Wen 丨 Nameless Hao
Editor丨Nameless Hao
In crop varieties, nutrient deficiency can seriously impair plant growth and development processes, resulting in a decrease in final yield, and root structure remodeling is considered a key factor supporting nutrient-poor soil environments.
Adequate nitrogen (N) supply plays an important role in maintaining crop productivity on nutrient-deficient soils, and excessive application of nitrogen fertilizer may contaminate the soil and increase the production costs for growers.
To address this issue, crop breeders have made great efforts to improve the nitrogen use efficiency of crops, and this article summarizes the latest advances in QTL/genes, regulatory pathways and hormonal crosstalk involved in legume root growth and development.
We have improved nitrogen access by regulating the progress of microbio-root symbiosis through QTL/genes, and understanding the molecular mechanisms that regulate root structure in response to nitrogen availability may help strengthen the root system of legumes and promote environmentally friendly and sustainable agriculture.
Insights from legumes
To make agriculture sustainable, pulses play an important role in promoting food security and environmental maintenance, often intercropping with cereals to increase land productivity through soil improvement.
In crop rotation, pulses serve as solid N2 plants to help diversify growing systems, and while pulses stabilize nitrogen, about 50% of soil nitrogen still needs to be used efficiently by plants to increase crop yields and maintain optimal levels.
Nitrogen is a large amount of essential minerals that plants need to produce enough energy to maintain vegetative growth and achieve economic food production, and roots are the main organs of plants to obtain nutrients and water from the soil.
Soil media infiltrated by plant roots are highly heterogeneous in the distribution of minerals and reservoirs. Plants employ various strategies to alter their root structure in response to these heterogeneous distributions, which are influenced by various abiotic and biological factors.
These include water scarcity, nitrogen, potassium and phosphorus levels, and plants have evolved over time to survive low nitrogen levels, which may involve changes in root structure such as primary root (PR) length, lateral root (LR) system, and root hair structure.
A large number of genes are involved in the modification of the root system to enhance the access to minerals in different plant species, such as maize, Arabidopsis, soybean and rice.
In addition, regulatory proteins and transcription factors also contribute to the regulatory network associated with the root system to improve nutrient use efficiency, and the potential of assisted breeding to improve nutrient profitability is also highlighted by many nitrogen-hungry root QTLs in different crops, including pulses, rice, maize and soybeans.
The importance of pulses in conservation agriculture
About 180 million hectares of cereal and fodder pulses are grown, accounting for 12 to 15 percent of the planet's farmland, and pulses account for 27 percent of the world's total production of major crops and 33 percent of human dietary protein needs.
Pulses are grown to improve soil fertility and organic matter content, enhance soil water retention and nutrient cycling, and sustainable growth in pulses production is essential to feed the world's growing population.
Also by adopting sustainable agricultural production methods, we can minimize greenhouse gas emissions, food loss and waste, improve crop productivity and global supply chains, and provide nutritious food to communities suffering from hunger and malnutrition.
Building on this, the introduction of pulses into sustainable agriculture can provide a solid foundation for food safety and environmental quality, pulses have many advantages that fix nitrogen in the atmosphere, release high-quality organic matter into the soil, and improve soil carbon-to-nitrogen ratio.
Most importantly with a strong and deep root system that promotes mycorrhizal mineral dissolution, absorption, and water transport in deeper soil layers, a variety of legumes have been widely adopted in sustainable farming systems and conservation agriculture in Turkey, Australia, Brazil and North America.
How important nitrogen (N) plays in sustainable crop production
Sustainable agriculture refers to crop production that does not harm biodiversity, crop quality and the environment. Sustainable crop production relies on minimizing pesticide use through integrated pest management.
So it protects biodiversity, improves soil health, and ensures food quality and safety, and N, as a mineral element, is an important component of plant cells and directly promotes the formation of proteins in grain seeds.
It is closely related to crop yield, and its accumulation in plants is an important factor in crop productivity, considering this, NUE is a key parameter of crop yield, but unfortunately its use is relatively inefficient.
There are also some losses in the use efficiency subsurface, which are in the form of acid rain, soil acidification, polluted freshwater streams and even air pollution, which have a more serious impact on our natural environment and ultimately cause human health problems.
In recent years, the cultivation of various high-yield new varieties has mainly relied on the high input of nitrogen fertilizer, which has been introduced into the planting system according to the preferences of breeders and farmers, and the increase in nitrogen fertilizer application has brought various problems such as a decline in nitrogen fertilizer utilization.
Therefore, there is a current consensus that there is a need to balance the benefits of nitrogen application with the reduction of adverse effects of yield improvement by reducing nitrogen inputs and soil pollution, and that the development of varieties that can tolerate low nitrogen stress or improve NUE is a key goal for future crop breeding.
Achieving these goals requires a comprehensive knowledge and understanding of nitrogen metabolism under nitrogen deficiency, with root-related traits having a considerable impact on nitrogen uptake from soils, and the effects of nitrogen deficiency on root growth, length, and LR remain divergent.
We are able to grow new varieties with improved root structure under low nitrogen conditions that are a prerequisite for sustainable agriculture, plants that change their root systems to extract nitrogen from the soil, and they can adapt to low nitrogen supply by guiding root development towards nitrogen-rich patches and increasing root uptake surface area.
Given the low nitrate utilization and high fluidity within a given soil area, the definition of a more effective root system may not be specific, it may vary and depend on the soil type, plant species, and other environmental factors.
