously applied abscisic acidIn recent years, a number of the abscisic-acid-related maturation-specific gene products of cereal embryos have been identified and characterised in some detail. The maintenance 151.Correspandence to A. C. Cuming, Department of Genetics, University of Leeds, Leeds LS2 9JT, EnglandAbbreviutions. dpa, days post-anthesis; Em, early-methioninelabelled polypeptide; norflurazon, 4-chloro-5-(methylamino)-2-(a,ct,a,-trifluoro-rr?-...

S-adenosylmethionine decarboxylase (SAMDC) is involved in the biosynthesis of the polyamines, spermidine and spermine. Recently, we reported the isolation of a putative cDNA clone of the SAMDC clone of potato (Plant Mol Biol 20; 641-651). In order to confirm that the potato genes does encode SAMDC, a complementation experiment with a yeast strain that possesses a null mutation in the SAMDC gene was performed. The yeast strain contains a deletion-insertion mutation in the SAMDC gene and has an absolute requirement for the addition of exogenous spermidine for growth. When the full-length potato cDNA was expressed in the mutant yeast strain there was no longer a requirement for exogenous spermidine. Immunoblotting experiments suggest that the potato SAMDC gene product has an apparent molecular mass of 39 kDa. Expression of the SAMDC gene was high in the young and actively dividing tissues and low in the mature and non-dividing tissues of both vegetative and reproductive organs. Additionally, isolation and characterisation of the corresponding genomic clone is reported. The gene has one intron in its 5'-untranslated sequence but otherwise the transcribed portion is identical to the cDNA clone.