Comparison of growth, exogenous auxin sensitivity, and endogenous indole-3-acetic acid content in roots of Hordeum vulgare L. and an agravitropic mutant

Fluorescent reporter lines for auxin and cytokinin signalling in barley (Hordeum vulgare)

The phytohormones auxin and cytokinin control development and maintenance of plant meristems and stem cell systems. Fluorescent protein reporter lines that monitor phytohormone controlled gene expression programmes have been widely used to study development and differentiation in the model species Arabidopsis, but equivalent tools are still missing for the majority of crop species. Barley (Hordeum vulgare) is the fourth most abundant cereal crop plant, but knowledge on these important phytohormones in regard to the barley root and shoot stem cell niches is still negligible. We have now analysed the role of auxin and cytokinin in barley root meristem development, and present fluorescent protein reporter lines that allow to dissect auxin and cytokinin signalling outputs in vivo. We found that application of either auxin or cytokinin to barley seedlings negatively impacts root meristem growth. We further established a barley cytokinin reporter, TCSnew, which revealed significant cytokinin signalling in the stele cells proximal to the QC, and in the differentiated root cap cells. Application of exogenous cytokinin activated signalling in the root stem cell niche. Commonly employed auxin reporters DR5 or DR5v2 failed to respond to auxin in barley. However, analysis of putative auxin signalling targets barley PLETHORA1 (HvPLT1) is expressed in a similar pattern as its orthologue AtPLT1 from Arabidopsis, i.e. in the QC and the surrounding cells. Furthermore, the PINFORMED1 (HvPIN1) auxin efflux carrier was found to be expressed in root and shoot meristems, where it polarly localized to the plasma membrane. HvPIN1 expression is negatively regulated by cytokinin and its intracellular localisation is sensitive to brefeldinA (BFA). With this study, we provide the first fluorescent reporter lines as a tool to study auxin and cytokinin signalling and response pathways in barley.

From weeds to crops: genetic analysis of root development in cereals

Root development of Arabidopsis, Zea mays (maize) and Oryza sativa (rice) differs in both overall architecture and the anatomy of individual roots. In maize and rice, the post-embryonic shoot-borne root system becomes the major backbone of the root stock; in Arabidopsis, the embryonic root system formed by a simple primary root and its lateral roots remains dominant. Recently, several specific root mutants and root-specific genes have been identified and characterized in maize and rice. Interestingly, some of these mutants indicate that the formation of primary-, seminal-, crown- and lateral roots is regulated by alternative root-type-specific pathways. Further analyses of these unique pathways will contribute to the understanding of the complex molecular networks involved in cereal root formation.

Root gravitropism

When a plant root is reoriented within the gravity field, it responds by initiating a curvature which eventually results in vertical growth. Gravity sensing occurs primarily in the root tip. It may involve amyloplast sedimentation in the columella cells of the root cap, or the detection of forces exerted by the mass of the protoplast on opposite sides of its cell wall. Gravisensing activates a signal transduction cascade which results in the asymmetric redistribution of auxin and apoplastic Ca2+ across the root tip, with accumulation at the bottom side. The resulting lateral asymmetry in Ca2+ and auxin concentration is probably transmitted to the elongation zone where differential cellular elongation occurs until the tip resumes vertical growth. The Cholodny-Went theory proposes that gravity-induced auxin redistribution across a gravistimulated plant organ is responsible for the gravitropic response. However, recent data indicate that the gravity-induced reorientation is more complex, involving both auxin gradient-dependent and auxin gradient-independent events.

Basis for changes in the auxin-sensitivity of Avena sativa (oat) leaf-sheath pulvini during the gravitropic response

During the gravitropic response, auxin-sensitivity of the lower flanks of leaf-sheath pulvini of Avena sativa (oat) is at least 1000-fold higher than those of the upper flanks and non-gravistimulated pulvini. When the pulvini are treated with 1 mM Ca2+, a 10-fold increase in auxin-sensitivity of the pulvini is observed. Related to this difference in auxin-sensitivity, in vitro activation of the vanadate-sensitive H(-)-ATPase by IAA was observed. Results show that the activation of the H(+)-ATPase by IAA is probably mediated by soluble protein factors and that the H(+)-ATPase prepared from the lower flanks is activated by IAA with a 1000-fold higher auxin-sensitivity as compared with that from the upper flanks of the graviresponding pulvini. Ammonium sulfate fractionation experiments show that these soluble protein factors are in the 30 to 60% fraction. Auxin-binding assays reveal that lower flanks contain more high-affinity soluble auxin-binding sites (kD; on the order of 10(-9) M) and less low-affinity soluble auxin-binding sites (kD; on the order of 10(-6) M) than upper flanks. It is concluded that differential auxin-sensitivity of graviresponding oat-shoot pulvini is achieved by the modulation of affinities of auxin-binding sites in upper and lower flanks of the pulvini, that Ca2+ is involved in such modulation, and that one of the probable cellular functions of these auxin binding sites is the activation of the proton pump on the plasma membranes.