From the perspective of modeling techniques, it can be proposed that the efficient capture of nitrate is the result of a trade-off between the total soil volume explored and the rate of nitrogen acquisition.
However, superior root structure helps plants explore soil and promote nutrient uptake, and improvement in root structure has been widely documented and is considered an important strategy to improve nitrogen uptake in nitrogen-deficient environments, which may accelerate root performance in low-nitrogen soils.
N is a component of organic compounds in plants, including protein, chlorophyll and nucleic acids, and nitrogen deficiency is a major factor inhibiting plant growth and development, resulting in a significant decline in crop yield and productivity.
Plants use different forms of nitrogen in the rhizosphere, including nitrates, organic compounds (containing soluble nitrogen), and ammonium, and in Arabidopsis, the roots adapt to different levels and types of nitrogen supply, including stimulating root branching, inhibiting LR initiation at high carbon-to-nitrogen ratios, and so on.
In response to low nitrogen supply, nutrient acquisition is enhanced primarily through root growth and deeper root uptake of nitrate, one of the most mobile nutrient molecules in soil.
Genotypes with deeper roots can absorb nitrogen from nitrogen-deficient soils more efficiently, and in order to efficiently obtain nitrogen in intensive planting systems, an ideal type with a root structure with strong LR, deeper roots and strong nitrate response is proposed.
A study based on field trials showed that extensive and deep root systems are a prerequisite for high nitrogen consumption in maize, and in legumes, changes in root structure promote nitrogen and phosphorus acquisition and nodule formation, with significant positive correlations between nitrogen content, number of nodules and root structure.
Legumes root structure in response to hormonal remodeling of N
Plant endogenous hormones play a decisive role in regulating root growth and LR formation, and auxins and cytokinins have been found to antagonize or synergistically interact with each other during plant growth and development.
Auxin and cytokinin have a positive effect on rhizome growth, in contrast, both hormones have hostile functions during LR development.
Auxin is an important hormone that plays an important role in nitrogen-mediated root growth and regulation of plant development, and high nitrogen levels are thought to reduce the accumulation of local auxin, suggesting that auxin is a core element of stem root signaling of nitrogen availability.
Higher bud nitrogen levels are thought to inhibit germ-root auxin trafficking, resulting in lower LR numbers in Arabidopsis, whereas studies on alfalfa tribulus have shown that higher nitrogen levels in buds improve auxin bud-root transport.
This response is further emphasized by the sunn-1 alfalfa mutant, which has insensitive auxin transport from stem to root regardless of nitrogen concentration.
In sunn-1, no correlation between LR regulation and N-mediated auxin transport was observed, whereas SUNN-mediated auxin clade motility is only suitable for nitrate-dependent LR remodeling.
The above description also shows that the same does not apply to nodulation, suggesting that auxin-mediated nitrogen regulation of nodules acts locally on the root, and auxin-mediated root growth also includes interactions with other hormones such as ethylene.
Ethylene is a gaseous plant hormone that is a positive regulator of roots and is involved in local nitrate-mediated root development, and in many plant species, higher nitrate levels promote the progression of root ethylene.
On the other hand, high ethylene limits LR development and nodule formation, while low ethylene levels promote LR growth and nodulation, and many rhizobia species break down aminocyclopropane by producing ethylene precursors, aminocyclopropane deaminase to promote nodulation.
In addition, ethylene can also control the location of nodulation, because higher ethylene levels in the phloem on the contrary favor the formation of nodulation. Ethylene regulation of nodules and LR is most likely to occur through regulation of cell cycle pathways.
Ethylene affects the cell cycle by interacting with cytokinins, which are also involved in nitrate-mediated root development. Cytokinins, another important plant hormone, regulate the cell cycle and mediate nitrogen state between roots and shoots through phosphate delivery pathways.
Nitrate supply improves cytokinin synthesis in nitrogen-deficient roots, where nitrate enters the bud and signals the nitrogen state of the root. As the bud nitrogen supply decreases, cytokinins are moved back to the roots to signal lower nitrogen levels in the buds.
Cytokinins act directly on LR-creating cells to inhibit LR initiation, however, once LR differentiation occurs, higher cytokinin levels stimulate LR elongation.
Higher nitrates also trigger LR growth, and further studies of nitrate-cytokinin crosstalk during LR formation may be interesting.
In legumes, the application of exogenous cytokinins triggers multiple genes associated with nodulation, as cytokinins act as upstream components of the nodulation pathway, alfalfa and lotus cytokinin mutants reduce nodule formation, and mutants with repair functions show increased nodule formation.
The root structure of soybeans is a key feature in maintaining yields in nitrogen-deficient environments, and plants with strong root systems can efficiently extract nutrients from the soil and enable them to maintain normal growth in nitrogen-deficient conditions.
It has been proven that better subsoil exploration can improve chickpea yields under adversity due to stronger, deeper roots, and several root traits were studied to determine the importance of root structure to maintain legume yields in nutrient-limited environments.
Root traits directly affect legume productivity under nitrogen-deficient conditions, suggesting that root structure is a trait needed for further breeding programs, and although many studies have recently been conducted to determine the QTL/genetic diversity that influence root structure, there have been fewer attempts to utilize these resources in breeding programs.
Efforts are under way to introduce this genomic region into a variety of other fine legume strains, which are rich in root structure genetic resources that have not yet been exploited in crop stress breeding programmes to develop better strains.