Because lignin limits the use of wood for fiber, chemical, and energy production, strategies for its downregulation are of considerable interest. We have produced transgenic aspen (Populus tremuloides Michx.) trees in which expression of a lignin biosynthetic pathway gene Pt4CL1 encoding 4-coumarate:coenzyme A ligase (4CL) has been downregulated by antisense inhibition. Trees with suppressed Pt4CL1 expression exhibited up to a 45% reduction of lignin, but this was compensated for by a 15% increase in cellulose. As a result, the total lignin–cellulose mass remained essentially unchanged. Leaf, root, and stem growth were substantially enhanced, and structural integrity was maintained both at the cellular and whole-plant levels in the transgenic lines. Our results indicate that lignin and cellulose deposition could be regulated in a compensatory fashion, which may contribute to metabolic flexibility and a growth advantage to sustain the long-term structural integrity of woody perennials.

Biotechnology on genetic control of lignin biosynthesis in trees -Repression of lignin biosynthesis promotes cellulose accumulation and growth in transgenic trees-

Secondary thickening in pteridophytes.-The known cases of secondary thickening in recent Pteridophyta have been brought together by HILL23 in a useful resume. After stating the criteria for secondary growth, Botrychium, which has a distinct cambium, and Ophioglossum, which lacks a definite layer, are described, followed by Angiopteris and Marattia, in which a cambium forms a few xylem elements. CORMACK'S observations on the secondary wood in the nodes of Equisetum are cited, though no reference is made to the cambium in the young cone as reported by J]FFREY.24 The other cases of secondary growth include Psilotum, Selaginella spinulosa, and several species of Isoetes, especially I. hystrix, which may show a cambium outside the vascular cylinder.-M. A.

The Cell Wall

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Mechanisms of Plant Responses and Adaptation to Soil Salinity.

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

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Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance.

Polysaccharides are among the most important members of the biopolymer family. They are natural macromolecules composed of monosaccharides. To date, more than 300 kinds of natural polysaccharide compounds have been identified. They are present in plants, animals, and microorganisms, and they engage in a variety of physiological functions. In the 1950s, due to the discovery of their immunoregulatory and anti-tumor activities, polysaccharides became a popular topic of research in pharmacology, especially in immunopharmacology. Plants are an important source of natural polysaccharides. Pharmacological and clinical studies have shown that plant polysaccharides have many functions, such as immune regulation, anti-tumor activity, anti-inflammatory activity, anti-viral functions, anti-radiation functions, and a hypoglycaemic effect. The immunomodulatory effects of plant polysaccharides have received much attention. Polysaccharides with these effects are also referred to as biological response modifiers (BRMs), and research on them is one of the most active areas of polysaccharide research. Thus, we summarize immunomodulatory effects of botanical polysaccharides isolated from different species of plants on the macrophage. The primary effect of botanical polysaccharides is to enhance and/or activate macrophage immune responses, including increasing reactive oxygen species (ROS) production, and enhancing secretion of cytokines and chemokines. Therefore, it is believed that botanical polysaccharides have significant therapeutic potential, and represent a new method for discovery and development of novel immunomodulatory medicine.

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Advances in Research on Immunoregulation of Macrophages by Plant Polysaccharides

To survive in a saline environment, halophytes have evolved many strategies to resist salt stress. The salt glands of recretohalophytes are exceptional features for directly secreting salt out of a plant. Knowledge of the pathway(s) of salt secretion in relation to the function of salt glands may help us to change the salt-tolerance of crops and to cultivate the extensive saline lands that are available. Recently, ultrastructural studies of salt glands and the mechanism of salt secretion, particularly the candidate genes involved in salt secretion, have been illustrated in detail. In this review, we summarize current researches on salt gland structure, salt secretion mechanism and candidate genes involved, and provide an overview of the salt secretion pathway and the asymmetric ion transport of the salt gland. A new model recretohalophyte is also proposed.

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Progress in Studying Salt Secretion from the Salt Glands in Recretohalophytes: How Do Plants Secrete Salt?

With the expansion of saline land worldwide, it is essential to establish a model halophyte to study the salt-tolerance mechanism. The salt glands in the epidermis of Limonium bicolor (a recretohalophyte) play a pivotal role in salt tolerance by secreting excess salts from tissues. Despite the importance of salt secretion, nothing is known about the molecular mechanisms of salt gland development. In this study, we applied RNA sequencing to profile early leaf development using five distinct developmental stages, which were quantified by successive collections of the first true leaves of L. bicolor with precise spatial and temporal resolution. Specific gene expression patterns were identified for each developmental stage. In particular, we found that genes controlling salt gland differentiation in L. bicolor may evolve in a trichome formation, which was also confirmed by mutants with increased salt gland densities. Genes involved in the special ultrastructure of salt glands were also elucidated. Twenty-six genes were proposed to participate in salt gland differentiation. Our dataset sheds light on the molecular processes underpinning salt gland development and thus represents a first step towards the bioengineering of active salt-secretion capacity in crops.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pce.12514

Comparative transcriptome analysis of developmental stages of the Limonium bicolor leaf generates insights into salt gland differentiation.

