Starvation, osmotic stress and desiccation tolerance lead to expression of different genes of the regulatory β and γ subunits of the SnRK1 complex in germinating seeds of Medicago truncatula

The SnRK1 complex, a metabolic regulator of nutrient deficit in plants, consists of a catalytic alpha-subunit and two regulatory subunits, beta and gamma that exist as several isoforms. To obtain insight into the developmental and stress conditions that regulate the expression of these regulatory subunits, four beta subunits MtAKINbeta1-beta4, and three gamma subunits MtAK-INbetagamma, MtSNF4b and MtAKINgamma were identified and characterized in seeds of M. truncatula. Their transcripts were found to accumulate differentially in vegetative and seed tissues and appear to be differentially modulated during germination and the imposition of stress. MtAKINgamma and MtAKINbeta3 showed identical patterns of expression upon osmotic shock, whereas transcripts of MtAKINgamma and MtAKINbeta1 were strongly up-regulated upon starvation of the radicles. Addition of glucose during the starvation process reversed this effect. MtAKINbeta2 and MtSNF4b were specifically induced upon the re-induction of desiccation tolerance in germinated, desiccation-sensitive radicles, whereas only MtSNF4b expression was repressed by an inhibitor of abscisic acid synthesis. A second gamma subunit, MtAKINbetagamma, was transiently expressed early during the induction of desiccation tolerance, and its expression could be modulated by blocking the respiratory ATP production by cyanide. The transcriptional regulation of the subunit isoforms showing identical expression profiles in germinating seeds appears to be under the control of different factors.