Avoiding transport bottlenecks in an expanding root system: Xylem vessel development in fibrous and pioneer roots under field conditions

? Premise of the study: Root systems develop to effectively absorb water and nutrients and to rapidly transport these materials to the transpiring shoot. In woody plants, roots can be born with different functions: fibrous roots are primarily used for water and nutrient absorption, whereas pioneer roots have a greater role in transport. Because pioneer roots extend rapidly in the soil and typically quickly produce fibrous roots, they need to develop transport capacity rapidly so as to avoid becoming a bottleneck to the absorbed water of the developing fibrous roots and, as we hypothesized, immediately activate a specific type of autophagy at a precise time of their development. ? Methods: Using microscopy techniques, we monitored xylem development in Populus trichocarpa roots in the first 7 d after emergence under field conditions. ? Key results: Newly formed pioneer roots contained more primary xylem poles and had larger diameter tracheary elements than fibrous roots. While xylogenesis started later in pioneer roots than in fibrous, it was completed at the same time, resulting in functional vessels on the third to fourth day following root emergence. Programmed cell death was responsible for creating the water conducting capacity of xylem. Although the early xylogenesis processes were similar in fibrous and pioneer roots, secondary vascular development proceeded much more rapidly in pioneer roots. ? Conclusions: Compared to fibrous roots, rapid development of transport capacity in pioneer roots is not primarily caused by accelerated xylogenesis but by larger and more numerous tracheary elements and by rapid initiation of secondary growth.     

<Emphasis Type="Italic">Picea asperata</Emphasis> pioneer and fibrous roots have different physiological mechanisms in response to soil freeze-thaw in spring

About 70 % of the total land area in the world are affected by soil freeze and thaw (FT) cycles. Root is the first organ of plant to sense soil environment and it is unclear how it copes with the soil FT. Based on the different functions of firstorder pioneer and fibrous roots in woody plants, we hypothesize that pioneer and fibrous roots respond differently. The experiment was conducted in a growth chamber using Picea asperata seedlings. We designed the FT based on field observation data. The physiological responses in fibrous and pioneer roots were examined. Fibrous roots had higher root vitality and N content, whereas pioneer roots exhibited higher total nonstructural saccharide content. The accumulation of O₂ ^- under FT treatment was similar in the two types of roots. Pioneer roots showed higher osmolyte (especially proline) content, whereas fibrous roots had higher peroxidase activity. The present study confirmed that fibrous roots have stronger metabolism ability, whereas pioneer roots are the key storage organs. FT in the temperature range from -5 to 5 °C are mild and do not cause serious injury to roots. Pioneer roots have higher tolerance to soil FT in spring than fibrous roots. The roots have different strategies to FT: fibrous roots increase the antioxidant system, whereas pioneer roots accumulate more osmolytes. Such knowledge can help us to understand how roots of woody plants cope with soil FT.     

Arginase activity in the cotyledons of soybean seedlings

The influence of naphthylacetic acid, abscisic acid, gibberellic acid and kinetin on the formation of aerenchyma in seedling roots of Zea mays L. cv. Capella has been studied in relation to reported changes of their concentration in poorly aerated roots, which readily form aerenchyma, and to the effects of these hormones on the production of ethylene, a major factor promoting aerenchyma formation. Because the absence of nitrate accelerates aerenchyma formation in aerated roots, their influence on these roots was compared. The growth regulators were added to roots growing in non-aerated and aerated nutrient solutions, and aerenchyma formation and the production and endogenous concentration of ethylene were measured. Naphthylacetic acid prevented aerenchyma formation in both aerated roots without nitrate and in non-aerated roots although it enhanced the ethylene concentration of the roots. Abscisic acid also prevented aerenchyma formation, but without affecting the ethylene concentration. Gibberellic acid promoted aerenchyma formation in aerated roots only, but ethylene production in both aerated and non-aerated roots. Kinetin promoted aerenchyma formation in both aerated and non-aerated roots. It stimulated ethylene production in aerated roots, but slightly inhibited it in non-aerated roots. Co²⁺ and Ag⁺, which suppress ethylene production and action, respectively, reduced the promoting effects of gibberellic acid, but not those of kinetin. It is concluded that the effects of the plant growth regulators on aerenchyma formation in maize roots were, with a possible exception for gibberellic acid, not the result of altered ethylene concentrations in the roots. Their influence on aerenchyma formation is discussed in relation to their reported actions on cell membranes.     

