Fucoidans of Moroccan Brown Seaweed as Elicitors of Natural Defenses in Date Palm Roots

Fucoidans from Moroccan brown seaweed Bifurcaria bifurcata and Fucus spiralis were tested for their elicitor activity after their purification and complete characterization. The fucoidans of B. bifurcata (BBF) and of F. spiralis (FSF) were extracted and purified then characterized by infrared spectroscopy, proton nuclear magnetic resonance spectroscopy and size exclusion chromatography. The results show that BBF and FSF are mainly sulfated with 45.49 and 49.53% (w/w) sulfate, respectively. Analysis of neutral sugars determined by gas chromatography-mass spectrometry showed that FSF and BBF were mainly composed of 64% and 91% fucose and 20% and 6% galactose, respectively, with a few other sugars such as glucose (8% in FSF), rhamnose (1% in BBF) and mannose (8% in FSF and, 2% in BBF). The eliciting activity of these sulfated polysaccharides in stimulating the natural defenses of the date palm was evaluated through the activity of phenylalanine ammonia-lyase (PAL), and the increase in phenols and lignin content in the roots. The results obtained clearly show that the two fucoidans early and intensely stimulate the natural defenses of the date palm after 24 h of treatments. This remarkable elicitor effect seems to be linked to the sulfated groups compared to non-sulfate alginates extracted from the same algae. These results open promising perspectives for a biological control approach against date palm diseases.

Elicitor-Induced Production of Biomass and Pharmaceutical Phenolic Compounds in Cell Suspension Culture of Date Palm (Phoenix dactylifera L.)

Plants that synthesize bioactive compounds that have high antioxidant value and elicitation offer a reliable in vitro technique to produce important nutraceutical compounds. The objective of this study is to promote the biosynthesis of these phenolic compounds on a large scale using elicitors in date palm cell suspension culture. Elicitors such as pectin, yeast extract (YE), salicylic acid (SA), cadmium chloride (CdCl2), and silver nitrate (AgNO3) at 50, 100, and 200 mg/L concentrations are used. The effects of elicitors on cell culture were determined in terms of biomass [packed cell volume (PCV), fresh and dry weight], antioxidant activity, and phenolic compounds (catechin, caffeic acid, kaempferol, apigenin) were determined using high-performance liquid chromatography (HPLC). Results revealed that enhanced PCV (12.3%), total phenolic content [317.9 ± 28.7 mg gallic acid equivalents (GAE)/100 g of dry weight (DW)], and radical scavenging activity (86.0 ± 4.5%) were obtained in the 50 mg/L SA treated cell culture of Murashige and Skoog (MS) medium. The accumulation of optimum catechin (26.6 ± 1.3 µg/g DW), caffeic acid (31.4 ± 3.8 µg/g DW), and kaempferol (13.6 ± 1.6 µg/g DW) was found in the 50 mg/L SA-treated culture when compared to the control. These outcomes could be of great importance in the nutraceutical and agronomic industries.

Hyperspectral Reflectance of Light-Adapted Leaves Can Predict Both Dark- and Light-Adapted Chl Fluorescence Parameters, and the Effects of Chronic Ozone Exposure on Date Palm (Phoenix dactylifera)

High-throughput and large-scale measurements of chlorophyll a fluorescence (ChlF) are of great interest to investigate the photosynthetic performance of plants in the field. Here, we tested the capability to rapidly, precisely, and simultaneously estimate the number of pulse-amplitude-modulation ChlF parameters commonly calculated from both dark- and light-adapted leaves (an operation which usually takes tens of minutes) from the reflectance of hyperspectral data collected on light-adapted leaves of date palm seedlings chronically exposed in a FACE facility to three ozone (O3) concentrations (ambient air, AA; target 1.5 × AA O3, named as moderate O3, MO; target 2 × AA O3, named as elevated O3, EO) for 75 consecutive days. Leaf spectral measurements were paired with reference measurements of ChlF, and predictive spectral models were constructed using partial least squares regression. Most of the ChlF parameters were well predicted by spectroscopic models (average model goodness-of-fit for validation, R2: 0.53-0.82). Furthermore, comparing the full-range spectral profiles (i.e., 400-2400 nm), it was possible to distinguish with high accuracy (81% of success) plants exposed to the different O3 concentrations, especially those exposed to EO from those exposed to MO and AA. This was possible even in the absence of visible foliar injury and using a moderately O3-susceptible species like the date palm. The latter view is confirmed by the few variations of the ChlF parameters, that occurred only under EO. The results of the current study could be applied in several scientific fields, such as precision agriculture and plant phenotyping.

