Principal Investigator
Researcher
Email:dychao@cemps.ac.cn
Personal Web: http://sippe.ac.cn/dycao/

National Key Laboratory of Plant Molecular Genetics

Our research interests mainly lie in understanding the genetic and molecular basis underlying plant ion homeostasis and local adaption. Owing to its sessile nature, plant has to evolve delicate mechanisms to maintain appropriate level of mineral nutrient and water during its direct exposure to complicated soil environment. Therefore, the molecular machinery in regulating mineral elements and water use efficiency is very important for local adaptation of plants. 

Our interests are then related to two areas: ionomics and abiotic stress.  

In the ionomics area, I am mainly interested in the genetic basis underlying the natural variation in leaf and grain ionome of Arabidopsis thaliana and crops, the evolution driven forces forming the variation, and the role of the natural variation in local adapation and speciation. Meanwhile, I am also interested in the molecular mechanisms of ion homeostasis, and try to engineer high mineral nutrient, low heavy metal and high fertilizer use efficiency crops according to obtained information. 

In the abiotic stress area, I am mainly interested in salt tolerance of A. thaliana and rice. After screening hundreds of natural accessions of A. thaliana and rice, I identified several extreme salt and drought tolerance accessions of A. thaliana and rice. I am now interested in mapping the causal QTLs/genes responsible for the tolerance traits of those accessions, and revealing the molecular mechanisms of plant adaptation to local adverse environments. 

By using thousands of natural accessions of A. thaliana, rice, corn and soybean, I have identified more than 100 QTLs and cloned 8 causal genes controlling plant ionomics and local adaptation. I am now engaging in cloning and functional analysis of more QTLs and revealing the molecular mechanisms of plant adaptation to local adverse environments. I have published 33 papers on journals such as Science, Nature Cell Biology, PLoS Biology, Genes & Development and Plant Cell. These papers have been cited for about 2100 times and my h-index has reached to 20. Below are some of our representative work published recently:    

1) Characterization of a choline transporter in vesicle trafficking and ion homeostasis of plant. 

The choline derivative phosphatidylcholine (PC) is a major component of eukaryotic membranes, which is synthesized on ER and transported to plasma membrane and other organelles via Golgi and vesicles. During the process of membrane trafficking, PC content is gradually decreased on the cytoplasmic side of the bilayer membrane from ER to plasma membrane and PC is thus predominantly presented on the out layer of plasma membrane. The significance of this phenomena was unknown and generated a question that if choline and PC have additional roles in eukaryotic cells.    

During a genetic screening, we isolated an A. thaliana mutant sic1 with significant ionomic defect and identified the causal gene that encodes a choline transporter CTL1. We found that this gene controls ionome homeostasis by regulating the secretory trafficking of proteins required for plasmodesmata (PD) development, as well as the transport of some ion transporters. The mutation of CTL1 disturbs choline homeostasis and membrane constitution and thus impairs vesicle trafficking. We further established that the defect on vesicle trafficking affect subcellular localization of PD and ion transporters such as NRAMP1. The PD defect not only affects ion transport itself, but also leads to ectopic expression of some ion transporters. Characterizing choline transporter-like 1 (CTL1) as a new regulator of protein sorting may enable researchers to understand not only ion homeostasis in plants but also vesicle trafficking in general.  

This study was published on PLoS Biology (Gao et al., 2017). After our paper was published, EurekAlert reported our work in its “News Release” and the journal of PLoS Biology also published a comment paper named “Lipids at the crossroad: Shaping biological membranes heterogeneity defines trafficking pathways” in its primers section (Boutté, 2018).  

