Na+ transporter HKT1;2 reduces flower Na+ content and considerably mitigates the decline in tomato fruit yields under saline conditions

Genes encoding HKT1-like Na transporters play a key role in the salinity tolerance mechanism in Arabidopsis and other plant species by retrieving Na from the xylem of different organs and tissues. In this study, we investigated the role of two HKT1;2 allelic variants in tomato salt tolerance in relation to vegetative growth and fruit yield in plants subjected to salt treatment in a commercial greenhouse under real production conditions. We used two near-isogenic lines (NILs), homozygous for either the Solanum lycopersicum (NIL17) or S. cheesmaniae (NIL14) allele, at HKT1;2 loci and their respective RNAi-Sl/ScHKT1;2 lines. The results obtained show that both ScHKT1;2- and SlHKT1;2-silenced lines display hypersensitivity to salinity associated with an altered leaf Na/K ratio, thus confirming that HKT1;2 plays an important role in Na homeostasis and salinity tolerance in tomato. Both silenced lines also showed Na over-accumulation and a slight, but significant, reduction in K content in the flower tissues of salt-treated plants and consequently a higher Na/K ratio as compared to the respective unsilenced lines. This altered Na/K ratio in flower tissues is associated with a sharp reduction in fruit yield, measured as total fresh weight and number of fruits, in both silenced lines under salinity conditions. Our findings demonstrate that Na transporter HKT1;2 protects the flower against Na toxicity and mitigates the reduction in tomato fruit yield under salinity conditions.We wish to thank Emilio Jaime Fernández (La Mayora, IHMS-CSIC) and Elena Sánchez Romero (EEZ-CSIC) for their technical assistance, the Scientific Instrumentation Service at EEZ-CSIC for their ICP-OES mineral analysis and Michael O'Shea for proofreading the text. The study was funded by EU-cofinanced grants from Agencia Estatal de Investigación, Spanish Ministry of Economy, Industry and Competition (AGL2013-41733-R and AGL2017-82452-C2-1R to A.B., AGL2017-82452-C2-2R to M.J.A.) and the University of Granada (ACCESP2018-4 to JAT)