The Brachypodium distachyon cold-acclimated plasma membrane proteome is primed for stress resistance

Abstract

In order to survive sub-zero temperatures, some plants undergo cold acclimation where low, non-freezing temperatures and/or shortened day lengths allow cold hardening and survival during subsequent freeze events. Central to this response is the plasma membrane, where low-temperature is perceived and cellular homeostasis must be preserved by maintaining membrane integrity. Here, we present the first plasma membrane proteome of cold-acclimated Brachypodium distachyon, a model species for the study of monocot crops. A time course experiment investigated cold acclimation-induced changes in the proteome following two-phase partitioning plasma membrane enrichment and label-free quantification by nano-liquid chromatography mass spectrophotometry. Two days of cold acclimation were sufficient for membrane protection as well as an initial increase in sugar levels, and coincided with a significant change in the abundance of 154 proteins. Prolonged cold acclimation resulted in further increases in soluble sugars and abundance changes in more than 680 proteins, suggesting both a necessary early response to low-temperature treatment, as well as a sustained cold acclimation response elicited over several days. A meta-analysis revealed that the identified plasma membrane proteins have known roles in low-temperature tolerance, metabolism, transport, and pathogen defense as well as drought, osmotic stress and salt resistance suggesting crosstalk between stress responses, such that cold acclimation may prime plants for other abiotic and biotic stresses. The plasma membrane proteins identified here present keys to an understanding of cold tolerance in monocot crops and the hope of addressing economic losses associated with modern climate-mediated increases in frost events.

Cold Acclimation in Brachypodium Is Accompanied by Changes in Above-Ground Bacterial and Fungal Communities

“…After transfer to 4 • C, the leaf microbiome was impacted by the temperature shift and also showed changes in the relative abundance of potential pathogens. These observations reflect the results of network analysis of hundreds of plasma membrane proteome changes after CA that showed crosstalk between pathways for low temperature stress and disease and defence [6]. Brachypodium responds to CA by diverting resources away from growth and to the stress response.…”

“…Surface-sterilized Brachypodium seeds of an inbred line (ecotype: Bd21) (RIKEN, Wak ō, Japan) were sown in the inoculated potting soil and grown in a temperature-controlled chamber (Conviron GEN2000, Queen's University Phytotron, Kingston, ON, Canada) on a 20 h light (~100 µmol m −2 s −1 ; 22 C) light cycle. Brachypodium that had been grown under standard conditions for three weeks (Figure S2) were then cold acclimated by transferring the plants to a low temperature chamber (Coldmatic Refrigeration, Etobicoke, ON, Canada) (4 • C, 12 h light as indicated above; 12 h dark) for 6 days [6]. Plants maintained at standard conditions until time of use were considered the non-acclimated controls.…”

“…Compared to the rhizosphere, which is relatively protected from rapid abiotic and biotic stresses, leaves are exposed to daily temperature fluctuations, visible and ultraviolet light, herbivore and mechanical damage, and arguably more pathogens. Within two days of the shift to CA conditions, the Brachypodium leaf membrane is protected from freezeinduced electrolyte leakage, contains elevated levels of soluble sugars, and shows changes in the abundance profiles of hundreds of proteins [6]. The leaf community response was also rapid, as revealed by numerous abundance changes in the bacterial and fungal microbiota, as well as in the proportion of individual core taxa, as supported by the distinct groupings shown in PCoAs (Figures 3 and 5; Table 2).…”

“…In addition, Streptomyces sp. M2 synthesizes cryoprotective soluble sugars that coincidentally increase rapidly in CA Brachypodium [6,80]. The synthesis of the osmoprotectant proline may also benefit host plants, as inoculation of sugarcane with Streptomyces increased proline content and drought tolerance [81].…”

“…The perennial grass and model cereal, Brachypodium distachyon (hereinafter, Brachypodium), is capable of cold acclimation, reaching peak freezing tolerance after two days Plants 2021, 10, 2824 2 of 18 at 4 • C, and is associated with changes in the abundance of multiple plasma membrane proteins at 2-6 days [6]. In turn, these proteins are involved in complex crosstalk networks that prime the Brachypodium defensive response to a variety of abiotic and pathogenic stresses.…”