Activation of acid growth in germinating horse chestnut seeds

The validity of the acid-growth hypothesis is proved for the case of cell elongation initiation in germinating seeds of horse chestnut (Aesculus hippocastanum L.), the embryo axes of which are known to extend during the first stages of germination only by cell elongation. During seed imbibition, H⁺-ion excretion was firstly low; it increased several times prior to radicle emergence and was maintained at a high level during growth initiation and further cell elongation. Cell wall acidification and radicle emergence were enhanced in the presence of 0.02 mM fusicoccin, thus indicating the involvement of the plasma membrane H⁺-ATPase in the execution of acid growth. The presence of this enzyme and its activator (14-3-3 protein) in microsomal fractions obtained from radicles and hypocotyls of the embryo axes during and after initiation of cell elongation was demonstrated immunochemically. It is supposed that the initiation of cell elongation at early germination occurs via the activation of the plasma membrane H⁺-ATPase and results in the acidification of cell walls, leading to their higher extensibility, in accordance with the hypothesis of acid growth.     

Cell elongation as an inseparable component of growth in terrestrial plants

The review is dedicated to the role of cell elongation in plant growth and morphogenesis. The ratios of cell division to elongation, cell competence for the initiation of elongation, main features of the metabolism of elongating cells, and physiological processes realizing elongation have been considered on the examples of seed germination and growth of roots, stems, and leaves. A special attention was paid to the vacuole as a specific feature of plant cells, pathways of its formation, and its role in maintenance of ion and water homeostasis in the elongating cell. The plant can modify its morphology according to changes in the environmental conditions via cell elongation.     

The Seventeenth International Conference on Plant Growth Substances (Brno,Czech Republic,July 1–6, 2001)

Russian Journal of Plant Physiology -     

Transition from hormonal to nonhormonal regulation as exemplified by seed dormancy release and germination triggering

It is generally believed that seed dormancy release is terminated by germination and that this process is controlled by phytohormones. Most attention was paid to gibberellins (GAs) because treatment with GAs is most frequently applied for seed dormancy breaking. The review characterizes the hormonal regulation of seed dormancy and its release, as exemplified by arabidopsis seeds possessing non-deep physiological dormancy. Dormancy release occurs under the influence of low temperature and/or illumination with red light. Two main trends are typical of this process: (1) a decrease in ABA content and blocking of signal transduction from ABA, and (2) GA synthesis and activation of GA signaling pathway. Dormancy release ends with the GA-induced syntheses of some proteins, enzymes in particular, required for the start of germination. Quiescent seeds are capable of realizing the germination program without hormonal induction, due to nothing but seed hydration. In imbibing seeds, the triggering role of water lies in the successive activation of basic metabolic systems after attaining the water content thresholds characteristic of these systems and in preparing cells of embryo axial organs for germination. Thus, seed dormancy release is controlled by phytohormones, whereas subsequent germination manifesting itself as the initiation of cell elongation in embryo axes is controlled by water inflow.     

Acid vacuolar invertase in dormant and germinating seeds of the horse chestnut

A high water content is maintained in the tissues of the axial organs of horse chestnut seeds after the fruit is shed and down to the time the seeds germinate. The plant cell vacuoles, features of whose metabolism can influence the cells’ preparation to initiate growth in germination, are preserved. It was shown that the activity of acid invertase and its capacity to hydrolyze both sucrose and raffinose remain stable throughout the period of dormancy and the transition to germination, as do the molecular weight of its subunits (63 and 65 kDa) and multimer (500 to 550 kDa). The activity of the enzyme increases when the seeds swell under optimal conditions for germination; this is associated with the synthesis of new molecules of the enzyme in long-lived mRNA templates. The storability of the enzyme in the vacuoles of dormant seeds, together with the increase in its activity when seeds coming out of dormancy swell, ensures the rapid hydrolysis of sucrose issuing from the seeds’ cotyledons, thus leading to increased osmotic pressure and, as a result, the beginning of cell elongation, i.e., germination.     