Use of Alginate Extracted from Moroccan Brown Algae to Stimulate Natural Defense in Date Palm Roots

Our study aimed to search for seaweed polysaccharides able to stimulate date palm defense mechanisms. Extraction, purification, characterization, and elicitor activity of sodium alginate (FSSA and BBSA) from Moroccan brown seaweeds Fucus spiralis and Bifurcaria bifurcata were investigated. FSSA and BBSA were characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR) and size exclusion chromatography (HPLC-SEC). The mannuronic acid/guluronic acid (M/G) ratio of FSSA was M/G= 0.92 indicating that FSSA contained 48% and 52% of mannuronic and guluronic acids respectively, and the M/G ratio of BBSA was 0.47 indicating that BBSA contained 32% and 68% of mannuronic and guluronic acids respectively. Elicitor activity of FSSA and BBSA was carried out by developing an innovative study model on the date palm. The elicitor capacities were evaluated by investigating phenolic metabolism including phenylalanine ammonia-lyase (PAL) activity and total polyphenol content in seedling roots of date palm maintained in alginates solution (FSSA and BBSA) at different concentrations. The results obtained show that the PAL activity and the phenolic compound content were significantly stimulated with 1 mg.mL-1 of FSSA and BBSA; after 1 day of treatment with FSSA, and after 12 hours of treatment with BBSA. These results show clearly those alginates extracted from Moroccan brown algae induced in date palm roots the stimulation of natural defense mechanisms.

Silicon-mediated alleviation of combined salinity and cadmium stress in date palm (Phoenix dactylifera L.) by regulating physio-hormonal alteration

We investigated the physio-molecular effects of separate and combined cadmium (Cd; 200 μM) and salinity (NaCl; 100 mM) stress on date palm during silicon (Si; 1.0 mM) applications. The results showed that exogenous Si led to significant improvements in plant growth, as well as physiology when compared with non-Si-treated seedling under stressed (Cd/NaCl) conditions. Interestingly, Si application led to lower metal (Cd) uptake and enhanced plant macronutrient uptake under combined stress, in turn, alleviating the combined salinity- and Cd-induced oxidative stress by lowering the lipid peroxidation rate, and peroxidase and catalase activities. Furthermore, ascorbate peroxidase level and the cytosolic Cu/Zn superoxide dismutase expression were significantly enhanced by Si application under combined stress. We further analyzed the effect of Si on modulation of stress-related hormonal crosstalk. Si markedly downregulated endogenous salicylic acid, jasmonic acid, and abscisic acid under NaCl stress and combined NaCl-Cd stress. However, during Cd toxicity alone, Si showed varying accumulation of these phytohormones. The results suggest that hindering the Cd uptake and enhancing silicon accumulation ultimately led to improvement of biomass and efficiency of the antioxidant system for alleviating combined stress. Moreover, higher transcript accumulation of PROLINE TRANSPORTER 2 and GAPDH and downregulation of ABA RECEPTOR by Si treatment under combined stress in date palm seedlings indicate the stress-ameliorative role of Si. The study provides evidence of the positive influence of Si on alleviating the combined toxicity of Cd and NaCl in date palm and can be further extended for field trials in Cd- and salinity-affected areas.

