Desert plant bacteria reveal host influence and beneficial plant growth properties.

Deserts, such as those found in Saudi Arabia, are one of the most hostile places for plant growth. However, desert plants are able to impact their surrounding microbial community and select beneficial microbes that promote their growth under these extreme conditions. In this study, we examined the soil, rhizosphere and endosphere bacterial communities of four native desert plants Tribulus terrestris, Zygophyllum simplex, Panicum turgidum and Euphorbia granulata from the Southwest (Jizan region), two of which were also found in the Midwest (Al Wahbah area) of Saudi Arabia. While the rhizosphere bacterial community mostly resembled that of the highly different surrounding soils, the endosphere composition was strongly correlated with its host plant phylogeny. In order to assess whether any of the native bacterial endophytes might have a role in plant growth under extreme conditions, we analyzed the properties of 116 cultured bacterial isolates that represent members of the phyla Proteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. Our analysis shows that different strains have highly different biochemical properties with respect to nutrient acquisition, hormone production and growth under stress conditions. More importantly, eleven of the isolated strains could confer salinity stress tolerance to the experimental model plant Arabidopsis thaliana suggesting some of these plant-associated bacteria might be useful for improving crop desert agriculture.

Different mechanisms of the metalliferous Zygophyllum fabago shoots and roots to cope with Pb toxicity.

Lead (Pb) remains classified as a priority pollutant. Zygophyllum fabago is considered an early colonizer of heavy metal-polluted soils under semiarid conditions, but physiological mechanisms underlying this colonizing capacity remain unclear. In order to characterize Z. fabago plants' performance on Pb-contaminated soils, we evaluated how Pb influenced root and shoot growth, carbon metabolism, and oxidative status. For that, 30-day-old seedlings from one population colonizing a mine tailing ("Mercader") at Murcia (southeast Spain) were exposed to 500-µM Pb(NO3)2 for 1 week. Results showed that this high dose of Pb induced no plant mortality nor senescence, though promoting plant nanism. Besides the efficiency of roots to accumulate Pb, shoots also demonstrate a high efficiency to translocate and accumulate this metal. Pb exposure decreased Zn uptake to the aerial part and reduced net photosynthetic rate (A), RuBisCO activity, chlorophyll, and soluble sugar contents in shoots. Moreover, in shoots, Pb exposure increased the levels of O2- and decreased antioxidant capacity, culminating with a loss of cell membrane integrity (electrolyte leakage) and increased protein oxidation. Compared to controls, exposed roots had less Mn and Zn levels, and despite the rise in H2O2 levels, they were able to modulate non-protein thiols presenting a robust defense capacity. This capacity may support the roots' ability to maintain cell membrane integrity (electrolyte leakage) with regard to control. Principal component analysis (PCA) contributed to elucidate how this species adjusts physiological mechanisms to cope with Pb toxicity, showing that roots and shoots evolved different antioxidant defenses, which demonstrates the importance of organ specificity in the response of Z. fabago to heavy metals.

Silicon improves photosynthesis and strengthens enzyme activities in the C3 succulent xerophyte Zygophyllum xanthoxylum under drought stress.

One main strategic adaptive mechanism adopted by succulent xerophyte species, resistance to drought stress is absorbing and accumulating large amounts of sodium (Na+) from poor and dry soil which was stored in photosynthesizing branches as well as leaves as major osmoregulators, while still accumulating and storing a great deal of silicon (Si) in roots to resist to arid environments. To understand the possible adaptive strategies underlying how Si accumulation stimulates growth and ameliorates the adverse environmental impacts of drought stress on the C3 succulent xerophyte Zygophyllum xanthoxylum, plants grown for 3 weeks were suffered different K2SiO3 concentrations (1.5-7.5mM) (3-15mM KCl as control) treatments in sand culture experiments. Plants were also treated with different osmotic stresses caused by polyethylene glycol (PEG 6000) and drought stress (maintain water content about 30% of field water capacity) (30% of FWC) with or without additional 2.5mMK2SiO3 (5mMKCl as control) treatment in sand culture and pot experiments, respectively. We found that 2.5mMK2SiO3 (5mMKCl as control) resulted in optimal plant growth and alleviated adverse influences of drought stress on Z. xanthoxylum, by strengthening the activities of superoxide dismutase, peroxidase and catalase, reducing membrane lipid peroxidation and decreasing soluble sugar and free proline concentrations, concomitantly, increasing tissue water content, leaf area and chlorophyll a concentration. The result of ion analysis indicated that the Si absorption of Z. xanthoxylum was markedly induced by drought stress and that the 2.5mMK2SiO3 (5mMKCl as control) treatment significantly increased the aboveground and root Si concentration under different osmotic stresses and 30% of field water capacity compared with the drought and drought with 5mMKCl treatments. Although the K+ concentration in root in the drought with 2.5mMK2SiO3 treatment was no significant changes compared with the drought treatment, K+ concentration in aboveground and root in drought with 2.5mMK2SiO3 treatment were significantly decreased by 42% and 65.2% compared with drought with 5mMKCl treatment under 30% of FWC, indicating that Si replaced the function of K+, thus stimulating the growth and mitigating adverse effects of Z. xanthoxylum under water deficit. These findings showed that the positive roles of Si in the drought tolerance of Z. xanthoxylum might be due to the ability of plant to accumulate a great quantity of Si and utilize it as an osmoregulator to copy with water deficit, which was coupled with an obvious improvement in photosynthetic activity and anti-oxidative enzyme activities.

