In order to survive severe winters without snow cover, boreal tree species have evolved superior freezing adaptation mechanisms. The freezing resistance of living cells in their aerial parts is well-controlled seasonally and exceeds -30°C in mid-winter. Although freeze–thawing can cause functional failure of their water transport system, boreal tree species have abilities to solve the problem and resume growth in spring. In this review, I introduce two topics on freezing responses of boreal tree species: (1) freezing adaptation mechanisms of xylem parenchyma cells which avoid intracellular freezing by deep supercooling, and (2) winter embolism formation mechanisms by which hydraulic architectures, vessels and tracheids, lose their function. In addition, possible relationship between freezing behavior of xylem parenchyma and their roles in embolism repair in early-spring is discussed.

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Abscisic acid (ABA) is a plant hormone important for environmental stress responses. We identified the Raf-like protein kinase ARK as an upstream regulator of subclass III sucrose non-fermenting1-related kinase2, known as a key activator of ABA signaling in Physcomitrella patens. Phosphopeptide mapping revealed that ARK is phosphorylated at a specific Ser residue in the activation loop of the kinase domain. Substitution of the Ser residue to Ala caused a decrease in ABA-induced gene expression while that to Asp caused an enhancement of the gene expression. From these results, we conclude that phosphorylation of ARK is important for its kinase activity and a response to ABA.

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Larvae of the sleeping Chironomid have an ability of the extreme desiccation tolerance, so-called anhydrobiosis. Transcriptome data of Pv11 cells, the desiccation tolerant cultured cell line derived from the insect, were analyzed to identify anhydrobiosis-related lncRNA (long non-coding RNA). The lncRNA is known to transcriptionally regulate expression of other coding genes. As a result, we identified a total of 1586 putative lncRNAs, of which 5 lncRNAs might correlate to expressions of anhydrobiosis-related genes.

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Freezing tolerance in plants is enhanced on exposure to non-freezing temperatures. This phenomenon is called cold acclimation (CA). It has been reported that Ca2+ acts as a second messenger of the CA process, especially in response to cooling. Many studies have focused on the effect of CA with rapid cooling and constant low-temperature on gene expression and freezing tolerance. An experimental system of 1°C temperature fluctuating and day-and-night cyclic cold acclimation (FC–CA) was developed. This system was applied to three Arabidopsis accessions: Col- 0, Fl-3, and Kas-1. Although the average temperature was higher in FC–CA than CA, both treatments tended to induce almost the same freezing tolerance. To observe the role of the Ca2+ signal during both CA treatments, Ca2+ channel blockers, ruthenium red, and LaCl3, were sprayed. Results demonstrated that the chemicals inhibited the increase of freezing tolerance, especially for FC–CA in Col-0 and Kas-1. These results suggest that FC–CA may induce more frequent Ca2+ signals and enhance freezing tolerance. Moreover, FC–CA revealed the possibility that the Ca2+ signaling pathway depends on the accessions. Thus, FC–CA experiment may help to determine the effect of Ca2+ signals during CA in the field with temperature fluctuation.

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Antifreeze protein (AFP) inhibits the ice-crystal growth. Several fishes, insects, plants, and fungi can improve survival using AFP in subzero environments. Although many biophysical and biochemical characteristics of AFP are identified, it is poorly understood how AFP within living animals contributes to the cold tolerance behavior. Here, we observe the in vivo AFP function for cold tolerance by using the nematode Caenorhabditis elegans. Using transgenic worms expressing AFP at cell-specifically, we show that a fungal AFP contributes to increase the survival rate and protect the cells from several cold shocks. Furthermore, we also show that thermal hysteresis which is a parameter for an antifreeze activity has a key for in vivo AFP function.

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Celosia cristata callus was induced from segments of a young seedling and two types of culture lines, one is red pigment producing callus (red callus), the other is non-pigment producing callus (white callus), were established by repeated selection procedure. The two types callus cells were cryopreserved by pre-freezing method. Although white callus could be successfully cryopreserved using DMSO and glucose as cryoprotectants, red callus was difficult for cryopreservation. By using ethylene glycol instead of DMSO red callus could be cryopreserved although its survival rate was low. Pre-culture with high concentrations of sucrose was effective for white callus preservation.

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Characteristics of deep root system of drought tolerance in cowpea (Vigna unguiculata (L.) Walp) were evaluated by using ‘basket’ method and gene analysis. The ratio of the deep root number and dry weight per total lateral roots in drought tolerant cultivar ‘205’ was significantly higher than those of drought sensitive ‘555’. These results suggested that drought tolerant cultivar has deeper rooting traits than the sensitive one. The drought tolerance of each cultivar was mainly regulated by BRrelated gene, BZR1, which relates to lateral root development. Furthermore, the expression of genes such as MSBP1 and Hy5 as well as BZR1 of lateral roots was significantly enhanced in the deep roots compared to the shallow roots in the drought tolerant cultivar. Taken together, the deep root system in cowpea may contribute to its drought tolerance through enhanced water uptake from deep soil layer.

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