Jocelyn E. Malamy

Salicylic acid: a likely endogenous signal in the resistance response of tobacco to viral infection

Some cultivars of tobacco are resistant to tobacco mosaic virus (TMV) and synthesize pathogenesis-related (PR) proteins upon infection. In a search for the signal or signals that induce resistance or PR genes, it was found that the endogenous salicylic acid levels in resistant, but not susceptible, cultivars increased at least 20-fold in infected leaves and 5-fold in uninfected leaves after TMV inoculation. Induction of PRl genes paralleled the rise in salicylic acid levels. Since earlier work has demonstrated that treatment with exogenous salicylic acid induces PR genes and resistance, these findings suggest that salicylic acid functions as the natural transduction signal.

An Auxin-Dependent Distal Organizer of Pattern and Polarity in the Arabidopsis Root

Root formation in plants involves the continuous interpretation of positional cues. Physiological studies have linked root formation to auxins. An auxin response element displays a maximum in the Arabidopsis root and we investigate its developmental significance. Auxin response mutants reduce the maximum or its perception, and interfere with distal root patterning. Polar auxin transport mutants affect its localization and distal pattern. Polar auxin transport inhibitors cause dramatic relocalization of the maximum, and associated changes in pattern and polarity. Auxin application and laser ablations correlate root pattern with a maximum adjacent to the vascular bundle. Our data indicate that an auxin maximum at a vascular boundary establishes a distal organizer in the root.

The SCARECROW Gene Regulates an Asymmetric Cell Division That Is Essential for Generating the Radial Organization of the Arabidopsis Root

In the Arabidopsis root meristem, initial cells undergo asymmetric divisions to generate the cell lineages of the root. The scarecrow mutation results in roots that are missing one cell layer owing to the disruption of an asymmetric division that normally generates cortex and endodermis. Tissue-specific markers indicate that a heterogeneous cell type is formed in the mutant. The deduced amino acid sequence of SCARECROW (SCR) suggests that it is a member of a novel family of putative transcription factors. SCR is expressed in the cortex/endodermal initial cells and in the endodermal cell lineage. Tissue-specific expression is regulated at the transcriptional level. These results indicate a key role for SCR in regulating the radial organization of the root.

The salicylic acid signal in plants

Plants are one of the world’s richest sources of natural medicines. The use of plants and plant extracts for healing dates back to earliest recorded history. Today, such plant-derived medicines as quinine, digitalis, opiates and morphine are widely used, while new natural chemicals such as the putative anti-cancer drug taxol from yew tree bark are being characterized and developed.

Temperature-Dependent Induction of Salicylic Acid and Its Conjugates during the Resistance Response to Tobacco Mosaic Virus Infection.

Increases in endogenous salicylic acid (SA) levels and induction of several families of pathogenesis-related genes (PR-1 through PR-5) occur during the resistance response of tobacco to tobacco mosaic virus infection. We found that at temperatures that prevent the induction of PR genes and resistance, the increases in SA levels were eliminated. The addition of exogenous SA to infected plants at these temperatures was sufficient to induce the PR genes but not the hypersensitive response. However, when the resistance response was restored by shifting infected plants to permissive temperatures, SA levels increased dramatically and preceded PR-1 gene expression and necrotic lesion formation associated with resistance. SA was also found in a conjugated form whose levels increased in parallel with the free SA levels. The majority of the conjugates appeared to be SA glucosides. The same glucoside was formed when plants were supplied with exogenous SA. These results provide further evidence that endogenous SA signals the induction of certain defense responses and suggests additional complexity in the modulation of this signal.

Transcriptional Profile of the Arabidopsis Root Quiescent Center

The self-renewal characteristics of stem cells render them vital engines of development. To better understand the molecular mechanisms that determine the properties of stem cells, transcript profiling was conducted on quiescent center (QC) cells from the Arabidopsis thaliana root meristem. The AGAMOUS-LIKE 42 (AGL42) gene, which encodes a MADS box transcription factor whose expression is enriched in the QC, was used to mark these cells. RNA was isolated from sorted cells, labeled, and hybridized to Affymetrix microarrays. Comparisons with digital in situ expression profiles of surrounding tissues identified a set of genes enriched in the QC. Promoter regions from a subset of transcription factors identified as enriched in the QC conferred expression in the QC. These studies demonstrated that it is possible to successfully isolate and profile a rare cell type in the plant. Mutations in all enriched transcription factor genes including AGL42 exhibited no detectable root phenotype, raising the possibility of a high degree of functional redundancy in the QC.

