Barley Root Angle Mutation Holds Key To Drought Resistant Cereals

Feeding a growing population under climate change demands innovative solutions, and surprisingly, a key answer might lie beneath the soil. Groundbreaking research published in the prestigious journal PNAS has pinpointed a single gene, ENHANCED GRAVITROPISM 2 (EGT2), as a master controller of how deep roots grow in barley and wheat, two of the world’s most vital cereal crops.

Understanding and harnessing this gene offers a powerful new strategy for breeding crops that can better withstand drought and find nutrients hidden deep underground. The angle at which roots grow downwards, known scientifically as the root growth angle (RGA), is far more than just a curiosity; it’s a critical survival trait.

Roots spreading shallowly near the surface excel at grabbing nutrients like phosphorus that don’t move much in the soil. Conversely, roots that dive steeply downwards act like nature’s boreholes, tapping into precious deep water reserves and mobile nutrients like nitrogen that wash down over time.

Previous research, like the discovery of the DRO1 gene in rice, showed that engineering steeper roots could boost drought yields by an impressive 18%. This new work on EGT2 builds on that promise, revealing a distinct genetic pathway specifically in barley and wheat, opening exciting doors for improving these essential grains.

Mutant Roots Grow Steeper Naturally

The journey to EGT2 began with a special barley plant, a mutant called egt2-1, identified because its roots grew unusually steeply. Scientists meticulously compared this mutant to normal barley plants using various methods, painting a clear picture of its unique behavior.

When grown on simple germination paper, in specialized see-through containers called rhizotrons, or even visualized non-invasively within real soil using advanced Magnetic Resonance Imaging (MRI), the difference was undeniable.

At just 7 days old, the main roots (seminal roots) of the *egt2-1* mutant grew at a significantly sharper angle – roughly 30 degrees compared to the normal 45 degrees in wild-type barley. This wasn’t just a small difference; statistical analysis confirmed it was highly significant, based on studying 40 plants per group.

Crucially, this steepness wasn’t limited to the main roots; the smaller side roots (lateral roots) also grew much steeper by 14 days old. Furthermore, experiments proved this wasn’t just passive growth; the mutant roots actively responded much more strongly to gravity.

When scientists rotated growing roots sideways by 90 degrees, the *egt2-1* mutant roots bent back downwards dramatically faster and further than normal roots, nearly reaching vertical within 3 days while normal roots lagged behind.

Importantly, this dramatic change only affected the root angle; the number of roots and how fast they grew remained normal, showing that EGT2 specifically fine-tunes the direction of growth in response to gravity.

Finding the Root Angle Gene

Finding the exact gene responsible for this dramatic root angle change required sophisticated detective work. First, scientists used a technique called Bulked Segregant Analysis (BSA) with a population of plants descended from crossing the mutant with normal barley.

This powerful approach narrowed down the location of the mutant gene to a specific 312 million base-pair region on barley chromosome 5H.

Next, they read the entire DNA sequence (Whole Genome Sequencing) of the mutant plant.  Comparing it to the normal barley sequence revealed several mutated genes in that region, but one stood out: a gene predicted to produce a protein containing a STERILE ALPHA MOTIF (SAM) domain.

This SAM domain is crucial because similar domains in other proteins often help them interact with each other. The mutation in the *egt2-1* mutant was a single letter change in the DNA code (G447A), causing the cellular machinery to stop building the protein too early, right before this important SAM domain, essentially creating a broken protein.

To prove beyond doubt that this specific gene was responsible, the team used CRISPR/Cas9 gene editing, often called “genetic scissors.” They deliberately created a new mutation (*egt2-2*) in a different barley variety, Golden Promise, deleting a chunk of DNA including the gene’s start signal.

Sure enough, roots of these CRISPR-edited plants grew just as steeply as the original mutant, confirming the gene’s critical role. The discovery didn’t stop with barley. Recognizing wheat’s global importance, the researchers found and combined mutations in the equivalent EGT2 genes within durum wheat (future) (used for pasta).

