Title: Enhanced salt tolerance and photosynthetic performance: Implication of ɤ-amino butyric acid application in salt-exposed lettuce (Lactuca sativa L.) plants

Gamma-Amino Butyric Acid (GABA) is a substantial component of the free amino acid pool with low concentration in plant tissues. Enhanced GABA content occurs during plant growth and developmental processes like seed germination. GABA level, basically, alters in response to many endogenous and exogenous stimuli. In the current study, GABA effects were studied on germination, photosynthetic performance and oxidative damages in salt-exposed lettuce plants. Three NaCl (0, 40 and 80 mM) and two GABA (0 and 25 μM) concentrations were applied on lettuce during two different developmental (seed germination and seedlings growth) stages. Negative effects of salinity on germination and plant growth were removed by GABA application. GABA significantly reduced mean germination time (MGT) in salt-exposed lettuce seeds. Although, salinity caused a significant decline in maximum quantum yield of photosystem II (Fv/Fm) during distinct steps of plant growth, GABA application improved Fv/Fm particularly on high salinity level. GABA decreased specific energy fluxes per reaction center (RC) for energy absorption and dissipation, while enhanced-electron transport flux in photosynthetic apparatus of lettuce plants was observed in GABA-supplemented plants. Moreover, decline in non-photochemical quenching (NPQ) and quenching coefficients (qP, qL, qN) by salt stress were recovered by GABA application. Elevated electrolyte leakage considerably decreased by GABA exposure on salt-treated plants. Although, proline level increased by NaCl treatments in a concentration dependent manner, combined application of salt with GABA caused a significant reduction in proline content. Catalase; EC 1.11.1.6 (CAT), l-ascorbate peroxidase; EC 1.11.1.11 (APX), and superoxide dismutase; EC 1.15.1.1 (SOD) activities were increased by GABA exposure in salt-supplemented plants that resulted in regulated hydrogen peroxide level. In conclusion, a multifaceted role for GABA is suggested for minimizing detrimental effects of salinity on lettuce through improvement of photosynthetic functionality and regulation of oxidative stress.

Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses

Nodules harboring nitrogen-fixing rhizobia are a well-known trait of legumes, but nodules also occur in other plant lineages, with rhizobia or the actinomycete Frankia as microsymbiont. It is generally assumed that nodulation evolved independently multiple times. However, molecular-genetic support for this hypothesis is lacking, as the genetic changes underlying nodule evolution remain elusive. We conducted genetic and comparative genomics studies by using Parasponia species (Cannabaceae), the only nonlegumes that can establish nitrogen-fixing nodules with rhizobium. Intergeneric crosses between Parasponia andersonii and its nonnodulating relative Trema tomentosa demonstrated that nodule organogenesis, but not intracellular infection, is a dominant genetic trait. Comparative transcriptomics of P. andersonii and the legume Medicago truncatula revealed utilization of at least 290 orthologous symbiosis genes in nodules. Among these are key genes that, in legumes, are essential for nodulation, including NODULE INCEPTION (NIN) and RHIZOBIUM-DIRECTED POLAR GROWTH (RPG). Comparative analysis of genomes from three Parasponia species and related nonnodulating plant species show evidence of parallel loss in nonnodulating species of putative orthologs of NIN, RPG, and NOD FACTOR PERCEPTION Parallel loss of these symbiosis genes indicates that these nonnodulating lineages lost the potential to nodulate. Taken together, our results challenge the view that nodulation evolved in parallel and raises the possibility that nodulation originated ∼100 Mya in a common ancestor of all nodulating plant species, but was subsequently lost in many descendant lineages. This will have profound implications for translational approaches aimed at engineering nitrogen-fixing nodules in crop plants.

CRISPR/Cas9-Mediated Mutagenesis of Four Putative Symbiosis Genes of the Tropical Tree Parasponia andersonii Reveals Novel Phenotypes

Parasponia represents five fast-growing tropical tree species in the Cannabaceae and is the only plant lineage besides legumes that can establish nitrogen-fixing nodules with rhizobium. Comparative analyses between legumes and Parasponia allows identification of conserved genetic networks controlling this symbiosis. However, such studies are hampered due to the absence of powerful reverse genetic tools for Parasponia. Here, we present a fast and efficient protocol for Agrobacterium tumefaciens-mediated transformation and CRISPR/Cas9 mutagenesis of Parasponia andersonii. Using this protocol, knockout mutants are obtained within 3 months. Due to efficient micro-propagation, bi-allelic mutants can be studied in the T0 generation, allowing phenotypic evaluation within 6 months after transformation. We mutated four genes - PanHK4, PanEIN2, PanNSP1, and PanNSP2 - that control cytokinin, ethylene, or strigolactone hormonal networks and that in legumes commit essential symbiotic functions. Knockout mutants in Panhk4 and Panein2 displayed developmental phenotypes, namely reduced procambium activity in Panhk4 and disturbed sex differentiation in Panein2 mutants. The symbiotic phenotypes of Panhk4 and Panein2 mutant lines differ from those in legumes. In contrast, PanNSP1 and PanNSP2 are essential for nodule formation, a phenotype similar as reported for legumes. This indicates a conserved role for these GRAS-type transcriptional regulators in rhizobium symbiosis, illustrating the value of Parasponia trees as a research model for reverse genetic studies.