At moderate concentrations, ROS have a physiological role in a number of plant signaling processes, such as photosynthesis and respiration regulation, cellular metabolic fluxes and plant organogenesis 8, 9. The superoxide anion is converted to H 2O 2 by superoxide dismutase and later, detoxified to water by peroxidases. As the seed starts to absorb water, the germination process begins which relieves the seed from desiccation and hypoxia but it is accompanied with the release of reactive oxygen species (ROS) among other by-products 7. The organ has low concentration of oxygen, i.e., it is in hypoxic condition. Seed is a complex plant organ, which by itself is an embryonic plant in the inactive state. Some seed proteins are known as biologically active viz., lectins, enzymes and enzyme inhibitors, and play their functional roles in different pathways of plant physiology 6.

However, it was later found that mostly vicilins and legumins degrade since they are localized in membrane-bound vesicles 2, 3 whereas albumins and lectins remain longer 4 since they are present in the cytoplasm 5. Earlier, it was believed that seed proteins, which are mainly composed of nitrogen, degrade to supply nutrition to developing embryo during germination. The seed proteins have been segregated on the basis of solubility into albumins, globulins, prolamines, and glutelins 1. Proteins constitute approximately 10 to 40% of seed dry weight in cereals, legumes and oilseeds. Seeds have a good storage of carbohydrates, fats, and proteins. The determined peroxidase activity of DC25 may be responsible for rescuing germinating seeds from oxidative stress. The identification of redox-sensitive cysteine and inhibition of peroxidase activity by iodoacetamide facilitated characterization of the possible active site. The protein exhibited intrinsic peroxidase activity, which could be inhibited by binding of a heme analog. Comparative study of the protein with other seed hemopexins revealed the presence of four conserved water molecules in between the blades which cross-link them and maintain the tertiary structure. While the structure exhibited homology with 2S albumins, variations in the loops connecting the outermost strands and the differences in surface-charge distribution may be relevant for distinct functions. The high-resolution structure of the protein revealed a four-bladed β-propeller hemopexin-type fold containing pseudo four-fold molecular symmetry at the central channel.

It was crystallized and structure determined at 1.28 Å resolution using x-ray crystallography. The protein was monomeric and highly thermostable in extreme conditions of pH and salt. We have screened the seed proteome of Dolichos which lead to identification and purification of a protein, DC25. However, many proteins remain yet to be functionally characterized. It is increasingly evident that seed proteins exhibit specific functions in plant physiology.

Me and She Square by Chandan Sharma