Impact of Antioxidants on <i>in vitro </i>Rooting and Acclimatization of Two Egyptian Dry Date Palm Cultivars

BACKGROUND AND OBJECTIVES

Date palm (Phoenix dactylifera L.) is propagated vegetatively via offshoots, which is limited by either the offshoots numbers produced from a superior selected plant, or the occurrence of these offshoots only during the juvenile phase of the date palm life cycle. As a result, the in vitro propagation could be considered an alternative technique for large scale propagation of date palm. Obtaining well-developed root system is considered the most important step in establishment of reliable protocol for successful production of date palm and subsequently successful acclimatization of the in vitro derived plants. The aim of this study is to obtain a well-developed root system through using different antioxidants, with detecting the similarity between the in vitro derived plants and the mother plants using RAPD, long RAPD and ISSR techniques.

MATERIALS AND METHODS

Individual plantlets obtained from maturation of somatic embryos in vitro of about 5-7 cm in length with 2-3 leaves were used as plant material. Plantlets were cultured on half strength MS liquid medium supplemented with 0.5 mg L-1 thiamine-HCl+2.0 mg L-1 glycine+0.1 mg L-1 biotin+40 g L-1 sucrose+ 0.1mg L-1 NAA with different concentrations from either AgNO3 or citric or ascorbic acids (0.0, 0.5, 1, 2 mg L-1). Growth development, root number and root length (cm) were evaluated at the end of the second subculture (12 weeks). Data were reported as Mean±Standard deviation (SD). Data were subjected to one way-analysis of variance (p<0.05). Results were processed by Excel (2010).

RESULTS

Among the different antioxidants with different concentrations used, generally it was found that 2 mg L-1 of each agent gave the highest values of growth development, roots number and roots length. However, using 2 mg L-1 AgNO3 gave the best results with all parameters. Regarding the response of date palm cultivar, it was remarkable that Bartamoda showed relatively better results than Sakkoty cultivar. According to PCR reactions, the results of RAPD, long RAPD and ISSR profile of tissue culture-derived plantlets grown on a medium supplemented with 2 mg L-1 AgNO3 obviously revealed high similarity to mother plants.

CONCLUSION

It could be concluded that using 2 mg L-1 AgNO3 gave the best results for growth development, root numbers and length of the two cultivars but Bartamoda showed relatively better results than Sakkoty cultivar. The tissue culture-derived plantlets on this medium (2 mg L-1 AgNO3) revealed high similarity to mother plant as a result to RAPD, long RAPD and ISSR profiles.

Metabolomic analysis of date palm seedlings exposed to salinity and silicon treatments

Silicon is known to promote plant growth as well as stress tolerance of plants. The current study was undertaken to assess the growth promoting effect of silicon on date palm seedling development as well as its ability to abate some of the negative effects of salinity. In this study, date palm seedlings were treated with silicon and sodium chloride salts, and the effect of these salts on some physiological parameters of the plants was determined. In addition, a global nontargeted metabolomics analysis was performed for the leaf and root tissues using liquid chromatography-mass spectrometry (LC-MS). The results showed that under non-stress conditions, silicon treatment enhanced the growth of the date palm seedlings, however, under salinity, silicon slightly mitigates the negative effects of salt stress on the date palm seedlings although it enhances the potassium accumulation under this condition. The global metabolomics analysis has identified a total of 1,101 significant differentially accumulated (p, q ≤ 0.05) metabolites in leaves and roots under silicon, salinity or their combination. A differential pairwise metabolic profile comparison revealed the accumulation of distinct metabolites in response to silicon and salinity treatments such as antioxidant compounds pyridoxine, cepharanthine, allithiamine, myristic acid and boldine; osmoregulators such as mucic acid; along with the accumulation of detoxification intermediates such as S-D-lactoylglutathione, beta-cyano-L-alanine and gamma-glutamyl-conjugates. In addition, histochemical analyses revealed that application of silicon significantly (p ≤ 0.05) enhanced the formation of the Casparian strip. Identification of the differentially accumulated metabolites could offer an insight into how silicon is able to promote growth and salinity tolerance in date palms.

