In a recent study published in New Phytologist, researchers in Professor CHU Chengcai’s group from Institute of Genetics and Developmental Biology, Chinese Academy of Sciences demonstrates that cytokinin phytohormones could provide a key for breeding Zn-enriched rice.
Zinc (Zn) deficiency is a critical problem in human nutrition. Currently, around two billion people worldwide suffer from Zn deficiency because their plant-based diets are not a sufficient source of this essential element. Deficiency of Zn raises the risk of growth stunting, infections, and even death. It is estimated that around 800,000 people die annually due to Zn deficiency, of which 450,000 are under the age of five. This is especially the case in developing countries, where cereal-based diets that are low in Zn content provide the principal daily calorie intake of the population.
Consequently, the simultaneous improvement of grain yield and quality is a big challenge for breeding. For decades, the primary objective of modern agriculture and breeding programs has focused on improving productivity and yield of crops. On the other side, an equally important, but largely overlooked objective in breeding programs is nutritional improvement of crops.
Rice is a major source of food for nearly half population worldwide, and breeding Zn-enriched rice cultivars has been strongly advocated. During the last decade, numerous attempts have been made, such as increasing the expression of genes encoding metal transporters and metal chelators. However, until now engineering approaches have not been so successful, mainly because either increased Zn concentration has been accompanied by a growth and/or yield penalty, or because the Zn-rich of engineered crops has not translated from the greenhouse to the field.
Professor CHU’s work provides a possibility that precise spatial control of cytokinin metabolism using CRISPR/Cas9 gene-editing technology could be employed in nutritional improvement of crops.
Researchers performed analyses with cytokinin-related transgenic lines and mutants to provide direct evidence that cytokinins occupy a key position in controlling Zn balance in the plant cell. Transporters responsible for Zn uptake and chelators for the internal transport of Zn are negatively controlled by cytokinins. Notably, reduced cytokinin levels attenuate the regulatory effects of cytokinin signaling and alleviate the repression of Zn transporters. Additionally, deficiency in cytokinins leads to formation of an enlarged root system which enables plants to efficiently obtain sparingly available nutrients from the soil.
The results showed that cytokinin synthesis is regulated in a highly dynamic way in response to Zn status and plants can sense Zn availability to coordinate cytokinin biosynthesis and Zn uptake. Subsequently, fine-tuning of cytokinin metabolism by root-specific expression of a cytokinin degradation enzyme displayed enhanced root development without affecting shoot parts, which improve both Zn nutrient and yield traits. These will increase the likelihood of farmer adoption, since such crops deliver benefits for both consumers and farmers.
This research was supported by grants from the Chinese Academy of Sciences, Ministry of Science and Technology of China, and the State Key Laboratory of Plant Genomics.