Biochemical changes in Glomus fasciculatum colonized roots of Lycopersicon esculentum in presence of Meloidogyne incognita

Glasshouse experiments were conducted to elicit biochemical substantiation for the observed difference in resistance to nematode infection in roots colonized by mycorrhiza, and susceptibility of the fresh flush of roots of the same plant that escaped mycorrhizal colonization. Tomato roots were assayed for their biochemical profiles with respect to total proteins, total phenols, indole acetic acid, activities of polyphenol oxidase, phenylalanine ammonia lyase and indole acetic acid oxidase. The roots of the same plant (one set) received Glomus fasciculatum and G. fasciculatum plus juveniles of Meloidogyne incognita separately; and half the roots of second set of plants received G. fasciculatum while the other half of roots did not receive any treatment. Roots colonized by G. fasciculatum recorded maximum contents of proteins and phenols followed by that of the roots that received G. fasciculatum plus M. incognita. However, IAA content was lowest in the roots that received mycorrhiza or mycorrhiza plus juveniles of root-knot nematode and correspondingly. Roots that received juveniles of root-knot nematode recorded maximum IAA content and per cent increase over healthy check and mycorrhiza-inoculated roots. The comparative assay on the activities of PPO, PAL and IAA oxidase enzymes in treated and healthy roots of tomato, indicated that PAL and IAA oxidase activities were maximum in G. fasciculatum colonized roots followed by the roots that received mycorrhiza plus juveniles of root-knot nematode, while the activity of PPO was minimum in these roots. The roots that received juveniles of root-knot nematode recorded minimum PAL and IAA oxidase activities and maximum PPO activity. Since the roots of same plant that received mycorrhiza and that did not receive mycorrhiza; and the plant that received nematode alone and mycorrhiza plus nematode recorded differential biochemical contents of proteins, total phenols and IAA, and differential activities of enzymes under study, it was evident that the biochemical defense response to mycorrhizal colonization against root-knot nematodes was localized and not systemic. This explained for the response of plant that differed in root galling due to nematode infection in presence of mycorrhizal colonization. The new or fresh roots which missed mycorrhizal colonization, got infected by nematodes and developed root galls.     

Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus

Proteoid roots develop in Lupinus albus L. in response to nutrient stress, especially P. Proteoid roots excrete citrate and thus increase the availability of P, Fe, and Mn in the rhizosphere. In an effort to understand citrate synthesis and organic acid metabolism in proteoid roots of lupin, we have evaluated in vitro enzyme activities of citrate synthase (CS), malate dehydrogenase (MDH), and phosphoenolpyruvate carboxylase (PEPC) in proteoid and normal roots of plants grown with or without P. Organic acid concentrations, respiration rates, and dark 14CO2-labeling patterns were also determined. The in vitro specific activities of CS, MDH, and PEPC and in vivo dark 14CO2 fixation were higher in proteoid roots compared to normal roots, particularly under P stress. Western blot analysis showed that PEPC enzyme protein was more highly expressed in -P proteoid roots compared to other tissues. The majority of the fixed 14C was found in organic acids, predominantly malate and citrate. A larger fraction of citrate was labeled in P- stressed proteoid roots compared to other root tissue. Respiration rates of proteoid roots were 31% less than those of normal roots. The data provide evidence for increased synthesis of citrate in proteoid roots compared to normal roots, particularly under P stress. A portion of the carbon for citrate synthesis is derived from nonautotrophic CO2 fixation via PEPC in proteoid roots.     

