Ice Encasement Injury to Microsomal Membranes from Winter Wheat Crowns : I. Comparison of Membrane Properties after Lethal Ice Encasement and during a Post-Thaw Period

The functional and physical properties of cellular membranes isolated from Triticum aestivum, cvs Norstar and Fredrick, were altered coincident with changes in composition after a lethal ice-encasement stress and further during a 6 hour post-thaw period. Crowns encased in ice for a duration which inhibited regrowth, exhibited enhanced rates of electrolyte leakage. Furthermore, the recovery of total microsomal protein and phospholipid declined, suggesting that some membrane degradation had been induced during the anoxic stress. The microviscosity of microsomes and liposomes prepared from such membranes increased during stress, and this was correlated with a 2- to 4-fold increase in the free fatty acid levels in the microsomal fraction. There was, however, only a relatively minor change in fatty acid unsaturation during the ice-encasement stress. The process continued during a 6 hour aerobic post-thaw treatment, but the pattern was somewhat different. During this phase, the leakage of electrolytes was further increased and the recovery of microsomal protein and phospholipid continued to decline, indicating general degradation; but, in contrast to the anoxic phase, the degree of fatty acid unsaturation declined markedly, indicating lipid peroxidation.     

Early and late heat shock proteins in wheats and other cereal species

Coleoptiles and roots of 3-day-old seedlings from five cereal species (Triticum aestivum L., T. durum Desf., Hordeum vulgare L., Secale cereale L., and Triticale) respond to heat shock at 40°C by synthesizing a new set of 13 strong bands (as revealed by one-dimensional sodium dodecyl sulfate gel electrophoresis) as well as some 20°C proteins. Heat shock proteins (HSPs) belong to three different size groups: high molecular mass HSPs in the 103 to 70 kilodalton range, intermediate molecular mass HSPs in the 62 to 32 kilodalton range, and low molecular mass HSPs about 17 to 16 kilodalton in size. At the beginning of the heat shock coleoptiles show a reduced ability to synthesize intermediate molecular mass HSPs but after 4 hours at 40°C they exhibit fully developed HSP patterns identical to that found in roots. Synthesis of early HSPs declines after 7 hours of treatment followed by the appearance of a new set of 12 protein bands (late HSPs) in the ranges 99 to 83, 69 to 35, and 15 to 14 kilodaltons. After 12 hours at 40°C, three other late HSPs of 89, 45, and 38 kilodalton are induced. The induction of late HSPs after 7 hours at 40°C appears to be associated with an enhancement of radioactive methionine incorporation into proteins.     

An evaluation of the recycling in measurements of photorespiration

All measurements of photorespiration and gross photosynthesis in leaves, whether using isotopes or not, are underestimated because of the recycling of O₂ or CO₂. On the basis of a simple diffusion model, we propose a method for the calculation of the recycling and the corresponding underestimation of the measurements. This procedure can be applied when the stomatal resistance is known, and allows for a correction of certain results in the literature. It is found that measurements of the photorespiratory CO₂ release are usually underestimated by 20 to 100%, which sets the estimated rate of CO₂ photorespired at 30 to 50% of the net photosynthesis in C3 plants under normal conditions. In water stress studies, the correction of the photorespiration is still more important (1.5-3.3) because the stomata are closed more. Analysis of the diffusion of O₂ shows that its recycling is low and that the underestimation of photorespiration with ¹⁸O₂ is negligible.     

Replication of cauliflower mosaic virus DNA in leaves and suspension culture protoplasts of cotton

Cauliflower mosaic virus (CaMV) replicated in protoplasts and in inoculated leaves of the non-host, cotton (Gossypium hirsutum, L.). Protoplasts prepared from suspension-cultured cotton cells were infected by incubation with liposome-encapsulated CaMV virions. During a 1-week culture period the amount of CaMV nucleic acid as detected by nucleic acid hybridization in the protoplasts increased significantly regardless of whether or not the protoplasts contained vacuoles. In leaves inoculated with CaMV virions or CaMV DNA, viral DNA sequences were found by leaf skeleton hybridization to be located in small circular areas. DNA extracted from ultracentrifugal pellets of homogenates of inoculated leaves contained circular, gapped CaMV DNA only when inocula contained CaMV virions, CaMV DNA, or partial nested dimer CaMV plasmid DNA. When plants had been heavily watered, the CaMV DNA recovered contained degraded CaMV DNA. The results suggest that the host range limitation for CaMV is not due to an inability to replicate or spread locally in inoculated leaves.     

