Evolution and Transcriptional Modulation of Lipoxygenase Genes Under Heat, Drought, and Combined Stress in Brassica rapa

In plants, Lipoxygenase (LOX) genes play crucial roles in tolerance to abiotic as well as biotic stress. Although they have been characterized in various plants, little is known about the evolution and role of LOX under high temperature, drought and their combination in Brassica. In this study, we identified 13 divergent LOX genes which are distributed among six linkage groups (LG) of Brassica rapa. These genes are unevenly distributed among all the three sub-genomes as moderately gene fractionized (MF1), most gene fractionized (MF2), and least gene fractionized (LF) with the differential loss and with the highest loss of genes on the LF. The substitution analysis (Ka/Ks) exhibited a signature of purifying selection of paralogous genes. Protein sequence analysis showed the presence of two canonical iron-binding signature regions along with PLAT/LH2 domain in all the identified LOX proteins indicating greater functional conservation. LOX expression pattern differed significantly under conditions of heat, drought and their combination which was also assessed by lipid peroxidation (MDA) assay and relative water content (RWC). Under heat stress, BraA.LOX5.a was found to be over expressed (6.16-fold) whereas under drought and combined stress, BraA.LOX2.a was identified as the major transcript with 15.76-fold over expression under drought and 18.67-fold under combined stress. These genes showed a significant positive correlation with biochemical stress indicator MDA. The 5′ upstream sequences of LOX genes were found to be highly divergent and showed the presence of different regulatory elements. This study provides a basis for conducting further characterization of stress-specific LOX protein and its involvement in stress-related signalling pathway.