The association between GABA-shunt and circadian rhythm directs salt stress responses in Nicotiana tabaccum L.

Abstract

Gamma amino butyric acid (GABA) is a neurotransmitter inhibitor molecule functioning in the central nervous system of mammals found in all the suprachiasmatic nucleus (SCN) and has a critical role in refining circadian rhythm since the SCN sends signals to peripheral clocks for synchronization. GABA may function in plants in a similar way to animals in relation to circadian rhythm. Previously we found that GABA biosynthesis was affected at transcriptional level in Nicotiana tabaccum under drought stress during 24 h periodicity. In the light of these findings we hypothesized that GABA might be an internal cue for circadian rhythm and disruption of this interaction by different stresses might affect phase rhythms in tobacco. We conducted experiments with N. tabaccum plants under salt stress in 72 h periodicity. Physiological parameters along with GABA content GAD and GABA-T activities were measured and the transcript profiles of genes related to the morning central and evening loops of the circadian rhythm were also analyzed. According to our findings under control conditions GABA-shunt was found to be associated with CCA1 PRR7 TOC PRR3 and GI while H2O2 content was negatively correlated with GABA catabolism. Salt stress reversed the daily rhythm of GABA biosynthesis (day and night cycle) as compared to the control plants. The morning and the night/central genes' transcript abundances were impaired during day-night transitions in salt-stressed groups. Moreover under salt stress GABA catabolism was associated more with the morning phase genes and the correlation between GABA-shunt and the circadian rhythm was adversely affected. It is possible that increased H2O2 content under salt stress may also play a regulatory role in this interplay.