An actin-depolymerizing factor from the halophyte smooth cordgrass, Spartina alterniflora (SaADF2), is superior to its rice homolog (OsADF2) in conferring drought and salt tolerance when constitutively overexpressed in rice

Authors: SENGUPTA, SONALI - LSU Agcenter; MANGU, VENKATA - LSU Agcenter; SANCHEZ, LUIS - Centro De Investigaciones Biotecnologicas Del Ecuador; BEDRE, RENESH - LSU Agcenter; JOSHI, ROHIT - Jawaharlal Nehru University; Rajasekaran, Kanniah - Rajah; BAISAKH, NIRANJAN - LSU Agcenter

Submitted to: Plant Biotechnology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2018
Publication Date: 5/31/2018
Citation: Sengupta, S., Mangu, V., Sanchez, L., Bedre, R., Joshi, R., Rajasekaran, K., Baisakh, N. 2018. An actin-depolymerizing factor from the halophyte smooth cordgrass, Spartina alterniflora (SaADF2), is superior to its rice homolog (OsADF2) in conferring drought and salt tolerance when constitutively overexpressed in rice. Plant Biotechnology Journal. 17(1):188-205. https://doi.org/10.1111/pbi.12957.
DOI: https://doi.org/10.1111/pbi.12957

Interpretive Summary: Aflatoxin contamination of food and feed crops is often exacerbated under conditions of extreme environmental stress. Naturally occurring resistance genes to abiotic stress (e.g., drought, salt) were identified in a Louisiana smooth cordgrass that grows well in marshy conditions. One such gene called actin depolymerizing factor (SaADF2) was expressed in rice and transgenic plants demonstrated significant resistance to drought and salinity. This gene from cordgrass was also found be to be superior to the native gene found in rice. Transgenic rice also showed better growth, relative water content, photosynthetic and agronomic yield than controls. Biochemical basis of the nature of resistance is explained in this study. Availability of genes such as SaADF2 is important in genetic engineering to improve abiotic stress (to drought and salt) in crops susceptible to aflatoxin contamination. The findings in this study are useful to biotechnologists, biochemists and molecular breeders to improve agronomic traits as well as food and feed safety.

Technical Abstract: Actin depolymerizing factors (ADFs) maintain the cellular actin network dynamics by regulating severing and disassembly of actin filaments in response to environmental cues. Ectopic expression of an ADF isolated from a monocot halophyte, Spartina alterniflora (SaADF2) imparted significantly higher level of drought and salinity tolerance when expressed in rice than its rice homologue OsADF2. SaADF2 differs from OsADF2 by a few amino acid residues, including a substitution in the regulatory phosphorylation site Serine-6, which accounted for its weak interaction with OsCDPK (calcium dependent protein kinase), thus resulting in an increased efficacy of SaADF2 and enhanced cellular actin dynamics. Indeed, SaADF2 overexpression preserved the actin filament organization better in rice protoplasts under desiccation stress. The predicted tertiary structure of SaADF2 showed a longer F-loop compared with OsADF2 that contributed to higher actin-binding affinity and rapid F-actin depolymerization in vitro by SaADF2. Rice transgenics constitutively overexpressing SaADF2 (SaADF2-OE) showed better growth, relative water content, photosynthetic and agronomic yield than wild-type and OsADF2 overexpressers (OsADF2-OE). SaADF2-OE plants preserved intact grana structure after prolonged drought stress, whereas the WT and OsADF2-OE plants presented highly damaged and disorganized grana stacking. Possible role of ADF2 in transactivation role was hypothesized from the comparative transcriptome analyses which showed significant differential expression of downstream stress-related genes including interacting partners in overexpresser lines. Identification of complex, differential interactome decorating or regulating stress-modulated cytoskeleton driven by ADF isoforms will lead us to key pathways that could be potential target for genome engineering to improve abiotic stress tolerance in agricultural crops.