Expression of the high-affinity K+ transporter 1 (PpHKT1) gene from almond rootstock ‘Nemaguard’ improved salt tolerance of transgenic Arabidopsis

Abstract

Soil salinity affects plant growth and development, which directly impact yield. Plants deploy many mechanisms to cope with, or mitigate, salt stress. One of such mechanism is to control movement of ions from root to shoot by regulating the loading of Na+ in the transpiration stream. The high-affinity K+ transporter 1 (HKT1) is known to play a role in the removal of Na+ from the xylem and bring it back to the root. As almond is a salt-sensitive crop, the rootstock plays an important role in successful almond cultivation in salt-affected regions. We currently lack knowledge on the molecular mechanisms involved in salt tolerance of almond rootstocks. In this study, we complemented the Arabidopsis athkt1 knockout mutant with HKT1 ortholog (PpHKT1) from the almond rootstock ‘Nemaguard’. Arabidopsis transgenic lines that were generated in athkt1 background with the constitutive promoter (PpHKT1OE2.2) and the native promoter (PpHKT1NP6) were subjected to different salt treatments. Both transgenic lines survived salt concentrations up to 120 mM NaCl, however, the mutant athkt1 died after 18 days under 120 mM NaCl. At 90 mM NaCl, the dry weight of athkt1 decreased significantly compared to the transgenic lines. Both transgenic lines showed significantly longer lateral roots compared to the athkt1 mutant at 80 mM NaCl treatment. The transgenic lines, PpHKT1OE2.2 and PpHKTNP6 had lower electrolyte leakage and higher relative water content compared to athkt1, suggesting that transgenic plants coped well with increased salt concentration by maintaining the integrity of the membranes. The expression analyses showed that PpHKT1 was induced in PpHKT1OE2.2 and PpHKTNP6 lines under salt treatment, which confirmed that both over-expression and native expression of PpHKT1 in the Arabidopsis mutant can complement salt tolerance function.

How Does Proline Treatment Promote Salt Stress Tolerance During Crop Plant Development?

“…Na + /H + is an antiporter plasma membrane transporter, encoded by an SOS1 gene, that pumps Na + from root cells to leaves, boosting salt stress tolerance (Zhu and Gong, 2014;Bargaz et al, 2015). High-affinity K + transporter (HKT) is another transporter that mediates salt tolerance in various plant species through regulation of the transport of salt ions from root to shoot (Kaundal et al, 2019;Thouin et al, 2019). In view of the important roles of these two transporters in plant salt tolerance, it would be interesting to investigate how exogenous proline can regulate the SOS1 and HKT gene expression under salt stress and their relationship with salt tolerance.…”

Investigation of Salt Tolerance Mechanisms Across a Root Developmental Gradient in Almond Rootstocks

“…The observed increased expression of all three SOS genes in E1 under salt stress ( Figures 8A-C) strongly suggests that minimizing salt entry into cells, at the level of the plasma membrane, contributes to the salinity tolerance of E1, in addition to minimizing salt entry via apoplastic barriers. The sodium transporter HKTs are known to control the retrieval of sodium from the xylem and reduce sodium accumulation in shoots in response to salinity stress (Rus et al, 2001;Davenport et al, 2007;Kaundal et al, 2019;Khan et al, 2020). The upregulation of HKT1 in all three rootstocks ( Figure 8E) indicates that this transporter is conserved in salt stress response.…”

“…Worldwide almond production is a rapidly growing sector, with a 45% global increase in production from 2017 to 2018. 1 However, the low salinity tolerance of almond plants is a significant hurdle negatively affecting almond production (Zrig et al, 2011;Kaundal et al, 2019). Therefore, selection of rootstocks with improved salinity tolerance is required to maintain the current production rate despite the trend of increasing soil salinity.…”

“…HKT and AKT therefore play a role in regulating Na + entry into the roots by selectively transporting K + under salt stress (Rus et al, 2001;Apse and Blumwald, 2007). A recent study showed that PpHKT1, the HKT1 ortholog in almond rootstock "Nemaguard" (Prunus persica × Prunus davidiana), is able to rescue athkt1 salt-sensitive phenotype (Kaundal et al, 2019), suggesting a conserved role in almond.…”

Morphological, Physiological, Biochemical, and Transcriptome Studies Reveal The Importance of Different Component Traits During Salinity Stress in Prunus

“…For example, the most widely used rootstock 'Nemaguard', a peach-based rootstock, is known for its high yields and resistance to root nematodes 22 . Previously, it was shown that the 'Nemaguard' HKT1 ortholog was able to restore salinity tolerance in transgenic Arabidopsis hkt1 mutants 23 . Varying levels of expression of more than 20 saltstress responsive genes associated with Na + homeostasis (e.g.…”