Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs

Background and AimsRoot proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition.MethodsNear-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg⁻¹ soil.ResultsBoth WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients.ConclusionsIn addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.     

An oligogalacturonide‐derived molecular probe demonstrates the dynamics of calcium‐mediated pectin complexation in cell walls of tip‐growing structures

Pectic homogalacturonan (HG) is one of the main constituents of plant cell walls. When processed to low degrees of esterification, HG can form complexes with divalent calcium ions. These macromolecular structures (also called egg boxes) play an important role in determining the biomechanics of cell walls and in mediating cell‐to‐cell adhesion. Current immunological methods enable only steady‐state detection of egg box formation in situ. Here we present a tool for efficient real‐time visualisation of available sites for HG crosslinking within cell wall microdomains. Our approach is based on calcium‐mediated binding of fluorescently tagged long oligogalacturonides (OGs) with endogenous de‐esterified HG. We established that more than seven galacturonic acid residues in the HG chain are required to form a stable complex with endogenous HG through calcium complexation in situ, confirming a recently suggested thermodynamic model. Using defined carbohydrate microarrays, we show that the long OG probe binds exclusively to HG that has a very low degree of esterification and in the presence of divalent ions. We used this probe to study real‐time dynamics of HG during elongation of Arabidopsis pollen tubes and root hairs. Our results suggest a different spatial organisation of incorporation and processing of HG in the cell walls of these two tip‐growing structures.     

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.     

Influence of plant growth regulators on the growth and essential oil content of cultured Lavandula dentata plantlets

Micropropagation of L. dentata L. through shoot-tips (ca. 5 mm) was achieved successfully. Micropropagated plantlets were cultured without plant regulators in the culture medium (control) or in media containing 0.1 mg l⁻¹ of 6-benzyladenine (BA) and/or indole-3-butyric acid (IBA). These plantlets were examined for their essential oil content and composition in relation to growth rate and density and secretory or postsecretory stage of glandular hairs at five weeks of culture. The growth rates of these plantlets were not always correlated with their essential oil content (0.26%–0.65% v/w). However, all cultured plantlets showed a positive correlation between oil accumulation and the percentage of glandular hairs in secretory stage. Quantitative changes in the major monoterpene components (1,8-cineole, fenchol, borneol and camphor) and sesquiterpene content of plantlet oil, were also observed in response to the effect of varying growth regulator concentration in the culture medium. The influence of this effect on HMG-CoA reductase activity of cultured plantlets is also discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

紫花野菊叶上毛状体的发育形态学研究

利用光镜和扫描电镜研究了紫花野菊叶表面毛状体结构和发育。较详细地描述和讨论了这两种毛的个体发育过程。     

Time-series analysis of measurements on living cells illustrated by analysis of particle movement in the cytoplasm of tomato root hairs

Summary Within a given root hair, the velocity of particle movement, here equated with cytoplasmic streaming, is usually very variable, making it difficult to distinguish experimentally-induced changes in velocity from those due to natural variability. Time-series analysis has been combined with piece-wise linear regression to provide an objective means of assessing the results of experiments in which the rate of streaming was recorded repeatedly from the same root hair. Ordinarily, regression and analysis of variance assume that the values in a data set are not strongly autocorrelated. These methods are applicable to sets of single observations (thus, they cannot be used with data obtained by repeated sampling of the same cell). By contrast, time-series analysis can take account of autocorrelations within the data and allows trends in the noise structure to be separated from treatment effects. The statistical methods described in the present paper are illustrated using the results from experiments in which the effect of -naphthalene acetic acid on streaming velocity in tomato root hairs was recorded. The conclusions reached are in accord with published accounts of variation in protoplasmic streaming and the known behaviour of cells in response to auxins. Our results also provide an explanation for the failure of some workers to observe any consistent changes in streaming velocity in response to exogenous auxin. The method described makes use of well documented statistical techniques and could be applied to other investigations in which sequential measurements of quantifiable parameters, such as fluorometric determination of intracellular pH or concentrations of calcium, are made on intact living cells.Abbreviations NAA -naphthalene acetic acid - ARMA auto-regressive moving average models - AIC akaike information criterion - d.f. degrees of freedom     

Influence of minerals on cytoplasmic streaming in root hairs of intact wheat seedlings (Triticum aestivum L.)

The calcium dependency of the cytoplasmic streaming of wheat root hairs was demonstrated by adding the Ca-Ionophore A 23187. Within three minutes the streaming velocity was decreased dramactically. The influence of ammonium on the cytoplasmic streaming is highly pH-dependent. While at a pH of 9.0 an inhibitory effect was observed even at low ammonium concentrations (0.5 mM) no effect could be measured at a pH of 6.5. Nitrate, independently of medium pH had no effect on the cytoplasmic streaming. The same is true for aluminium. It is suggested that at pH 9 ammonium permiates the plasmalemma as NH₃. Due to higher cytoplasmic pH ( 7.5), NH₃ is protonated leading to an increase in cytoplasmic pH. Ammonium may displace sorbed calcium leading to an increase in the free cytoplasmic calcium responsible for the cessation of the streaming. Alternative explanations are discussed.Abbreviations HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulfonic acid