Selective gene expression allows the halophyte Mesembryanthemum crystallinum to survive a salt stress. To broaden our understanding of the environmental cues initiating diverse stress responses in this higher plant, unstressed and 0.4 M NaCl‐stressed plants were compared to plants treated with several concentrations of copper (CuSO4), an increasingly relevant environmental heavy metal pollutant. Comparisons of control and copper‐stressed plants included germination, chlorophyll content, accumulation of proline, heat shock protein (HSP) 60 and a Crassulacean acid metabolism (CAM)‐specific marker enzyme, phosphoenolpyruvate carboxylase (PEPCase). In germination and whole plant tests, M. crystallinum was significantly more tolerant to copper than Arabidopsis thaliana. Mature M. crystallinum plants stressed with 50 ppm CuSO4 for 48 h became dehydrated. These plants produced a 4‐fold increase in proline concentration and accumulated both the CAM‐specific PEPCase and HSP 60 compared to controls. Higher levels of copper stress resulted in a 10‐fold increase in leaf proline content, 10‐fold HSP 60 accumulation but no detectable PEPCase protein compared to unstressed controls. HSP 60 did not accumulate under NaCl stress. Concurrent with copper‐induced genetic responses to stress, copper was accumulated and concentrated in leaves (3 500 ppm). Together, these results suggest that this halophyte copes with copper metal exposure through distinct genetic mechanisms.

Pod shatter in oilseed rape is associated with the degradation of the pectin-rich middle lamella at the site of dehiscence. It has been reported that, accompanying pod development, there is an increase in the activity of polygalacturonase (EC 3.2.1.15) and that this rise is restricted to the tissue undergoing cell separation. Using a PCR strategy a fragment of a polygalacturonase encoding an mRNA that is up-regulated specifically in the dehiscence zone tissue during pod development has been cloned. A full length clone (SAC66) complementary to this mRNA has been isolated from a dehiscence zone cDNA library and sequenced. The mRNA encoded by SAC66 shares significant amino acid homology with endopolygalacturonases from fruit of Actinidia deliciosa and Lycopersicon esculentum. The transcript size of the SAC66 mRNA is 1.7 kb. Northern analysis has revealed that expression of SAC66 mRNA increases at 30 d after anthesis (DAA), reaching a plateau at 45 DAA, and that the up-regulation is restricted to the site where dehiscence takes place.

Metallothioneins (MTs) are metal-binding proteins that confer heavy metal tolerance and accumulation in yeast. To augment higher plant metal sequestration, the yeast metallothionein (CUP 1) was introduced into tobacco plants. The CUP 1 gene expression and copper and cadmium phytoextraction were determined. To confirm transformation, selfed and kanamycin-resistant third generation plants were subjected to DNA blot and polymerase chain reaction (PCR) analysis. A 4 mM CuSO(4) stress for 7 days resulted in a decline in CUP 1 transcripts versus nonstress conditions. Despite low mRNA levels, CUP 1 transformants accumulated up to seven times more copper in older versus younger leaves during copper stress. Pooled leaves of transgenic plants grown in soils from copper stamp-sands contained two to three times the copper content as that of the control plants. Unlike some previous reports featuring MT overexpression in plants, CUP 1 seedlings did not significantly sequester or demonstrate tolerance to CdCl(2). Using this transgenic approach, yeast CUP 1 expression under nonstressed conditions contributed to copper metal phytoextraction during a subsequent copper challenge. This strategy could be incorporated into plants designed for enhanced phytoremediation of metal contaminants.

No abstract

No abstract