Matsuo Uemura

Cold Acclimation of Arabidopsis thaliana (Effect on Plasma Membrane Lipid Composition and Freeze-Induced Lesions)

Maximum freezing tolerance of Arabidopsis thaliana L. Heyn (Columbia) was attained after 1 week of cold acclimation at 2[deg]C. During this time, there were significant changes in both the lipid composition of the plasma membrane and the freeze-induced lesions that were associated with injury. The proportion of phospholipids increased from 46.8 to 57.1 mol% of the total lipids with little change in the proportions of the phospholipid classes. Although the proportion of di-unsaturated species of phosphatidylcholine and phosphatidylethanolamine increased, mono-unsaturated species were still the preponderant species. The proportion of cerebrosides decreased from 7.3 to 4.3 mol% with only small changes in the proportions of the various molecular species. The proportion of free sterols decreased from 37.7 to 31.2 mol%, but there were only small changes in the proportions of sterylglucosides and acylated sterylglucosides. Freezing tolerance of protoplasts isolated from either nonacclimated or cold-acclimated leaves was similar to that of leaves from which the protoplasts were isolated (-3.5[deg]C for nonacclimated leaves; -10[deg]C for cold-acclimated leaves). In protoplasts isolated from nonacclimated leaves, the incidence of expansion-induced lysis was [less than or equal to]10% at any subzero temperature. Instead, freezing injury was associated with formation of the hexagonal II phase in the plasma membrane and subtending lamellae. In protoplasts isolated from cold-acclimated leaves, neither expansion-induced lysis nor freeze-induced formation of the hexagonal II phase occurred. Instead, injury was associated with the “fracture-jump lesion,” which is manifested as localized deviations of the plasma membrane fracture plane to subtending lamellae. The relationship between the freeze-induced lesions and alterations in the lipid composition of the plasma membrane during cold acclimation is discussed.

A contrast of the plasma membrane lipid composition of oat and rye leaves in relation to freezing tolerance

The lipid composition of the plasma membrane isolated from leaves of spring oat (Avena sativa L. cv Ogle) was vastly different from that of winter rye (Secale cereale L. cv Puma). The plasma membrane of spring oat contained large proportions of phospholipids (28.8 mol% of the total lipids), cerebrosides (27.2 mol%), and acylated sterylglucosides (27.3 mol%) with lesser proportions of free sterols (8.4 mol%) and sterylglucosides (5.6 mol%). In contrast, the plasma membrane of winter rye contained a greater proportion of phospholipids (36.6 mol%), and there was a lower proportion of cerebrosides (16.4 mol%); free sterols (38.1 mol%) were the predominant sterols, with lesser proportions of sterylglucosides (5.6 mol%) and acylated sterylglucosides (2.9 mol%). Although the relative proportions of individual phospholipids, primarily phosphatidylcholine and phosphatidylethanolamine, and the molecular species of these two phospholipids were similar in oat and rye, the relative proportions of di-unsaturated species of these two phospholipids were substantially lower in oat than in rye. The relative proportions of sterol species in oat were different from those in rye; the molecular species of cerebrosides were similar in oat and rye, with only slight differences in the proportions of the individual species. After 4 weeks of cold acclimation, the proportion of phospholipids increased significantly in both oat (from 28.8 to 36.8 mol%) and rye (from 36.6 to 43.3 mol%) as a result of increases in the proportions of phosphatidylcholine and phosphatidylethanolamine. For both oat and rye, the relative proportions of di-unsaturated species increased after cold acclimation, but the increase was greater in rye than in oat. In both oat and rye, this increase occurred largely during the first week of cold acclimation. During the 4 weeks of cold acclimation, there was a progressive decrease in the proportion of cerebrosides in the plasma membrane of rye (from 16.4 to 10.5 mol%), but there was only a small decrease in oat (from 27.2 to 24.2 mol%). In both oat and rye, there were only small changes in the proportions of free sterols and sterol derivatives during cold acclimation. Consequently, the proportions of both acylated sterylglucosides and cerebrosides remained substantially higher in oat than in rye after cold acclimation. The relationship between these differences in the plasma membrane lipid composition of oat and rye and their freezing tolerance is presented.

Auxin Response in Arabidopsis under Cold Stress: Underlying Molecular Mechanisms

To understand the mechanistic basis of cold temperature stress and the role of the auxin response, we characterized root growth and gravity response of Arabidopsis thaliana after cold stress, finding that 8 to 12 h at 4°C inhibited root growth and gravity response by ∼50%. The auxin-signaling mutants axr1 and tir1, which show a reduced gravity response, responded to cold treatment like the wild type, suggesting that cold stress affects auxin transport rather than auxin signaling. Consistently, expression analyses of an auxin-responsive marker, IAA2-GUS, and a direct transport assay confirmed that cold inhibits root basipetal (shootward) auxin transport. Microscopy of living cells revealed that trafficking of the auxin efflux carrier PIN2, which acts in basipetal auxin transport, was dramatically reduced by cold. The lateral relocalization of PIN3, which has been suggested to mediate the early phase of root gravity response, was also inhibited by cold stress. Additionally, cold differentially affected various protein trafficking pathways. Furthermore, the inhibition of protein trafficking by cold is independent of cellular actin organization and membrane fluidity. Taken together, these results suggest that the effect of cold stress on auxin is linked to the inhibition of intracellular trafficking of auxin efflux carriers.

