Abstract
In Arabidopsis thaliana (Arabidopsis), the microRNA399 (miR399)/PHOSPHATE2 (PHO2) expression module is central to the response of Arabidopsis to phosphate (PO4) stress. In addition, miR399 has been demonstrated to also alter in abundance in response to salt stress. We therefore used a molecular modification approach to alter miR399 abundance to investigate the requirement of altered miR399 abundance in Arabidopsis in response to salt stress. The generated transformant lines, MIM399 and MIR399 plants, with reduced and elevated miR399 abundance respectively, displayed differences in their phenotypic and physiological response to those of wild-type Arabidopsis (Col-0) plants following exposure to a 7-day period of salt stress. However, at the molecular level, elevated miR399 abundance, and therefore, altered PHO2 target gene expression in salt-stressed Col-0, MIM399 and MIR399 plants, resulted in significant changes to the expression level of the two PO4 transporter genes, PHOSPHATE TRANSPORTER1;4 (PHT1;4) and PHT1;9. Elevated PHT1;4 and PHT1;9 PO4 transporter levels in salt stressed Arabidopsis would enhance PO4 translocation from the root to the shoot tissue which would supply additional levels of this precious cellular resource that could be utilized by the aerial tissues of salt stressed Arabidopsis to either maintain essential biological processes or to mount an adaptive response to salt stress.
Highlights
In recent decades, soil salinization has become a major environmental concern with approximately 20% of the world’s land usable for agriculture adversely affected by saline soils [1,2]
It has been reported previously that the constitutive expression of the eTM specific to miR399 in Arabidopsis failed to result in the generated transformant lines displaying any phenotypic differences to wild-type Arabidopsis plants [39,40], we subsequently employed RT-qPCR to confirm that the eTM transgene introduced into the MIM399 transformant line did result in the molecular alteration of the miR399/PHO2 expression module
Elevated PHO2 gene expression in response to a reduced miR399 level was expected in the MIM399 transformant line considering that we have previously demonstrated the predominant mode of miR399-directed regulation of PHO2 expression in Arabidopsis vegetative tissues to be the target messenger RNA (mRNA)
Summary
Soil salinization has become a major environmental concern with approximately 20% of the world’s land usable for agriculture adversely affected by saline soils [1,2]. A high abundance of salt ions in the aerial tissues leads to ionic toxicity, but can enhance osmotic pressure, reduce turgor pressure, and promote the generation of reactive oxygen species (ROS), with elevated levels of ROS leading to the onset of oxidative stress [11,12,13] Together, this promotes the closure of leaf stomata, reduces the rate of photosynthesis, decreases the uptake of nutrients and water from the soil, and represses carbon assimilation, all of which in turn result in the termination of cell elongation and expansion, while simultaneously promoting the premature transition from vegetative to reproductive development and the early onset of senescence [5,9,14,15]
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