Qing Ma

Nitrate Excretion Transporter 1 (NAXT1/NPF2.7) is known to regulate NO₃⁻ transport in Arabidopsis, a salt-sensitive glycophyte that exhibits a significant reduction in the NO₃⁻ content under salt stress. However, its role in the NO₃⁻ homeostasis and salt tolerance of xerophytes, which exhibit strong stress tolerance, remains unclear. In the present study, we cloned the NPF2.7 homolog (PcNPF2.7) from the xerophyte Pugionium cornutum, which exhibits stable NO₃⁻ content in the shoot under salt stress, and investigated its function in ion homeostasis and salt tolerance. PcNPF2.7 was specifically expressed in the stele tissue of roots and localized to the plasma membrane; its expression level in the roots was significantly induced by NaCl and NaNO₃ treatments. PcNPF2.7 overexpression driven by a stelar-specific promoter significantly increased NO₃⁻ accumulation and reduced Na⁺ levels in the shoots of Arabidopsis under 75 mM NaCl or NaNO₃ treatments, resulting in an enhanced salt tolerance. Furthermore, PcNPF2.7 overexpression significantly induced AtHKT1;1, which mediates the unloading of Na⁺ from xylem in the roots. Taken together, our findings showed that PcNPF2.7 facilitates the transport of NO₃⁻ from the roots to the shoots and indirectly reduces Na⁺ accumulation in the shoot, therefore contributing to the salt tolerance in plants.

Background/Objectives: Desert plants exhibit remarkable resilience to extreme environments, and their capacity for population establishment is noteworthy. However, the adaptation process mechanisms of those plants to harsh habitats, particularly concerning intraspecific differentiation and genetic diversity, remain poorly understood, and a comprehensive framework is lacking. Zygophyllum loczyi Kanitz, an annual or biennial desert herb, demonstrates significant phenotypic plasticity across diverse habitats. Methods: Using mixed-effects models, this study examined 20 populations from four deserts to assess phenotypic variation and predict trait_environment relationships. Results: The findings indicated substantial inter-population phenotypic differentiation in Z. loczyi, with greater variation observed between deserts than within them. Traits such as blade length, petal length, sepal length, and stamen length were influenced by environmental conditions. Mixed-effects model prediction showed that the growth location of Z. loczyi significantly impacted its phenotypic traits. The characteristics of the four desert populations displayed varying responses to temperature and moisture changes, with the most pronounced response noted in the Gurbantunggut desert (Gt) population, indicating that survival stress has an important influence on the performance of plants. The single nucleotide polymorphisms result further confirmed that the differentiation and genetic diversity of the Gt population displayed the highest selection pressure, resulting the small effective size of the population. Conclusions: This study uncovers the adaptive mechanism of Z. loczyi to habitat through investigating the inter-population phenotypic differentiation and genetic diversity and provides new insight into local adaptation and evolutionary processes in the desert environment.

Summary: Uncovering the mechanisms underlying stress‐resistant traits in xerophytes thriving in harsh environments can aid the genetic improvement of crops. The xerophyte Zygophyllum xanthoxylum features high Na⁺ accumulation in leaves, mediated by the vacuolar antiporter ZxNHX1. Co‐expression of ZxNHX1 and vacuolar H⁺‐PPase gene ZxVP1‐1 has been demonstrated to enhance the stress resistance and biomass of alfalfa. However, it remains unknown if ZxNHX1 outperforms its homologues from the Na⁺‐excluding and stress‐sensitive glycophytes such as Arabidopsis in enhancing plant stress resistance and yield. Here, we found that expression of ZxNHX1 conferred superior growth under salt stress in alfalfa, compared to the Arabidopsis homologue AtNHX1. When expressed in yeast, ZxNHX1 displays stronger Na⁺/H⁺ but weaker K⁺/H⁺ exchange activity than AtNHX1. Under both K⁺ sufficient and deficient conditions, an Arabidopsis atnhx1‐1 mutant expressing ZxNHX1 accumulated higher Na⁺ and lower K⁺ concentrations, with more Na⁺ being sequestered into vacuoles and a larger proportion of K⁺ retained in the cytosol. This optimized cellular ion distribution ensures energy‐conserving osmotic adjustment, leading to stronger stress resistance and higher biomass than plants expressing AtNHX1. Moreover, ZxNHX1 governed the root uptake and root‐to‐leaf transport of Na⁺ at the whole‐plant level, whereas AtNHX1 acted mainly in K⁺ transport processes. We also identified a polar residue Thr265 in a membrane‐spanning region of ZxNHX1 that influences its Na⁺ and K⁺ selectivity. These findings reveal a new energy‐conserving, Na⁺‐based osmotic adjustment mechanism that can enhance crop stress resistance without sacrificing yield, providing an effective way for utilizing saline soils to expand crop production into marginal lands.