Insight into the expression variation of metal-responsive genes in the seedling of date palm (Phoenix dactylifera)

Phytochelatin synthase and metallothionein gene expressions were monitored via qPCR in order to investigate the molecular mechanisms involved in Cd and Cr detoxification in date palm (Phoenix dactylifera). A specific reference gene validation procedure using BestKeeper, NormFinder and geNorm programs allowed selection of the three most stable reference genes in a context of Cd or Cr contamination among six reference gene candidates, namely elongation factor α1, actin, aldehyde dehydrogenase, SAND family, tubulin 6 and TaTa box binding protein. Phytochelatin synthase (pcs) and metallothionein (mt) encoding gene expression were induced from the first days of exposure. At low Cd stress (0.02 mM), genes were still up-regulated until 60th day of exposure. At the highest metal concentrations, however, pcs and mt gene expressions decreased. pcs encoding gene was significantly up-regulated under Cr exposure, and was more responsive to increasing Cr concentration than mt encoding gene. Moreover, exposure to Cd or Cr influenced clearly seed germination and hypocotyls elongation. Thus, the results have proved that both analyzed genes participate in metal detoxification and their expression is regulated at transcriptional level in date palm subjected to Cr and Cd stress. Consequently, variations of expression of mt and pcs genes may serve as early-warning biomarkers of metal stress in this species.

Biochemical and Genetical Responses of Phoenix dactylifera L. to Cadmium Stress

The cadmium (Cd), a heavy metal, causes toxicity, which leads to hampering the growth and development of the plant. The molecular and biochemical approaches were used for the investigation of antioxidant system response and genotoxicity in date palm (Phoenix dactylifera L.) cv. Sagai in pot experiment having Cd. The root length was more affected than the shoot length as more accumulation of Cd occurs in roots. Fresh weights of root and shoot were reduced significantly in treated plants as compared to the control. The proline content was increased at low concentration of Cd (300 µM-CdCl2) than the medium and high concentrations (600 and 900 µM-CdCl2), respectively. The thiobarbituric acid reactive substances (TBARS) content was increased at 600 and 900 µM-CdCl2 compared to the plants treated at 300 µM-CdCl2 and controls. Antioxidant enzymatic assay was performed under Cd stress and compared with control plants. The catalase (CAT) and superoxide dismutase (SOD) activities were found to be high in plants treated with CdCl2 at 300 µM compared to at 600 and 900 µM-CdCl2, respectively. The genotoxicity of Cd was assessed using the inter-simple sequence repeat (ISSR) marker where all treated and control plants were clustered into three main groups based on genetic similarity. P. dactylifera plants were found to be more divergent at high Cd stress as compared to control and plants treated at low concentration of Cd.

The desert plant Phoenix dactylifera closes stomata via nitrate-regulated SLAC1 anion channel

Date palm Phoenix dactylifera is a desert crop well adapted to survive and produce fruits under extreme drought and heat. How are palms under such harsh environmental conditions able to limit transpirational water loss? Here, we analysed the cuticular waxes, stomata structure and function, and molecular biology of guard cells from P. dactylifera. To understand the stomatal response to the water stress phytohormone of the desert plant, we cloned the major elements necessary for guard cell fast abscisic acid (ABA) signalling and reconstituted this ABA signalosome in Xenopus oocytes. The PhoenixSLAC1-type anion channel is regulated by ABA kinase PdOST1. Energy-dispersive X-ray analysis (EDXA) demonstrated that date palm guard cells release chloride during stomatal closure. However, in Cl^- medium, PdOST1 did not activate the desert plant anion channel PdSLAC1 per se. Only when nitrate was present at the extracellular face of the anion channel did the OST1-gated PdSLAC1 open, thus enabling chloride release. In the presence of nitrate, ABA enhanced and accelerated stomatal closure. Our findings indicate that, in date palm, the guard cell osmotic motor driving stomatal closure uses nitrate as the signal to open the major anion channel SLAC1. This initiates guard cell depolarization and the release of anions together with potassium.

