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

Integrated Physiological and Transcriptomic Analyses Revealed Improved Cold Tolerance in Cucumber (Cucumis sativus L.) by Exogenous Chitosan Oligosaccharide

Cucumber (Cucumis sativus L.), sensitive to cold stress, is one of the most economically important vegetables. Here, we systematically investigated the roles of exogenous glycine betaine, chitosan, and chitosan oligosaccharide in alleviating cold stress in cucumber seedlings. The results showed that 50 mg·L−1 chitosan oligosaccharide had the best activity. It effectively increases plant growth, chlorophyll content, photosynthetic capacity, osmotic regulatory substance content, and antioxidant enzyme activities while reducing relative electrical conductivity and malondialdehyde levels in cucumber seedlings under cold stress. To reveal the protective effects of chitosan oligosaccharide in cold stress, cucumber seedlings pretreated with 50 mg·L−1 chitosan oligosaccharide were sampled after 0, 3, 12, and 24 h of cold stress for transcriptome analysis, with distilled water as a control. The numbers of differentially expressed genes in the four comparison groups were 656, 1274, 1122, and 957, respectively. GO functional annotation suggested that these genes were mainly involved in “voltage-gated calcium channel activity”, “carbohydrate metabolic process”, “jasmonic acid biosynthetic”, and “auxin response” biological processes. KEGG enrichment analysis indicated that these genes performed important functions in “phenylpropanoid biosynthesis”, “MAPK signaling pathway—plant”, “phenylalanine metabolism”, and “plant hormone signal transduction.” These findings provide a theoretical basis for the use of COS to alleviate the damage caused by cold stress in plant growth and development.

Cold-inducible promoter-driven knockdown of Brachypodium antifreeze proteins confers freezing and phytopathogen susceptibility

The model forage crop, Brachypodium distachyon, has a cluster of ice recrystallization inhibition (BdIRI) genes, which encode antifreeze proteins that function by adsorbing to ice crystals and inhibiting their growth. The genes were targeted for knockdown using a cold-induced promoter from rice (prOsMYB1R35) to drive miRNA. The transgenic lines showed no apparent pleiotropic developmental defects but had reduced antifreeze activity as assessed by assays for ice-recrystallization inhibition, thermal hysteresis, electrolyte leakage, and leaf infrared thermography. Strikingly, the number of cold-acclimated transgenic plants that survived freezing at -8°C was reduced by half or killed entirely, depending on the line, compared with cold-acclimated wild type plants. In addition, more leaf damage was apparent at subzero temperatures in knockdowns after infection with an ice nucleating pathogen, Pseudomonas syringae. Although antifreeze proteins have been studied for almost 60 years, this is the first unequivocal demonstration of their function by knockdown in any organism, and their dual contribution to freeze protection as well as pathogen susceptibility, independent of obvious developmental defects. These proteins are thus of potential interest in a wide range of biotechnological applications from cryopreservation, to frozen product additives, to the engineering of transgenic crops with enhanced pathogen and freezing tolerance.

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

Shifts in microbiota undoubtedly support host plants faced with abiotic stress, including low temperatures. Cold-resistant perennials prepare for freeze stress during a period of cold acclimation that can be mimicked by transfer from growing conditions to a reduced photoperiod and a temperature of 4 °C for 2-6 days. After cold acclimation, the model cereal, Brachypodium distachyon, was characterized using metagenomics supplemented with amplicon sequencing (16S ribosomal RNA gene fragments and an internal transcribed spacer region). The bacterial and fungal rhizosphere remained largely unchanged from that of non-acclimated plants. However, leaf samples representing bacterial and fungal communities of the endo- and phyllospheres significantly changed. For example, a plant-beneficial bacterium, Streptomyces sp. M2, increased more than 200-fold in relative abundance in cold-acclimated leaves, and this increase correlated with a striking decrease in the abundance of Pseudomonas syringae (from 8% to zero). This change is of consequence to the host, since P. syringae is a ubiquitous ice-nucleating phytopathogen responsible for devastating frost events in crops. We posit that a responsive above-ground bacterial and fungal community interacts with Brachypodium's low temperature and anti-pathogen signalling networks to help ensure survival in subsequent freeze events, underscoring the importance of inter-kingdom partnerships in the response to cold stress.