2) Discovering a new mechanism of plant adaptation to saline habitat  

Soil salinization affects crop yield and quality, and is becoming a big problem for modern agriculture. Some plants have evolved delicate mechanisms to deal with salt stress during their colonization in saline habitat. Identifying the genetic basis underlying these mechanisms is not only critical for understanding natural selection and evolutionary mechanisms, but also helps to resolve the soil salinization problem. Previous studies have established that the natural accessions accumulated high sodium in the leaves are enriched in the coastal and saline areas (Baxter et al., 2010). Weak expression of AtHKT1 encoding a sodium transporter is responsible for the high leaf sodium in these accessions (Rus et al., 2006), and was then hypothesized to drive local adaptation to saline environments in A. thaliana. However, high leaf sodium was believed to be a salt-sensitive symbol and AtHKT1 was previously found to be essential for salt tolerance of A. thaliana (Rus et al., 2006; Berthomieu et al., 2003). These contradictive results challenge if and how AtHKT1 is involved in saline adaptation of A. thaliana.  

By using forward and reverse genetics, we uncovered that AtHKT1 plays a positive role in local adaptation. We further used reciprocal grafting experiment to establish that shoot is more important for salt tolerance of Tsu-1 while root plays a major role in salt tolerance of the inland A. thaliana accession Col-0. Further analysis showed that the expression level of AtHKT1 in Tsu-1 stems is much higher than in Col-0 stems under salt stress, indicating that AtHKT1 is hyper-functional in stems but hypo-functional in roots of the salt tolerant accessions. Ionomics analysis revealed that the floral sodium content is significant in Tsu-1 then in Col-0 upon salt stress, establishing that AtHKT1 in Tsu-1 is more efficient in retrieving sodium flow to the flower, an essential but salt sensitive organ for productivity. Together with other evidences, these results demonstrated that the coastal accessions have evolved a new mechanism by upregulating expression of AtHKT1 in shoots to reduce sodium toxicity to the flowers to adapt to saline habitats. This study not only addresses an important scientific question of evolution biology, but may also open a new angle for engineering salt tolerance crops. This study was published on PLoS Genetics (An et al., 2017).  

3) Identification of the real arsenate reductases in plants and uncovering their roles in arsenic accumulation. 

Inorganic arsenic is a carcinogen and its ingestion in foods such as rice presents a significant risk to human health.  Arsenate is chemically similar to phosphate and therefore cannot be distinguished by plants. To recognize arsenic from phosphorus, plants chemically reduce arsenate to arsenite. However, the arsenate reductase required for this reduction is unknown. Using genome-wide association (GWA) mapping of loci controlling natural variation in arsenic accumulation in Arabidopsis thaliana, we identified the missing arsenate reductase, which we named High Arsenic Content1 (HAC1). The HAC1 protein accumulates in the epidermis, the outer cell layer of the root, and also in the pericycle cells surrounding the central vascular tissue. Plants lacking HAC1 lose their ability to efflux arsenite from roots, leading to both increased transport of arsenic into the central vascular tissue and on into the shoot. HAC1 therefore functions to reduce arsenate to arsenite in the outer cell layer of the root, facilitating efflux of arsenic as arsenite back into the soil to limit its accumulation in the root and transport to the shoot. Arsenate reduction by HAC1 in the pericycle may play a role in limiting arsenic loading into the xylem. Loss of HAC1 encoded arsenic reduction leads to a 300-fold increase in arsenic accumulation in shoots causing an increased sensitivity to arsenate toxicity (Chao et al., 2014).  

By using forward genetics, we further studied the orthologs of AtHAC1 in rice, namely OsHAC1;1 and OsHAC1;2. We confirmed that both OsHAC1;1 and OsHAC1;2 encode arsenate reductases in rice, that predominantly expressed in root epidermis, pericycle cells, cortex, and endodermis cells. Knocking out OsHAC1;1 or OsHAC1;2 decreased the reduction of arsenate to arsenite in roots, reducing arsenite efflux to the external medium. Loss of arsenite efflux led to increased As accumulation in shoots. Greater effects were observed in a double mutant of the two genes. In contrast, overexpression of either OsHAC1;1 or OsHAC1;2 increased arsenite efflux, reduced As accumulation in leaves and grains, and enhanced arsenate tolerance. These results suggest HAC1 has application potential for engineering low arsenic crops.