A Comparison of Freezing Injury in Oat and Rye: Two Cereals at the Extremes of Freezing Tolerance

A detailed analysis of cold acclimation of a winter rye (Secale cereale L. cv Puma), a winter oat (Avena sativa L. cv Kanota), and a spring oat cultivar (Ogle) revealed that freezing injury of leaves of nonacclimated seedlings occurred at -2[deg]C in both the winter and spring cultivars of oat but did not occur in winter rye leaves until after freezing at -4[deg]C. The maximum freezing tolerance was attained in all cultivars after 4 weeks of cold acclimation, and the temperature at which 50% electrolyte leakage occurred decreased to -8[deg]C for spring oat, -10[deg]C for winter oat, and -21[deg]C for winter rye. In protoplasts isolated from leaves of nonacclimated spring oat, expansion-induced lysis was the predominant form of injury over the range of -2 to -4[deg]C. At temperatures lower than -4[deg]C, loss of osmotic responsiveness, which was associated with the formation of the hexagonal II phase in the plasma membrane and subtending lamellae, was the predominant form of injury. In protoplasts isolated from leaves of cold-acclimated oat, loss of osmotic responsiveness was the predominant form of injury at all injurious temperatures; however, the hexagonal II phase was not observed. Rather, injury was associated with the occurrence of localized deviations of the plasma membrane fracture plane to closely appressed lamellae, which we refer to as the “fracture-jump lesion.” Although the freeze-induced lesions in the plasma membrane of protoplasts of spring oat were identical with those reported previously for protoplasts of winter rye, they occurred at significantly higher temperatures that correspond to the lethal freezing temperature.

Alterations in detergent-resistant plasma membrane microdomains in Arabidopsis thaliana during cold acclimation.

Microdomains in the plasma membrane (PM) have been proposed to be involved in many important cellular events in plant cells. To understand the role of PM microdomains in plant cold acclimation, we isolated the microdomains as detergent-resistant plasma membrane fractions (DRMs) from Arabidopsis seedlings and compared lipid and protein compositions before and after cold acclimation. The DRM was enriched in sterols and glucocerebrosides, and the proportion of free sterols in the DRM increased after cold acclimation. The protein-to-lipid ratio in the DRM was greater than that in the total PM fraction. The protein amount recovered in DRMs decreased gradually during cold acclimation. Cold acclimation further resulted in quantitative changes in DRM protein profiles. Subsequent mass spectrometry and Western blot analyses revealed that P-type H(+)-ATPases, aquaporins and endocytosis-related proteins increased and, conversely, tubulins, actins and V-type H(+)-ATPase subunits decreased in DRMs during cold acclimation. Functional categorization of cold-responsive proteins in DRMs suggests that plant PM microdomains function as platforms of membrane transport, membrane trafficking and cytoskeleton interaction. These comprehensive changes in microdomains may be associated with cold acclimation of Arabidopsis.

Calcium-Dependent Freezing Tolerance in Arabidopsis Involves Membrane Resealing via Synaptotagmin SYT1

Plant freezing tolerance involves the prevention of lethal freeze-induced damage to the plasma membrane. We hypothesized that plant freezing tolerance involves membrane resealing, which, in animal cells, is accomplished by calcium-dependent exocytosis following mechanical disruption of the plasma membrane. In Arabidopsis thaliana protoplasts, extracellular calcium enhanced not only freezing tolerance but also tolerance to electroporation, which typically punctures the plasma membrane. However, calcium did not enhance survival when protoplasts were exposed to osmotic stress that mimicked freeze-induced dehydration. Calcium-dependent freezing tolerance was also detected with leaf sections in which ice crystals intruded into tissues. Interestingly, calcium-dependent freezing tolerance was inhibited by extracellular addition of an antibody against the cytosolic region of SYT1, a homolog of synaptotagmin known to be a calcium sensor that initiates exocytosis. This inhibition indicates that the puncture allowing the antibody to flow into the cytoplasm occurs during freeze/thawing. Thus, we propose that calcium-dependent freezing tolerance results from resealing of the punctured site. Protoplasts or leaf sections isolated from Arabidopsis SYT1-RNA interference (RNAi) plants lost calcium-dependent freezing tolerance, and intact SYT1-RNAi plants had lower freezing tolerance than control plants. Taken together, these findings suggest that calcium-dependent freezing tolerance results from membrane resealing and that this mechanism involves SYT1 function.

