A number of plant genes have been found to protect plants against salt and drought stress. Some of these are summarized in Plant Soil (2017) 410:335–356 DOI 10.1007/s11104-016-3007-x. In addition to overproducing some plant genes it is possible to treat stressed plants with plant growth-promoting bacteria that overproduce cytokinin, IAA or ACC deaminase. Thus, also have a look at the following: Ali, S. and Glick, B.R. 2019. Plant-bacterial interactions in management of plant growth under abiotic stresses. In: New and Future Developments in Microbial Biotechnology and Bioengineering. (Ed.) J.S. Singh, Elsevier. pp. 21-45. and Ali, S., Charles, T.C. and Glick, B.R. 2014. Amelioration of damages caused by high salinity stress by plant growth-promoting bacterial endophytes. Plant Physiol. Biochem. 80:160-167.

Amongst the defences that are widely used by plants to protect themselves against salt stress, there are:

1) exclusion of salt from the shoot, which is much more sensitive than the root.

2) Storing of salt in the vacuole, (which is relatively hardy), and excluding it from the cytoplasm.

3) Synthesis of high levels of compatible solutes to protect cell components from salt toxicity, (for example, by stabilising proteins that would otherwise be destabilised by salts, and by scavenging oxygen radicals), and to manage osmolarity in regions of the plant and cell that are too sensitive to use inorganic ions to raise osmolarity.

Plants use these defences to varying extents. For example, Puniran-Hartley (2021) found that maize had a strong ability to exclude salt from the whole shoot, allowing it to make good growth under mild salt stress, but it quickly died when subjected to higher salt levels because it was unable to restrict salt to old leaves in order to protect young leaves. Puniran-Hartley (2021) found that across plant species representing a range of salt tolerance levels, the ability to restrict Na to old leaves and thus protect young leaves was the trait most consistently associated with salt tolerance. Conversely, salt tolerant plants tend to have low K levels in old leaves, to conserve it for young leaves.

So far, genetic engineering has failed to give practical salt-tolerant varieties. The reasons for this are reviewed by Puniran-Hartley (2021, Section 1.1.8). On the other hand, conventional breeding is slow, but is making progress on salt tolerance, (aided by more modern methods, such as marker-assisted selection, and embryo rescue to allow crosses with more distant relatives).

Aside from breeding work, plant salt tolerance may be improved to some degree by supporting the plants with chemicals supplied exogenously, (such as compatible solutes, other anti-oxidants, and precursors to metabolites), though again this has not been used much in practice. As Prof. Emeritus Bernard Glick has mentioned to you, inocculation with plant growth promoting rhizobia (PGPR) can also raise salt tolerance.

At present, the most effective way available to manage salinity is not to breed salt tolerance into glycophytic crops, but rather to either manage the salt level, or grow a more salt-hardy species. For example, drip irrigation is effective at temporarily pushing salt out of the root zone, allowing crops to be grown in soil that is otherwise to saline for them. As an example of using salt tolerant species, saline land in Western Australia is often planted with native species of Amaranthaceae, (especially Atriplex nummularia var. Anameka), which profitably uses the land by yielding feed for sheep at times of year when feed is scarce.

You can find Puniran-Hartley (2021) at this link: https://figshare.utas.edu.au/articles/thesis/Understanding_the_role_of_compatible_solutes_in_adaptive_responses_of_plants_to_salinity_and_waterlogging/23246987?file=46314154

When it comes to improving salt tolerance in crops, one strategy that’s really gaining traction is the use of plant growth-promoting rhizobacteria (PGPR). These microbes live around plant roots and help their hosts in several ways: they can produce hormones that stimulate growth, reduce stress-induced ethylene through ACC deaminase activity, improve nutrient uptake, and even help plants maintain a healthier balance of sodium and potassium under saline conditions. In short, they give plants the extra support they need to handle the osmotic and oxidative stress caused by salt.

What’s even more exciting is the new attention being paid to root exudates-the cocktail of sugars, amino acids, organic acids, and secondary metabolites that roots release into the soil. These compounds aren’t just waste products; they’re active signals that shape the microbial community in the rhizosphere. Under salt stress, plants actually change the composition of their exudates, which in turn can recruit helpful PGPR. This means that plants may, in a sense, “call for help” through their exudates when they’re under stress. So while applying PGPR is already a proven eco-friendly strategy, digging deeper into how root exudates drive these interactions opens up a fascinating new direction. If we can understand and perhaps even manipulate this biochemical dialogue, we could design smarter ways to strengthen salt tolerance in crops. For a deeper dive, you might want to check out the review Article Root exudation as a strategy for plants to deal with salt st...

- it’s a great resource that pulls together the findings on this topic.