Hydrogen sulfide (H2S) is an important gaseous molecule in a number of plant developmental processes and stress responses. Triticum aestivum L. (LM 15) is a salt-sensitive wheat cultivar that was used here to examine the effect of exogenous H2S on seedling growth, Na+ and K+ concentration, and Na+ transportation under NaCl stress. The results showed that pretreatment with 0.05 mM sodium hydrosulfide (NaHS), a H2S donor, for 12 h significantly alleviated the growth inhibition of wheat seedlings under 100 mM NaCl. In addition, pretreatment with NaHS decreased the Na+ concentration, Na+/K+ ratio, the selective absorption capacity for K+ over Na+ and Na+ efflux ratio, and increased the selective transport capacity for K+ over Na+ under salt stress. Additional experiments with CaCl2 (an inhibitor of nonselective cation channels—NSCCs), TEA+ (an inhibitor of low affinity K+ transporter) or amiloride (an inhibitor of salt overly sensitive 1—SOS1) showed that NSCCs and SOS1 were the major pathways by which H2S reduced Na+ concentration in wheat seedlings. These results showed that exogenous H2S alleviated growth inhibition by maintaining a lower Na+ concentration in wheat seedlings under NaCl stress via the regulation of NSCCs and SOS1 pathways.

Exogenous hydrogen sulfide alleviates salt stress in wheat seedlings by decreasing Na+ content

Limonium bicolor, a typical recretohalophyte that lives in saline environments, excretes excessive salt to the environment through epidermal salt glands to avoid salt stress. The aim of this study was to screen for L. bicolor genes involved in salt secretion by high-throughput RNA sequencing. We established the experimental procedure of salt secretion using detached mature leaves, in which the optimal salt concentration was determined as 200 mM NaCl. The detached salt secretion system combined with Illumina deep sequencing were applied. In total, 27,311 genes were annotated using an L. bicolor database, and 2040 of these genes were differentially expressed, of which 744 were up-regulated and 1260 were down-regulated with the NaCl versus the control treatment. A gene ontology enrichment analysis indicated that genes related to ion transport, vesicles, reactive oxygen species scavenging, the abscisic acid-dependent signaling pathway and transcription factors were found to be highly expressed under NaCl treatment. We found that 102 of these genes were likely to be involved in salt secretion, which was confirmed using salt-secretion mutants. The present study identifies the candidate genes in the L. bicolor salt gland that are highly associated with salt secretion. In addition, a salt-transporting pathway is presented to explain how Na(+) is excreted by the salt gland in L. bicolor. These findings will shed light on the molecular mechanism of salt secretion from the salt glands of plants.

The transcriptome of NaCl-treated Limonium bicolor leaves reveals the genes controlling salt secretion of salt gland

The CCCH type zinc finger proteins are a super family involved in many aspects of plant growth and development. In this study, we investigated the response of one CCCH type zinc finger protein AtZFP1 (At2g25900) to salt stress in Arabidopsis. The expression of AtZFP1 was upregulated by salt stress. Compared to transgenic strains, the germination rate, emerging rate of cotyledons and root length of wild plants were significantly lower under NaCl treatments, while the inhibitory effect was significantly severe in T-DNA insertion mutant strains. At germination stage, it was mainly osmotic stress when treated with NaCl. Relative to wild plants, overexpression strains maintained a higher K+, K+/Na+, chlorophyll and proline content, and lower Na+ and MDA content. Quantitative real-time PCR analysis revealed that the expression of stress related marker genes KIN1, RD29B and RD22 increased more significantly in transgenic strains by salt stress. Overexpression of AtZFP1 also enhanced oxidative and osmotic stress tolerance which was determined by measuring the expression of a set of antioxidant genes, osmotic stress genes and ion transport protein genes such as SOS1, AtP5CS1 and AtGSTU5. Overall, our results suggest that overexpression of AtZFP1 enhanced salt tolerance by maintaining ionic balance and limiting oxidative and osmotic stress.

Fang Yuan

Papers

Progress in Studying Salt Secretion from the Salt Glands in Recretohalophytes: How Do Plants Secrete Salt?

Comparative transcriptome analysis of developmental stages of the Limonium bicolor leaf generates insights into salt gland differentiation.

Exogenous hydrogen sulfide alleviates salt stress in wheat seedlings by decreasing Na+ content

The transcriptome of NaCl-treated Limonium bicolor leaves reveals the genes controlling salt secretion of salt gland

The CCCH zinc finger protein gene AtZFP1 improves salt resistance in Arabidopsis thaliana