Up‐regulation of arginine decarboxylase gene expression and accumulation of polyamines in mustard (Brassica juncea)in response to stress

Arginine decarboxylase (ADC) is a key enzyme involved in the synthesis of polyamines, which have been implicated in a wide range of plant responses, including stress. However, regulation of polyamine levels in relation to ADC in response to stress at the molecular level is not well understood. In an attempt to address this question, we first cloned two cDNAs in mustard (Brassica juncea[L.] Czern & Coss var. Indian Mustard), designated MADC2 and MADC3, encoding predicted ADC. MADC2 and MADC3 encode polypeptides of 692 and 680 amino acid residues, respectively. A comparison of deduced amino acid sequence revealed that both were highly homologous to MADC1 (77%), a mustard ADC, and other plant ADCs (63-84%). Northern analysis revealed that ADC transcripts in mustard were generally more abundant in stem and root but were barely detectable in leaf. However, ADC expression in the leaf was up-regulated differentially in response to stress such as chilling, salt and mannitol and to treatments with exogenous polyamines. While chilling induced expression of all three ADC genes, salt predominantly resulted in increased accumulation of MADC3 transcript. Leaves exhibited a similar response to exogenous putrescine, spermidine and spermine, all of which stimulated accumulation of MADC2 and MADC3 transcripts but not MADC1. Furthermore, exogenous putrescine also increased the endogenous levels of spermidine and spermine, while a higher endogenous putrescine and spermidine content was detected in leaf incubated with exogenous spermine. Leaves also responded to chilling, salt and mannitol by increasing the levels of the cellular polyamine content, in which the level of spermine in free and conjugated forms increased most profoundly.