Alcohol dehydrogenase activity in the roots of marsh plants in naturally waterlogged soils

The aim of this work was to discover whether oxygen tensions in the roots of marsh plants in flooded soils are high enough to allow fully acrobic metabolism. Activity of alcohol dehydrogenase (ADH), a protein synthesised in anoxic plants, was measured in roots of marsh plants growing in habitats where the availability of oxygen to the roots would be expected to differ. Roots of Carex riparia in standing water had ADH activities about 2.5 times higher than those of phosphofructokinase, and comparable to ADH activities of Poa trivialis, Urtica dioica and Ranunculus repens roots in dry soil. Removal of the oxygen supply via aerenchyma to Carex roots caused a 30-fold increase in ADH activity relative to that of phosphofructokinase. There was no change in ADH activity with depth in Carex roots in waterlogged soil, but in Filipendula ulmaria roots activity was 14 times higher below 10 cm depth than near the surface. Urtica roots in waterlogged soil had alcohol dehydrogenase activities 26 times higher than roots in dry soil, but for Poa and Ranunculus roots this figure was only 1.7 and 4.2, respectively. These results indicate that the oxygen tensions in the roots of marsh plants in waterlogged soil differ considerably among species. Ethanol was the major product of fermentation in roots of all species studied. There was no correlation between ADH activity and the rate of ethanol production under anoxia of Urtica roots. The physiological significance of high ADH activities in roots is thus unclear.Abbreviations ADH alcohol dehydrogenase - PFK phosphofructokinase - PFP pyrophosphate:fructose 6-phosphate phosphotransferase     

Early events in the life of apple roots: variation in root growth rate is linked to mycorrhizal and nonmycorrhizal fungal colonization

Early events of mycorrhizal and nonmycorrhizal fungal colonization in newly-emerging roots of mature apple (Malus domestica Borkh) trees were characterized to determine the relationship of these events to fine root growth rate and development. New roots were traced on root windows to measure growth and then collected and stained to quantify microscopically the presence of mycorrhizal and nonmycorrhizal fungal structures. Most new roots were colonized by either mycorrhizal or nonmycorrhizal fungi but none less 25 days old were ever internally colonized by both. Compared to nonmycorrhizal colonization, mycorrhizal colonization was associated with faster growing roots and roots that grew for a longer duration, leading to longer roots. While either type of fungi was observed in roots as soon as 3 days after root emergence, intraradical colonization by mycorrhizal fungi was generally faster (peaking at 7 to 15 days) than that by nonmycorrhizal fungi and often occurred more frequently in younger roots. Only 15 to 35% of the roots had no fungal colonization by 30 days after emergence. This study provides the first detailed examination of the early daily events of mycorrhizal and nonmycorrhizal fungal colonization in newly emerging roots under field conditions. We observed marked discrimination of roots between mycorrhizal and nonmycorrhizal fungi and provide evidence that mycorrhizal fungi may select for faster growing roots and possibly influence the duration of root growth by non-nutritional means.     

Detection of Polypeptides Involved in Early Stages of Nodulation in Soybean Roots

Soluble proteins extracted from the roots of nodulating soybean[Glycine max (L.) Merr. cv. T202] and from roots of the non-nodulatingisoline rj₁ of cv. T202 (cv. T201), which had been inoculatedwith Bradyrhizobium japonicum, were analyzed by two-dimensionalpolyacrylamide gel electrophoresis and silver staining, in anattempt to identify polypeptides involved in early stages ofnodulation. Almost identical patterns of polypeptides were generatedby extracts of 3-day-old roots of uninoculated T201 and T202and of inoculated T201 and T202, but a unique spot, correspondingto a polypeptide of 38 kDa was detected in the case of inoculatedroots of T202. Western blotting analysis using "inoculated-T202-rootspecific" antiserum, prepared by titration of antiserum againstproteins from inoculated T202 roots with proteins from inoculatedT201 roots, revealed spots corresponding to polypeptides of26,27, and 33 kDa that were detectable only in the extractsof roots of inoculated T202. However, no unique polypeptidespots were detected in the case of roots of inoculated T201and T202, as compared with those from uninoculated T201 andT202 roots by Western blotting analysis using "inoculated-T201-rootspecific" antiserum prepared by titration of antiserum againstproteins from inoculated T201 roots with proteins from uninoculatedT201 roots. (Received May 27, 1991; Accepted September 30, 1991)     

