ROS Regulation and Abiotic Stress Tolerance Genes in crops

Antioxidant Genes

Superoxide Dismutase (SOD)

SOD (E.C.1.15.1.1) belongs to the family of metallo enzymes onmipresentinall aerobic organisms.Under environmental stresses, SOD forms the first line of defense against ROS-induced damages. The SOD catalyzes the removal of Of^- by dismutating it in to 0_? and H₂0₂. This removes the possibility of OH' formation by the Haber-Weiss reaction. SODs are classified in to three isozymes based on the metal ion it binds, Mn-SOD (localized in mitochondria), Fe-SOD (localized in chloroplasts), and Cu/Zn-SOD (localized incytosol, peroxisomes, and chloroplasts) (Mittler, 2002). SOD has been found to be upregulated by abiotic stress conditions (Boguszewska et al., 2010).

The rice (japonica) genome has eight genes that encode putative SODs, including two cytosolic copper-zinc SODs (cCuZn-SODl and cCtiZn-SOD2), one putative Cu Zn-SOD-like (CuZn-SOD-L), one plastidic SOD (pCuZn-SOD), two iron SODs (Fe-SOD2 and Fe-SOD3), and one manganese SOD (Mn-SODl) (Natheto/., 2014). Overexpression of Mn-SODl showed less mitochondrial O-,' under stress and reduced the stress induction of OsAOXla/b in rice plants (Li et a!., 2013).

Catalase (CAT)

CAT(E.C.1.11.1.6) is a tetramericheme-containing enzyme responsible for catalysing the dismutation of H,0₂ in to H₂0 and 0₂. It has high affinity for H-,0₂, but lesser specificity for organic peroxides (R-O-O-R). It has a very high turnover rate (6xl0⁶ molecules of Н₇Сц to H-,0 and min^-1) and is unique amongst antioxidant enzymes in not requiring a reducing equivalent. Peroxisomes are the hot spots of Н,Сц production due to (3-oxidation of fatty acids, photo respiration, purine catabolism and oxidative stress (Mittler, 2002). However, recent reports suggest that CAT is also found in other sub cellular compartments such as the cytosol, chloroplast and the mitochondria, though significant CAT activity is yet to be seen (Mhamdi et al., 2010). Angiospenns have been reported to have three CAT genes. CAT1 is expressed in pollen sand seeds (localized in peroxisomes and cytosol), CAT2 pre dominantly expressed in photosynthetic tissues but also in roots and seeds (localized in peroxisomes and cytosol) and finally CAT3 is found to be expressed in leaves and vascular tissues (localized in the mitochondria). Stressful conditions demand greater energy generation and expenditure of the cell. This is fiilfilled by increased catabolism which generates H-,0,. CAT removes the H₇0₂ in an energy efficient way.

Ascorbate Peroxidase (APX)

APX (E.C. 1.1.11.1) is an integral component of the Ascorbate- Glutathione (ASC- GSH) cycle. While CAT pre dominantly scavenges H₂0, in the peroxisomes, APX performs the same function in the cytosol and the chloroplast. The APX reduces H^O, to H,0 and DHA, using Ascorbicacid (AA) as a reducing agent.

The APX family comprises of five isoforms based on different amino acids and locations, viz., cytosolic, mitochondrial, peroxisomal, and chloroplastid (stromal and thylakoidal) (Shanna and Dubey, 2004). Since APX is widely distributed and has a better affinity for H₂0, than CAT, it is a more efficient scavenger of H₂0, at times of stress.

There are eight ascorbate peroxidase (APX) genes in rice, including two cytosolic APXs (OsAPXl and OsAPX2), two chloroplastic APXs ('OsAPX7 and OsAPX8), two mitochondrial APXs (OsAPXS and OsAPX6) and two peroxisomal APXs (OsAPXS and OsAPX4) (Teixeira et al., 2004, 2006). Cytosolic ascorbate peroxidase (APXs) i.e. OsAPXl and OsAPX2, have vital roles in abiotic stress resistance in rice (Sato et al, 2011 Zhang et al., 2013).

Interestingly, rice mutants double silenced for cytosolic APXs (APXl/2s) exhibit significant changes in the redox status indicated by higher H-,0, levels and increased glutathione and ascorbate redox states, triggering alterations in the ROS signaling networks and making the mutants able to cope with abiotic stress similar to non-transformed plants (Bonifacio et al., 2011).