Expression of AoNHX1 increases salt tolerance of rice and Arabidopsis, and bHLH transcription factors regulate AtNHX1 and AtNHX6 in Arabidopsis.

KEY MESSAGE: Expression of AoNHX1 from the mangrove Avicennia increases salt tolerance of rice and Arabidopsis, and specific bHLH transcription factors regulate AtNHX1 and AtNHX6 in Arabidopsis to mediate the salinity response. Improving crop plants to better tolerate soil salinity is a challenging task. Mangrove trees such as Avicennia officinalis have special adaptations to thrive in high salt conditions, which include subcellular compartmentalization of ions facilitated by specialized ion transporters. We identified and characterized two genes encoding Na+/H+ exchangers AoNHX1 and AoNHX6 from Avicennia. AoNHX1 was present in the tonoplast, while, AoNHX6 was localized to the ER and Golgi. Both NHXs were induced by NaCl treatment, with AoNHX1 showing high expression levels in the leaves and AoNHX6 in the seedling roots. Yeast deletion mutants (ena1-5Δ nha1Δ nhx1Δ and ena1-5Δ nha1Δ vnx1Δ) complemented with AoNHX1 and AoNHX6 showed increased tolerance to both NaCl and KCl. Expression of AoNHX1 and AoNHX6 in the corresponding Arabidopsis mutants conferred enhanced NaCl tolerance. The underlying molecular regulatory mechanism was investigated using AtNHX1 and AtNHX6 in Arabidopsis. We identified two basic helix-loop-helix (bHLH) transcription factors AtMYC2 and AtbHLH122 as the ABA-mediated upstream regulators of AtNHX1 and AtNHX6 by chromatin immunoprecipitation. Furthermore, expression of AtNHX1 and AtNHX6 transcripts was reduced in the atmyc2 and atbhlh122 mutants. Lastly, transgenic rice seedlings harboring pUBI::AoNHX1 showed enhanced salt tolerance, suggesting that this gene can be exploited for developing salt-tolerant crops.