Phosphorus (P) uptake and growth of a root hairless barley mutant (bald root barley, brb) and wild type in low- and high-P soils

The recently isolated root‐hairless mutant of barley (Hordeum vulgare L), bald root barley, brb offers a unique possibility to quantify the importance of root hairs in phosphorus (P) uptake from soil. In the present study the ability of brb and the wild‐type, barley genotype Pallas producing normal root hairs to deplete P in the rhizosphere soil was investigated and the theory of diffusion and mass flow applied to compare the predicted and measured depletion profiles of diffusible P. Pallas depleted twice as much P from the rhizosphere soil as brb. The P depletion profile of Pallas uniformly extended to 0.8 mm from the root surface, which was equal to the root hair length (RHL). The model based on the theory of diffusion and mass flow explained the observed P‐depletion profile of brb, and the P depletion outside the root‐hair zone of Pallas, suggesting that the model is valid only for P movement in rhizosphere soil outside the root‐hair zone. In low‐P soil (P in soil solution 3 µm ) brb did not survive after 30 d, whereas Pallas continued to grow, confirming the importance of root hairs in plant growth in a P‐limiting environment. In high‐P soil (P in soil solution 10 µm ) both brb and Pallas maintained their growth, and they were able to produce seeds. At the high‐P concentration, RHL of the Pallas was reduced from 0.80 ± 0.2 to 0.68 ± 0.14 mm. In low‐P soil, P‐uptake rate into the roots of Pallas was 4.0 × 10^?7 g mm^?1 d^?1 and that of brb was 1.9 × 10^?7 g mm^?1 d^?1, which agreed well with the double amount of P depleted from the rhizosphere soil of Pallas in comparison with that of brb. In high‐P soil, the P uptake rates into the roots of brb and Pallas were 3.3 and 5.5 × 10^?7 g mm^?1 d^?1, respectively. The results unequivocally confirmed that in a low‐P environment, root hairs are of immense importance in P acquisition and plants survival, but under high‐P conditions they may be dispensable. The characterization of phenotypes brb and Pallas and the ability to reproduce seeds offers a unique possibility of molecular mapping of QTLs and candidate genes conferring root‐hair formation and growth of barley.     

Influence of maize root mucilage on soil aggregate stability

This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg⁻¹ dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO₂ was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO₂ evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO₂ up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.     

Phosphorus (P) acquisition of cereal cultivars in the field at three levels of P fertilization

Low phosphorus (P) availability in soils and diminishing P reserves emphasize the need to create plants that are more efficient P users. Knowledge of P efficient germplasm among the existing cereal varieties may serve as the basis for improving soil P use by selection and breeding. We had identified some cereal cultivars (winter wheat: Kosack and Kraka; winter barley: Hamu and Angora; spring barley: Canut, Alexis, Salka, Zita;) which differed (p<0.05) in P depletion from thin slices (0.2 mm) of the rhizosphere soil under controlled conditions. In the present study, the same cultivars were studied under field conditions at three levels of P supply (no-P, 10 and 20 kg P ha^-1) and the differences in P uptake as found in the previous work were confirmed. Under both conditions, the variation between the cultivars was greatest in soil without P fertilizers (no-P) for about 30 years. The variation in P uptake with most cultivars disappeared when 10 kg P ha^-1 was applied. Root development did not differ between the cultivars much, but there was wide, consistent variation in their root hairs, regardless of growth media (solution, soil column and field). Increase in soil P level reduced the length of root hairs. The variation in root hairs between the cultivars was largest in no-P soil. When 10 kg P ha^-1 was applied, the root hair lengths did not differ between the cultivars. Barley cultivars with longer root hairs depleted more P from the rhizosphere soil and also absorbed more P in the field. The relationship between root hairs and phosphorus uptake of the wheat cultivars was less clear. The wide variation in P uptake among the barley cultivars in the field and its relationship to the root hair development confirms that root hair length may be a suitable plant characteristic to use as criterion for selecting barley cultivars for P efficiency, especially in low-P soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

A root hairless barley mutant for elucidating genetic of root hairs and phosphorus uptake