Phylogeographical structure inferred from cpDNA sequence variation of Zygophyllum xanthoxylon across north-west China.

Zygophyllum xanthoxylon, a desert species, displaying a broad east-west continuous distribution pattern in arid Northwestern China, can be considered as a model species to investigate the biogeographical history of this region. We sequenced two chloroplast DNA spacers (psbK-psbI and rpl32-trnL) in 226 individuals from 31 populations to explore the phylogeographical structure. Median-joining network was constructed and analysis of AMOVA, SMOVA, neutrality tests and distribution analysis were used to examine genetic structure and potential range expansion. Using species distribution modeling, the geographical distribution of Z. xanthoxylon was modeled during the present and at the Last Glacial Maximum (LGM). Among 26 haplotypes, one was widely distributed, but most was restricted to either the eastern or western region. The populations with the highest levels of haplotype diversity were found in the Tianshan Mountains and its surroundings in the west, and the Helan Mountains and Alxa Plateau in the east. AMOVA and SAMOVA showed that over all populations, the species lacks phylogeographical structure, which is speculated to be the result of its specific biology. Neutrality tests and mismatch distribution analysis support past range expansions of the species. Comparing the current distribution to those cold and dry conditions in LGM, Z. xanthoxylon had a shrunken and more fragmented range during LGM. Based on the evidences from phylogeographical patterns, distribution of genetic variability, and paleodistribution modeling, Z. xanthoxylon is speculated most likely to have originated from the east and migrated westward via the Hexi Corridor.

ZxNHX controls Na⁺ and K⁺ homeostasis at the whole-plant level in Zygophyllum xanthoxylum through feedback regulation of the expression of genes involved in their transport.

BACKGROUND AND AIMS: In order to cope with arid environments, the xerohalophyte Zygophyllum xanthoxylum efficiently compartmentalizes Na(+) into vacuoles, mediated by ZxNHX, and maintains stability of K(+) in its leaves. However, the function of ZxNHX in controlling Na(+) and K(+) homeostasis at the whole-plant level remains unclear. In this study, the role of ZxNHX in regulating the expression of genes involved in Na(+) and K(+) transport and spatial distribution was investigated. METHODS: The role of ZxNHX in maintaining Na(+) and K(+) homeostasis in Z. xanthoxylum was studied using post-transcriptional gene silencing via  Agrobacterium-mediated transformation. Transformed plants were grown with or without 50 mm NaCl, and expression levels and physiological parameters were measured. KEY RESULTS: It was found that 50 mm NaCl induced a 620 % increase in transcripts of ZxSOS1 but only an 80 % increase in transcripts of ZxHKT1;1 in roots of wild-type (WT) plants. Consequently, the ability of ZxSOS1 to transport Na(+) exceeded that of ZxHKT1;1, and Na(+) was loaded into the xylem by ZxSOS1 and delivered to the shoots. However, in a ZxNHX-silenced line (L7), the capacity to sequester Na(+) into vacuoles of leaves was weakened, which in turn regulated long-distance Na(+) transport from roots to shoots. In roots of L7, NaCl (50 mm) increased transcripts of ZxSOS1 by only 10 %, whereas transcripts of ZxHKT1;1 increased by 53 %. Thus, in L7, the transport ability of ZxHKT1;1 for Na(+) outweighed that of ZxSOS1. Na(+) was unloaded from the xylem stream, consequently reducing Na(+) accumulation and relative distribution in leaves, but increasing the relative distribution of Na(+) in roots and the net selective transport capacity for K(+) over Na(+) from roots to shoots compared with the WT. Silencing of ZxNHX also triggered a downregulation of  ZxAKT1 and ZxSKOR in roots, resulting in a significant decrease in K(+) accumulation in all the tissues in plants grown in 50 mm NaCl. These changes led to a significant reduction in osmotic adjustment, and thus an inhibition of growth in ZxNHX-silenced lines. CONCLUSIONS: The results suggest that ZxNHX is essential for controlling Na(+), K(+) uptake, long-distance transport and their homeostasis at whole-plant level via feedback regulation of the expression of genes involved in Na(+), K(+) transport. The net result is the maintenance of the characteristic salt accumulation observed in Z. xanthoxylum and the regulation of its normal growth. A model is proposed for the role of ZxNHX in regulating the Na(+) transport system in Z. xanthoxylum under saline conditions.

