Plants use twisting roots to avoid too much salt in the soil

Plants do not have the ability to move from one place to another like animals. However, their roots and shoots can still respond to changes in their environment by growing in different directions. These movements, called tropisms, help plants survive and adjust to challenges like light, gravity, water, or salt.

One such response is halotropism, which helps roots grow away from areas with high salt levels in the soil. This is important because too much salt, especially sodium, can damage plant cells and reduce crop yields.

Halotropism allows plants to avoid damage by bending their roots away from salty soil. Although this behavior has been known for several years, scientists did not fully understand how it worked at the cellular and molecular levels.

In a 2022 study published in the journal Developmental Cell, researchers led by Bo Yu, Wenna Zheng, Lu Xing, and others focused on how halotropism happens in the model plant Arabidopsis thaliana. Their research showed that the process is not controlled by the common plant hormones auxin or cytokinin.

Instead, halotropism is driven by a stress hormone called abscisic acid (ABA), changes in cell structure caused by microtubules, and a protein called SP2L. This article explains their findings in clear and simple English, while keeping all the key facts, experimental details, and results.

Understanding the Basics: What Is Halotropism?

Halotropism is the movement of roots away from salt, particularly sodium ions (Na⁺). It is a type of negative tropism, meaning that the root avoids rather than grows toward the stimulus. Earlier studies showed that halotropism is triggered by sodium and not by other salt components like chloride.

Scientists believed that plant hormones like auxin and cytokinin might control this behavior, as they do for other tropisms like phototropism and gravitropism. However, this new research shows that halotropism works differently.

To study halotropism, the scientists used a method called the split-agar assay. In this setup, the plant root grows between two types of agar medium—one normal, and the other containing 200 mM sodium chloride (NaCl). In this condition, the roots of normal Arabidopsis plants bent away from the salty side, confirming the presence of halotropism.

To test whether the known salt stress pathways are involved, the researchers used Arabidopsis mutants called sos and moca1. These mutants are sensitive to salt because they have problems removing sodium from their cells. Surprisingly, these mutants showed even stronger halotropism than normal plants.

This suggested that halotropism is not controlled by the classic SOS pathway, but may instead be triggered directly by sodium stress at the root tip. This idea was confirmed when they used a fluorescent dye called CoroNa Green to show that sodium accumulated more in the root tips of the mutants than in wild-type plants.

Auxin and Cytokinin Are Not Required for Halotropism

Auxin is a well-known plant hormone that controls many types of directional growth, including bending in response to light and gravity. This idea comes from the Cholodny-Went theory, which says that an uneven distribution of auxin on one side of the root or shoot causes that side to grow differently, leading to bending. The researchers wanted to find out if halotropism followed the same rule.

They tested several auxin-related mutants, including aux1, pin2, and pin3/4/7, which are involved in auxin transport, as well as yuc mutants, which have reduced auxin biosynthesis. To their surprise, all of these mutants still showed halotropism.

In fact, some even bent more than wild-type plants, though this was likely due to weakened gravitropism. They also used NPA, a chemical that blocks auxin transport, and saw the same result—halotropism still occurred.

Next, they tested the role of cytokinin, a hormone involved in hydrotropism, the response to water gradients. Cytokinin-related mutants like ahk2-5/cre1-2, ahp1/2/3, and arr16/arr17 all responded normally to salt.

They also tested the miz1 mutant, which is known to affect hydrotropism, but it also showed normal root bending. These results confirmed that halotropism is independent of both auxin and cytokinin signaling.

ABA: The Central Hormone in Halotropism

The researchers next focused on abscisic acid, a hormone that helps plants respond to drought, salt, and other stresses. Using a special sensor called ABACUS, they measured ABA levels in root tips after salt exposure.

Within two hours, ABA levels increased significantly, suggesting that ABA is one of the earliest signals involved in halotropism.

They then used Arabidopsis mutants that were unable to produce or respond to ABA. These included the nced3/5 mutant, which cannot synthesize ABA properly; the snrk2.2/3/6 mutant, which cannot signal ABA; and the pyl duodecuple mutant, which lacks ABA receptors.

All three showed reduced or almost no root bending in the split-agar assay. This confirmed that ABA is not only involved but is essential for halotropism to happen.

