Eiichi Tanimoto

Progressive Inhibition by Water Deficit of Cell Wall Extensibility and Growth along the Elongation Zone of Maize Roots Is Related to Increased Lignin Metabolism and Progressive Stelar Accumulation of Wall Phenolics

Water deficit caused by addition of polyethylene glycol 6000 at −0.5 MPa water potential to well-aerated nutrient solution for 48 h inhibited the elongation of maize (Zea mays) seedling primary roots. Segmental growth rates in the root elongation zone were maintained 0 to 3 mm behind the tip, but in comparison with well-watered control roots, progressive growth inhibition was initiated by water deficit as expanding cells crossed the region 3 to 9 mm behind the tip. The mechanical extensibility of the cell walls was also progressively inhibited. We investigated the possible involvement in root growth inhibition by water deficit of alterations in metabolism and accumulation of wall-linked phenolic substances. Water deficit increased expression in the root elongation zone of transcripts of two genes involved in lignin biosynthesis, cinnamoyl-CoA reductase 1 and 2, after only 1 h, i.e. before decreases in wall extensibility. Further increases in transcript expression and increased lignin staining were detected after 48 h. Progressive stress-induced increases in wall-linked phenolics at 3 to 6 and 6 to 9 mm behind the root tip were detected by comparing Fourier transform infrared spectra and UV-fluorescence images of isolated cell walls from water deficit and control roots. Increased UV fluorescence and lignin staining colocated to vascular tissues in the stele. Longitudinal bisection of the elongation zone resulted in inward curvature, suggesting that inner, stelar tissues were also rate limiting for root growth. We suggest that spatially localized changes in wall-phenolic metabolism are involved in the progressive inhibition of wall extensibility and root growth and may facilitate root acclimation to drying environments.

Regulation of Root Growth by Plant Hormones—Roles for Auxin and Gibberellin

Plant hormones are important biotic factors to regulate root growth. Among the seven kinds of plant hormones, auxin and gibberellin (GA) are strong accelerators of shoot growth, but these are not always accelerators for root growth. The classical views of root-growth regulation by auxin and gibberellin are summarized and current theory of the regulation mechanism is described in this review. The concentration-dependent deceleration of root growth is a key to understanding the auxin action on roots, since the endogenous concentration of indole-3-acetic acid (IAA) is inversely proportional to the growth rate. As massive IAA is transported from shoots to roots by polar transport, the influx speed of IAA mainly controls IAA levels in root cells. The classical view of IAA transport in roots has been supported by recent discoveries of IAA-carrier proteins such as AUX1, PINs and MDRs. The role of plasma membrane-located H+-ATPase and its regulation by IAA has also been described for the acid growth phenomenon caused by the acidification of root cell walls. Compared to auxins, GA functions in roots are less remarkable. Nevertheless, GA also plays an indispensable role in the normal development of roots, since artificial GA-depletion causes abnormal expansion and suppression of root elongation. The GA-requirement for normal root growth was unveiled by the use of chemical inhibitors and mutants of GA biosynthesis. GA function that keeps root morphology long and slender is ascribed to the arrangement of cortical microtubules, cellulose microfibrils and unknown additional factor(s). Cross talks among plant hormones were recently found in the signal transduction pathways mainly in aerial organs. GA and IAA de-repress gene expression by degrading the gene-repressing proteins via the ubiquitin-mediated proteasome system. Another interaction of IAA and GA in growth regulation is the enhancement of GA1 level by IAA. Since the final biochemical steps of growth regulation take place in cell walls, possible cross talks are also conceivable in cell wall formation and modification.

Promotion of leaf sheath growth by gibberellic acid in a dwarf mutant of rice

Abstract. The mechanism of gibberellin (GA)-induced leaf sheath growth was examined using a dwarf mutant of rice (Oryza sativa L. cv. Tan-ginbozu) treated in advance with an inhibitor of GA biosynthesis. Gibberellic acid (GA3) enhanced the growth of the second leaf sheath, but auxins did not. Measurement of the mitotic index and cell size revealed that cell elongation rather than cell division is promoted by GA3. Gibberellic acid increased the extensibility of cell walls in the elongation zone of the leaf sheath. It also increased the total amount of osmotic solutes including sugars in the leaf sheath, but did not increase the osmotic concentration of the cell sap, due to an accompanying increase in cell volume by water absorption. In the later stage of GA3-induced growth, starch granules completely disappeared from leaf sheath cells, whereas dense granules remained in control plants. These findings indicate that GA enhances cell elongation by increasing wall extensibility, osmotic concentration being kept unchanged by starch degradation.

Gibberellin Requirement for the Normal Growth of Roots

Gibberellin (GA) strongly promotes shoot growth whereas it shows little effect on root elongation in GA-deficient dwarf plants (cf. Fig. 1) and in rosette plants.1 Although roots of dwarf maize and lettuce respond to exogenous GA in some experimental conditions,2,3 roots of these plants elongate normally without GA application. These phenomena suggest that roots do not require GA or require less GA than shoots. Thus, studies on the role of GA in root growth have been limited as compared with those in shoots.4–6 In order to evaluate GA requirements for root growth, the effects of the interaction of ancymidol7 and GA3 on the elongation growth of roots and shoots have been studied in lettuce seedlings and in dwarf and normal pea plants.

