A study released by the Chinese Academy of Sciences has unveiled that a new breeding strategy to create climate-smart crops rapidly has seen significant progress. The new varieties created using this strategy show higher yields under normal climatic conditions and when being exposed to heat stresses they rescue and limit yield losses both for staple grain and vegetable crops.
The research looks at the year 2050 with the assumption that the world population will increase to ten billion people by then, and it is assumed that to feed the population, farm productivity must increase by 60%. However, with current crop production combined with climate impacts, it is not feasible to see such increases.
The researchers estimate that an increase of 2 degrees C, at the moment we are heading for at least 2 degrees C, will result in yield losses ranging between 3-13%. So, in order to ensure global food security and overcome breeding bottlenecks, scientists are working against the clock to develop ‘climate-smart’ crops which can achieve higher yields under normal conditions and stable yields when under heat stresses.
The science of crop breeding
The physiological basis of crop yield and quality is the source-sink relationship with source tissues, such as leaves, being net producers of photoassimilates – mainly carbohydrates. However, reversely sink tissues, such as fruits, seeds, and roots, are net importers that use or store photoassimilates. The cell wall invertase gene (CWIN) is the crucial gene that regulates the source-sink relationship in plants. The enzyme encoded by this gene uploads and converts sucrose (carbohydrates), which is transported from leaves into glucose and fructose within sink organs, and here, these sugars can be directly absorbed and utilised. Not only are these sugars essential nutrients for the development of fruits and seeds, but they significantly influence the sweetness of fruits and the quality of rice grains.
Whenever grains are exposed to heat stress, it represses CWIN activity. As a result, the source-sink balance is disrupted, which means an inadequate energy supply in the sink organs, reduced reproductive development, and a loss of yields.
The process of the study
The researchers conceptualised a strategy based on climate-responsive optimisation of carbon partitioning to sinks (CROCS) by rationally manipulating the expression of CWIN genes in fruit and cereal crops.
With precision, they created a ten-bp heat shock element (HSE) into promoters of CWIN genes in elite rice and tomato cultivars by using a self-developed high-efficiency prime-editing tool. This HSE insertion endowed CWINs with heat-responsive upregulation in controlled and field environments to enhance carbon partitioning to rice grains and tomato fruits.
A pathway to climate-smart crops
Yield tests were carried out at several locations and across various seasons on tomatoes under various cultivation conditions such as greenhouses and open fields. Positive results showed under normal conditions these yields increased by 14-47%, and under heat stress they increased per-plot yield by 26-33% over controls and rescued 56-100% of fruit yield losses that would be created by those heat stresses. Additionally, the aspects of fruit quality such as uniformity and sugar content were significantly improved compared to unmodified controls.
Additionally, researchers found that rice cultivars improved by this strategy and not only showed a yield increase of 7-13% under normal conditions but also showed a 25% grain yield increase over controls under heat stress conditions. Specifically, up to 41% of heat-induced grain losses were rescued in rice.
The researchers concluded CROCS to be an efficient, versatile and prime-editing-based system for rapid crop improvement. It paves the way to rapidly create climate-smart crops by targeting the insertion of environment-responsive cis-regulatory elements. It also provides practical gene-editing tools and operational procedures that are feasible to study how a plant responds to stress.
The researchers added that this breeding strategy has been applied to other key crops such as soybeans, wheat and corn.