Comparative root transcriptome analysis of two soybean cultivars with different cadmium sensitivities reveals the underlying tolerance mechanisms.

Soybean can provide rich protein and fat and has great economic value worldwide. Cadmium (Cd) is a toxic heavy metal to organisms. It can accumulate in plants and be transmitted to the human body via the food chain. Cd is a serious threat to soybean development, particularly root growth. Some soybean cultivars present tolerant symptoms under Cd stress; however, the potential mechanisms are not fully understood. Here, we optimized RNA-seq to identify the differentially expressed genes (DEGs) in Cd-sensitive (KUAI) and Cd-tolerant (KAIYU) soybean roots and compared the DEGs between KAIYU and KUAI. A total of 1506 and 1870 DEGs were identified in the roots of KUAI and KAIYU, respectively. Through Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and gene function analyses, we found that genes related to antioxidants and sequestration were responsible for Cd tolerance in KAIYU. In addition, overexpression of Glyma11g02661, which encodes a heavy metal-transporting ATPase, significantly improved Cd tolerance in transgenic hairy roots. These results provide a preliminary understanding of the tolerance mechanisms in response to Cd stress in soybean root development and are of great importance in developing Cd-resistant soybean cultivars by using the identified DEGs through genetic modification.

Evolution and Expression Characteristics of Receptor-Like Cytoplasmic Protein Kinases in Maize, Rice and Arabidopsis.

Receptor-like cytoplasmic protein kinases (RLCKs) are involved in various activities in plant growth and development. We have totally identified 162, 160, and 402 RLCK genes in maize, rice, and Arabidopsis genomes, respectively. Phylogenetic analyses divided 724 RLCK genes into 15 subfamilies and similar structural patterns of kinase activity sites and functional sites were observed within the subfamilies. Furthermore, the structural patterns of intron/exon in the same subfamilies were similar, implicating their close evolutionary relationship. Chromosome distribution indicated that segmental duplication of RLCK genes might be a major mechanism contributing to the expansion of the RLCK superfamilies in maize, rice, and Arabidopsis, respectively. The analysis of the synteny relationship and gene structure indicated that the evolution of most RLCKs in maize were prior to rice and Arabidopsis. Most of the ratio of Ka/Ks is inferior to one, suggesting that RLCK genes have experienced the negative selection in maize, rice and Arabidopsis. Duplication time revealed that the maize was the earliest emergence among these three species. The expression profiles showed that there are some specifically expressed RLCK genes in maize root, leaf, ear, and tassel. These specific expression genes may participate in the developmental regulation of these maize tissues. Our results will be useful in providing new insights into evolution of RLCKs and revealing the regulatory network of maize, rice, and Arabidopsis development.

GmIDL2a and GmIDL4a, Encoding the Inflorescence Deficient in Abscission-Like Protein, Are Involved in Soybean Cell Wall Degradation during Lateral Root Emergence.

The number of lateral roots (LRs) of a plant determines the efficiency of water and nutrient uptake. Soybean is a typical taproot crop which is deficient in LRs. The number of LRs is therefore an important agronomic trait in soybean breeding. It is reported that the inflorescence deficient in abscission (IDA) protein plays an important role in the emergence of Arabidopsis LRs. Previously, the genes which encode IDA-like (IDL) proteins have been identified in the soybean genome. However, the functions of these genes in LR development are unknown. Therefore, it is of great value to investigate the function of IDL genes in soybean. In the present study, the functions of two root-specific expressed IDL genes, GmIDL2a and GmIDL4a, are investigated. The expressions of GmIDL2a and GmIDL4a, induced by auxin, are located in the overlaying tissue, where LRs are initiated. Overexpression of GmIDL2a and GmIDL4a increases the LR densities of the primary roots, but not in the elder root. Abnormal cell layer separation has also been observed in GmIDL2a- and GmIDL4a-overexpressing roots. These results suggest that the overlaying tissues of GmIDL2a- and GmIDL4a-overexpressing roots are looser and are suitable for the emergence of the LR primordium. Further investigation shows that the expression of some of the cell wall remodeling (CWR) genes, such as xyloglucan endotransglucosylase/hydrolases, expansins, and polygalacturonases, are increased when GmIDL2a and GmIDL4a are overexpressed in hairy roots. Here, we conclude that GmIDL2a and GmIDL4a function in LR emergence through regulating soybean CWR gene expression.

ZmSTK1 and ZmSTK2, encoding receptor-like cytoplasmic kinase, are involved in maize pollen development with additive effect.

Pollen germination and pollen tube growth are important physiological processes of sexual reproduction of plants and also are involved in signal transduction. Our previous study reveals that ZmSTK1 and ZmSTK2 are two receptor-like cytoplasmic kinases (RLCK) homologs in Zea mays as members of receptor-like protein kinase (RLK) subfamily, sharing 86% identity at the amino acid level. Here, we report that ZmSTK1 and ZmSTK2, expressed at late stages of pollen development, regulate maize pollen development with additive effect. ZmSTK1 or ZmSTK2 mutation exhibited severe pollen transmission deficiency, which thus influenced pollen fertility. Moreover, the kinase domains of ZmSTKs were cross-interacted with C-terminus of enolases detected by co-immunoprecipitation (Co-IP) and yeast two-hybrid system (Y2H), respectively. Further, the detective ZmSTK1 or ZmSTK2 was associated with decreased activity of enolases and also reduced downstream metabolite contents, which enolases are involved in glycolytic pathway, such as phosphoenolpyruvate (PEP), pyruvate, ADP/ATP, starch, glucose, sucrose and fructose. This study reveals that ZmSTK1 and ZmSTK2 regulate maize pollen development and indirectly participate in glycolytic pathway.