Klaus Palme

Sterol-dependent endocytosis mediates post-cytokinetic acquisition of PIN2 auxin efflux carrier polarity

The polarization of yeast and animal cells relies on membrane sterols for polar targeting of proteins to the plasma membrane, their polar endocytic recycling and restricted lateral diffusion. However, little is known about sterol function in plant-cell polarity. Directional root growth along the gravity vector requires polar transport of the plant hormone auxin. In Arabidopsis, asymmetric plasma membrane localization of the PIN–FORMED2 (PIN2) auxin transporter directs root gravitropism. Although the composition of membrane sterols influences gravitropism and localization of two other PIN proteins, it remains unknown how sterols contribute mechanistically to PIN polarity. Here, we show that correct membrane sterol composition is essential for the acquisition of PIN2 polarity. Polar PIN2 localization is defective in the sterol-biosynthesis mutant cyclopropylsterol isomerase1-1 (cpi1-1) which displays altered sterol composition, PIN2 endocytosis, and root gravitropism. At the end of cytokinesis, PIN2 localizes initially to both newly formed membranes but subsequently disappears from one. By contrast, PIN2 frequently remains at both daughter membranes in endocytosis-defective cpi1-1 cells. Hence, sterol composition affects post-cytokinetic acquisition of PIN2 polarity by endocytosis, suggesting a mechanism for sterol action on establishment of asymmetric protein localization.

Mechanical induction of lateral root initiation in Arabidopsis thaliana

Lateral roots are initiated postembryonically in response to environmental cues, enabling plants to explore efficiently their underground environment. However, the mechanisms by which the environment determines the position of lateral root formation are unknown. In this study, we demonstrate that in Arabidopsis thaliana lateral root initiation can be induced mechanically by either gravitropic curvature or by the transient bending of a root by hand. The plant hormone auxin accumulates at the site of lateral root induction before a primordium starts to form. Here we describe a subcellular relocalization of PIN1, an auxin transport protein, in a single protoxylem cell in response to gravitropic curvature. This relocalization precedes auxin-dependent gene transcription at the site of a new primordium. Auxin-dependent nuclear signaling is necessary for lateral root formation; arf7/19 double knock-out mutants normally form no lateral roots but do so upon bending when the root tip is removed. Signaling through arf7/19 can therefore be bypassed by root bending. These data support a model in which a root-tip-derived signal acts on downstream signaling molecules that specify lateral root identity.

Mutation of the Rice Narrow leaf1 Gene, Which Encodes a Novel Protein, Affects Vein Patterning and Polar Auxin Transport

The size and shape of the plant leaf is an important agronomic trait. To understand the molecular mechanism governing plant leaf shape, we characterized a classic rice (Oryza sativa) dwarf mutant named narrow leaf1 (nal1), which exhibits a characteristic phenotype of narrow leaves. In accordance with reduced leaf blade width, leaves of nal1 contain a decreased number of longitudinal veins. Anatomical investigations revealed that the culms of nal1 also show a defective vascular system, in which the number and distribution pattern of vascular bundles are altered. Map-based cloning and genetic complementation analyses demonstrated that Nal1 encodes a plant-specific protein with unknown biochemical function. We provide evidence showing that Nal1 is richly expressed in vascular tissues and that mutation of this gene leads to significantly reduced polar auxin transport capacity. These results indicate that Nal1 affects polar auxin transport as well as the vascular patterns of rice plants and plays an important role in the control of lateral leaf growth.

Auxin as a Model for the Integration of Hormonal Signal Processing and Transduction

The regulation of plant growth responds to many stimuli. These responses allow environmental adaptation, thereby increasing fitness. In many cases, the relay of information about a plants environment is through plant hormones. These messengers integrate environmental information into developmental pathways to determine plant shape. This review will use, as an example, auxin in the root of Arabidopsis thaliana to illustrate the complex nature of hormonal signal processing and transduction. It will then make the case that the application of a systems-biology approach is necessary, if the relationship between a plants environment and its growth/developmental responses is to be properly understood.

Blue shift of CdSe/ZnS nanocrystal-labels upon DNA-hybridization.

Luminescence color multiplexing is one of the most intriguing benefits, which might occur by using semiconductor Quantum Dots (QDs) as labels for biomolecules. It was found, that the luminescence of QDs can be quenched, and replaced by a luminescence peak at approximately 460 nm on hybridization with certain regions of Arabidopsis thaliana tissue. This effect is site selective, and it is unclear whether it occurs due to an energy transfer process, or due to quenching and scattering of the excitation light. The article describes methods for phase-transfer of differently coloured, hydrophobically ligated QDs, coupling of DNA strands to the QDs surface, and hybridization of the labelled DNA to different cell types of Arabidopsis thaliana. The reason for the luminescence blue-shift was studied systematically, and narrowed down to the above mentioned causes.

Organogenic nodule development in hop (Humulus lupulus L.): Transcript and metabolic responses

BackgroundHop (Humulus lupulus L.) is an economically important plant forming organogenic nodules which can be used for genetic transformation and micropropagation. We are interested in the mechanisms underlying reprogramming of cells through stress and hormone treatments.ResultsAn integrated molecular and metabolomic approach was used to investigate global gene expression and metabolic responses during development of hops organogenic nodules.Transcript profiling using a 3,324-cDNA clone array revealed differential regulation of 133 unigenes, classified into 11 functional categories. Several pathways seem to be determinant in organogenic nodule formation, namely defense and stress response, sugar and lipid metabolism, synthesis of secondary metabolites and hormone signaling. Metabolic profiling using 1H NMR spectroscopy associated to two-dimensional techniques showed the importance of metabolites related to oxidative stress response, lipid and sugar metabolism and secondary metabolism in organogenic nodule formation.ConclusionThe expression profile of genes pivotal for energy metabolism, together with metabolites profile, suggested that these morphogenic structures gain energy through a heterotrophic, transport-dependent and sugar-degrading anaerobic metabolism. Polyamines and auxins are likely to be involved in the regulation of expression of many genes related to organogenic nodule formation. These results represent substantial progress toward a better understanding of this complex developmental program and reveal novel information regarding morphogenesis in plants.

A New Gene for Auxin Synthesis

There is much interest in understanding the pathways that trigger biosynthesis of the plant hormone auxin. In this issue, Stepanova et al. (2008) and Tao et al. (2008) reveal that a small family of tryptophan aminotransferases catalyze formation of indole-3-pyruvic acid (IPA) from L-tryptophan (L-Trp), the first step in a pathway for auxin biosynthesis.