Principal Investigator
Researcher
Email:zhangpeng@cemps.ac.cn
Personal Web: http://www.cassavabiotech.org.cn

National Key Laboratory of Plant Molecular Genetics

Researchers in my group use molecular tools to answer basic scientific questions related to production constraints of cassava (Manihot esculenta Crantz) and sweet potato (Ipomoea batatas Lam.), and to develop biotechnologies for bridging the gap between scientific discoveries and applications. By exploring cassava and sweet potato germplasms and focusing on key biological questions, our goals are to make cassava and sweet potato more productive, better sources of industrial applications and profitable to grow in marginal lands in order to guarantee food security and nutrition.  

We seek different biotechnological approaches for increased yield and starch production, enhanced disease and abiotic stress resistance, and improved nutrition in cassava and sweet potato. The systems for their genetic transformation and CRISPR/Cas9-based genome editing, which have been well-established in our group, provide essential molecular tools to study the two vegetatively propagated root crops. Our major objectives include: (1) Better understanding of ‘Source-Sink’ relationship in root crops; (2) Development of the molecular regulatory models of storage root development and starch accumulation; (3) Investigation of the molecular stress responses and genetic improvement under cold, drought, infertile soils and post-harvest physiological deterioration; (4) Molecular breeding of novel cultivars with high yield, novel starches and/or better nutrition. Importantly, intensive collaboration with domestic and global partners makes our team more dynamic, energetic and active in the research communities of root crops. Ongoing projects include: 

(1) Genome-wide germplasm evaluation for exploration of key trait genes in cassava and sweet potato. 

(2) Molecular regulation of sugar and starch metabolism on yield in cassava and sweet potato. 

(3) Regulatory mechanism of storage root development under the crosstalk between root cambium cell differentiation and lignin biosynthesis. 

(4) Deciphering the feature stress responses to cold, drought, nutrient deficiency and post-harvest physiological deterioration of storage roots in cassava and/or sweet potato. 

(5) Molecular breeding of cassava and sweet potato for value-added traits and biofortification. 

Cassava and sweet potato are major tropical root crops. The studies of polyploid genome decoding, storage root development, starch metabolism and response to various abiotic stresses have been conducted using biotechnological approaches as our major research activities over the past five years. 

(1) Disentangling sweet potato polyploid genome and tracing its evolutionary origin  

The sweet potato, having 90 chromosomes and being a hexaploid organism, was sequenced and assembled using our own-bred ‘Taizhong 6’. A novel haplotyping method based on single nucleotide polymorphism was developed to determine all six haplotypes within its genome individually. The high-quality genome sequences of the hexaploid species offer better precision for sweet potato research than its diploid references. The study showed that quite a number of genes have accumulated deleterious mutations on different alleles. This leads to the assumption that the selection pressure on the redundant chromosomes is much lower and hence that the ploidization event can drive an evolutionary advantage. This is a major breakthrough of the scientific community in the field of polyploid genomes. The study, which was also commented by the journal Nature Plants with a title of ‘Disentangling a polyploid genome’, aroused the great interest and attention of the public and media. 

(2) Regulatory mechanism of post-harvest deterioration of storage roots  

PPD is the result of a rapid oxidative burst, which leads to discoloration of the vascular tissues due to oxidation of phenolic compounds. Co-expression of cytosolic MeCu/ZnSOD and peroxisomal MeCAT1 in cassava dramatically improved ROS scavenging ability, leading to reduced H₂O₂ accumulation, improved abiotic stress resistance and delayed PPD occurrence. Further study showed that melatonin delays cassava PPD by directly or indirectly maintaining ROS homoeostasis and the accumulation of endogenous melatonin and the transcript levels of melatonin biosynthesis genes changed dynamically during the PPD process. This finding suggested that endogenous melatonin acts as a signal modulator for maintaining cassava PPD progression and that manipulation of melatonin biosynthesis genes through genetic engineering might prevent cassava root deterioration. These findings imply that modification of ROS scavenging enzymes through genetic engineering and thereby controlling the levels of cellular ROS is an effective approach to obtaining abiotic stress tolerant plants. In addition, our studies confirm the current model of oxidative burst as a key player in initiating ROS and it is known that enhanced ROS scavenging capacity represses PPD occurrence.  

