None of that sideways, wandering roots looking for water and nutrients only near the surface – these go straight down
The ability of plant roots to efficiently access water and nutrients ensures strong plant health and resilience to weather events such as heat waves and drought.
Researchers from the Universities of Bonn in Germany and Bologna in Italy as well as colleagues from Great Britain have now discovered a mutant in barley in which the roots grow straight downwards and not the typical growth pattern laterally or outwards.
Varying the angle of root growth can affect the way roots anchor in and explore different layers of soil to absorb nutrients and water. This could open up opportunities for breeding more drought-resistant varieties.
The mutated root system was discovered during genetic work with the barley variety Morex at the University of Bologna.
Silvio Salvi, an adjunct professor in the university’s Faculty of Agricultural and Food Sciences and Technologies, said the group produced about 4,000 independent mutant lines. The collection, named TILLMore, aimed to create mutant plants to identify the genetic and physiological basis of plant traits that are important for crop adaptation.
Barley is one of the most important types of grain with uses in malting, food, feed and animal feed. According to Statistics Canada, barley production in 2020 was 10.7 million tons with an average yield of 71.8 bushels per acre in western Canada.
The researchers compared the genome of the mutated barley with normally grown barley plants. They discovered that the mutation is on chromosome number five, which they named “Enhanced Gravitropism 2” or egt2. It basically means an improved orientation towards gravity.
The researchers then confirmed that the egt2 properties were retained when plants grew in the soil.
The rarity of this mutation is significant as many mutants are more likely to have short or missing roots than roots that grow at different angles.
âIn this context, rare means that only a few such root angle mutants have been found so far,â says Frank Hochholdinger, Institute for Crop Science and Resource Conservation at the University of Bonn.
Ultimately, the researchers were able to show that the mutated gene is responsible for the vertical growth of the roots by creating a mutation in normal barley plants using the CRISPR / Cas 9 gene scissors. They grew the small barley plants in seed paper or soil and recorded the root angles with a scanner and special software.
They also grew barley in special “flower pots” that fit in an MRI scanner that they could use to look through the soil and record the growth of the roots. To prove the sensitivity of the genetic mutation to the influence of gravity, they placed the roots of the barley seedlings at a 90-degree angle to the direction of gravity, which means that the roots grow significantly in the direction of the influence of gravity compared to normal barley seedlings.
The scientists also worked with researchers from the John Innes Center in Norwich, UK, and were able to show that a similar mutant also exists in wheat plants.
âWhile the degree of hypergravitropism is very different between different types of barley, the homologous mutation in wheat also showed a hypergravitropic phenotype,â says Hochholdinger. “This suggests that both evolution and domestication play a role in the manifestation of the root angle in cereals and especially barley.”
The influence of evolution and domestication is a question that researchers continue to investigate.
“In another study, we examined a collection of around 500 lines of barley, including wild barley from around the world, for root gravitropism,” said Salvi. âInterestingly, lines with hypergravitropic roots similar to our egt2 mutant were extremely rare (two out of 500 lines). Rather, a continuous range of variation in root gravitropism was observed. We are currently testing whether supergravitropic or super shallow roots are associated with dry or moist environments. “
“Steeper roots are an advantage when it comes to tapping water resources and mobile nutrients at greater depths,” says Hochholdinger.
âConversely, a broadly growing root system penetrates a larger volume of soil and can thus access nutrients over a larger area and gives the plants better stability. Which root system offers the better conditions for good yields depends on the respective location. In drier regions these could be steeper roots, while in areas with less nutrients it could be the flatter ones. “
Salvi said that if all factors are equal, in relatively mild drought conditions it would be beneficial to have deeper roots where the soil water table is lower than in otherwise fertile soil. But the devil is in the details when it comes to complications.
“First, egt2 works by changing the angle of root growth so that while the maximum root depth is likely to increase, the effective volume of soil explored by the roots could decrease due to the narrower root growth of all root types,” said Salvi. âThis could affect the efficiency of chemical nutrient absorption. Second, a narrower (or wider) root system certainly affects the competitive processes between plants and plants, so the planting density may need to be considered and possibly adjusted. Thirdly, egt2 probably works via an as yet unexplained mechanism of altered cell wall stiffness, which could have important compromise effects. “