Biochemical and Gene Expression Analyses in Different Poplar Clones: The Selection Tools for Afforestation of Halomorphic Environments

Halomorphic soils cover a significant area in the Vojvodina region and represent ecological and economic challenges for agricultural and forestry sectors. In this study, four economically important Serbian poplar clones were compared according to their biochemical and transcriptomic responses towards mild and severe salt stress to select the most tolerant clones for afforestation of halomorphic soils. Three prospective clones of Populus deltoides (Bora-B229, Antonije-182/81 and PE19/66) and one of hybrid genetic background P. nigraxP. deltoides, e.g., P. x euramericana (Pannonia-M1) were hydroponically subjected to NaCl as a salt stress agent in a concentration range from 150 mM to 450 mM. Plant responses were measured at different time periods in the leaves. Biochemical response of poplar clones to salt stress was estimated by tracking several parameters such as different radical scavenging capacities (estimated by DPPH, FRAP and ABTS assays), accumulation of total phenolic content and flavonoids. Furthermore, accumulation of two osmolytes, glycine betaine and proline, were quantified. The genetic difference of those clones has been already shown by single nucleotide polymorphisms (SNPs) but this paper emphasized their differences regarding biochemical and transcriptomic salt stress responses. Five candidate genes, two putative poplar homologues of GRAS family TFs (PtGRAS17 and PtGRAS16), PtDREB2 of DREB family TFs and two abiotic stress-inducible genes (PtP5SC1, PtSOS1), were examined for their expression profiles. Results show that most salt stress-responsive genes were induced in clones M1 and PE19/66, thus showing they can tolerate salt environments with high concentrations and could be efficient in phytoremediation of salt environments. Clone M1 and PE19/66 has ABA-dependent mechanisms expressing the PtP5CS1 gene while clone 182/81 could regulate the expression of the same gene by ABA-independent pathway. To improve salt tolerance in poplar, two putative GRAS/SCL TFs and PtDREB2 gene seem to be promising candidates for genetic engineering of salt-tolerant poplar clones.