This paper reports a new barley mutant missing root hairs. The mutant was spontaneously discovered among the population of wild type (Pallas, a spring barley cultivar), producing normal, 0.8 mm long root hairs. We have called the mutant bald root barley (brb). Root anatomical studies confirmed the lack of root hairs on mutant roots. Amplified Fragment Length Polymorphism (AFLP) analyses of the genomes of the mutant and Pallas supported that the brb mutant has its genetic background in Pallas. The segregation ratio of selfed F₂ plants, resulting from mutant and Pallas outcross, was 1:3 (–root hairs:+root hairs), suggesting a monogenic recessive mode of inheritance.In rhizosphere studies, Pallas absorbed nearly two times more phosphorus (P) than the mutant. Most of available inorganic P in the root hair zone (0.8 mm) of Pallas was depleted, as indicated by the uniform P depletion profile near its roots. The acid phosphatase (Apase) activity near the roots of Pallas was higher and Pallas mobilised more organic P in the rhizosphere than the mutant. The higher Apase activity near Pallas roots also suggests a link between root hair formation and rhizosphere Apase activity. Hence, root hairs are important for increasing plant P uptake of inorganic as well as mobilisation of organic P in soils.Laboratory, pot and field studies showed that barley cultivars with longer root hairs (1.10 mm), extracted more P from rhizosphere soil, absorbed more P in low-P field (Olsen P=14 mg P kg^–1 soil), and produced more shoot biomass than shorter root hair cultivars (0.63 mm). Especially in low-P soil, the differences in root hair length and P uptake among the cultivars were significantly larger. Based on the results, the perspectives of genetic analysis of root hairs and their importance in P uptake and field performance of cereals are discussed.     

金边黄杨与大叶黄杨光合特性的比较

在田间条件下,比较金边黄杨与大叶黄杨光合生理特性的结果表明：（1）金边黄杨的叶绿素含量极显著低于大叶黄杨,类胡萝卜素和花色素苷含量无显著差异;（2）金边黄杨的光合能力显著低于大叶黄杨,表现量子效率、羧化效率分别为大叶黄杨的54％和50％;（3）二者的净光合速率日变化均为单峰曲线,金边黄杨净光合速率和蒸腾速率日积分值分别为大叶黄杨的60％和75％。     

What are the implications of variation in root hair length on tolerance to phosphorus deficiency in combination with water stress in barley (Hordeum vulgare)?

Background and Aims

Phosphorus commonly limits crop yield and is frequently applied as fertilizer; however, supplies of quality rock phosphate for fertilizer production are diminishing. Plants have evolved many mechanisms to increase their P-fertilizer use efficiency, and an understanding of these traits could result in improved long-term sustainability of agriculture. Here a mutant population is utilized to assess the impact of root hair length on P acquisition and yield under P-deficient conditions alone or when combined with drought.

Methods

Mutants with various root hair phenotypes were grown in the glasshouse in pots filled with soil representing sufficient and deficient P treatments and, in one experiment, a range of water availability was also imposed. Plants were variously harvested at 7 d, 8 weeks and 14 weeks, and variables including root hair length, rhizosheath weight, biomass, P accumulation and yield were measured.

Key Results

The results confirmed the robustness of the root hair phenotypes in soils and their relationship to rhizosheath production. The data demonstrated that root hair length is important for shoot P accumulation and biomass, while only the presence of root hairs is critical for yield. Root hair presence was also critical for tolerance to extreme combined P deficit and drought stress, with genotypes with no root hairs suffering extreme growth retardation in comparison with those with root hairs.

Conclusions

The results suggest that although root hair length is not important for maintaining yield, the presence of root hairs is implicit to sustainable yield of barley under P-deficient conditions and when combined with extreme drought. Root hairs are a trait that should be maintained in future germplasm.     

Signal transmission during gravitropic curvature of primary roots of Zea mays

Primary roots of Zea mays cv. Ageotropic are nonresponsive to gravity and elongate approximately 0.80 mm h^?1. Applying mucilage-like material (K-Y Jelly) to the terminal 1.5 cm of these roots induces graviresponsiveness and slow elongation 28% (i.e. from 0.80 to 0.58mm h^?1). Applying mucilage-like material to one side of the terminal 1.5 cm of the root induces curvature toward the mucilage, irrespective of the root's orientation to gravity. Applying a 2-mm-wideband of mucilage-like material to a root's circumference 8 to 10 mm behind the root cap neither induces gravicurvature nor affects elongation significantly. Similarly, applying mucilage-like material to only the root cap does not significantly affect elongation or graviresponsiveness. Gravicurvature of mutant roots occurs only when mucilage-like material is applied to the root/root-cap junction. Reversing the caps of wild-type and mutant roots produces gravitropic responses characteristic of the root cap rather than the host root. These results are consistent with the suggestion that gravitropic effectors are growth inhibitors that move apoplastically through mucilage between the root cap and root.     

