Sugar Beet (Beta vulgaris) Guard Cells Responses to Salinity Stress: A Proteomic Analysis

Abstract

Soil salinity is a major environmental constraint affecting crop growth and threatening global food security. Plants adapt to salinity by optimizing the performance of stomata. Stomata are formed by two guard cells (GCs) that are morphologically and functionally distinct from the other leaf cells. These microscopic sphincters inserted into the wax-covered epidermis of the shoot balance CO2 intake for photosynthetic carbon gain and concomitant water loss. In order to better understand the molecular mechanisms underlying stomatal function under saline conditions, we used proteomics approach to study isolated GCs from the salt-tolerant sugar beet species. Of the 2088 proteins identified in sugar beet GCs, 82 were differentially regulated by salt treatment. According to bioinformatics analysis (GO enrichment analysis and protein classification), these proteins were involved in lipid metabolism, cell wall modification, ATP biosynthesis, and signaling. Among the significant differentially abundant proteins, several proteins classified as “stress proteins” were upregulated, including non-specific lipid transfer protein, chaperone proteins, heat shock proteins, inorganic pyrophosphatase 2, responsible for energized vacuole membrane for ion transportation. Moreover, several antioxidant enzymes (peroxide, superoxidase dismutase) were highly upregulated. Furthermore, cell wall proteins detected in GCs provided some evidence that GC walls were more flexible in response to salt stress. Proteins such as L-ascorbate oxidase that were constitutively high under both control and high salinity conditions may contribute to the ability of sugar beet GCs to adapt to salinity by mitigating salinity-induced oxidative stress.

Salinity Effects on Guard Cell Proteome in Chenopodium quinoa

“…A comparison of differentially abundant proteins in quinoa and sugar beet [ 23 ] displayed that only 5% of up and downregulated proteins in quinoa are shared with those in sugar beet. Aspartic protease and non-specific lipid-transfer proteins showed enhanced abundance while catalase and cationic peroxidase showed lower abundances in GCs of both salt-treated plants.…”

“…It has been proposed that ROS function as signaling molecules to promote stomatal closure in response to various biotic and abiotic stress conditions [ 65 ]. In our previous study on sugar beet GCs [ 23 ], a higher concentration of H 2 O 2 was detected in the GCs under saline conditions compared to control, which was mitigated by ascorbic acid and resulted in higher stomatal conductance and slower response of stomata to dark conditions. In the present study, inhibition of aspartic proteinases impaired stomatal opening due to excessive accumulation of oxidant molecules in the GCs.…”

“…Salt stress was imposed 3 weeks after planting by adding 300 mM NaCl to irrigation water over a period of 3 weeks. GC-enriched epidermal peels were prepared as it was previously described [ 23 , 68 ]. Briefly, fully expanded leaves of well-watered 3–4 week-old quinoa were grinded in a Grindomix blender with a basic solution and crushed ice, then it was passed through a nylon mesh and rinsed with ice-cold distilled water.…”

“…Protein samples from four biological replicates per treatment were extracted essentially as published elsewhere [ 23 ]. Following homogenization by grinding in liquid nitrogen, proteins were precipitated using TCA-acetone containing 0.07% ( v / v ) 2-mercaptoethanol, followed by two acetone washes and resuspension in a denaturation buffer (7 M urea etc).…”

“…Our study applying proteomics to investigate the effects of salt stress on the GCs of sugar beet revealed a significant proportion of differentially expressed proteins related to different abiotic stress e.g., salinity, drought, and oxidative stresses, as well as some proteins related to biotic stress [ 23 ]. Salt stress also altered the abundances of some proteins related to signaling, cell wall modification and ATP biosynthesis, indicating the high impact of salt on GCs in sugar beet.…”