Downregulation of stress-associated protein 1 (PagSAP1) increases salt stress tolerance in poplar (Populus alba x P-glandulosa)

Downregulating PagSAP1 expression increases salt tolerance in poplar and leads to the alterations in the regulation of genes involved in maintaining cellular ionic homeostasis. Plants have evolved defense mechanisms to cope with unfavorable climate conditions such as salinity, drought, and extreme temperatures. Such adaptive strategies include fine-tuning of stress-associated proteins (SAPs). Here, we investigated the molecular and physiological characteristics of the novel gene PagSAP1 from hybrid poplar (Populus alba x P. glandulosa) in response to salt stress. PagSAP1 encodes a putative protein containing an A20 and AN1 zinc-finger domain at its N- and C-terminus, respectively. Salt stress significantly downregulated the expression of PagSAP1 in roots, but not leaves, of poplar at the seven-leaf stage. Compared to control plants, knockdown (RNAi) transgenic poplar plants showed strong tolerance to salt stress, while overexpression (OX) of PagSAP1 increased salt sensitivity. Analysis of Ca2+, Na+, and K+ contents in roots and leaves following salt treatment (150 mM NaCl) revealed that RNAi plants accumulated more Ca2+ and K+, and less Na+ than PagSAP1 OX plants. The RNAi lines exhibited increased expression of genes involved in maintaining cellular ionic homeostasis across the plasma membrane, such as salt overly sensitive 3 (SOS3), SOS1, high-affinity K+ transporter 1 (HKT1), H+-ATPase, AAA-type ATPase, and Arabidopsis K+ channel 2 (AKT2). Taken together, these results indicate that PagSAP1 represents a promising candidate gene for engineering trees with improved salt tolerance, which would increase their suitability for planting in marginal lands, such as reclaimed areas.