Abstract
Soil alkalization severely limits plant growth and development, however, the mechanisms of alkaline response in plants remain largely unknown. In this study, we performed physiological and transcriptomic analyses using two alfalfa cultivars (Medicago sativa L.) with different sensitivities to alkaline conditions. The chlorophyll content and shoot fresh mass drastically declined in the alkaline-sensitive cultivar Algonquin (AG) following alkaline treatment (0–25 mM Na2CO3 solution), while the alkaline-tolerant cultivar Gongnong NO.1 (GN) maintained relatively stable growth and chlorophyll content. Compared with AG, GN had higher contents of Ca2+ and Mg2+; the ratios of Ca2+ and Mg2+ to Na+, proline and soluble sugar, as well as higher enzyme activities of peroxidase (POD) and catalase (CAT) under the alkaline conditions. Furthermore, transcriptomic analysis identified three categories of alkaline-responsive differentially expressed genes (DEGs) between the two cultivars: 48 genes commonly induced in both the cultivars (CAR), 574 genes from the tolerant cultivar (TAR), and 493 genes from the sensitive cultivar (SAR). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that CAR genes were mostly involved in phenylpropanoid biosynthesis, lipid metabolism, and DNA replication and repair; TAR genes were significantly enriched in metabolic pathways, such as biosynthesis of amino acids and secondary metabolites including flavonoids, and the MAPK signaling pathway; SAR genes were specifically enriched in vitamin B6 metabolism. Taken together, the results identified candidate pathways associated with genetic variation in response to alkaline stress, providing novel insights into the mechanisms underlying alkaline tolerance in alfalfa.
Highlights
Soil salinization and alkalization are important ecological and agricultural problems that seriously inhibit plant growth and limit worldwide crop yields (Julkowska and Testerink 2015)
Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that CAR genes were mostly involved in phenylpropanoid biosynthesis, lipid metabolism, and DNA replication and repair; tolerant cultivar (TAR) genes were significantly enriched in metabolic pathways, biosynthesis of secondary metabolites, MAPK signaling pathway, and flavonoid and amino acid biosynthesis; the sensitive cultivar (SAR) genes were enriched in vitamin B6 metabolism
We show that pathways involved in homeostasis of ions and redox status, biosynthesis of phenylpropanoids, flavonoids, and amino acids, and MAPK activation are important for alkaline tolerance in alfalfa seedlings
Summary
Soil salinization and alkalization are important ecological and agricultural problems that seriously inhibit plant growth and limit worldwide crop yields (Julkowska and Testerink 2015). In addition to high salinity, SA stress is characterized by high pH that destroys the integrity of root cell membranes (Ding et al 2019; Zhang et al 2017), disrupts the root ion homeostasis (Tester and Davenport 2003), and reduces photosynthesis and photosynthetic electron transport (An et al 2016; Li et al 2010) To cope with these stresses, plants have evolved molecular, physiological, and ecological strategies for either avoiding or tolerating the stresses, including accumulating inorganic ions (Ashrafi et al 2018; Yang et al 2007) and osmotic substances to reduce osmotic damage (Ashrafi et al 2014; Li et al 2010; Rokebul et al 2017) and activating antioxidant enzymes to scavenge the excessive ROS (Apel and Hirt 2004; Zhang et al 2017)
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