Nickel and zinc effects,accumulation and distribution in ruderal plants Lepidium ruderale and Capsella bursa-pastoris

Ruderal plants can grow in polluted areas, but little is known about heavy metal accumulation and distribution in them. Here Ni and Zn accumulation, distribution and effects were investigated in Lepidium ruderale and Capsella bursa-pastoris grown at 5–30 µM Ni(NO₃)₂ or 10–80 µM Zn(NO₃)₂. Metal contents were measured by flame atomic absorption spectrophotometry and tissue distribution of metals was studied histochemically. Ni was more toxic than Zn for both plants. When metal-induced growth-inhibiting effects were compared at various metal concentrations in solution, L. ruderale was more tolerant to Ni, whereas C. bursa-pastoris to Zn. However, when compared at similar Zn or Ni contents in roots, root growth of C. bursa-pastoris was more tolerant than that of L. ruderale. On the contrary, at similar Zn or Ni contents in shoots, shoot growth of L. ruderale was more tolerant. Both plants are excluders maintaining low metal levels in shoots. In roots, Ni located in protoplasts while Zn was also detected in cell walls. Metal accumulation in root apices resulted in growth inhibition. Ni accumulation in root cortex constrained metal translocation into central cylinder and then to shoots, where it located only in conductive tissues and epidermis, particularly in leaf trichomes of C. bursa-pastoris. Zn was translocated to shoots more actively and distributed in all shoot tissues, being accumulated in leaf vascular bundles and epidermis. To conclude, these patterns of Ni and Zn distribution are aimed at metal sequestration in roots and leaf epidermis, thus keeping mesophyll from metal penetration and pigment degradation.     

The Role of Water Uptake in the Transition of Recalcitrant Seeds from Dormancy to Germination

In recalcitrant seeds of horse chestnut (Aesculus hippocastanum L.) maintaining a high water content during winter, dormancy is determined by the presence and influence of the seed coat, while the axial organs of the embryos excised from these seeds are not dormant. Such axial organs were capable for active water uptake and rapid fresh weight increase, so that their fresh weights exceeded those in intact seeds at the time of radicle protrusion. Fructose plays an essential role in the water uptake as a major osmotically active compound. ABA interferes with the water uptake by the axial organs and thus delays the commencement of their growth. The manifestation of seed response to ABA during the entire dormancy period indicates the presence of active ABA receptors and the pathways of its signal transduction. The content of endogenous ABA in the embryo axes doubled in the middle of dormancy period, which coincided with a partial suppression of water uptake by the axes. During seed dormancy release and imbibition before radicle protrusion, the level of endogenous ABA in axes declined gradually. Application of exogenous ABA can imitate dormancy by limiting water absorption by axial organs. Fusicoccin A (FC A) treatment neutralized completely this ABA effect. Endogenous FC-like ligands were detected in the seed axial organs during dormancy release and germination. Apparently, endogenous FC stimulates water uptake via the activation of plasmalemmal H⁺-ATPase, acidification of cell walls, their loosening, and turgor pressure reduction. FC can evidently counteract the ABA-induced suppression of water uptake by controlling the activity of H⁺-ATPase. It is likely that, in dormant intact recalcitrant seeds, axial organs, maintaining a high water content, are competent to elevate their water content and to start their preparation for germination under the influence of FC when coat-imposed dormancy becomes weaker.     

Proteins of Axial Organs of Dormant and Germinating Horse Chestnut Seeds: 1. General Characterization

This is the first characterization of proteins from axial organs of recalcitrant horse chestnut seeds during deep dormancy, dormancy release, and germination. We demonstrated that, during the entire period of cold stratification, axial organs were enriched in easily soluble albumin-like proteins and almost devoid of globulins. About 80% of the total protein was found in the cytosol. Approximately one third of cytosolic proteins were heat-stable polypeptides, which were major components of total proteins. Heat-stable proteins comprised three groups of polypeptides with mol wts of 52–54, 24–25, and 6–12 kD with a predominance of low-molecular-weight proteins. The polypeptide patterns of heat-stable and thermolabile proteins differed strikingly. Heat-stable proteins accumulated in axes during the late seed maturation, comprising more than 30% of the total protein in axes of mature seeds. The polypeptide patterns of the total protein of axial organs and its particular fractions did not change in the course of seed dormancy and release. At early germination, the content of heat-stable proteins in axes decreased and their polypeptide pattern changed both in the cytosol and cell structures. We believe that at least some heat-stable proteins can function as storage proteins in the axes. Localization of storage proteins in the cells of axial organs and the role of heat-stable proteins in recalcitrant seeds are discussed.