Wheat plants lacking functional EGT2 genes in both their genomes also developed significantly steeper roots compared to normal wheat, powerfully demonstrating that this gene’s function in controlling root angle is evolutionarily conserved between these two major cereal crops.

Beyond Auxin: Root Angle Control

Understanding where and how the EGT2 protein functions was the next crucial step. Using a technique called in situ hybridization, scientists visualized where the EGT2 gene is active.

They found it is expressed throughout the critical root tip regions: the root cap (where gravity is sensed), the meristem (where cells divide), and the elongation zone (where cells stretch, causing bending). This widespread presence suggests EGT2 plays a role in the signaling process across these zones.

Interestingly, rotating roots sideways didn’t change EGT2‘s activity level or pattern; it wasn’t switched on or off by gravity itself, hinting its action is likely constant, perhaps involving interactions between proteins.

One of the most surprising findings was that EGT2 operates independently of the well-known plant hormone auxin.

Auxin is a major player directing root growth and gravitropism. However, experiments showed that treating roots with auxin or chemicals that block auxin transport affected the egt2 mutant and normal plants identically.

Even more convincingly, detailed analysis of which genes were turned on or off (RNA sequencing) in mutant versus normal roots revealed no significant differences in genes directly involved in making, moving, or responding to auxin. This clearly showed EGT2 works outside the main auxin pathway.

So, how does it work? Using incredibly precise laser capture microdissection to isolate RNA from tiny, specific root regions (cap, meristem, elongation zone) followed by RNA sequencing revealed the answer.

The key changes occurred in the elongation zone, where gravity signals are physically executed through uneven cell stretching. Here, the mutant showed seven genes coding for “expansins” significantly turned down. Expansins are proteins that loosen plant cell walls, allowing cells to expand.

This down-regulation perfectly aligns with the steeper growth: reduced expansin activity might subtly alter how cells elongate in response to gravity cues, favoring the downward bend. Supporting this, other gene changes pointed towards altered cell wall processes and calcium signaling, further implicating the elongation zone as EGT2’s key point of action.

Gene Key to Drought-Proof Crops

The discovery of the EGT2 gene is far more than just an interesting scientific finding; it represents a potent, practical tool for improving our most important food crops. Its significance lies in several key areas.

• Firstly, it defines a completely novel pathway for controlling root gravitropism in cereals, distinct from the dominant auxin mechanism scientists previously focused on.
• Secondly, its function is conserved between barley and wheat, the world’s 4th and 1st most important cereals, meaning strategies developed using this gene could have broad impact.
• Thirdly, and crucially, mutations in EGT2 produce a highly specific, beneficial change: steeper angles for both main and side roots, without negatively affecting other vital traits like root number, length, or shoot growth.

This precision is ideal for breeding. Steeper root growth angles are strongly linked to improved drought tolerance, as deeper roots can access water unavailable to shallower systems when the topsoil dries out.

This makes EGT2 a prime target for developing crops resilient to the more frequent and severe droughts predicted with climate change. How can we use this knowledge? Plant breeders can now screen diverse barley and wheat collections for natural variations in the EGT2 gene associated with steeper rooting.

Alternatively, CRISPR/Cas9 gene editing provides a direct route to create non-functional egt2 versions in high-performing crop varieties, replicating the beneficial mutant root angle demonstrated in the research.

Furthermore, EGT2 variants could potentially be combined with other valuable traits, like genes for nutrient efficiency or disease resistance, creating super-resilient crops. In essence, the EGT2 gene gives scientists and breeders a powerful genetic lever.

By understanding and manipulating this single point of control, we can engineer barley and wheat varieties with root systems intrinsically designed to delve deeper into the soil, seeking the water and nutrients essential for growth even under challenging conditions. This breakthrough offers tangible hope for securing grain production and global food security (russiaukraine) in an increasingly uncertain climate.