(3) Dynamic responses of tropical plants to abiotic stresses and nutrient use 

Cassava and sweet potato are adapted to a wide range of environmental stimuli such as drought and infertile soils but sensitive to extreme cold as well as salt. Our previous study showed the featured expression patterns of the cold responsive genes in cassava. Among them, CBF-mediated cold response network was intensively studied. Cassava MeCBF1 is a typical CBF transcription factor mediating cold responses but its low expression in apical buds along with a retarded response causes inefficient upregulation of downstream cold-related genes, rendering cassava chilling-sensitive. Overexpression of AtCBF3 rendered cassava increasingly tolerant to cold and drought stresses but showed a varied regulation pattern of CBF regulon from that of cassava CBFs. It is proposed that negative regulators play a crucial role in the low-temperature response signaling pathway in cassava and several candidates have been identified for further investigation. As a response to cold induced stress in cassava, an increase in transcripts and enzyme activities of ROS scavenging genes and the accumulation of total soluble sugars were also observed. Transgenic cassava having improved ROS scavenging confirmed the tolerance to cold, drought and leaf abscission. 

The transcription of type I H⁺-pyrophosphatase (H⁺-PPase) gene IbVP1 in sweet potato plants was strongly induced by Fe deficiency and auxin, improving Fe acquisition via increased rhizosphere acidification and auxin regulation. It promotes plant growth via altered carbohydrate metabolism, improved auxin polar transport and increased rhizosphere acidification. Improved antioxidant capacities were also detected in the transgenic plants which showed improved stress resistance. The work reveals the novel regulatory mechanism of sweet potato adapting to infertile soils and iron efficient uptake, which provides new technologies for nutrient utilization and iron biofortification in crops.

(4) Constructing the “Source-Sink” relationship in root crops underlying the key regulation mechanism in starch metabolism and storage root development  

Using the first cassava T-DNA insertion mutant storage root delay (srd), we revealed that the key gene causing the retarded plant and storage root growth was the α-glucan, water dikinase 1 (GWD1) gene, which is involved in starch phosphorylation. Repression of the GWD1 expression resulted in starch excess phenotypes with reduced photosynthetic capacity and decreased levels of soluble saccharides in their leaves. These leaves contained starch granules having greatly increased amylose content and type C semi-crystalline structures with increased short chains that suggested storage starch. The study also confirmed that starch degradation in cassava is catalyzed by β-amylase in collaboration with GWD1 function by phosphorylation of starch. These results suggest that GWD1 regulates transient starch morphogenesis and storage root growth by decreasing photo-assimilation partitioning from the source to the sink and by starch mobilization in root crops.  

Much attention has been paid to the developmental regulation of the storage roots globally. Our recent study confirmed that lignification in fibrous roots by upregulation of lignin biosynthesis in the Lc-transgenic lines exerts adverse effect on storage root development in sweet potato. The intrinsic competition for carbohydrate source between lignin biosynthesis and starch accumulation affects the storage root development in sweet potato. The study provides direct evidence whereby the transition from lignin biosynthesis to starch biosynthesis is crucial for storage root development in sweet potato. It provides a clue for further investigation of regulatory crosstalk between cambium cell differentiation and lignin biosynthesis in storage roots, especially being involved in the NAC-domain transcript factors. 

(5) Germplasm evaluation, breeding and enhancement for value-added traits 

Starches and healthy components of cassava and sweet potato have a wide range of food and bioindustrial applications. To further extend their application potential and reveal their metabolism regulation, understanding the intrinsic relationship of these products and development of value-added traits are required. Both cassava and sweet potato with altered starches of different amylose/amylopectin ratio have been developed by down-regulating the expression of starch biosynthesis genes GBSSI, BEI and BEII. Those starches showed altered structural and physico-chemical properties, including morphology, chain length distribution, endothermic enthalpy, crystallinity and amylogram patterns. In addition to providing ideal materials for studying the mechanisms of starch biosynthesis and starch granule formation in the root crops, these novel waxy or high-amylose cassava and sweet potato lines enhance the potential in various industrial applications and promote their industrialization.  

Breeding bio-fortified crops is an efficient approach to reducing hidden hunger, specifically vitamins deficiency in the Africa regions where the staple people’s food is cassava and sweetpotato. In collaboration with ETH Zurich, increased bioavailable vitamin B6 in field-grown transgenic cassava for dietary sufficiency have been developed by co-expressing two enzymes, PDX1 and PDX2 that are involved in the synthesis of the vitamin from Arabidopsis. In purple sweet potato, anthocyanin components and accumulation is strongly relied on the glycosylation of anthocyanidin. An anthocyanidin 3-O-glucoside-2″-O-glucosyltransferase was revealed to catalyze the transfer of glucose to glucosylated anthocyanins in the cytosol and demonstrated that Thr-138 is the key amino acid residue for UDP glucose recognition. More importantly, new released farmer-preferred varieties ‘Taizhong 6’ and ‘Taizhong 11’, which contains high carotenoids and anthocyanins, respectively, passed through the national and provincial accreditation and have brought more than 100 million RMB incomes for local farmers. Based on these contributions, our team was awarded the Outstanding Project Award of Three Agricultural Systems S&T Golden Bridge Award in 2015.