Effect of seed priming with spermine/spermidine on transcriptional regulation of stress-responsive genes in salt-stressed seedlings of an aromatic rice cultivar

Gobindobhog, the indigenous aromatic rice cultivar from West Bengal, India, is well-known for its magnificent flavor and intense pleasant aroma. Earlier observation established the highly salt-susceptible nature of this cultivar and the potential role of exogenous spermidine (Spd) and spermine (Spm) in the amelioration of salt stress. In the present study, the effect of seed priming with Spd and Spm on salt-stressed Gobindobhog seedlings has been studied with respect to the regulation of expression of genes involved in multiple metabolic pathways controlling salt tolerance. The expression profiling of key genes encoding non-enzymatic and enzymatic antioxidants (ANS, CAT, SOD, APX, GR), osmolyte (P5CS, PDH, BADH1), ABA biosynthetic enzyme (NCED3), transcription factors (TRAB-1, WRKY-71), LEA (Osem), ion transporter (NHX1), PA metabolic enzymes (SAMDC, SPDS, SPMS, DAO, PAO), enzyme for RuBisCo (RbcS), and content of endogenous PAs, responsible for stress tolerance were studied both in the shoots and roots of Gobindobhog seedlings. Both Spm and Spd priming enhanced the expression of antioxidant genes in shoots and roots with respect to stressed seedlings. However, the enhanced expression was more pronounced with Spm priming. The genes for osmolyte and ABA biosynthesis were significantly enhanced by both the PAs, together with increased expression of ABA-inducible transcription factors and LEA gene in shoots and roots of stressed seedlings. However, better expression was noted with Spm. Priming also altered the expression of ion transporter gene NHX1 during stress. Salt stress did not increase the (Spm + Spd)/Put ratio; however, Spm pre-treatment slightly altered this ratio and also led to increased expression of PA biosynthetic genes and decreased expression of PA catabolic genes like DAO and PAO in shoots. Seed pre-treatment with Spd however decreased the (Spm + Spd)/Put ratio, as well as leading to higher expression of PAO in shoots, thereby indicating that the overall levels of Spm and Spd did not increase and that the biosynthesis and degradation processes were mutually regulated. However, the expression of DAO and PAO was decreased in roots. The metabolic readjustment in the seedlings brought about by the coordinated expression of a diverse array of genes during pre-treatment could significantly restore the down regulated RbcS expression noted under stress. Our results highlight the potentiality of priming technique in improving the overall performance of the aromatic cultivar during salt stress by controlling stress-tolerant determinants of multiple metabolic pathways.