Abstract
Zinc salts occurring in soils can exert an osmotic stress toward plants. However, being zinc a heavy metal, some more specific effects on plant metabolisms can be forecast. In this work, lettuce has been used as a model to investigate salt and zinc stresses at proteome level through a shotgun tandem MS proteomic approach. The effect of zinc stress in lettuce, in comparison with NaCl stress, was evaluated to dissect between osmotic/oxidative stress related effects, from those changes specifically related to zinc. The analysis of proteins exhibiting a fold change of 3 as minimum (on log 2 normalized abundances), revealed the involvement of photosynthesis (via stimulation of chlorophyll synthesis and enhanced role of photosystem I) as well as stimulation of photophosphorylation. Increased glycolytic supply of energy substrates and ammonium assimilation [through formation of glutamine synthetase (GS)] were also induced by zinc in soil. Similarly, protein metabolism (at both transcriptional and ribosomal level), heat shock proteins, and proteolysis were affected. According to their biosynthetic enzymes, hormones appear to be altered by both the treatment and the time point considered: ethylene biosynthesis was enhanced, while production of abscisic acid was up-regulated at the earlier time point to decrease markedly and gibberellins were decreased at the later one. Besides aquaporin PIP2 synthesis, other osmotic/oxidative stress related compounds were enhanced under zinc stress, i.e., proline, hydroxycinnamic acids, ascorbate, sesquiterpene lactones, and terpenoids biosynthesis. Although the proteins involved in the response to zinc stress and to salinity were substantially the same, their abundance changed between the two treatments. Lettuce response to zinc was more prominent at the first sampling point, yet showing a faster adaptation than under NaCl stress. Indeed, lettuce plants showed an adaptation after 30 days of stress, in a more pronounced way in the case of zinc.
Highlights
Salt stress and NaCl in particular, is probably the most common abiotic stress affecting crop production and reducing yields
The different treatments affected the production of biomass, as plant mean weight was significantly reduced under saline conditions (Figure 1), whereas the detrimental effect of zinc on yield was less evident (ANOVA at alpha = 0.05, Student– Newman–Keuls post hoc test)
Metals analysis in plant leaves, investigated through inductively coupled plasma atomic emission spectroscopy, revealed that zinc and sodium concentration increased as a consequence of the corresponding treatment
Summary
Salt stress and NaCl in particular, is probably the most common abiotic stress affecting crop production and reducing yields. The proteins identified in different studies are involved in the major plant metabolic processes, such as photosynthesis, energy metabolism, ROS scavenging, and ion homeostasis, protein synthesis, nitrogen assimilation as well as the secondary oxidative stress (Parida and Das, 2005; Zhang et al, 2012). In Arabidopsis thaliana plants exposed to zinc at subtoxic levels, Barkla et al (2014) found that the enzymes involved in one carbon metabolism and protein synthesis were involved in acclimation to heavy metal stress. At higher concentration, this species showed alteration of proteins related to oxidative stress, proteasome, and energy metabolism (Fukao et al, 2009). Most of the work in agricultural species work has been done in sugar beet, demonstrating the imbalance of photosystems, oxidative stress, alteration of carboxylates trafficking, and low photosynthesis rates as a consequence of stomatal and mesophyll conductance to CO2 (Sagardoy et al, 2009, 2010, 2011)
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