Comparative study of plant defense response in volicitin treated and mechanically wounded Zea mays plants under the action of the calcium channel blocker Gadolinium(III)

Zea mays plants were treated with varying concentrations of Gd3+ and further subjected to insect elicitor (IE) and mechanical wounding (MW) treatment. Transcript accumulation of genes involved in plant defense and calcium signaling studied by semi-quantitative PCR are differentially regulated. The AOC (allene oxide cyclase) gene playing an important role in Jasmonic acid (JA) synthesis is down-regulated with higher Gd3+ concentration in volicitin treated plants whereas the mechanically wounded plants show a rise in AOC expression till 1mM of Gd3+ but goes down when subjected to 10mM Gd. These results suggest that the JA synthesis and plant defense response could be primarily regulated by apoplastic Ca2+ reserves for insect elicitor (IE) induced defense response whereas the mechanical wounding induced defense response might be using the intracellular Ca2+ reserves as its primary source of Ca2+. A stronger possibility exists that Ca2+ signaling is not involved in MW induced defense response. Further expression of AOC and important Ca2+ regulatory protein calmodulin (gene CAM) suggest that, in plant cells the calcium channels on the plasma membranes are saturated by 1mM Gd3+ treatment. A sudden variation in expression of genes at 10mM Gd3+ concentration suggests that Gd3+ ions might show membrane permeability. Due permeability of plasma membrane to Gd3+, the Gd3+ ions can enter the cells and block the Ca2+ channels on the intracellular organelle. The gene expression data for calcium binding protein CCD1 and calcium dependent protein kinase isoform AK1 shows that they do have a regulatory function in IE mediated plant defense response. Rise in transcript accumulation of CCD1 and AK1 genes under high Gd3+ stress (10mM) gives rise to a possibility of high protein synthesis in order to maximize Ca2+ utilization in Ca2+ starved conditions. Though a lot of variations were observed, the average expression data for the CAM gene suggests a down-regulated expression of the calmodulin transcribing gene at 10mM suggesting a lowered cytosolic calcium concentration. Surprisingly the gene expression data from Inositol (1, 3, 4) - Triphosphate kinase (gene IPK3) which plays an important role in JA signaling does not show any noticeable changes in transcript accumulation.