One group of antifreeze proteins (AFPs) is composed of two chitinases that accumulate in the apoplast of winter rye leaves during cold acclimation. In this study, the 28- and 35-kDa chitinase-AFPs were localized in nonacclimated and cold-acclimated rye leaves by immunoelectron microscopy with an antiserum produced against the purified winter rye 35-kDa chitinase-AFP. In cold-acclimated winter rye leaves, labelled chitinase-AFPs were abundant in the walls of epidermal, parenchymal sheath and mesophyll cells and xylem vessels, while less label was present in walls of vascular parenchyma cells. In contrast, chitinase labelling was essentially absent in the nonacclimated cells except in xylem vessels. As shown by RNA blotting, the transcripts of chitinase-AFPs accumulated to a high level in rye leaves during cold acclimation, to a lesser extent in crowns and were not detectable in roots. mRNA transcripts of the 28-kDa chitinase-AFP were localized in rye leaves by in situ hybridization. The chitinase-AFP transcripts were found in the same cell types as the protein itself. We conclude that all metabolically active cell types in cold-acclimated winter rye leaves and crowns are able to synthesize chitinase-AFPs and secrete them into adjacent cell walls, where they may interact with ice to delay its propagation through the plant and modify its growth.
"Interestingly, recent studies support that cold signals are closely linked to pathogen resistance. A group of pathogenesis-related (PR) proteins, including endochitinases, b-1,3-glucanases, and thaumatin-like proteins, is synthesized after exposure to low temperatures in overwintering grasses (Hon et al., 1995; Pihakaski-Maunsbach et al., 2001). The PR proteins not only play a role in pathogen resistance but are also able to resist freezing (Snider et al., 2000). "
[Show abstract][Hide abstract] ABSTRACT: Cold signals interact with other environmental cues to modulate plant developmental processes. Recent studies have shown that many Pathogenesis-Related (PR) genes are induced and disease resistance is enhanced after exposure to low temperatures, linking cold signals with pathogenesis in plants. However, the underlying molecular mechanisms and signaling schemes are largely unknown. Here, we demonstrate that cold stimulates proteolytic activation of a plasma membrane-tethered NAC (NAM/ATAF1/2/CUC2) transcription factor NTL6. The transcriptionally active NTL6 protein enters the nucleus, where it induces a subset of PR genes by directly binding to a conserved sequence in the promoters of cold-responsive PR genes, such as PR1, PR2, and PR5. While transgenic plants overexpressing an active NTL6 form exhibited enhanced disease resistance, RNAi plants with reduced NTL6 activity were more susceptible to pathogen infection at low temperatures. Accordingly, cold induction of PR1 disappeared in the RNAi plants. Consistent with the close relationship between cold and pathogenesis, cold-acclimated plants showed enhanced resistance to pathogen infection. In this signaling cascade, controlled activation of the membrane-tethered, dormant NTL6 transcription factor serves as a molecular link that incorporates cold signals into pathogen resistance responses. However, the NTL6-mediated cold induction of the PR genes is independent of salicylic acid (SA). The PR genes were still induced by SA in the NTL6 RNAi plants. Cold regulation of the PR genes through the membrane-mediated transcriptional control is thought to be an adaptive process that ensures quick plant responses to incoming pathogens that frequently occur during cold seasons.
The Plant Journal 11/2009; 61(4):661-71. DOI:10.1111/j.1365-313X.2009.04091.x · 5.97 Impact Factor
"To become freezing-tolerant, overwintering cereals must first undergo a complex adjustment of plant morphology and metabolism known as cold acclimation . As winter rye (Secale cereale) plants acclimate to cold temperatures, they secrete pathogenesis-related (PR) proteins, including endochitinases, b-1,3-gluca- nases, and thaumatin-like proteins into intercellular spaces and the xylem (Hon et al., 1995; Antikainen et al., 1996; Pihakaski-Maunsbach et al., 1996, 2001). These apoplastic PR proteins are thought to provide a preemptive defense against psychrophilic fungi, such as snow molds (Snider et al., 2000), that are able to infect overwintering cereals at low temperatures and reduce their winter survival (Ergon et al., 1998). "
[Show abstract][Hide abstract] ABSTRACT: During cold acclimation, winter rye (Secale cereale) plants accumulate pathogenesis-related proteins that are also antifreeze proteins (AFPs) because they adsorb onto ice and inhibit its growth. Although they promote winter survival in planta, these dual-function AFPs proteins lose activity when stored at subzero temperatures in vitro, so we examined their stability in solutions containing CaCl2, MgCl2, or NaCl. Antifreeze activity was unaffected by salts before freezing, but decreased after freezing and thawing in CaCl2 and was recovered by adding a chelator. Ca2+ enhanced chitinase activity 3- to 5-fold in unfrozen samples, although hydrolytic activity also decreased after freezing and thawing in CaCl2. Native PAGE, circular dichroism, and Trp fluorescence experiments showed that the AFPs partially unfold after freezing and thawing, but they fold more compactly or aggregate in CaCl2. Ruthenium red, which binds to Ca(2+)-binding sites, readily stained AFPs in the absence of Ca2+, but less stain was visible after freezing and thawing AFPs in CaCl2. We conclude that the structure of AFPs changes during freezing and thawing, creating new Ca(2+)-binding sites. Once Ca2+ binds to those sites, antifreeze activity, chitinase activity and ruthenium red binding are all inhibited. Because free Ca2+ concentrations are typically low in the apoplast, antifreeze activity is probably stable to freezing and thawing in planta. Ca2+ may regulate chitinase activity if concentrations are increased locally by release from pectin or interaction with Ca(2+)-binding proteins. Furthermore, antifreeze activity can be easily maintained in vitro by including a chelator during frozen storage.
"Recent studies have shown that a class of pathogenesis-related (PR) proteins such as endo-chitinase and b-1,3-glucanase accumulated in cold-acclimated rye apoplast [11,16]. Cold-induced accumulation of this class of PR proteins has been correlated with enhanced resistance to pathogens  . Interestingly, cold-induced PR proteins from rye showed antifreeze activity and are thought to be bifunctional in cold acclimation. "
[Show abstract][Hide abstract] ABSTRACT: A novel cDNA clone, Tad1, was isolated from crown tissue of winter wheat after differential screening of cold acclimation-induced genes. The Tad1 cDNA encoded a 23kDa polypeptide with a potential N-terminal signal sequence. The putative mature sequence showed striking similarity to plant defensins or gamma-thionins, representing low molecular size antipathogenic polypeptides. High levels of Tad1 mRNA accumulation occurred within one day of cold acclimation in crown tissue and the level was maintained throughout 14 days of cold acclimation. Similar rapid induction was observed in young seedlings treated with low temperature but not with exogenous abscisic acid. In contrast to defensins from other plant species, neither salicylic acid nor methyl jasmonate induced expression of Tad1. The recombinant mature form of TAD1 polypeptide inhibited the growth of the phytopathogenic bacteria, Pseudomonas cichorii; however, no antifreeze activity was detected. Collectively, these data suggested that Tad1 is induced in cold-acclimated winter wheat independent of major defense signaling(s) and is involved in low temperature-induced resistance to pathogens during winter hardening.
Biochemical and Biophysical Research Communications 11/2002; 298(1):46-53. DOI:10.1016/S0006-291X(02)02391-4 · 2.30 Impact Factor
Irina Fridlyanskaya, Larisa Alekseenko, Nikolay Nikolsky
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