CRISPR/Cas9-Engineered Large Fragment Deletion Mutations in Arabidopsis CEP Peptide-Encoding Genes Reveal Their Role in Primary and Lateral Root Formation

MhIDA small peptides modulate the growth and development of roots in Malus hupehensis

KEY MESSAGE

MhIDA small peptides promote apple root growth by enhancing auxin synthesis and cell wall remodeling gene expression, revealing a peptide-based strategy to improve root architecture. Although small peptides have been well documented as crucial regulators of plant growth and development, the molecular mechanisms underlying lateral root morphogenesis in Malus hupehensis remain poorly understood. In this research, exogenous application of 1 µM MhIDA-Like family peptides increased primary root (PR) length by 14.31-19.96% and lateral root (LR) number by 124.54-149.08%. MhIDA, predominant expression in the root tip and lateral root primordium, demonstrated the most substantial promoting effects on PR elongation, LR number and density when the treatment concentration reached 1 µM. Furthermore, similar effects were found in MhIDA-overexpression transgenic apple seedlings, with the number and density of transgenic LRs increase by 80.52 and 126.86%, respectively, compared with wild-type seedlings. More importantly, 1 µM MhIDA treatment induced significant hormonal alterations, with the content of auxin, salicylic acid and gibberellic acid increasing by 1.5-fold, 1.4-fold, and 2.1-fold, respectively, compared to control. The qRT-PCR results showed that MhIDA could induce the expression of auxin synthesis genes (MhTAA1 and MhYUCC1) that were up-regulated by about twofold, and the cell wall remodeling-related genes (MhEXP17, MhXTR6, MhPGAZAT and MhPGLR) were upregulated by about 2- to 4-fold after 1 µM MhIDA treatment, thereby regulating LR emergence and formation of Malus hupehensis. Overall, these findings suggested the MhIDA peptide can promote the growth and development of roots, laying the foundation for cultivating apple rootstocks with strong roots and higher resistance to abiotic stress.

Peptide hormones in plants

Peptide hormones are defined as small secreted polypeptide-based intercellular communication signal molecules. Such peptide hormones are encoded by nuclear genes, and often go through proteolytic processing of preproproteins and post-translational modifications. Most peptide hormones are secreted out of the cell to interact with membrane-associated receptors in neighboring cells, and subsequently activate signal transductions, leading to changes in gene expression and cellular responses. Since the discovery of the first plant peptide hormone, systemin, in tomato in 1991, putative peptide hormones have continuously been identified in different plant species, showing their importance in both short- and long-range signal transductions. The roles of peptide hormones are implicated in, but not limited to, processes such as self-incompatibility, pollination, fertilization, embryogenesis, endosperm development, stem cell regulation, plant architecture, tissue differentiation, organogenesis, dehiscence, senescence, plant-pathogen and plant-insect interactions, and stress responses. This article, collectively written by researchers in this field, aims to provide a general overview for the discoveries, functions, chemical natures, transcriptional regulations, and post-translational modifications of peptide hormones in plants. We also updated recent discoveries in receptor kinases underlying the peptide hormone sensing and down-stream signal pathways. Future prospective and challenges will also be discussed at the end of the article.

The pathogen-induced peptide CEP14 is perceived by the receptor-like kinase CEPR2 to promote systemic disease resistance in Arabidopsis

Secreted plant peptides that trigger cellular signaling are crucial for plant growth, development, and adaptive responses to environmental stresses. In Arabidopsis (Arabidopsis thaliana), the C-TERMINALLY ENCODED PEPTIDE (CEP) family is a class of secreted signaling peptides that is phylogenetically divided into 2 groups: group I (CEP1-CEP12) and group II (CEP13-CEP15). Several group I CEP peptides regulate root architecture and nitrogen starvation responses, whereas the biological activity and roles of group II CEPs remain unknown. Here, we report that a group II CEP peptide, CEP14, functions as a pathogen-induced elicitor of Arabidopsis immunity. In response to infection by the bacterial pathogen Pseudomonas syringae, CEP14 expression was highly induced via the salicylic acid pathway in Arabidopsis leaves and roots. In the absence of a pathogen attack, treatment of Arabidopsis plants with synthetic CEP14 peptides was sufficient to trigger immune responses. Genetic and biochemical analyses demonstrated that the receptor-like kinase CEP RECEPTOR 2 (CEPR2) perceives CEP14 to trigger plant immunity. The SOMATIC EMBRYOGENESIS RECEPTOR KINASES (SERKs) BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and SERK4 also participated in CEP14 perception by forming CEP14-induced complexes with CEPR2. Overexpression of CEP14 largely enhanced Arabidopsis resistance to P. syringae, while CEP14 or CEPR2 mutation significantly attenuated Arabidopsis systemic resistance to P. syringae. Taken together, our data reveal that the pathogen-induced CEP14 peptide, which is perceived by the CEPR2-BAK1/SERK4 receptor complexes, acts as an endogenous elicitor to promote systemic disease resistance in Arabidopsis.