Photomorphogenesis of Plant Roots in Vitro

Photomorphogenetic phenomena of excised roots of various plants cultured aseptically in White medium were studied. Continuous white fluorescent light (3000 lx) could generally inhibit extension growth of the primary roots and lateral roots, depress emergence of lateral roots and raise of fresh weight of roots. If greening of the roots could be induced by light, however, the light would increase the fresh weight and formation of lateral roots. Addition of 75 mg/l inositol significantly improved root development. Continuous irradiation stimulated greening of excised roots from some plants (e. g. cucumber, bird rape, India mustard and maize) depending on their hereditary potency. Chlorophyll a, chlorophyll b and carotenoids accumulated slowly. After 1 day or 5 days under the light, chlorophyll or carotenoids could be detected separately. Total amount of plastid pigments per unit of fresh weight in roots was much lower than in leaves. More sucrose applied could pro- mote synthesis of chlorophyll and growth of roots. Chloroplasts appeared in small parenchyma cells located in the stele. They did not come frorn proplastid, but were transformed from a kind of leueoplast-amyloplast. Electron microscopic photographes indicated that starch grains diminished gradually while lamella structure appeared first within enveloped and then filled inner-space of plastid. Many thylakoids stacked to form irregular-shaped gram. These chloroplasts seemed to be different from those in mesophyll. Special inhibitor of carotenoid biosynthesis (norflurazon) prevented chlorophyll synthesis with high efficiency while another photosynthetic inhibitor (motoxuron) increased chlorophyll accumulation in isolated roots.     

Iron deficiency induces changes in metabolism of citrate in lateral roots and cluster roots of Lupinus albus

This paper describes the first measurement of enzyme activities in cluster roots under –Fe stress, at different stages of cluster root development and function. In Lupinus albus L., Cluster roots are produced both under iron- and phosphorus-deficient conditions. In both cases the structure is similar, but the level of exudation is much greater in iron-deficient plants. Much work has been done on the enzyme kinetics of P-deficient cluster roots, but none on enzyme activities of Fe-deficient cluster roots. The enzymes investigated were citrate synthase (EC 4.1.3.7), aconitase (EC 4.2.1.3), isocitrate dehydrogenase [IDH(NAD) (EC 1.1.1.41) and IDH (NADP) (EC 1.1.1.42)] and lactate dehydrogenase (LDH) (EC 1.1.1.27). In cluster roots, citrate synthase activity was initially lower than in lateral roots but, after 5 days, recovered to the lateral root level. Cluster root aconitase levels initially increased, but fell sharply on day 3, and no activity was detected after day 5. IDH (NAD) levels were much lower in cluster roots than in laterals, dropping to a low on day 3, and then rising throughout development. IDH (NADP) levels were always higher in cluster roots than in lateral roots, increasing throughout development. LDH levels in cluster roots fell throughout development. Internal tissue concentrations of citrate were markedly higher in –Fe laterals than in +Fe lateral roots and in cluster roots. Cluster root levels of citrate increased dramatically after day 3. Results are discussed within the context of previous work on enzyme kinetics under –P, and the importance of a block in aconitase activity is highlighted.     

Amyloplasts are necessary for full gravitropic sensitivity in roots of Arabidopsis thaliana

The observation that a starchless mutant (TC7) of Arabidopsis thaliana (L.) Heynh. is gravitropic (T. Caspar and B.G. Pickard, 1989, Planta 177, 185–197) raises questions about the hypothesis that starch and amyloplasts play a role in gravity perception. We compared the kinetics of gravitropism in this starchless mutant and the wild-type (WT). Wild-type roots are more responsive to gravity than TC7 roots as judged by several parameters: (1) Vertically grown TC7 roots were not as oriented with respect to the gravity vector as WT roots. (2) In the time course of curvature after gravistimulation, curvature in TC7 roots was delayed and reduced compared to WT roots. (3) TC7 roots curved less than WT roots following a single, short (induction) period of gravistimulation, and WT, but not TC7, roots curved in response to a 1-min period of horizontal exposure. (4) Wild-type roots curved much more than TC7 roots in response to intermittent stimulation (repeated short periods of horizontal exposure); WT roots curved in response to 10 s of stimulation or less, but TC7 roots required 2 min of stimulation to produce a curvature. The growth rates were equal for both genotypes. We conclude that WT roots are more sensitive to gravity than TC7 roots. Starch is not required for gravity perception in TC7 roots, but is necessary for full sensitivity; thus it is likely that amyloplasts function as statoliths in WT Arabidopsis roots. Furthermore, since centrifugation studies using low gravitational forces indicated that starchless plastids are relatively dense and are the most movable component in TC7 columella cells, the starchless plastids may also function as statoliths.Abbreviations S2 story two - S3 story three - WT wild-type     

