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This study was designed to elucidate the physiological responses of three Lotus forage accessions to alkaline stress, and the influence of inoculating with Pantoea eucalypti endophyte strain on alkaline stress mitigation. A diploid L. corniculatus (Lc) accession, L. tenuis (Lt), and the interspecific hybrid Lt × Lc obtained from these two parental lines were exposed to alkaline stress (pH 8.2). Both Lt and the Lt × Lc hybrid are alkaline-tolerant compared to Lc, based on observations that dry mass was not reduced under stress, and there were no chlorosis symptoms on leaf blades. In all three Lotus accessions, Fe 2+ concentration under stress decreased in aerial parts and simultaneously increased in roots. Inoculation with P. eucalypti considerably increased Fe 2+ content in shoots of all three Lotus forage species under alkaline treatment. Photochemical efficiency of PSII was affected in Lc accession only when exposed to alkaline treatment. However, when cultivated under alkalinity with inoculation, plants recovered and had photosynthetic parameters equivalent to those in the control treatment. Together, the results highlight the importance of inoculation with P. eucalypti, which contributes significantly to mitigating alkaline stress. All results provide useful information for improving alkaline tolerance traits of Lotus forage species and their interspecific hybrids., (© 2024 Wiley-VCH GmbH. Published by John Wiley & Sons Ltd.)

Premise of the Study: To maximize benefits from symbiosis, legumes must limit physiological inputs into ineffective rhizobia that nodulate hosts without fixing nitrogen. The capacity of legumes to decrease the relative fitness of ineffective rhizobia-known as sanctions-has been demonstrated in several legume species, but its mechanisms remain unclear. Sanctions are predicted to work at the whole-nodule level. However, whole-nodule sanctions would make the host vulnerable to mixed-nodule infections, which have been demonstrated in the laboratory and observed in natural settings. Here, we present and test a cell-autonomous model of legume sanctions that can resolve this dilemma., Methods: We analyzed histological and ultrastructural evidence of sanctions in two legume species, Acmispon strigosus and Lotus japonicus. For the former, we inoculated seedlings with rhizobia that naturally vary in their abilities to fix nitrogen. In the latter, we inoculated seedlings with near-isogenic strains that differ only in the ability to fix nitrogen., Key Results: In both hosts, plants inoculated with ineffective rhizobia exhibited evidence for a cell autonomous and accelerated program of senescence within nodules. In plants that received mixed inoculations, only the plant cells harboring ineffective rhizobia exhibited features consistent with programmed cell death, including collapsed vacuoles, ruptured symbiosomes, and bacteroids that are released into the cytosol. These features were consistently linked with ultrastructural evidence of reduced survival of ineffective rhizobia in planta., Conclusions: Our data suggest an elegant cell autonomous mechanism by which legumes can detect and defend against ineffective rhizobia even when nodules harbor a mix of effective and ineffective rhizobial genotypes., (© 2017 Botanical Society of America.)

In the generally bee-pollinated genus Lotus a group of four species have evolved bird-pollinated flowers. The floral changes in these species include altered petal orientation, shape and texture. In Lotus these characters are associated with dorsiventral petal identity, suggesting that shifts in the expression of dorsal identity genes may be involved in the evolution of bird pollination. Of particular interest is Lotus japonicus CYCLOIDEA 2 (LjCYC2), known to determine the presence of papillate conical cells on the dorsal petal in L. japonicus. Bird-pollinated species are unusual in not having papillate conical cells on the dorsal petal. Using RT-PCR at various stages of flower development, we determined the timing of expression in all petal types for the three putative petal identity genes (CYC-like genes) in different species with contrasting floral morphology and pollination syndromes. In bird-pollinated species the dorsal identity gene, LjCYC2, is not expressed at the floral stage when papillate conical cells are normally differentiating in bee-pollinated species. In contrast, in bee-pollinated species, LjCYC2 is expressed during conical cell development. Changes in the timing of expression of the above two genes are associated with modifications in petal growth and lateralisation of the dorsal and ventral petals in the bird-pollinated species. This study indicates that changes in the timing, rather than spatial distribution, of expression likely contribute to the modifications of petal micromorphology and petal size during the transition from bee to bird pollination in Macaronesian Lotus species., (© 2017 German Botanical Society and The Royal Botanical Society of the Netherlands.)

