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Background Ramie is an important fiber-producing crop in China, and its fibers are widely used as textile materials. Fibers contain specialized secondary cellular walls that are mainly composed of cellulose, hemicelluloses, and lignin. Understanding the mechanism underlying the secondary wall biosynthesis of fibers will benefit the improvement of fiber yield and quality in ramie. Results Here, we performed a proteomic analysis of the bark from the top and middle parts of the stem, where fiber growth is at different stages. We identified 6971 non-redundant proteins from bast bark. Proteomic comparison revealed 983 proteins with differential expression between the two bark types. Of these 983 proteins, 46 were identified as the homolog of known secondary wall biosynthetic proteins of Arabidopsis, indicating that they were potentially associated with fiber growth. Then, we proposed a molecular model for the secondary wall biosynthesis of ramie fiber. Furthermore, interaction analysis of 46 candidate proteins revealed two interacting networks that consisted of eight cellulose biosynthetic enzymes and seven lignin biosynthetic proteins, respectively. Conclusion This study sheds light on the proteomic basis underlying bast fiber growth in ramie, and the identification of many candidates associated with fiber growth provides important basis for understanding the fiber growth in this crop.

A total of 249 sites from 197 proteins showed a differential ubiquitination level in the fiber development of ramie barks. The function of two differentially ubiquitinated proteins for fiber growth was demonstrated. Ubiquitination is one of the most common post-translational modifications of proteins, and it plays essential roles in plant growth and development. However, the involvement of ubiquitination in the growth of plant fibers remains largely unknown. We compared the ubiquitylome of the top and middle stems of ramie bark, with different fiber growth stages. We identified 249 differentially ubiquitinated sites in 197 proteins in fiber-developing barks in the stems and found that seven were homologs of Arabidopsis proteins associated with fiber growth. Overexpression of the differentially ubiquitinated proteins, RWA3 homolog whole_GLEAN_10024150 and MYB protein whole_GLEAN_10015497, significantly promoted fiber growth in transgenic Arabidopsis, indicating their involvement in this process. We also found that the abundance of these proteins decreased when their ubiquitination levels increased and vice versa in the fiber-developing bark. These results indicated that the abundance of these two proteins was adjusted through ubiquitin-dependent degradation. Collectively, our findings provide important insights into the involvement of ubiquitination in the growth of ramie fibers.

The rice MtN3/saliva/SWEET gene family consists of 21 paralogs. However, their functions in physiological processes are largely unknown, although at least three of the 21 paralogs are used by pathogenic bacteria to infect rice. Here, we report the evolutionary features, transcriptional characteristics, and putative functions in sugar transport of this gene family. The wild rice accessions in this study included those with AA, BB, CC, BBCC, CCDD, EE, and GG genomes, which appeared approximately 0.58-14.6 million years ago. The structures, chromosomal locations, phylogenetic relationships, and homologous distribution among the accessions suggest that the number of rice MtN3/saliva/SWEET paralogs gradually increased as the Oryza genus evolved, and one third of the paralogs may have originated recently. These paralogs are differentially expressed in vegetative and reproductive tissues, in the leaf senescence process, and in signaling dependent on gibberellic acid, cytokinin, or 1-naphthalene acetic acid (an analog of auxin), suggesting that they may be associated with multiple physiological processes. Four paralogs could transport galactose in yeast, which suggests that they may have a similar function in rice. These results will help to elucidate their roles and biochemical functions in rice development, adaptation to environment, host-pathogen interaction, and so forth.