Sugar utilization and anoxia tolerance in rice roots acclimated by hypoxic pretreatment

Although most cereal roots cannot elongate under anoxic conditions, primary roots of three-day-old rice (Oryza sativa L.) seedlings were able to elongate during a 24-h period of anoxia. Hypoxic pretreatment (H-PT) increased the elongation of their roots. Sucrose synthase (EC 2.4.1.13), glucokinase (EC 2.7.1.2), fructokinase (EC 2.7.1.4), pyruvate decarboxylase (EC 4.1.1.1) and alcohol dehydrogenase (EC 1.1.1.1) activities were increased by anoxia in both H-PT and non-pretreated (N-PT) roots. However, these activities were greater in the H-PT roots than in the N-PT roots. The average rate of production of ethanol for the initial 6h after the onset of anoxia was 3.7 and 1.4 micromolg(-1) fresh weight h(-1) for the H-PT and N-PT roots, respectively, suggesting that ethanolic fermentation may increase more quickly in the H-PT roots than in the N-PT roots. Roots of the seedlings lost ATP and total adenine nucleotides in anoxia, however, the H-PT roots maintained higher levels of ATP and total adenine nucleotides compared to the N-PT roots. These results show that rice roots are able to utilize the set of enzymes involved in the metabolism of soluble sugars under anoxia. The ability to maintain an active fermentative metabolism for production of ATP by fueling the glycolytic pathway with fermentable carbohydrate is probably greater in H-PT than in N-PT roots.     

The behaviour of roots encountering cracks in soil

Summary Experimental methods are described for observing the behaviour of roots encountering cracks in soil. The proportions of roots which enter a second soil block after crossing a crack of known width were measured. Soil strength was measured with a penetrometer.Results are presented for the proportions of seminal roots of wheat and primary lateral roots of pea which enter moulded soil of various strengths after crossing cracks. Results are also presented for the proportions of seminal roots of pea, rape and safflower which enter undisturbed soil after crossing cracks.It was found that, in all cases, the proportion of roots penetrating the second soil block decreased with increasing crack width and increasing soil strength. Also, a smaller proportion of thinner roots penetrated the second soil block than thicker roots under similar conditions. Root diameter in the cracks was influenced by both crack width and soil strength, and an empirical equation is presented to describe this effect.     

Relation of fine root distribution to soil C in a Cunninghamia lanceolata plantation in subtropical China

Background and aims

Growth and distribution of fine roots closely depend on soil resource availability and affect soil C distribution in return. Understanding of relationships between fine root distribution and soil C can help to predict the contribution of fine root turnover to soil C accumulation.

Methods

A study was conducted in a subtropical Cunninghamia lanceolata plantation to assess the fine root mass density (FRMD), fine root C density (FRCD) of different fine root groups as well as their relations with soil C.

Results

The FRMD and FRCD of short-lived roots, dead roots and herb roots peaked in the 0–10 cm soil layer and decreased with soil depth, while FRMD, FRCD of long-lived roots peaked in the 10–20 cm soil layer. Soil C was positively related to FRMD and FRCD of total fine roots (across all three soil layers), dead roots (0–10 cm) and herb roots (10–20 cm) as well as FRCD of short-lived roots (20–40 cm) (P <0.05).

Conclusions

Soil C was mainly affected by herb roots in upper soil layers and by woody plant roots in deeper soil layers.     