The phytochelatin synthase gene in date palm (Phoenix dactylifera L.): Phylogeny, evolution and expression

We studied date palm phytochelatin synthase type I (PdPCS1), which catalyzes the cytosolic synthesis of phytochelatins (PCs), a heavy metal binding protein, in plant cells. The gene encoding PdPCS1 (Pdpcs) consists of 8 exons and 7 introns and encodes a protein of 528 amino acids. PCs gene history was studied using Notung phylogeny. During evolution, gene loss from several lineages was predicted including Proteobacteria, Bilateria and Brassicaceae. In addition, eleven gene duplication events appeared toward interior nodes of the reconciled tree and four gene duplication events appeared toward the external nodes. These latter sequences belong to species with a second copy of PCs suggesting that this gene evolved through subfunctionalization. Pdpcs1 gene expression was measured in seedling hypocotyls exposed to Cd, Cu and Cr using quantitative real-time polymerase chain reaction (qPCR). A Pdpcs1 overexpression was evidenced in P. dactylifera seedlings exposed to metals suggesting that 1-the Pdpcs1 gene is functional, 2-there is an implication of the enzyme in metal detoxification mechanisms. Additionally, the structure of PdPCS1 was predicted using its homologue from Nostoc (cyanobacterium, NsPCS) as a template in Discovery studio and PyMol software. These analyses allowed us to identify the phytochelatin synthase type I enzyme in date palm (PdPCS1) via recognition of key consensus amino acids involved in the catalytic mechanism, and to propose a hypothetical binding and catalytic site for an additional substrate binding cavity.

Plant Regeneration from Somatic Embryogenic Suspension Cultures of Date Palm

Somatic embryogenesis is one of the most important technologies for plant regeneration of elite date palm cultivars. Recently, considerable progress has been made in the development and optimization of this technique from embryogenic cell suspension cultures. This chapter describes a procedure for the rapid development of a large number of somatic embryos from embryogenic cell suspension cultures. An efficient plant regeneration protocol via somatic embryogenesis from cell suspension cultures starting with shoot-tip explants to plantlet acclimatization also is fully described. Low concentrations of 6-benzylaminopurine (BAP) to 0.3 mg/L and high rate of subcultures each 7 days lead to improve the establishment and multiplication of somatic embryos in suspension cultures by limiting oxidative browning, associated with high total phenols and peroxidase activities. The detailed morphological observations have revealed the cells destined to become somatic embryos. Activated charcoal (AC) at 0.15 g/L has a positive effect on growth rate of somatic embryos by reducing tissue and medium browning, phenolics, and peroxidase activity.

Cryopreservation of Date Palm Pro-Embryonic Masses Using the D Cryo-plate Technique

In this chapter, we describe a cryopreservation (liquid nitrogen, -196 °C) protocol developed for long-term storage of date palm pro-embryonic masses (PEMs), which uses the recently established D cryo-plate technique. Clumps of PEMs (3-5 mm in size) were dissected from PEM cultures and placed on pretreatment medium containing 171 g/L sucrose for 3 days. Clumps were placed in the wells of aluminum cryo-plates in which they were made to adhere using droplets of 3% calcium alginate. PEMs were treated for 20 min with a loading solution containing 184 g/L glycerol and 136.8 g/L sucrose. They were then dehydrated for 90-120 min in the air current of a laminar airflow cabinet and immersed directly in liquid nitrogen. For rewarming, the cryo-plates holding the PEMs were immersed for 15 min in an unloading solution containing 410.4 g/L sucrose. The PEMs were then detached from the cryo-plates, placed for 3 days in the dark on posttreatment medium containing 102.6 g/L sucrose, and transferred on recovery medium under light conditions. Using this protocol, 74.6 and 95.8% recovery were achieved with the PEMs of the two cultivars tested, Sukkari and Sultany.

In Vitro Conservation of Date Palm Shoot-Tip Explants and Callus Cultures Under Minimal Growth Conditions

Date palm fruit production has great economic significance for many countries. There is a fundamental necessity to conserve valuable date palm germplasm, but there are various problems with in vivo and ex situ conservation. In vitro storage has several advantages over conventional germplasm conservation methods. The in vitro technique offers a developed method of slow-growth storage, which is considered as an alternate solution for short- and medium-term storage of date palm germplasm under controlled conditions. Minimal growth conditions for germplasm conservation are generally achieved by reducing growth rate through modification of environmental growing conditions and culture, by using low temperatures, and the addition of growth retardants and osmotic agents. This chapter describes a protocol for short-term in vitro conservation of date palm shoot-tip and callus cultures under slow-growth storage conditions, using sucrose as an osmotic agent and abscisic acid (ABA) as a growth retardant at 15 °C for 12 months.