A novel root gravitropism mutant of Arabidopsis thaliana exhibiting altered auxin physiology

A root gravitropism mutant was isolated from the DuPont Arabidopsis thaliana T-DNA insertional mutagenesis collection. This mutant has reduced root gravitropism, hence the name rgr1. Roots of rgr1 are shorter than those of wild-type, and they have reduced lateral root formation. In addition, roots of rgr1 coil clockwise on inclined agar plates, unlike wild-type roots which grow in a wavy pattern. The rgr1 mutant has increased resistance, as measured by root elongation, to exogenously applied auxins (6-fold to indole-3-acetic acid, 3-fold to 2,4-dichlorophenoxyacetic acid, and 2-fold to napthyleneacetic acid). It is also resistant to polar auxin transport inhibitors (2-fold to triiodobenzoic acid and 3- to 5-fold to napthylphthalamic acid). The rgr1 mutant does not appear to be resistant to other plant hormone classes. When grown in the presence of 10(-7) M 2,4-dichlorophenoxyacetic acid, rgr1 roots have fewer root hairs than wild type. All these rgr1 phenotypes are Mendelian recessives. Complementation tests indicate that rgr1 is not allelic to previously characterized agravitropic or auxin-resistant mutants. The rgr1 locus was mapped using visible markers to 1.4 +/- 0.6 map units from the CH1 locus at 1-65.4. The rgr1 mutation and the T-DNA cosegregate, suggesting that rgr1 was caused by insertional gene inactivation.

Approaches to understanding auxin action

The effects of auxin on plant growth and development have been studied for decades, but the molecular mechanisms of auxin action remain unknown. These mechanisms have primarily been investigated by characterization of auxin physiology mutants and analysis of auxin-binding proteins and auxin-regulated genes. These efforts are now converging, since some mutants have recently been shown to have altered expression of specific auxin-binding proteins and auxin-regulated genes. The features of these proteins and genes are providing the first tantalizing clues to the organization of auxin signal transduction pathways.

Genetic approaches to auxin action

Answers to long-standing questions concerning the molecular mechanism of auxin action and auxin's exact functions in plant growth and development are beginning to be uncovered through studies using mutant and transgenic plants. We review recent work in this area in vascular plants. A number of conclusions can be drawn from these studies. First, auxin appears essential for cell division and viability, as auxin auxotrophs isolated in tissue culture are dependent on auxin for growth and cannot be regenerated into plants even when auxin is supplied exogenously. Secondly, plants with transgenes that alter auxin levels are able to regulate cellular auxin concentrations by synthesis and conjugation; wild-type plants are probably also capable of such regulation. Thirdly, the phenotypes of transgenic plants with altered auxin levels and of mutant plants with altered sensitivity to auxin confirm earlier physiological studies which indicated a role for auxin in regulation of apical dominance, in development of roots and vascular tissue, and in the gravitropic response. Finally, the cloning of a mutationally identified gene important for auxin action, along with accumulating biochemical evidence, hints at a major role for protein degradation in the auxin response pathway.

Mutants of Nicotiana plumbaginifolia with specific resistance to auxin

We have isolated nine independent auxin-resistant mutants of Nicotiana plumbaginifolia by culturing M2 seedlings in the presence of indole-3-acetic acid ethyl ester or 1-naphthaleneacetic acid at concentrations which significantly inhibit hypocotyl elongation of the wild type. The mutations were induced by treating seed with ethyl methanesulphonate and were found in the course of screening 10,000 individual M2 families. Auxin resistance was in all cases the result of a mutation at a single, nuclear locus. The dominance relationships of two of the mutants could be defined as recessive or dominant; all other mutants showed partial dominance. In contrast to previously described mutants of Arabidopsis and N. plumbaginifolia, all of the present mutants were specifically resistant to auxin; the mutants were cross-resistant to several auxins, but showed no increased resistance to cytokinin, abscisic acid, ethylene or 1-amino-cyclopropane-1-carboxylic acid. The importance of the choice of the selection criterion for the isolation of specific resistance traits is discussed.