Root Cap Mucilage and Extracellular Calcium as Modulators of Cellular Growth in Postmitotic Growth Zones of the Maize Root Apex*

Abstract: The control of maize root growth by root cap mucilage and extracellular calcium (Ca) was examined. Special attention was paid to the influence of these factors on cellular aspects of root growth, such as cell shape and organization of the microtubular (MT) cytoskeleton. Externally supplied Ca impaired the transition of early post-mitotic cells from a more-or-less apolar mode of expansion to a strictly anisotropic mode of elongation accompanied by their more rapid growth. However, this inhibitory effect of Ca was not associated with any re-arrangement of the cortical MTs, their transverse arrays, with respect to the root axis, being maintained under these conditions. Root mucilage, collected from donor root caps and placed around root tips, exerted a similar effect on cell shapes as did externally supplied Ca. In contrast, roots grown in a medium of low Ca content, or from which the root cap mucilage was continually removed, had more elongated cell shapes in their post-mitotic growth regions when compared to the control roots. These findings are consistent with a notion that Ca is present in the root cap mucilage in physiologically relevant amounts and can mediate growth responses in both the PIG region and the apical part of the elongation zone. Integrating several known effects of Ca ions on growth at the root apex, a hypothesis is proposed that a Ca-mediated and MT-independent control of cell growth in the PIG region might be involved in morphogenetic root movements (e.g. gravitropism), and that root growth responses could be initiated by an asymmetric distribution of extracellular calcium, or root cap slime, around the growing root tip.     

The effect of abscisic acid on root and shoot growth of cauliflower plants

Addition of abscisic acid (ABA) to the nutrient solution increased the root to shoot ratio of hydroponically-grown cauliflower plants by reducing the dry weight of the shoot and increasing that of the root. At concentrations higher than 10^–7 M, ABA increased root branching and root hair formation. Root extension was inhibited in plants kept continuously in solutions containing high ABA concentrations but following removal from the ABA solution root elongation was increased in comparison with plants given no ABA treatment. This elongation was greatest in plants with increased root branching caused by higher ABA concentrations.     

Tribology of the root cap in maize (Zea mays) and peas (Pisum sativum)

Frictional resistance to a penetrating body can account for more than 80% of the total resistance to penetration of soil. We measured the frictional resistance between growing root caps of maize and pea and ground and smooth glass surfaces, which was linearly correlated to load, allowing calculation of the coefficient of kinetic friction and adhesion. Coefficients of kinetic friction between the root caps and the ground and smooth glass surfaces were approximately 0.04 and 0.02, respectively, the first measurements of the frictional properties of root tips at rates approaching those of root elongation, and an order of magnitude smaller than those previously reported. Results suggest that roots are well designed for penetrating soil, and encounter only small frictional resistance on the root cap. These data provide important parameters for modelling soil stresses and deformation around growing root tips.     

The longevity and activity of the primary root of maize

The longevity of the main root cylinder and the laterals of the primary root of maize plants was determined under controlled greenhouse conditions by means of nuclear staining with acridine orange. The cortex of the main root was found to be alive for the whole life-span of the plant, whereas the life-span of the root hairs was only 2 to 3 days as evidenced by electronmicroscopical examination of cell integrity. The onset of senescence of laterals was observed at the older part of the main root at the 6-leaf stage of the plant. Senescence of 1st and 2nd order laterals commenced near the root tip a few days after their protrusion and advanced towards the basal region of the root. In any root segment death of the cortex cells preceeded that of the stele. At the late grain filling stage all laterals along the main root exhibited advanced senescence, but stainable nuclei were seen in the root tissues of the basal part of 1^st order laterals (both cortex and stele) as well as of the 2^nd order laterals which emerged from that root segment. The pattern of the dying of the root tissue is discussed with regard to the P-nutrition of the shoot system by the primary root.