Proteomics of the rice cell: systematic identification of the protein populations in subcellular compartments

Despite recent progress in sequencing the complete genome of rice (Oryza sativa), the proteome of this species remains poorly understood. To extend our knowledge of the rice proteome, the subcellular compartments, which include plasma membranes (PM), vacuolar membranes (VM), Golgi membranes (GM), mitochondria (MT), and chloroplasts (CP), were purified from rice seedlings and cultured suspension cells. The proteins of each of these compartments were then systematically analyzed using two-dimensional (2D) electrophoresis, mass spectrometry, and Edman sequencing, followed by database searching. In all, 58 of the 464 spots detected by 2D electrophoresis in PM, 43 of the 141 spots in VM, 46 of the 361 spots in GM, 146 in the 672 spots in MT, and 89 of the 252 spots in CP could be identified by this procedure. The characterized proteins were found to be involved in various processes, such as respiration and the citric acid cycle in MT; photosynthesis and ATP synthesis in CP; and antifungal defense and signal systems in the membranes. Edman degradation revealed that 60–98% of N-terminal sequences were blocked, and the ratios of blocked to unblocked proteins in the proteomes of the various subcellular compartments differed. The data on the proteomes of subcellular compartments in rice will be valuable for resolving questions in functional genomics as well as for genome-wide exploration of plant function.

Freezing Sensitivity in the sfr4 Mutant of Arabidopsis Is Due to Low Sugar Content and Is Manifested by Loss of Osmotic Responsiveness

Protoplasts were tested to determine whether the freezing sensitivity of the sfr4 (sensitive to freezing) mutant of Arabidopsis was due to the mutants deficiency in soluble sugars after cold acclimation. When grown under nonacclimated conditions,sfr4 protoplasts possessed freezing tolerance similar to that of wild type, with the temperature at which 50% of protoplasts are injured (LT50) of −4.5°C. In both wild-type andsfr4 protoplasts, expansion-induced lysis was the predominant lesion between −2°C and −4°C, but its incidence was low (approximately 10%); below −5°C, loss of osmotic responsiveness (LOR) was the predominant lesion. After cold acclimation, the LT50 was decreased to only −5.6°C forsfr4 protoplasts, compared with −9.1°C for wild-type protoplasts. Although expansion-induced lysis was precluded in both types of protoplasts, the sfr4 protoplasts remained susceptible to LOR. After incubation of seedlings in Suc solution in the dark at 2°C, freezing tolerance and the incidence of freeze-induced lesions in sfr4 protoplasts were examined. The freezing tolerance of isolated protoplasts (LT50 of −9°C) and the incidence of LOR were now similar for wild type and sfr4. These results indicate that the freezing sensitivity of cold-acclimated sfr4 is due to its continued susceptibility to LOR (associated with lyotropic formation of the hexagonal II phase) and associated with the low sugar content of its cells.

Cold Acclimation in Plants:Relationship Between the Lipid Composition and the Cryostability of the Plasma Membrane

Cold acclimation of plants requires an orchestration of many different, seemingly disparate processes. However, many of these processes that occur during cold acclimation ultimately contribute to the increased stability of cellular membranes during freeze-induced dehydration-the destabilization of which is the primary cause of the freezing injury. Among all cellular membranes, the plasma membrane is of primary importance to maintain its structural integrity because of the central role it plays during a freeze/ thaw cycle. We will describe here that there is a close association between the alterations of the plasma membrane lipid composition and the difference in the incidence of freeze-induced membrane lesions during cold acclimation. The stability of the plasma membrane during freezeinduced dehydration is also affected by factors associated with the endomembranes (the chloroplast envelope lipid composition) and the cytoplasm (the accumulation of sugars and the cold-regulated gene expression). Collectively, these results indicate that the structural integrity of the plasma membrane during freeze-induced dehydration is maintained by a complex but well-coordinated manner.

Effect of Cold Acclimation on the Lipid Composition of the Inner and Outer Membrane of the Chloroplast Envelope Isolated from Rye Leaves

The lipid composition of the inner and outer membranes of the chloroplast envelope isolated from winter rye (Secale cereale L. cv Puma) leaves was characterized before and after cold acclimation. In nonacclimated leaves the inner membrane contained high proportions of monogalactosyldiacylglycerols (MGDG, 47.9 mol% of the total lipids) and digalactosyldiacylglycerols (DGDG, 31.1 mol%) and a low proportion of phosphatidylcholine (PC, 8.1 mol%). The outer membrane contained a similar proportion of DGDG (30.0 mol%); however, the proportion of MGDG was much lower (20.1 mol%) and the proportion of PC was much higher (31.5 mol%). After 4 weeks of cold acclimation, the proportions of these lipid classes were significantly altered in both of the inner and outer membranes. In the inner membrane the proportion of MGDG decreased (from 47.9 to 38.4 mol%) and the proportion of DGDG increased (from 31.1 to 39.3 mol%), with only a slight change in the proportion of PC (from 8.1 to 8.8 mol%). In the outer membrane MGDG decreased from 20.1 to 14.8 mol%, DGDG increased from 30.0 to 39.9 mol%, and PC decreased from 31.5 to 25.4 mol%. Thus, both before and after cold acclimation, the proportion of MGDG was much higher in the inner membrane than in the outer membrane. In contrast, the proportion of PC was higher in the outer membrane than in the inner membrane. The relationship between the lipid composition of the inner and outer membranes of the chloroplast envelope and freeze-induced membrane lesions is discussed.