Growing world population reinforces the demand for increasing crop production. Yet, pitted against this expected and justifiable demand is the shrinkage of arable land. An appallingly large proportion of the total land comprises what are known as “problem soils”. On such soils, crop productivity is very low due to the presence of various constraints including acidity, salinity and sodic characteristics. In particular, sodic soils, with high alkalinity and elevated salt content, have been a matter of serious concern in respect to the world’s food security.
Encouragingly, there has been a worldwide effort to (genetically) engineer crops that would grow on sodic soils. Evidently, this requires identification of the causes for crops’ alkaline sensitivity.
Genetic diagnosis of the malady
One very widely adopted strategy to identify the factors and causes behind a biological phenomenon is gene knockout. In genomics, “knockout” implies the use of genetic engineering to remove one or more specific genes from an organism and study of the subsequent impact. Implementing the knockout strategy, researchers in Chinese laboratories have identified a sorghum gene SbAT1 (Alkaline Tolerance 1) whose knockout results in enhanced tolerance to alkaline stress (SCIENCE, 24 Mar 2023, Vol. 379, Issue 6638). In contrast, overexpression of the gene (that is, having an enhanced level of the gene-encoded protein) leads to an increased alkaline sensitivity of the plant.
Further, it was found that SbAT1encodes an ‘atypical’ G protein, whose homologs are present in other cereal crops including rice, maize and millet – AT1 gene knockout resulted in increased tolerance to alkaline stress in these crops as well.
The cue to these enlightening experiments came from the fact that there exists a remarkable variation in alkaline stress tolerance between species and, also, with a specific species. Sorghum originates from harsh environments of Africa and has, therefore, evolved higher alkaline tolerance compared to other cereal crops. Some varieties of sorghum can survive in a sodic soil with a pH as high as 10.0.
What causes alkaline sensitivity? – Alkaline stress induced ROS.
Reactive oxygen species (ROS) are produced in plant cells in response to biotic and abiotic stress. They reside in ionic states including hydroxyl radicals (– OH) and superoxide anions (O2 –) and / or molecular states including hydrogen peroxide (H2O2). In general, ROS at low levels are necessary for multiple biological processes such as proliferation and differentiation. However, at higher levels, ROS cause irreversible DNA damage. Alkaline stress induces an accumulation of H2O2 in the cytosol leading to cell death.
How do plants cope with the problem? – With the help of aquaporins.
Aquaporins (AQPs), sometimes called aquaporin water channels, are a family of integral membrane proteins found throughout the animal and plant kingdoms. They are embedded in the lipid bilayer. All aquaporins have a similar basic structure – each monomer consists of six transmembrane helical segments and two short helical segments that surround a narrow aqueous pore connecting the cytoplasmic and extracellular spaces. As the name suggests, aquaporins are well known to transport water; nevertheless, some members of the family can transport glycerol and even hydrogen peroxide.
Depending upon their substrate specificities, plant AQPs have been classified into five subfamilies – the plasma membrane intrinsic proteins (PIPs) constitute the largest subfamily. Further, among the PIPs, members of the PIP2 group are more efficient as water channels than those of PIP1. H2O2, that accumulates in the cell due to alkaline stress, is transported from the cytosol into the apoplast by the phosphorylated form of PIP2. The apoplast comprises the intercellular space, the cell walls, and the xylem. Antioxidants present in the apoplast can detoxify different types of ROS.
Where does G protein come in? – Negatively modulating phosphorylation of PIP2.
In animals as well as plants, heterotrimeric G proteins, comprising alpha-, beta- and gamma-subunits (Galpha, Gbeta and Ggamma), perceive extracellular stimuli via cell surface receptors and transmit signals to control different cellular activities. In sorghum, the Ggamma subunit AT1, together with Gbeta, negatively modulates the phosphorylation level of PIP2 leading to the variation in alkaline sensitivity/tolerance. Understandably, AT1 knockout releases the inhibition of PIP2 resulting in an increased H2O2 transport from the cytosol into the apoplast.
Based on such encouraging results obtained from the investigations on alkaline tolerance of sorghum and other crops, researchers have suggested that genetically engineered crops with knockouts of ATI homologs or using crop variants with low Ggamma level could substantially improve crop yield in sodic lands.