Because of the high energy consumed during symbiotic nitrogen fixation, legumes must balance growth and symbiotic nodulation. Both lateral roots and nodules form on the root system and the developmental coordination of these organs according to reduced nitrogen (N) availability remains elusive. We show that the Compact Root Architecture 2 (MtCRA2) receptor-like kinase is essential to promote the initiation of early symbiotic nodulation and to inhibit root growth in response to low-N. MtCEP1 peptides can activate MtCRA2 under N-starvation conditions, leading to a repression of MtYUC2 auxin biosynthesis gene expression, and therefore of auxin root responses. Accordingly, the compact root architecture phenotype of cra2 can be mimicked by an auxin treatment or by over-expressing MtYUC2, and conversely, a treatment with YUC inhibitors or a MtYUC2 knock-out rescues the cra2 root phenotype. The MtCEP1-activated CRA2 can additionally interact with and phosphorylate the MtEIN2 ethylene signaling component at Ser643 and Ser924, preventing its cleavage and therefore repressing ethylene responses, thus locally promoting the root susceptibility to rhizobia. In agreement, the cra2 low nodulation phenotype is rescued by an ein2 mutation. Overall, by reducing auxin biosynthesis and inhibiting ethylene signaling, the MtCEP1/MtCRA2 pathway balances root and nodule development under low-N conditions.

Plants can be considered an open system. Throughout their life cycle, plants need to exchange material, energy and information with the outside world. To improve their survival and complete their life cycle, plants have developed sophisticated mechanisms to maintain cellular homeostasis during development and in response to environmental changes. Autophagy is an evolutionarily conserved self-degradative process that occurs ubiquitously in all eukaryotic cells and plays many physiological roles in maintaining cellular homeostasis. In recent years, an increasing number of studies have shown that autophagy can be induced not only by starvation but also as a cellular response to various abiotic stresses, including oxidative, salt, drought, cold and heat stresses. This review focuses mainly on the role of autophagy in plant abiotic stress management.

The multimember CEP (C-terminally Encoded Peptide) gene family is a complex group that is involved in various physiological activities in plants. Previous studies demonstrated that MtCEP1 and MtCEP7 control lateral root formation or nodulation, but these studies were based only on gain of function or artificial miRNA (amiRNA)/RNAi approaches, never knockout mutants. Moreover, an efficient multigene editing toolkit is not currently available for Medicago truncatula. Our quantitative reverse transcription–PCR data showed that MtCEP1, 2, 4, 5, 6, 7, 8, 9, 12, and 13 were up-regulated under nitrogen starvation conditions and that MtCEP1, 2, 7, 9, and 12 were induced by rhizobial inoculation. Treatment with synthetic MtCEP peptides of MtCEP1, 2, 4, 5, 6, 8, and 12 repressed lateral root emergence and promoted nodulation in the R108 wild type but not in the cra2 mutant. We optimized CRISPR/Cas9 [clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9] genome editing system for M. truncatula, and thus created single mutants of MtCEP1, 2, 4, 6, and 12 and the double mutants Mtcep1/2C and Mtcep5/8C; however, these mutants did not exhibit significant differences from R108. Furthermore, a triple mutant Mtcep1/2/12C and a quintuple mutant Mtcep1/2/5/8/12C were generated and exhibited more lateral roots and fewer nodules than R108. Overall, MtCEP1, 2, and 12 were confirmed to be redundantly important in the control of lateral root number and nodulation. Moreover, the CRISPR/Cas9-based multigene editing protocol provides an additional tool for research on the model legume M. truncatula, which is highly efficient at multigene mutant generation.

Nucleus-encoded chloroplast proteins can be transported via the secretory pathway. The molecular mechanisms underlying the trafficking of chloroplast proteins between the intracellular compartments are largely unclear, and a cargo sorting receptor has not previously been identified in the secretory pathway. Here we report a cargo sorting receptor that is specifically present in Viridiplantae and mediates the transport of cargo proteins to the chloroplast. Using a forward genetic analysis, we identified a gene encoding a transmembrane protein (MtTP930) in barrel medic (Medicago truncatula). Mutation of MtTP930 resulted in impaired chloroplast function and a dwarf phenotype. MtTP930 is highly expressed in the aerial parts of the plant and is localized to the ER exit sites (ERESs) and Golgi. MtTP930 contains typical cargo sorting receptor motifs, interacts with Sar1, Sec12 and Sec24, and participates in coat protein II (COPII) vesicular transport. Importantly, MtTP930 can recognize the cargo proteins plastidial N-glycosylated nucleotide pyrophosphatase/ phosphodiesterase (MtNPP) and α-carbonic anhydrase (MtCAH) in the ER, and then transport them to the chloroplast via the secretory pathway. Mutation of a homolog of MtTP930 in Arabidopsis (Arabidopsis thaliana) resulted in a similar dwarf phenotype. Furthermore, MtNPP-GFP failed to localize to chloroplasts when transgenically expressed in Attp930 protoplasts, implying that these cargo sorting receptors are conserved in plants. These findings fill a gap in our understanding of the mechanism by which chloroplast proteins are sorted and transported via the secretory pathway.