Genetic responsiveness to copper in the Ice Plant, Mesembryanthemum crystallinum

Abstract

To better understand genetic responsiveness to stress, Mesembryanthemum crystallinum plants were challenged with copper and then analyzed for mRNAs corresponding to the salt inducible and constitutive forms of phospho enolpyruvate carboxylase (PEPCase) ( Ppc1 and Ppc2 respectively), and a senescence inducible cysteine protease ( Sep7). During the 200 μM copper stress of 6–7-week old adult plants, leaf transcripts of the CAM specific Ppc1 and Sep7 were approximately 5–10 times more abundant than those seen in unstressed plants. Constitutive Ppc2mRNA levels were unchanged relative to controls. Compared to unstressed plants, both Ppc1 and Sep7 transcripts were also quite abundant in leaves during NaCl and cytokinin treatment. Leaves of 800 μM copper sulfate treated plants did not contain detectable Ppc1, Ppc2 or Sep7 transcripts. Aerial tissues of 12–14-week old previously unstressed plants contained mostly flower buds and stems. Both these organs expressed Sep7 and Ppc2 transcripts at similar levels regardless of manipulation (no stress, salt or copper stress). As in younger tissues, mature bud and stem tissues responded to NaCl or copper stress by increasing Ppc1 mRNA levels. Only young copper stressed plants had enhanced copper sequestration and tissue lipid peroxidation, the latter indicative of free radical damage. Together, these results suggest that within a developmental context, the Ice Plant uses distinct genetic mechanisms quite akin to salt/drought stress in response to a moderate copper stress.