Growth and cell wall changes in rice roots during spaceflight

We analyzed the changes in growth and cell wall properties of roots of rice (Oryza sativa L. cv. Koshihikari) grown for 68.5, 91.5, and 136 h during the Space Shuttle STS-95 mission. In space, most of rice roots elongated in a direction forming a constant mean angle of about 55° with the perpendicular base line away from the caryopsis in the early phase of growth, but later the roots grew in various directions, including away from the agar medium. In space, elongation growth of roots was stimulated. On the other hand, some of elasticity moduli and viscosity coefficients were higher in roots grown in space than on the ground, suggesting that the cell wall of space-grown roots has a lower capacity to expand than the controls. The levels of both cellulose and the matrix polysaccharides per unit length of roots decreased greatly, whereas the ratio of the high molecular mass polysaccharides in the hemicellulose fraction increased in space-grown roots. The prominent thinning of the cell wall could overwhelm the disadvantageous changes in the cell wall mechanical properties, leading to the stimulation of elongation growth in rice roots in space. Thus, growth and the cell wall properties of rice roots were strongly modified under microgravity conditions during spaceflight.

Tall or short? Slender or thick? A plant strategy for regulating elongation growth of roots by low concentrations of gibberellin

BACKGROUND Since the plant hormone gibberellin (GA) was discovered as a fungal toxin that caused abnormal elongation of rice shoots, the physiological function of GA has mainly been investigated in relation to the regulation of plant height. However, an indispensable role for GA in root growth has been elucidated by using severely GA-depleted plants, either with a gene mutation in GA biosynthesis or which have been treated by an inhibitor of GA biosynthesis. The molecular sequence of GA signalling has also been studied to understand GA functions in root growth. SCOPE This review addresses research progress on the physiological functions of GA in root growth. Concentration-dependent stimulation of elongation growth by GA is important for the regulation of plant height and root length. Thus the endogenous level of GA and/or the GA sensitivity of shoots and roots plays a role in determining the shoot-to-root ratio of the plant body. Since the shoot-to-root ratio is an important parameter for agricultural production, control of GA production and GA sensitivity may provide a strategy for improving agricultural productivity. The sequence of GA signal transduction has recently been unveiled, and some component molecules are suggested as candidate in planta regulatory sites and as points for the artificial manipulation of GA-mediated growth control. CONCLUSIONS This paper reviews: (1) the breakthrough dose-response experiments that show that root growth is regulated by GA in a lower concentration range than is required for shoot growth; (2) research on the regulation of GA biosynthesis pathways that are known predominantly to control shoot growth; and (3) recent research on GA signalling pathways, including GA receptors, which have been suggested to participate in GA-mediated growth regulation. This provides useful information to suggest a possible strategy for the selective control of shoot and root growth, and to explain how GA plays a role in rosette and liana plants with tall or short, and slender or thick axial organs.

α-and β-Glycosidases in maize roots

Four glycosidases were analyzed in 10 mm apical segments prepared from growing roots (15 mm) of Zea mays L. The pH optima were found to be 5.8 for β-glucosidase, 4.4 for β-galactosidase, 6.4 for α-glucosidase and 6.0 for α-galactosidase. The β-glucosidase showed 4-fold higher activity than the β-galactosidase. The distribution of the β-glucosidase activity was signifcantly different from that of the β-galactosidase, α-glucosidase and α-galactosidase.

ROOT GROWTH INHIBITING, a Rice Endo-1,4-β-d-Glucanase, Regulates Cell Wall Loosening and is Essential for Root Elongation

The molecular mechanism involved in cell wall dynamics has not been well clarified, although it is quite important for organ growth. We characterized a rice mutant, root growth inhibiting (rt), which is defective in root elongation. The rt mutant showed a severe defect in cell elongation at the root-elongating zone with additional collapse of epidermal and cortex cells at the root tip caused by the defect in the smooth exfoliation of root cap cells. Consistent with these phenotypes, expression of the RT gene, which encodes a member of the membrane-anchored endo-1,4-β-d-glucanase, was specifically localized in the root-elongating zone and at the junction between epidermal and root cap cells. The enzymatic analysis of root extracts from the wild-type and rt mutant indicated that RT hydrolyzes noncrystalline amorphous cellulose. The cellulose content was slightly increased but the crystallinity of cellulose was decreased in the rt root. In addition, the hemicellulose composition was different between wild-type and rt roots. The total extensibility was significantly lower in the rt root explants. Based on these results, we concluded that RT is involved in the disassembly of the cell wall for cell elongation in roots as well as for root cap exfoliation from the epidermal cell layer by hydrolyzing the noncrystalline amorphous cellulose fibers of cellulose microfibrils resulting in loosening of the hemicellulose and cellulose interaction.

Breakdown of Cell Wall Polysaccharides in Rice Culms at the Early Ripening Stage

The senescence process of a tissue often includes the degradation of cell wall polysaccharides. A well-known example is ripening fruit, where the degradation of pectic polysaccharides causes tissue softening (Huber, 1983a, b). In rice, the senescence of culms is relevant to such important phenomena as carbohydrate turnover and lodging resistance. However, little is known about the degradation process of cell walls during culm senescence. In this study, we examined the degradation of cell walls in ripening rice culms and found that (1) the cell wall thickness decreases, and (2) hemicellulose is remobilized as the grain fi lling proceeds in the culm.

Effect of gibberellin and ancymidol on the growth and cell wall components of pea (Pisum sativum L.) roots

Externally applied gibberellin A3 (GA3) is known to show little effect on root elongation whereas it strongly promotes stem elongation in peas. GA3 was, however, found to enhance root elongation when root elongation was suppressed by ancymidol (Anc), an inhibitor of gibberellin (GA) biosynthesis. In a series of dose-response experiments, Anc was found to inhibit elongation and induced expansion of roots. These Anc effects were reversed when GA3 was given together with Anc.