Genetic control of root morphology in response to nitrogen across rapeseed diversity

Rapeseed (Brassica napus L.) is an oil-containing crop of great economic value but with considerable nitrogen requirement. Breeding root systems that efficiently absorb nitrogen from the soil could be a driver to ensure genetic gains for more sustainable rapeseed production. The aim of this study is to identify genomic regions that regulate root morphology in response to nitrate availability. The natural variability offered by 300 inbred lines was screened at two experimental locations. Seedlings grew hydroponically with low or elevated nitrate levels. Fifteen traits related to biomass production and root morphology were measured. On average across the panel, a low nitrate level increased the root-to-shoot biomass ratio and the lateral root length. A large phenotypic variation was observed, along with important heritability values and genotypic effects, but low genotype-by-nitrogen interactions. Genome-wide association study and bulk segregant analysis were used to identify loci regulating phenotypic traits. The first approach nominated 319 SNPs that were combined into 80 QTLs. Three QTLs identified on the A07 and C07 chromosomes were stable across nitrate levels and/or experimental locations. The second approach involved genotyping two groups of individuals from an experimental F2 population created by crossing two accessions with contrasting lateral root lengths. These individuals were found in the tails of the phenotypic distribution. Co-localized QTLs found in both mapping approaches covered a chromosomal region on the A06 chromosome. The QTL regions contained some genes putatively involved in root organogenesis and represent selection targets for redesigning the root morphology of rapeseed.

Research Progress of Small Plant Peptides on the Regulation of Plant Growth, Development, and Abiotic Stress

Small peptides in plants are typically characterized as being shorter than 120 amino acids, with their biologically active variants comprising fewer than 20 amino acids. These peptides are instrumental in regulating plant growth, development, and physiological processes, even at minimal concentrations. They play a critical role in long-distance signal transduction within plants and act as primary responders to a range of stress conditions, including salinity, alkalinity, drought, high temperatures, and cold. This review highlights the crucial roles of various small peptides in plant growth and development, plant resistance to abiotic stress, and their involvement in long-distance transport. Furthermore, it elaborates their roles in the regulation of plant hormone biosynthesis. Special emphasis is given to the functions and mechanisms of small peptides in plants responding to abiotic stress conditions, aiming to provide valuable insights for researchers working on the comprehensive study and practical application of small peptides.

CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant-microbe interactions

A growing understanding is emerging of the roles of peptide hormones in local and long-distance signalling that coordinates plant growth and development as well as responses to the environment. C-TERMINALLY ENCODED PEPTIDE (CEP) signalling triggered by its interaction with CEP RECEPTOR 1 (CEPR1) is known to play roles in systemic nitrogen (N) demand signalling, legume nodulation, and root system architecture. Recent research provides further insight into how CEP signalling operates, which involves diverse downstream targets and interactions with other hormone pathways. Additionally, there is emerging evidence of CEP signalling playing roles in N allocation, root responses to carbon levels, the uptake of other soil nutrients such as phosphorus and sulfur, root responses to arbuscular mycorrhizal fungi, plant immunity, and reproductive development. These findings suggest that CEP signalling more broadly coordinates growth across the whole plant in response to diverse environmental cues. Moreover, CEP signalling and function appear to be conserved in angiosperms. We review recent advances in CEP biology with a focus on soil nutrient uptake, root system architecture and organogenesis, and roles in plant-microbe interactions. Furthermore, we address knowledge gaps and future directions in this research field.