CRISPR/Cas9-Mediated Generation of Mutant Lines in Medicago truncatula Indicates a Symbiotic Role of MtLYK10 during Nodule Formation.

CRISPR/Cas9 systems are commonly used for plant genome editing; however, the generation of homozygous mutant lines in Medicago truncatula remains challenging. Here, we present a CRISPR/Cas9-based protocol that allows the efficient generation of M. truncatula mutants. Gene editing was performed for the LysM receptor kinase gene MtLYK10 and two major facilitator superfamily transporter genes. The functionality of CRISPR/Cas9 vectors was tested in Nicotiana benthamiana leaves by editing a co-transformed GUSPlus gene. Transformed M. truncatula leaf explants were regenerated to whole plants at high efficiency (80%). An editing efficiency (frequency of mutations at a given target site) of up to 70% was reached in the regenerated plants. Plants with MtLYK10 knockout mutations were propagated, and three independent homozygous mutant lines were further characterized. No off-target mutations were identified in these lyk10 mutants. Finally, the lyk10 mutants and wild-type plants were compared with respect to the formation of root nodules induced by nitrogen-fixing Sinorhizobium meliloti bacteria. Nodule formation was considerably delayed in the three lyk10 mutant lines. Surprisingly, the size of the rare nodules in mutant plants was higher than in wild-type plants. In conclusion, the symbiotic characterization of lyk10 mutants generated with the developed CRISPR/Cas9 protocol indicated a role of MtLYK10 in nodule formation.

Black Poplar (Populus nigra L.) Root Extracellular Trap, Structural and Molecular Remodeling in Response to Osmotic Stress.

The root extracellular trap (RET) consists of root-associated, cap-derived cells (root AC-DCs) and their mucilaginous secretions, and forms a structure around the root tip that protects against biotic and abiotic stresses. However, there is little information concerning the changes undergone by the RET during droughts, especially for tree species. Morphological and immunocytochemical approaches were used to study the RET of black poplar (Populus nigra L.) seedlings grown in vitro under optimal conditions (on agar-gelled medium) or when polyethylene glycol-mediated (PEG6000-infused agar-gelled medium) was used to mimic drought conditions through osmotic stress. Under optimal conditions, the root cap released three populations of individual AC-DC morphotypes, with a very low proportion of spherical morphotypes, and equivalent proportions of intermediate and elongated morphotypes. Immunolabeling experiments using anti-glycan antibodies specific to cell wall polysaccharide and arabinogalactan protein (AGP) epitopes revealed the presence of homogalacturonan (HG), galactan chains of rhamnogalacturonan-I (RG-I), and AGPs in root AC-DC cell walls. The data also showed the presence of xylogalacturonan (XGA), xylan, AGPs, and low levels of arabinans in the mucilage. The findings also showed that under osmotic stress conditions, both the number of AC-DCs (spherical and intermediate morphotypes) and the total quantity of mucilage per root tip increased, whereas the mucilage was devoid of the epitopes associated with the polysaccharides RG-I, XGA, xylan, and AGPs. Osmotic stress also led to reduced root growth and increased root expression of the P5CS2 gene, which is involved in proline biosynthesis and cellular osmolarity maintenance (or preservation) in aerial parts. Together, our findings show that the RET is a dynamic structure that undergoes pronounced structural and molecular remodeling, which might contribute to the survival of the root tip under osmotic conditions.