During their life cycle, plants often encounter a broad spectrum of exogenous constraints caused by environmental conditions that adversely affect growth, development and, ultimately, productivity. Exogenous stressors can be biotic or abiotic in nature, whether caused by other living organisms (such as pathogens), or by deficits/chemical-physical excesses (e.g., heavy metals, salt, drought, extreme temperatures). Abiotic constraints trigger a wide range of responses which acclimatize the organism to help ensure its survival; these responses range from changes in the rate of the basal metabolism [1
] to an altered expression of a specific set of genes, such as cell wall-related genes [2
Soil salinity is a threat to agriculture; an excess of salts leads to the formation of compacted soils which reduce the cellular respiration of roots, as well as the drainage of water and substances dissolved in the organic matter [7
]. Additionally, the water potential of the soil is altered, with consequent osmotic imbalances to the roots.
Many of the responses of plants to an excess of salts are related to those observed for water and osmotic stress; salinity stress is indeed due to ionic, osmotic and oxidative stresses.
One of the most immediate responses of plants to salinity is the synthesis of compatible solutes, called osmolytes, characterized by particular biochemical properties; they are highly soluble organic compounds that do not interfere with cell metabolism, even at high concentrations (recently reviewed in [8
]). Osmolytes stabilize the hydration shell of proteins in their native conformation. The synthesis and accumulation of compatible solutes is also a strategy used by plants to lower the osmotic potential of the cytosol. Osmolytes therefore have multiple roles; they contribute to maintaining an appropriate turgor pressure in the cell, they protect the proteins against misfolding, and they mitigate the toxic effects caused by reactive oxygen species (ROS), such as protein carbamylation and lipid peroxidation.
Salinity stress induces structural changes, as, for example, seen in potato plantlets [10
]; the chloroplasts were damaged and disorganized and the cell wall was thickened and ruptured. Also membrane stability is affected by salt stress, because of the accumulation of malondialdehyde, which originates from the decomposition of polyunsaturated fatty acids [11
]. Like other forms of stress, salinity also causes genotoxicity, e.g., DNA damage. The hyperosmolarity caused by an excess of salts triggers the appearance of DNA fragments of double-stranded DNA (dsDNA). As a matter of fact, studies carried out on barley showed that salt stress already triggered fragmentation of DNA (called DNA laddering) after 8 h and led to cell death [12
In the model plant thale cress, salt stress induces autophagy; mutants defective in autophagy, e.g., ATG2
, are indeed hypersensitive to salt stress [13
]. Autophagy is therefore a prerequisite for tolerance to salt stress. To better understand the impact of salinity on plants, molecular and physiological studies focused on both tolerant and susceptible plants are necessary. A recent transcriptomic study on the mangrove Avicennia officinalis
, which shows high salt tolerance, revealed that, in the roots exposed to salinity, genes involved in ethylene and auxin signaling were upregulated, while those related to abscisic acid signaling were downregulated [14
]. In particular, this study identified an important role of ethylene-responsive factors (ERF
s) in salt tolerance. Another recent study, based on proteomics, on a different plant species (a grass, Leymus chinensis
) tolerant to salinity, confirmed the (already reported) important role of peroxidase, superoxide dismutase and catalase in the tolerance mechanism [15
]. Rice is considered a salinity susceptible crop, together with wheat [8
]; however, among rice varieties, there are genotypes which are more tolerant to salinity. For example, the genotype Dongdao-4 increased catalase activity, accumulated more proline and soluble sugars, and therefore showed higher tolerance to salinity [16
Fiber crops like textile hemp and flax are important sources of raw materials for industry, as they provide, in a sustainable manner, great amounts of biomass in a relatively short time. Soil salinity can negatively affect the production of plant biomass and therefore has a strong impact on fiber crop productivity. Some studies are available in the literature on the effects that salinity exposure has on fiber crops. For example, in flax, NaCl exposure (50 mM NaCl) triggered the increased expression (log2
FC as compared to control >6.8) of a NAC47
gene, together with a β-glucosidase 17, a dehydrin and a cytochrome CYP82C4
]. More recently, the response to salinity (500 mM NaCl) was compared in the leaves of a seed vs. textile hemp variety using RNA sequencing (RNA-Seq) [18
]: this study demonstrated that the fiber variety, after salt stress, showed an enrichment in genes belonging to the spliceosome ontology and amino acid metabolism, while in the seed variety there was a predominance of genes related to fatty acid and amino acid metabolisms, as well as endoplasmic reticulum protein processing pathway. In the same work, 220 co-upregulated differentially expressed genes were identified, among which several transcription factors belonging to the MYB, NAC, GATA and HSF families.
Hemp was previously shown to be a valid model for carrying out molecular investigation focusing on the cell walls; the hypocotyl shows a temporal separation of elongation and secondary growth [19
], while adult plants have a basipetal gradient of lignification and of bast fiber developmental stages [20
In the present study, the effects of NaCl exposure on a fiber variety of Cannabis sativa
(cv. Santhica 27) were analyzed by means of targeted gene expression. The goal was to elucidate the effects that salt exposure triggers in hemp plantlets aged 15 days (a time-point characterized by cessation of bast fiber primary growth and the start of cell wall thickening [19
]), by monitoring genes involved in cell wall biosynthesis, as well as in the general response of plants to exogenous constraints in both leaves and hypocotyls. We provide a description of the events associated to NaCl exposure using a cell wall perspective, which has so far not been done to our knowledge.