Abstract
Salt stress is a common abiotic stress that limits the growth, development and yield of maize (Zea mays L.). To better understand the response of maize to salt stress and the mechanism by which exogenous glycine betaine (GB) alleviates the damaging effects of salt stress, the morphology, physiological and biochemical indexes, and root transcriptome expression profiles of seedlings of salt-sensitive inbred line P138 and salt-tolerant inbred line 8723 were compared under salt stress and GB-alleviated salt stress conditions. The results showed that under salt stress the growth of P138 was significantly inhibited and the vivo ion balance was disrupted, whereas 8723 could prevent salt injury by maintaining a high ratio of K+ to Na+. The addition of a suitable concentration of GB could effectively alleviate the damage caused by salt stress, and the mitigating effect on salt-sensitive inbred line P138 was more obvious than that on 8723. Transcriptome analysis revealed that 219 differentially expressed genes (DEGs) were up-regulated and 153 DEGs were down-regulated in both P138 and 8723 under NaCl treatment, and that 487 DEGs were up-regulated and 942 DEGs were down-regulated in both P138 and 8723 under salt plus exogenous GB treatment. In 8723 the response to salt stress is mainly achieved through stabilizing ion homeostasis, strong signal transduction activation, increasing reactive oxygen scavenging. GB alleviates salt stress in maize mainly by inducing gene expression changes to enhance the ion balance, secondary metabolic level, reactive oxygen scavenging mechanism, signal transduction activation. In addition, the transcription factors involved in the regulation of salt stress response and exogenous GB mitigation mainly belong to the MYB, MYB-related, AP2-EREBP, bHLH, and NAC families. We verified 10 selected up-regulated DEGs by quantitative real-time polymerase chain reaction (qRT-PCR), and the expression results were basically consistent with the transcriptome expression profiles. Our results from this study may provide the theoretical basis for determining maize salt tolerance mechanisms and the mechanism by which GB regulates salt tolerance.
Highlights
Salt stress is one of the main abiotic stresses limiting the growth and development of plants
To study the physiological responses of two maize inbred lines to salt stress and the mitigation effect of exogenous glycine betaine (GB), plant height, root length, aboveground fresh weight and underground fresh weight of these lines were measured. For both P138 and 8723, no significant difference in seedling growth was observed between seedlings grown under salt plus GB mitigation (T) treatment and those grown under normal conditions (CK), but P138 was obviously shorter under salt stress (S) treatment compared with CK, while 8723 showed little change, which indicated that GB had an obvious mitigation effect on the salt-sensitive maize genotype (Fig 1)
We identified a putative protein phosphatase 2C family protein gene (LOC 100382524) that was up-regulated in the abscisic acid (ABA) pathway of the mitogen-activated protein kinase (MAPK) cascade under the T treatment in P138, but was not identified under the salt stress. we suggest that the gene may be induced by exogenous GB, which enhances the salt tolerance of P138
Summary
Salt stress is one of the main abiotic stresses limiting the growth and development of plants. Plants have evolved a series of complex salt tolerance mechanisms involving complex physiological changes [6]. These mechanisms involve the expression and coordination of many genes, such as SKC1, a sodium transporter that participates in the response to salt stress by regulating the homeostasis of K+/Na+ [7], and OsMAPK33, a gene in the mitogen-activated protein kinase (MAPK) signal cascade that allows rice plants to cope with stress by maintaining cell osmotic balance [8]. A full understanding of the complex mechanisms by which plants respond to salt stress will help us to breeding plants that can better adapt to salt stress
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