Exogenous proline mediates alleviation of cadmium stress by promoting photosynthetic activity, water status and antioxidative enzymes activities of young date palm (Phoenix dactylifera L.)

The ability of exogenous compatible solutes, such as proline, to counteract cadmium (Cd) inhibitory effects in young date palm plants (Phoenix dactylifera L. cv Deglet Nour) was investigated. Two-year-old date palm plants were subjected for five months at different Cd stress levels (0, 10 and 30 mg CdCl2 kg(-1) soil) whether supplied or not with exogenous proline (20mM) added through the irrigation water. Different levels of Cd stress altered plant growth, gas exchanges and chlorophyll content as well as water status, but at different extent among them. In contrast, an increase of antioxidant enzymes activities of Cd-treated plants in association with high amounts of proline content, hydrogen peroxide (H2O2), thiobarbituric acid reactive substances (TBARS) and electrolyte leakage (EL) were observed. Interestingly, exogenous proline mitigated the adverse effects of Cd on young date palm. Indeed, it alleviated the oxidative damage induced by Cd accumulation and established better levels of plant growth, water status and photosynthetic activity. Moreover, proline-treated plants showed high antioxidant enzymes activities (superoxide dismutase, catalase and glutathione peroxydase) in roots and leaves as compared to Cd-treated plants.

Enzymatic saccharification and fermentation of cellulosic date palm wastes to glucose and lactic acid

The bioconversion of cellulosic wastes into high-value bio-products by saccharification and fermentation processes is an important step that can reduce the environmental pollution caused by agricultural wastes. In this study, enzymatic saccharification of treated and untreated date palm cellulosic wastes by the cellulases from Geobacillus stearothermophilus was optimized. The alkaline pre-treatment of the date palm wastes was found to be effective in increasing the saccharification percentage. The maximum rate of saccharification was found at a substrate concentration of 4% and enzyme concentration of 30 FPU/g of substrate. The optimum pH and temperature for the bioconversions were 5.0 and 50°C, respectively, after 24h of incubation, with a yield of 31.56mg/mL of glucose at a saccharification degree of 71.03%. The saccharification was increased to 94.88% by removal of the hydrolysate after 24h by using a two-step hydrolysis. Significant lactic acid production (27.8mg/mL) was obtained by separate saccharification and fermentation after 72h of incubation. The results indicate that production of fermentable sugar and lactic acid is feasible and may reduce environmental pollution by using date palm wastes as a cheap substrate.

Effect of potassium nitrate on antioxidants production of date palm (Phoenix dactylifera L.) in vitro

Antioxidants present in dates are necessary for all physiological processes of humans and animals. In Saudi Arabia date palm is a national fruit tree, produces millions of tons of dates for consumption and is considered a major source of antioxidants. The main aim of this study was to determine the role of potassium nitrate (KNO3) in the formation of antioxidants from explants collected from date palm cultivars in Al-Ahsa Oasis, Kingdom of Saudi Arabia (KSA) and to monitor the extent of its effect on growth and development of cells during callus formation stage via somatic embryogenesis. The results showed that full concentration of KNO3 was the best for callus formation in general. While, the half concentration of KNO3 played an important role for stimulating the explants to form phenolic compounds and the browning emergence for all the cultivars under investigation. On the other hand, the chemical analysis for measuring the phenolic compounds in the explants showed that all the explants formed antioxidants but with varying degrees. The highest mean of phenolic contents was found in those explants cultured with the half concentration of KNO3 for Shishi cv 2.053 ± 0.010a mg g(-1) and antioxidant activity by ABTS Inhibition and UM Trolox was 80.694 ± 0.439 and 801.575 ± 2.391, respectively.