Fluctuating temperatures terminate dormancy inCynara cardunculusseeds by turning off ABA synthesis and reducing ABA signalling, but not stimulating GA synthesis or signalling

Abstract

Fluctuating temperatures terminate seed dormancy in many species, including Cynara cardunculus (L.) var. sylvestris. Termination of physiological dormancy requires low ratios of abscisic acid (ABA)/gibberellins (GA). In a previous paper we have shown that physiological responses to fluctuating temperatures comprise a reduction of abscisic acid (ABA) content and sensitivity. However, a possible stimulation of GA synthesis was also suggested as part of the mechanism. That possible stimulation, as well as the identification of potential regulatory sites for ABA and GA metabolism and signalling involved in the termination of dormancy by fluctuating temperatures, are yet to be determined. In this study, we measured GA content and sensitivity in seeds incubated under constant and fluctuating temperatures. We also assessed the expression of several genes involved in ABA and GA metabolism and signalling. Our results show that fluctuating temperatures reduce ABA/GA ratios through a reduction in ABA accumulation during incubation but without altering GA synthesis as compared to that observed under constant temperatures. On the other hand, fluctuating temperatures did not increase sensitivity to GA. Fluctuating temperatures reduced the expression of CycaNCED and CycaABI5 (ABA synthesis and signalling genes) with a temporal pattern that coincides with the interruption of ABA accumulation that precedes germination of seeds incubated under fluctuating temperatures.However, fluctuating temperatures did not modify the expression of CycaCYP707A2 (ABA inactivation) as compared to that observed under constant temperatures. Consistent with our determinations of GA content and sensitivity, fluctuating temperatures did not modify the expression of GA synthesis (CycaGA3ox) and signalling genes (CycaRGL2 and CycaGAI) in relation to that observed at constant temperatures. These results show that fluctuating temperatures terminate dormancy in Cynara cardunculus seeds through an interruption in ABA accumulation and a reduction in ABA signalling exerted at the level of CycaNCED and CycaABI5 expression.

Dormancy and germination characteristics of Tarenaya hassleriana (Cleomaceae) seeds

“…Alternating temperatures generally promote seed germination more than constant temperatures (Baskin and Baskin, 2004;Forti et al, 2020). An alternating temperature treatment can increase the level of germination-promoting phytohormones and decrease the level of germination-inhibiting phytohormones (Huarte et al, 2014;Özden et al, 2021). Compared to the initial germination of freshly harvested T. hassleriana seeds at constant temperature , the 20 °C/30 °C alternating temperature treatment could effectively release seed dormancy and promote germination, regardless of alternating light or darkness, whereas the germination rate in alternating light was significantly higher than that in darkness (P < 0.05), being more than 90%, while the 15°C/25°C alternating temperature treatment did not show a significant effect on seed germination (Figure 2).…”

The Seed and the Metabolism Regulation

“…Indeed, alternating temperatures more than constant ones can promote germination via the interplay between ROS signaling and hormones [ 81 ]. In fact, it has been shown that fluctuating temperatures alleviate dormancy by reducing ABA synthesis and signaling [ 82 ]. In the absence of a change in ABA content, there was a decrease in ABA sensitivity in sunflower dormant seeds in response to constant temperature that induced dormancy alleviation [ 71 ].…”

Exogenous gibberellic acid shortening after-ripening process and promoting seed germination in a medicinal plant Panax notoginseng

“…The balance of GA and ABA is a critical factor in controlling seed dormancy and germination [ 4 ]. Fluctuating temperatures enhance the ratio of GA/ABA by decreasing ABA content in Cynara cardunculus seeds, and simultaneously the expression of Nine-cis-epoxycarotenoid dioxygenase ( NCED ) and ABA-INSENSITIVE5 ( ABI5 ) is inhibited [ 16 ]. In the Arabidopsis and tomato ( Lycopersicon esculentum M.), the mutants with defects in the gene encoding GA biosynthetic enzymes are unable to germinate [ 17 ].…”