The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O₂ evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.     

Influx of na, k, and ca into roots of salt-stressed cotton seedlings : effects of supplemental ca

High Na⁺ concentrations may disrupt K⁺ and Ca²⁺ transport and interfere with growth of many plant species, cotton (Gossypium hirsutum L.) included. Elevated Ca²⁺ levels often counteract these consequences of salinity. The effect of supplemental Ca²⁺ on influx of Ca²⁺, K⁺, and Na⁺ in roots of intact, salt-stressed cotton seedlings was therefore investigated. Eight-day-old seedlings were exposed to treatments ranging from 0 to 250 millimolar NaCl in the presence of nutrient solutions containing 0.4 or 10 millimolar Ca²⁺. Sodium influx increased proportionally to increasing salinity. At high external Ca²⁺, Na⁺ influx was less than at low Ca²⁺. Calcium influx was complex and exhibited two different responses to salinity. At low salt concentrations, influx decreased curvilinearly with increasing salt concentration. At 150 to 250 millimolar NaCl, ⁴⁵Ca²⁺ influx increased in proportion to salt concentrations, especially with high Ca²⁺. Potassium influx declined significantly with increasing salinity, but was unaffected by external Ca²⁺. The rate of K⁺ uptake was dependent upon root weight, although influx was normalized for root weight. We conclude that the protection of root growth from salt stress by supplemental Ca²⁺ is related to improved Ca-status and maintenance of K⁺/Na⁺ selectivity.     

Common signal transduction system shared by STE2 and STE3 in haploid cells of Saccharomyces cerevisiae: autocrine cell-cycle arrest results from forced expression of STE2

Induction of STE2 expression using the GAL1 promoter both in a wild-type MATalpha strain and in a MATalpha ste3 strain caused transient cell-cycle arrest and changes in morphology ('shmoo'-like phenotype) in a manner similar to alpha cells responding to alpha-factor. In addition, STE2 expressed in a MATalp[ha ste3 mutant allowed the cell to conjugate with alpha cells but at an efficiency lower than that of wil-type alpha cells. This result indicates that signal(s) generated by alpha-factor in alpha cells can be substituted by signal(s) generated by the interaction of alpha-factor with the expressed STE2 product. When STE2 or STE3 was expressed in a matalpha1 strain (insensitive to both alpha- and a-factors), the cell became sensitive to alpha- or a-factor, respectively, and resulted in morphological changes. These results suggest that STE2 and STE3 are the sole determinants for alpha-factor and a-factor sensitivity, respectively, in this strain. On the other hand, expression of STE2 in an a/alpha diploid cell did not affect the alpha-factor insensitive phenotype. Haploid-specific components may be necessary to transduce the alpha-factor signal. These results are consistent with the idea that STE2 encodes an alpha-factor receptor and STE3 encodes an a-factor receptor, and suggest that both alpha- and a-factors may generate an exchangeable signal(s) within haploid cells.     

Host-specific regulation of nodulation genes in Rhizobium is mediated by a plant-signal, interacting with the nodD gene product

We have identified a nodD gene from the wide host-range Rhizobium strain MPIK3030 (termed nodD1) which is essential for nodulation on Macroptilium atropurpureum (siratro). Experiments with nodA–lacZ gene fusions demonstrate that the MPIK3030 nodD1 regulates expression of the nodABC genes. Additionally, we used nodC–lacZ fusions of Rhizobium meliloti to show that the MPIK3030 nodD1 gene induces expression of these fusions by interacting with plant factors from siratro and from the non-host Medicago sativa (alfalfa). The R. meliloti nodD genes, however, only interact with alfalfa exudate. In line with these results, no complementation of MPIK3030 nodD1 mutants could be obtained on siratro with the R. meliloti nodD genes, while the MPIK3030 nodD1 can complement nodD mutants of R. meliloti on alfalfa. Furthermore, R. meliloti transconjugants harbouring the MPIK3030 nodD1 efficiently nodulate the illegitimate host siratro. When compared with other nodD sequences, the amino acid sequence of the MPIK3030 nodD1 shows a conserved aminoterminus, whereas the carboxy-terminus of the putative gene product diverges considerably. Studies on a chimeric MPIK3030/R. meliloti nodD gene indicates that the carboxy-terminal region is responsible for the interaction with plant factor(s) and may have evolved in different rhizobia specifically to interact with plant–host factors.