The signals controlling the abundance of transcripts up-regulated (pTIP27, pTIP31, and pTIP32) or down-regulated (pTIP20 and pTIP21) after harvest in asparagus (Asparagus officinalis L.) spears were examined. pTIP27 and pTIP31 are known to encode asparagine synthetase (AS) and a β-galactosidase (β-gal) homolog, respectively. The nucleotide sequences of pTIP20, pTIP21, and pTIP32 were determined, and they encode histone 3, histone 2B, and an unknown product, respectively. Changes in respiration, soluble sugars, and abundance of the five mRNAs were similar in the tips stored as 30-mm lengths or as part of 180-mm spears. We previously hypothesized that sugars may regulate the level of AS transcripts in asparagus tissue. Asparagus cell cultures were used to test the role of sugar status in regulating gene expression. Transcript abundance for AS, β-gal, and pTIP32 was low in cells in sugar-containing medium but increased within 12 h after transferring cells to a sugar-free medium. Histone 3 and histone 2B transcripts were, in general, abundant in cells on sugar-containing medium but declined in abundance when transferred to sugar-free medium. When cells were returned to sugar-containing medium the abundance of transcripts for histone 3 and histone 2B increased, whereas that for AS, β-gal, and pTIP32 decreased. Soluble sugar levels are known to decline rapidly in the tips of harvested spears. Metabolic regulation by sugar status may have a major influence on gene expression in asparagus spears and other tissues after harvest.
Hairy roots of Brassica napus (rape cv. Giant) were produced by cocultivating leaf and cotyledon explants with Agrobacterium rhizogenes strain A4T. The hairy roots grew prolifically on solid and in liquid media. Incorporation of ammonium sulphate or phosphinothricin (PPT) into the media reduced growth. PPT treatment reduced glutamine synthetase (GS) activity and increased the ammonia content of the hairy roots. We have found that PPT treatment also induces a loss of glutamine from the roots and this may influence root growth. To test this we grew hairy roots in a liquid medium containing 10 mM glutamine. This glutamine treatment overcame the PPT induced suppression of growth but also significantly increased GS activity, reduced ammonia accumulation and increased the levels of glutamate and asparagine.
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