Sodium chloride improves photosynthesis and water status in the succulent xerophyte Zygophyllum xanthoxylum.

Zygophyllum xanthoxylum, a C(3) woody species, is a succulent xerophyte that is well adapted to arid environments. Our previous investigations showed that Na(+) has a positive effect on the growth of Z. xanthoxylum under drought conditions, which was closely related to high Na(+) accumulation in leaves. To reveal the physiological mechanisms underlying how Na(+) accumulation improves the drought resistance of Z. xanthoxylum, 3-week-old seedlings were treated with a series of additional external NaCl concentrations (5-150 mM) in sand culture experiments. Seedlings were also subjected to water deficit (30% of field water capacity) in the presence or absence of additional NaCl (50 mM) in pot experiments. The results indicated that 50 mM NaCl could mitigate deleterious impacts of water deficit on the growth of Z. xanthoxylum, by improving the relative water content, inducing a significant drop in leaf water potential and, concomitantly, increasing leaf turgor pressure and chlorophyll concentrations resulting in an enhancement of overall plant photosynthetic activity (i.e., photosynthetic rate and water use efficiency). Furthermore, NaCl (50 mM) could alleviate the inhibitory effect of water deficit on the activity of photosystem II in Z. xanthoxylum. The contribution of Na(+) to the total osmotic potential varied from 8% in the control to 13% in plants subjected to water deficit and, surprisingly, to 28% in plants grown in the presence of 50 mM NaCl under water deficit; however, the contribution of K(+) significantly decreased from 13 to 8%. These findings suggest that, under arid environments, Z. xanthoxylum is able to accumulate a high concentration of Na(+) in its leaves and use it directly for osmotic adjustment, which was coupled with an improvement in leaf hydration and photosynthetic activity.

The ZxNHX gene encoding tonoplast Na(+)/H(+) antiporter from the xerophyte Zygophyllum xanthoxylum plays important roles in response to salt and drought.

Sodium (Na(+)) has been found to play important roles in the adaptation of xerophytic species to drought conditions. The tonoplast Na(+)/H(+) antiporter (NHX) proved to be involved in the compartmentalization of Na(+) into vacuoles from the cytosol. In this study, a gene (ZxNHX) encoding tonoplast Na(+)/H(+) antiporter was isolated and characterized in Zygophyllum xanthoxylum, a succulent xerophyte growing in desert areas of northwest China. The results revealed that ZxNHX consisted of 532 amino acid residues with a conserved binding domain ((78)LFFIYLLPPI(87)) for amiloride and shared high similarity (73-81%) with the identified tonoplast Na(+)/H(+) antiporters in other plant species. Semi-quantitative RT-PCR analysis showed that the mRNA level of ZxNHX was significantly higher in the leaf than in stem or root. The transcript abundance of ZxNHX in Z. xanthoxylum subjected to salt (5-150 mM NaCl) or drought (50-15% of field water capacity (FWC)) was 1.4-8.4 times or 2.3-4.4 times that of plants grown in the absence of NaCl or 70% of FWC, respectively. Leaf Na(+) concentration in plants exposed to salt or drought was 1.7-5.2 times or 1.5-2.2 times that of corresponding control plants, respectively. It is clear that there is a positive correlation between up-regulation of ZxNHX and accumulation of Na(+) in Z. xanthoxylum exposed to salt or drought. Furthermore, Z. xanthoxylum accumulated larger amounts of Na(+) than K(+) in the leaf under drought conditions, even in low salt soil. In summary, our results suggest that ZxNHX encodes a tonoplast Na(+)/H(+) antiporter and plays important roles in Na(+) accumulation and homeostasis of Z. xanthoxylum under salt and drought conditions.