Plants are directed to transport nutrients from their roots to their shoots through the development of pipe-like vessels
Scientists at the University of California, Riverside, have discovered genes in crop roots that help plants survive dry spells.
They do so by instructing the plants to do three things:
- Produce pipe-like vessels that shore up water and nutrients.
- Produce suberin, which acts as a barrier to hold in liquids.
- Encourage the growth of the meristem at the root tips, which dictates the specific properties of the actual root.
The discovery of these genes came about while the research team was creating a molecular atlas of crop roots, the place where plants first detect the threat of drought.
“(A molecular atlas) is like a Google map of what is happening in individual cells,” said Julia Bailey-Serres, professor of genetics and director of the Center for Plant Cell Biology. “It gives us their location and classifies their functions (such as) mineral transport, production of suberin and making sugars used to build cells.”
Bailey-Serres said that roots are the first responders to threats that involve water availability and interact with both good and bad microbes. They have different types of cells that can work together to transport water and minerals to the above-ground parts of the plant.
Her lab has been working on how rice responds to flooding. She collaborated with colleagues Siobhan Brady and Neelima Sinha to investigate how wild tomato species grow in stressful environments and the role of different cells in their roots. Both Brady and Sinha are professors of plant biology at the University of California, Davis and Sinha is an expert in plant growth and drought mechanisms.
The results of the study led to a greater understanding of root functions because it combined genetic data from tomato plants grown both indoors and in the field.
One of the functions the newly discovered genes direct plants to do is to transport water and nutrients from the roots to the shoots through the development of hollow, pipe-like vessels called xylem. These vessels are essential not only to protect plants against drought but also against salt and other environmental stresses. Without transport in xylem, plants cannot create their own food via photosynthesis.
“We found some of the expected players that control xylem development, but also some new and surprising (genes were found),” said Bailey-Serres.
The second set of genes discovered were those that directed an outer layer of the root to produce lignin and suberin. Suberin is a cell wall-associated biopolymer, the key substance in cork, and it surrounds plant cells in a thick layer, holding in water during a drought.
The news release stated that crops like tomatoes and rice have suberin in the roots. Apple fruits have suberin surrounding their outer cells. Anywhere it is found it prevents the plant from losing water. Lignin waterproofs cells and provides a form of mechanical support.
“Suberin and lignin are natural forms of drought protection and now that the genes that encode for them in this very specific layer of cells have been identified, these compounds can be enhanced,” said Bailey-Serres.
“I’m excited we’ve learned so much about the genes regulating this moisture barrier layer. It is important for being able to improve drought tolerance for crops.”
The meristem is the growing tip of each root and the source of all the cells that make up the root. Genes that encode for the meristem are similar between tomato, rice, and Arabidopsis, plant commonly used for lab testing.
“It’s the region that makes the rest of the root and serves as its stem cell niche,” she said. “It dictates the properties of the roots and how big they get. We found a very surprising conclusion in that the meristem is very conserved across three plant species that are evolutionarily distant from each other. In fact, the meristems of these species at the molecular level were more similar to each other than to other cells within their own species.”
The researchers said that they want plants to grow more but grow “smarter” in the face of stress. This might mean co-ordinating growth through meristems, or different types of meristems. They hope to see how plants can co-ordinate growth from below ground with what happens above ground. She said that the hidden half of a plant below ground is critical for breeders to consider if they want to grow a plant successfully.
“Being able to modify the meristem of a plant’s roots will help us engineer crops with more desirable properties.”
Modification strategies include finding the genes that provide stress resilience in existing lines of a crop. Once found, these can be moved to other lines of the crop for breeding.
“Finding the important genes can be hard,” she said. “Moving them around is relatively easy for tomato and rice. These genes can be ones that influence meristems, or other characteristics such as disease resistance or drought resilience.
“The second method is through genetic engineering. The use of CRISPR-based gene editing is another method and is not considered genetic engineering in the U.S.”
Bailey-Serres said breeding for more resilient crops has a bright future. As technology to study root development has become more accessible to researchers, funding agencies and biotechnology companies have been spending more money on trying to use the root data as an additional feature to breed more resistant crops with the bonus of high yields.
“This has definitely started to transition into breeding from a more holistic perspective in terms of the plant,” she said. “Some researchers look for aspects of root systems that are hard-wired in a particular crop or cultivar. We are interested in aspects of root systems that can change or be more plastic.”
Biotechnology companies are particularly interested in how roots cope with droughts and how they interact with good and bad microbes.
Researchers expect to next study how plants integrate a variety of different stresses for a positive outcome in the face of drought and how plants communicate what is happening below ground to adjust their growth above ground.
The study was recently published in the journal Cell.