Physiological Mechanisms Only Tell Half Story: Multiple Biological Processes are involved in Regulating Freezing Tolerance of Imbibed Lactuca sativa Seeds

Ganesh K Jaganathan,Yingying Han,Mengqi Shen,Baolin Liu,Chenxue Zhang,Danping Song,Xiaoyan Song,Qiang Zhou,Weijie Li

doi:10.1038/srep44166

Ganesh K Jaganathan, Yingying Han + Show 7 more

Open Access

https://doi.org/10.1038/srep44166

Copy DOI

Abstract

The physiological mechanisms by which imbibed seeds survive freezing temperatures in their natural environment have been categorized as freezing avoidance by supercooling and freezing tolerance by extracellular freeze-desiccation, but the biochemical and molecular mechanisms conferring seed freezing tolerance is unexplored. In this study, using imbibed Lactuca sativa seeds we show that fast cooled seeds (60 °C h−1) suffered significantly higher membrane damage at temperature between −20 °C and −10 °C than slow cooled (3 °Ch−1) seeds (P < 0.05), presumably explaining viability loss during fast cooling when temperature approaches −20 °C. Total soluble sugars increase in low temperature environment, but did not differ significantly between two cooling rates (P > 0.05). However, both SOD activity and accumulation of free proline were induced significantly after slow cooling to −20 °C compared with fast cooling. RNA-seq demonstrated that multiple pathways were differentially regulated between slow and fast cooling. Real-time verification of some differentially expressed genes (DEGs) revealed that fast cooling caused mRNA level changes of plant hormone and ubiquitionation pathways at higher sub-zero temperature, whilst slow cooling caused mRNA level change of those pathways at lower sub-zero ttemperatures. Thus, we conclude that imbibed seed tolerate low temperature not only by physiological mechanisms but also by biochemical and molecular changes.

Highlights

Research into identifying the mechanisms by which plants survive low temperature has established a multitude of molecular, biochemical, and physiological changes are involved in making a plant freezing tolerant
Because snow-cover provides a benign environment to the over-wintering seeds, the risk of mortality resulting from ice formation as a consequence of low temperature is minimized[16]
At the time of snow-melt in late winter, seeds in the soil seed banks are hydrated by the melting snow and subsequently exposed to nocturnal temperature plummeting to sub-zero levels

Summary

Introduction

Research into identifying the mechanisms by which plants survive low temperature has established a multitude of molecular, biochemical, and physiological changes are involved in making a plant freezing tolerant. One of the widely held assumptions is that plants adapted to a cold-environment escape the mortality associated risks of a generative phase by completing the reproduction in summer and shed the seeds before the onset of winter These seeds subsequently establish soil seed banks and over-winter under dense snow cover before beginning to germinate the summer[14,15]. Quest[20], Eurotia lanata (Pursh) Moq[21], Lactuca sativa[22,23,24], Pinus sylvestris[25] and Pyrus malus[26,27] These studies show that imbibed seeds survive low temperature either by freezing avoidance by supercooling or freezing tolerance by extracellular-freezing. The putative biochemical and molecular changes in seeds during low temperature survival in the imbibed state remain largely superficial, promoting the need for further investigation

Methods

Results

Conclusion