Hisashi Okamoto

Bio: Hisashi Okamoto is an academic researcher from Nagoya University. The author has contributed to research in topics: Hypocotyl & Osmotic shock. The author has an hindex of 14, co-authored 36 publications receiving 542 citations.

Auxin changes both the extensibility and the yield threshold of the cell wall of vigna hypocotyls

Abstract: Elongation of plant stem is governed by two simultaneous processes: irreversible yielding of the cell wall and uptake of water. Among many candidates for the parameters that redulate and/or restrict growth, we focused on the mechanical properties of the cell wall and determined those parameters that govern of IAA-induced growth by means of the pressure-jump method combined with the pressure-probe technique

A pressure-jump method as a new tool in growth physiology for monitoring physiological wall extensibility and effective turgor

Abstract: Nouvelle methode pour determiner l'extensibilite physiologique de la paroi cellulaire en un temps tres court: la pression de turgescence est modifiee de facon hydraulique par une methode de perfusion du xylene

Effect of infection by Phytophthora infestans on the membrane potential of potato cells

Abstract: The membrane potential (Ψm) of potato cells (cultivar Rishiri) infected by incompatible and compatible races of Phytophthora infestans was measured, while the infection process of the measured cells was observed under a microscope. The passive (Ψd) and respiration dependent electrogenic (Ψp) components of Ψm were also analysed using the anoxia method. Cytoplasm often gathered around the inserted microelectrode and sometimes disturbed the measurement. However, it was shown that the experimental system was suitable for the long-term measurement of fm during the infection process. At the moment of penetration of the host cell wall by both the incompatible and compatible races, no signal such as an action potential was observed in the Tm. Infection by the incompatible race caused little change of Tm for a while after penetration, but |Ψd| then began to decrease. |ΨP| did not decrease, but rather increased and apparently compensated for the decrease in |Ψd|, resulting in little or slow reduction of |Ψm|. Later |Ψp| also decreased and Ψm began to depolarize gradually and at about one to several hours after penetration, rapid depolarization took place to reach a steady level of −45 to −55 mV before the cell content became granulated. In some instances, slow and steady depolarization was followed by cell death. Ψd and Ψp of the uninfected cell did not change for more than 10 h after the beginning of the measurement. The compatible race had no effect on either Ψp or Ψd at least within 24 h infection.

Effects of auxin and anoxia on the cell wall yield threshold determined by negative pressure jumps in segments of cowpea hypocotyl.

Abstract: In this paper, we report the determination of Y' not by such an extrapolation but directly by the negative pressure jump technique, in combination with the results of a pressure-probe study. The effects of auxin (IAA) and the metabolic control exerted by anoxia on Y' have also been investigated

Microbial water stress.

Abstract: A fellow of my acquaintance, on seeing a colleague drink undiluted water (55.5 molal), has been known to comment in disapproval that water at such a concentration should not be used for that purpose and that its main function is for putting around the outside of boats. He conceded that dilution with a little salt is acceptable for boats but for no other purpose. The proponent of this philosophy is not a biologist and it is unlikely that many biologists would accept his generalization without some qualification. Nevertheless, it is a point of view. Another point of view with which all biologists might not agree, at least initially, is one which I wish to advance in this review. It is that, notwithstanding the indispensability of water in living systems and the unique properties of solvent water, quantitative variations in the amount of water available are of less direct microbiological significance than is generally conceded.

Involvement of superoxide anion generation in the hypersensitive response of potato tuber tissues to infection with an incompatible race of Phytophthora infestans and to the hyphal wall components

Abstract: Potato tuber tissue discs, which were aged after wounding in order to acquire hypersensitive reactivity, reduced extracellular cytochrome c and nitroblue tetrazolium (NBT) following inoculation with an incompatible, but not compatible, race of Phytophthora infestans . The cytochrome c -reducing activity rapidly increased from l to 4 h after inoculation along with an increase in the percentage of hypersensitively dead cells, and then decreased from the time when most of the penetrated cells had died. A localized activation of NBT reduction around invading hyphae of the incompatible, but not those of the compatible, race was observed at early stages of penetration before cell death. The reductive activity of the discs was also elicited by treatment with a hypersensitivity-eliciting substance, hyphal wall components of the fungus. Superoxide dismutase (SOD), an enzyme catalysing the conversion of the superoxide anion (O 2 − ) to H 2 O 2 and O 2 inhibited the enhanced reducing activity of the discs when added to the assay solution, indicating that cytochrome c and NBT may be reduced by O 2 − generated from the discs. Pre-infectional, vacuum infiltration of the discs with a solution containing SOD significantly delayed the occurrence of hypersensitive cell death caused by infection with the incompatible race as well as the accumulation of phytoalexin. Application of SH-binding reagents and NADP + , but not respiratory inhibitors, inhibited the elicitation of the reducing activity caused by infection with the incompatible race. These results indicate that an O 2 − -generating system may be activated in potato tissues during the incompatible interaction induced by invading fungi or fungal wall components, and also that the generation of O 2 − may be involved during hypersensitive cell death as a trigger of the sequence of resistance reactions.

Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport

Abstract: Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Ψ) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of Ψ gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low Ψ. In contrast, in leaves under reductions in Ψ of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low Ψ in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Ψ is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Ψ of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of Ψ in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment.

Relaxation in a high-stress environment: the molecular bases of extensible cell walls and cell enlargement.

Abstract: SUMMARY AND PERSPECTIVE The enlargement of plant cells involves the coordinate con- trol of wall synthesis and expansion, solute and water trans- port, membrane synthesis, Golgi secretion, ion transport, and many other processes. In this review, I have focused on the wall because it is the major control point for cell enlarge- ment. Some of the key processes that may be involved in wall enlargement are summarized in Figure 4. I offer the following speculative picture as a tentative working model for the control of wall expansion. The primary wall is initially secreted and assembled in a form that is me- chanically tough yet has “hot spots” where expansin can weaken microfibril-matrix adhesion. Expansin activity, which is modulated both by secretion of the protein to the wall and by changes in the pH and redox potential of the wall, in- duces the stress relaxation and polymer creep needed for wall enlargement and water uptake by the cell. By altering Synthesis & secretion secretion of wall polysaccharides and proteins

Assembly and enlargement of the primary cell wall in plants

Abstract: Growing plant cells are shaped by an extensible wall that is a complex amalgam of cellulose microfibrils bonded noncovalently to a matrix of hemicelluloses, pectins, and structural proteins. Cellulose is synthesized by complexes in the plasma membrane and is extruded as a self-assembling microfibril, whereas the matrix polymers are secreted by the Golgi apparatus and become integrated into the wall network by poorly understood mechanisms. The growing wall is under high tensile stress from cell turgor and is able to enlarge by a combination of stress relaxation and polymer creep. A pH-dependent mechanism of wall loosening, known as acid growth, is characteristic of growing walls and is mediated by a group of unusual wall proteins called expansins. Expansins appear to disrupt the noncovalent bonding of matrix hemicelluloses to the microfibril, thereby allowing the wall to yield to the mechanical forces generated by cell turgor. Other wall enzymes, such as (1-->4) beta-glucanases and pectinases, may make the wall more responsive to expansin-mediated wall creep whereas pectin methylesterases and peroxidases may alter the wall so as to make it resistant to expansin-mediated creep.