Phototropism and gravitropism in lateral roots of Arabidopsis

Gravitropism and, to a lesser extent, phototropism have been characterized in primary roots, but little is known about structural/functional aspects of these tropisms in lateral roots. Therefore, in this study, we report on tropistic responses in lateral roots of Arabidopsis thaliana. Lateral roots initially are plagiogravitropic, but when they reach a length of approximately 10 mm, these roots grow downward and exhibit positive orthogravitropism. Light and electron microscopic studies demonstrate a correlation between positive gravitropism and development of columella cells with large, sedimented amyloplasts in wild-type plants. Lateral roots display negative phototropism in response to white and blue light and positive phototropism in response to red light. As is the case with primary roots, the photoresponse is weak relative to the graviresponse, but phototropism is readily apparent in starchless mutant plants, which are impaired in gravitropism. To our knowledge, this is the first report of phototropism of lateral roots in any plant species.     

Ethylene Interacts with Auxin in Regulating Developmental Attenuation of Gravitropism in Flax Root

Previous research shows that gravity-sensing in flax (Linum usitatissimum) root is initiated during seed imbibition and precedes root emergence. In this study we investigated the developmental attenuation of flax root gravitropism post-germination and the involvement of ethylene. Gravity response deteriorated significantly from 3 to 11?h after root emergence, which occurred at around 19?h after imbibition (that is, from “age” 22 to 30?h). Although the root elongation rate increased from 22 to 30?h, the gravitropic curving rate declined steadily. Older roots were able to tolerate higher levels of exogenous IAA before inhibition of elongation and gravitropism occurred. The age-dependent effect of IAA on root growth and gravitropism suggests that young roots are more sensitive to auxin and respond to a smaller vertical auxin gradient than older roots upon horizontal gravistimulation. The ethylene synthesis inhibitor AVG (2-aminoethoxyvinyl glycine, 10?μM) or ethylene action inhibitor Ag⁺ (10?μM) stimulated gravitropic curvature of 30?h roots by 24 and 32%, respectively, but had no effect on 22?h roots, suggesting that as roots age, ethylene begins to play a role in root gravitropism. The auxin transport inhibitor NPA (N-naphthylphthalamic acid, 50?μM) reduced gravitropic curvature of 30?h roots by 24% but had no effect on 22?h roots. On the other hand, treating roots simultaneously with the auxin transport inhibitor and ethylene synthesis or action inhibitor stimulated gravitropic curvature of 30?h roots but not 22?h roots. Taken together, these data indicate that as roots develop, their weakened gravity response is due to decreased auxin sensitivity and possibly auxin transport regulated by ethylene.     

Defects in root development and gravity response in the aem1 mutant of rice are associated with reduced auxin efflux

The phytohormone auxin is involved in the regulation of a variety of developmental processes. In this report, we describe how the processes of lateral root and root hair formations and root gravity response in rice are controlled by auxin. We use a rice mutant aem1 (auxin efflux mutant) because the mutant is defective in these characters. The aem1 line was originally isolated as a short lateral root mutant, but we found that the mutant has a defect in auxin efflux in roots. The acropetal and basipetal indole-3-acetic acid (IAA) transports were reduced in aem1 roots compared to wild type (WT). Furthermore, gravitropic bending as well as efflux of radioactive IAA was impaired in the mutant roots. We also propose a unique distribution of endogenous IAA in aem1 roots. An immunoassay revealed a 4-fold-endogenous IAA content in the aem1 roots compared to WT, and the application of IAA to the shoot of WT seedlings mimicked the short lateral root phenotype of aem1, suggesting that the high content of IAA in aem1 roots impaired the elongation of lateral roots. However, the high level of IAA in aem1 roots contradicts the auxin requirement for root hair formation in the epidermis of mutant roots. Since the reduced development in root hairs of aem1 roots was rescued by exogenous auxin, the auxin level in the epidermis is likely to be sub-optimum in aem1 roots. This discrepancy can be solved by the ideas that IAA level is higher in the stele and lower in the epidermis of aem1 roots compared to WT and that the unique distribution of IAA in aem1 roots is induced by the defect in auxin efflux. All these results suggest that AEM1 may encode a component of auxin efflux carrier in rice and that the defects in lateral roots, root hair formation and root gravity response in aem1 mutant are due to the altered auxin efflux in roots.     

