Abstract
Previously, cell type-specific expression of AtHKT1;1, a sodium transporter, improved sodium (Na+) exclusion and salinity tolerance in Arabidopsis. In the current work, AtHKT1;1, was expressed specifically in the root cortical and epidermal cells of an Arabidopsis GAL4-GFP enhancer trap line. These transgenic plants were found to have significantly improved Na+ exclusion under conditions of salinity stress. The feasibility of a similar biotechnological approach in crop plants was explored using a GAL4-GFP enhancer trap rice line to drive expression of AtHKT1;1 specifically in the root cortex. Compared with the background GAL4-GFP line, the rice plants expressing AtHKT1;1 had a higher fresh weight under salinity stress, which was related to a lower concentration of Na+ in the shoots. The root-to-shoot transport of 22Na+ was also decreased and was correlated with an upregulation of OsHKT1;5, the native transporter responsible for Na+ retrieval from the transpiration stream. Interestingly, in the transgenic Arabidopsis plants overexpressing AtHKT1;1 in the cortex and epidermis, the native AtHKT1;1 gene responsible for Na+ retrieval from the transpiration stream, was also upregulated. Extra Na+ retrieved from the xylem was stored in the outer root cells and was correlated with a significant increase in expression of the vacuolar pyrophosphatases (in Arabidopsis and rice) the activity of which would be necessary to move the additional stored Na+ into the vacuoles of these cells. This work presents an important step in the development of abiotic stress tolerance in crop plants via targeted changes in mineral transport.
Highlights
Salinity stress affects crop production worldwide, by limiting plant growth and reducing potential yield
The encoded transporter appears to function in combination with HKT1;5, by retrieving Na+ from the shoot transpiration stream and sequestering Na+ in the sheath tissue [7]
To confirm that a gene-of-interest could be transactivated in the same cell-types as GAL4-GFP, the uidA gene was fused to a second UASGAL4 element and transformed into J1551. b-glucuronidase activity was detected in the same cell-types as the original GFP fluorescence pattern (Figure 1C) and both GFP and GUS expression was unaffected by NaCl treatment
Summary
Salinity stress affects crop production worldwide, by limiting plant growth and reducing potential yield. Plants have three mechanisms for tolerating soil salinity, including tolerance of the osmotic effects of salt; a tolerance in leaf tissues of the negative effects of sodium ions (Na+) on cellular function through compartmentalisation of Na+ in specific tissues, cells, or cellular organelles; and exclusion of Na+ from the sensitive shoot tissue [1]. Reduced activity of this transporter leads to increased transfer of Na+ from the root to the shoot, leading to a higher shoot Na+ concentration and a decrease in salinity tolerance [4]. The endogenous function of the family of HKT transporters and their role in Na+ transport and provision of salinity tolerance to plants has been reviewed recently [8]
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