Twisting and Cell Expansion Help the Root Bend

To understand how the root bends, the team looked closely at the shape and structure of root cells. Using a microscope and a fluorescent marker called EYFP-NPSN12, they observed the root epidermis during salt treatment.

In normal plants, the cells in the transition zone began to twist in a right-handed direction after exposure to salt. This twisting was not seen in ABA-deficient mutants, proving that ABA causes changes in how the cells grow.

This type of growth is called anisotropic expansion, where the cell grows more in one direction than another. It often depends on structures inside the cell called microtubules, which act like tracks guiding the direction of growth. The researchers suspected that microtubules were involved in halotropism and began investigating further.

SP2L and Microtubules: Changing the Direction of Growth

Microtubules are small protein filaments inside plant cells that help control how the cell expands. The researchers identified a microtubule-associated protein called SP2L as being important in this process.

They found that ABA activates a protein kinase called SnRK2.6, which then adds a phosphate group to SP2L at amino acid serine 406. This phosphorylation changes the behavior of SP2L and helps reorient the microtubules in the cell.

To test the role of SP2L, they studied Arabidopsis mutants called sp2l-2 and sp2l-4. These mutants failed to show root bending and also did not show the normal twisting of root cells.

They also created a version of SP2L where serine 406 was replaced with alanine (S406A), making it impossible to phosphorylate. Plants with this mutation also failed to respond to salt. This proved that SP2L, and specifically its phosphorylation at serine 406, is required for halotropism.

Microscopy showed that microtubules in normal plants changed from transverse (sideways) to longitudinal (up-and-down) direction about two hours after salt exposure. In the sp2l mutants, this change did not happen, and the cells failed to twist.

Cellulose Synthesis and the Role of CesA Proteins

The next step was to find out how these internal changes affect the cell wall. The researchers looked at the behavior of cellulose synthase proteins (CesA), which are responsible for producing cellulose in the cell wall.

They used fluorescent markers to track CesA3 and observed that its orientation also changed after salt treatment, following the direction of microtubules. This showed that microtubules guide the movement of cellulose-synthesizing enzymes, which in turn change the cell wall’s structure.

They further confirmed this by using the cellulose synthesis inhibitor isoxaben, which blocked halotropism.

Mutants for CesA genes (cesa1, cesa3, and cesa6) and for the protein CSI1, which links microtubules and CesA, also showed no root bending and no cell twisting. These findings proved that cellulose microfibril orientation, guided by microtubules and CesA proteins, is crucial for root bending.

The study included detailed measurements to support their conclusions. For example, ABA levels in root tips increased significantly within two hours of salt treatment. In the split-agar assay, over 70% of wild-type seedlings showed strong root bending, compared to less than 20% in ABA-defective mutants.

Microtubule orientation changed by about 40–60 degrees in the elongating epidermal cells. Root curvature was measured using ImageJ software, and all data were analyzed using ANOVA and Tukey’s test. Each experiment included between 17 and over 90 seedlings, and standard errors were reported for each value.

To see whether halotropism actually helps plants grow better in salty environments, the scientists grew normal and mutant plants for 21 days on split-agar with one side containing 200 mM NaCl.

Wild-type plants had healthy shoots and better overall growth. In contrast, the mutants sp2l-4, snrk2.2/3/6, and pyl duodecuple showed smaller shoots, more salt damage, and higher sodium content in their tissues. This suggested that their roots had grown into the salty area because they were unable to bend away from it.

They also observed that lateral roots, not just primary roots, showed halotropic bending. This means that the whole root system helps the plant escape salt and survive.

Conclusion

This study provides a complete and clear picture of how halotropism works in Arabidopsis. It shows that the process is independent of the classic auxin or cytokinin hormone systems. Instead, halotropism depends on ABA, a stress hormone that increases rapidly in root tips after salt exposure.

ABA activates a kinase called SnRK2.6, which phosphorylates the microtubule-associated protein SP2L. This triggers a reorientation of microtubules inside the root cells, guiding the movement of cellulose synthase proteins that change the structure of the cell wall. As a result, the cells twist and expand unevenly, bending the root away from the salt.

This new understanding has practical importance. By targeting the ABA–SP2L–microtubule pathway, scientists might be able to breed or engineer crops that can better survive in salty soils. This would be especially helpful in regions facing increasing soil salinity due to climate change, irrigation problems, or overuse of fertilizers.