Cellular differentiation in root caps of Zea mays that do not secrete mucilage

We quantified the structural changes accompanying cellular differentiation in root caps of Zea mays cv. Ageotropic to determine the developmental basis for the nongraviresponsiveness of their primary roots. Cells of the calyptrogen and columella of primary roots of the ageotropic mutant have structures indistinguishable from those of caps of primary roots of Z. mays cv. Kys the graviresponsive, wild-type parent of Z. mays cv. Ageotropic. However, the relative volumes of dictyosomes, dictyosome-derived vesicles and starch in the outermost peripheral cells of wild-type roots were significantly lower than were those in peripheral cells of mutant roots. This corresponds to a dramatic accumulation of starch and mucilage-filled vesicles in peripheral cells of mutant roots. Cellular differentiation in root caps of graviresponsive seminal roots of the Ageotropic mutant resembled that of primary and seminal roots of the wild-type cultivar, and differed significantly from that of primary roots of the mutant. We conclude that the mutation that blocks secretion of mucilage from peripheral cells of Ageotropic roots: (1) expresses itself late in cellular differentiation in root caps; (2) is expressed only in primary (but not seminal) roots of the Ageotropic mutant; and (3) is consistent with malfunctioning dictyosomes and dictyosome-derived vesicles being the cellular basis for agravitropism of primary roots of this mutant.     

The regulation of glycolysis and electron transport in roots

The respiration of roots and isolated root mitochondria was investigated in Phaseolus vulgaris L., Spinacea oleracea L.; Triticum aestivum L., and Zea mays L. Although the respiration of both intact roots and isolated mitochondria displayed resistance to cyanide and sensitivity to SHAM, the percentage resistance and inhibition in roots was not the same as that in the mitochondria, with the exception of wheat. Adding FCCP to roots stimulated oxygen uptake and equalized the effects of SHAM and cyanide on roots and mitochondria. In spinach and maize roots, FCCP stimulated both the cytochrome and alternative pathways, while in bean roots, only the alternative pathway was stimulated. FCCP had little effect on wheat root respiration rates. Potential in vivo rates of oxygen uptake were estimated by expressing rates obtained with isolated mitochondria on a fumarase activity basis, and fumarase activity on a root weight basis. In wheat roots the potential rate was approximately equal to the measured in vivo rate; in the other species the potential rates were substantially greater than measured rates, but approximately equal to uncoupled in vivo rates. Key glycolytic intermediates in roots were measured, and it was found that the phosphofructokinase and pyruvate kinase reactions were displaced far from equilibrium, the degree of displacement being approximately equal in roots with little, and roots with substantial, alternative path engagement. Thus, although glycolysis is controlled, the regulation of this pathway appears to be quite flexible. The results are discussed in terms of possible regulatory mechanisms.     

Secreted acid phosphatase is expressed in cluster roots of lupin in response to phosphorus deficiency

The roots of white lupin (Lupinus albus L. cv. Kievskij mutant) secrete acid phosphatase, S-APase, when they grow under conditions of low available phosphorus (P). S-APases hydrolyze organic phosphate compounds in the rhizosphere and supply inorganic phosphate to the plants. Low phosphorus availability also induces vigorous growth of cluster roots. In this study, the function of cluster roots was investigated with reference to S-APase secretion. White lupins were grown in hydroponic culture in a greenhouse under P-deficient and P-sufficient conditions. S-APase in the excised roots after treatment was detected by staining with 4-methylumbelliferone phosphate (MUP). Gene expression of S-APase in cluster and normal roots was also investigated. Activity was greatest in the roots of plants grown under conditions of P -deficiency, particularly in cluster roots. S-APase gene expression was induced by a decrease in internal P concentrations, and was especially high in cluster roots formed under conditions of P -deficiency. It was suggested that decrease of internal P concentration stimulated both of the S-APase expression and cluster root formation.