Revealing the genetic basis for stress-resistant traits in extremophile plants will yield important information for crop improvement. Zygophyllum xanthoxylum, an extant species of the ancient Mediterranean, is a succulent xerophyte that can maintain a favorable water status under desert habitats; however, the genetic basis of this adaptive trait is poorly understood. Furthermore, the phylogenetic position of Zygophyllales, to which Z. xanthoxylum belongs, remains controversial. In this study, we sequenced and assembled the chromosome-level genome of Z. xanthoxylum. Phylogenetic analysis showed that Zygophyllales and Myrtales form a separated taxon as a sister to the clade comprising fabids and malvids, clarifying the phylogenetic position of Zygophyllales at whole-genome scale. Analysis of genomic and transcriptomic data revealed multiple critical mechanisms underlying the efficient osmotic adjustment using Na⁺ and K⁺ as ‘cheap’ osmolytes that Z. xanthoxylum has evolved through the expansion and synchronized expression of genes encoding key transporters/channels and their regulators involved in Na⁺/K⁺ uptake, transport, and compartmentation. It is worth noting that ZxCNGC1; 1 (cyclic nucleotide-gated channels) and ZxCNGC1; 2 constituted a previously undiscovered energy-saving pathway for Na⁺ uptake. Meanwhile, the core genes involved in biosynthesis of cuticular wax also featured an expansion and up-regulated expression, contributing to the water retention capacity of Z. xanthoxylum under desert environments. Overall, these findings boost the understanding of evolutionary relationships of eudicots, illustrate the unique water retention mechanism in the succulent xerophyte that is distinct from glycophyte, and thus provide valuable genetic resources for the improvement of stress tolerance in crops and insights into the remediation of sodic lands.

In order to study the genetics of local adaptation in all main deserts of northwest China, whole genomes of 169 individuals were resequenced, which covers 20 populations of Zygophyllum loczyi (Zygophyllales: Zygophylaceae). We describe more than 15 million single nucleotide polymorphisms and numerous InDels. The expected heterozygosity and PIC values associated with local adaptation varied significantly across biogeographic regions. Variation in environmental factors contributes largely to the population genetic structure of Z. loczyi. Bayesian analysis performed with STRUCTURE defined four genetic clusters, while the results of principle component analysis were similar. Our results shows that the Qaidam Desert group appears to be diverging into two branches characterized by significant geographic separation and gene flow with two neighboring deserts. Geological data assume that it is possible that the Taklamakan Desert was the original distribution site, and Z. loczyi could have migrated later on and expanded within other desert areas. The above findings provide insights into the processes involved in biogeography, phylogeny, and differentiation within the northwest deserts of China.

Desert plants possess excellent water-conservation capacities to survive in extreme environments. Cuticular wax plays a pivotal role in reducing water loss through plant aerial surfaces. However, the role of cuticular wax in water retention by desert plants is poorly understood. We investigated leaf epidermal morphology and wax composition of five desert shrubs from north-west China and characterized the wax morphology and composition for the typical xerophyte Zygophyllum xanthoxylum under salt, drought and heat treatments. Moreover, we examined leaf water loss and chlorophyll leaching of Z. xanthoxylum and analysed their relationships with wax composition under the above treatments. The leaf epidermis of Z. xanthoxylum was densely covered by cuticular wax, whereas the other four desert shrubs had trichomes or cuticular folds in addition to cuticular wax. The total amount of cuticular wax on leaves of Z. xanthoxylum and Ammopiptanthus mongolicus was significantly higher than that of the other three shrubs. Strikingly, C31 alkane, the most abundant component, composed >71 % of total alkanes in Z. xanthoxylum, which was higher than for the other four shrubs studied here. Salt, drought and heat treatments resulted in significant increases in the amount of cuticular wax. Of these treatments, the combined drought plus 45 °C treatment led to the largest increase (107 %) in the total amount of cuticular wax, attributable primarily to an increase of 122 % in C31 alkane. Moreover, the proportion of C31 alkane within total alkanes remained >75 % in all the above treatments. Notably, the water loss and chlorophyll leaching were reduced, which was negatively correlated with C31 alkane content. Zygophyllum xanthoxylum could serve as a model desert plant for study of the function of cuticular wax in water retention because of its relatively uncomplicated leaf surface and because it accumulates C31 alkane massively to reduce cuticular permeability and resist abiotic stressors.