A common stress on plants is NaCl-derived soil salinity. Genus Lotus comprises model and economically important species, which have been studied regarding physiological responses to salinity. Leaf area ratio (LAR), root length ratio (RLR) and their components, specific leaf area (SLA) and leaf mass fraction (LMF) and specific root length (SRL) and root mass fraction (RMF) might be affected by high soil salinity. We characterised L. tenuis, L. corniculatus, L. filicaulis, L. creticus, L. burtii and L. japonicus grown under different salt concentrations (0, 50, 100 and 150 mm NaCl) on the basis of SLA, LMF, SRL and RMF using PCA. We also assessed effects of different salt concentrations on LAR and RLR in each species, and explored whether changes in these traits provide fitness benefit. Salinity (150 mm NaCl) increased LAR in L. burtii and L. corniculatus, but not in the remaining species. The highest salt concentration caused a decrease of RLR in L. japonicus Gifu, but not in the remaining species. Changes in LAR and RLR would not be adaptive, according to adaptiveness analysis, with the exception of SLA changes in L. corniculatus. PCA revealed that under favourable conditions plants optimise surfaces for light and nutrient acquisition (SLA and SRL), whereas at higher salt concentrations they favour carbon allocation to leaves and roots (LMF and RMF) in detriment to their surfaces. PCA also showed that L. creticus subjected to saline treatment was distinguished from the remaining Lotus species. We suggest that augmented carbon partitioning to leaves and roots could constitute a salt-alleviating mechanism through toxic ion dilution., (© 2016 German Botanical Society and The Royal Botanical Society of the Netherlands.)

A chlorate-resistant mutant (Ljclo1) of the model legume Lotus japonicus was identified that showed normal levels of nitrate reductase enzyme activity but had decreased uptake of nitrate, as determined from nitrate depletion and electrophysiological measurements. The data suggest that the mutant could be affected specifically in the low-affinity but not in the high-affinity nitrate transport system, and also showed decreased uptake of chlorate. Back-crosses of the mutant plant to the wild type indicated that it is affected in a single Mendelian recessive trait. Thus, the mutation produced in Ljclo1 may be related to some of the low-affinity nitrate transporters or to a regulatory mechanism associated with nitrate/chlorate uptake. Both size and chlorophyll content in young leaves of the mutant plants were significantly reduced compared to the wild type. In addition, nodulation performance of the mutant plants was similar to the wild type in the absence of any exogenous nitrate. However, the nodule:root biomass ratio in mutant plants was considerably reduced in the presence of 1-2 mm nitrate. The levels of several transcripts for nitrate transport and assimilation genes were determined for the wild type and mutant plants and were slightly different. The results suggest interdependence between nitrate uptake, plant growth and nodulation in Ljclo1 mutant plants., (© 2014 German Botanical Society and The Royal Botanical Society of the Netherlands.)

Saline, alkaline and mixed saline-alkaline conditions frequently co-occur in soil. In this work, we compared these plant stress sources on the legume Lotus tenuis, regarding their effects on shoot growth and leaf and stem anatomy. In addition, we aimed to gain insight on the plant physiological status of stressed plants. We performed pot experiments with four treatments: control without salt (pH = 5.8; EC = 1.2 dS·m(-1)) and three stress conditions, saline (100 mM NaCl, pH = 5.8; EC = 11.0 dS·m(-1)), alkaline (10 mM NaHCO3, pH = 8.0, EC = 1.9 dS·m(-1)) and mixed salt-alkaline (10 mM NaHCO3 + 100 mM NaCl, pH = 8.0, EC = 11.0 dS·m(-1)). Neutral and alkaline salts produced a similar level of growth inhibition on L. tenuis shoots, whereas their mixture exacerbated their detrimental effects. Our results showed that none of the analysed morpho-anatomical parameters categorically differentiated one stress from the other. However, NaCl- and NaHCO3 -derived stress could be discriminated to different extents and/or directions of changes in some of the anatomical traits. For example, alkalinity led to increased stomatal opening, unlike NaCl-treated plants, where a reduction in stomatal aperture was observed. Similarly, plants from the mixed saline-alkaline treatment characteristically lacked palisade mesophyll in their leaves. The stem cross-section and vessel areas, as well as the number of vascular bundles in the sectioned stem were reduced in all treatments. A rise in the number of vessel elements in the xylem was recorded in NaCl-treated plants, but not in those treated exclusively with NaHCO3., (© 2014 German Botanical Society and The Royal Botanical Society of the Netherlands.)