Down and out in Arabidopsis: the formation of lateral roots

The number and placement of lateral roots on the primary root is not predetermined, but can be drastically affected by the availability of water and nutrients in the soil. To respond to local and often transient conditions, the plant must be able to perceive environmental cues and respond by initiating lateral roots in appropriate positions. This involves recruiting small numbers of mature cells to resume dividing and enter a program of postembryonic organogenesis. Through the use of new mutants and molecular markers of cell differentiation, the molecular mechanisms involved in lateral root initiation and development are beginning to be understood.

Root System Architecture in Arabidopsis Grown in Culture Is Regulated by Sucrose Uptake in the Aerial Tissues

This article presents a detailed model for the regulation of lateral root formation in Arabidopsis thaliana seedlings grown in culture. We demonstrate that direct contact between the aerial tissues and sucrose in the growth media is necessary and sufficient to promote emergence of lateral root primordia from the parent root. Mild osmotic stress is perceived by the root, which then sends an abscisic acid–dependent signal that causes a decrease in the permeability of aerial tissues; this reduces uptake of sucrose from the culture media, which leads to a repression of lateral root formation. Osmotic repression of lateral root formation in culture can be overcome by mutations that cause the cuticle of a plants aerial tissues to become more permeable. Indeed, we report here that the previously described lateral root development2 mutant overcomes osmotic repression of lateral root formation because of a point mutation in Long Chain Acyl-CoA Synthetase2, a gene essential for cutin biosynthesis. Together, our findings (1) impact the interpretation of experiments that use Arabidopsis grown in culture to study root system architecture; (2) identify sucrose as an unexpected regulator of lateral root formation; (3) demonstrate mechanisms by which roots communicate information to aerial tissues and receive information in turn; and (4) provide insights into the regulatory pathways that allow plants to be developmentally plastic while preserving the essential balance between aboveground and belowground organs.

Identification of Quantitative Trait Loci That Regulate Arabidopsis Root System Size and Plasticity

Root system size (RSS) is a complex trait that is greatly influenced by environmental cues. Hence, both intrinsic developmental pathways and environmental-response pathways contribute to RSS. To assess the natural variation in both types of pathways, we examined the root systems of the closely related Arabidopsis ecotypes Landsberg erecta (Ler) and Columbia (Col) grown under mild osmotic stress conditions. We found that Ler initiates more lateral root primordia, produces lateral roots from a higher percentage of these primordia, and has an overall larger root system than Col under these conditions. Furthermore, although each of these parameters is reduced by osmotic stress in both ecotypes, Ler shows a decreased sensitivity to osmotica. To understand the genetic basis for these differences, QTL for RSS under mild osmotic stress were mapped in a Ler × Col recombinant inbred population. Two robust quantitative trait loci (QTL) were identified and confirmed in near-isogenic lines (NILs). The NILs also allowed us to define distinct physiological roles for the gene(s) at each locus. This study provides insight into the genetic and physiological complexity that determines RSS and begins to dissect the molecular basis for naturally occurring differences in morphology and developmental plasticity in the root system.

Root System Architecture

Abstract Plants develop most organs post-embryonically, which allows the incorporation of environmental information into decisions concerning when and where to produce new organs. This developmental plasticity is evident in the plant root system, which in dicotyledonous plants such as Arabidopsis thaliana is mostly comprised of lateral and adventitious roots that develop along the length of the primary root. The rate of primary root growth and the location, spacing and growth rate of lateral roots are influenced by the availability of environmental cues such as water and nutrients, which can have dramatic effects on the final architecture of the root system. These environmental responses must intersect with the intrinsic developmental programme of the plant, which is responsible for the general formation and maintenance of the root system. The final root system architecture of any plant is then the product of both intrinsic and environmental response pathways. Carbohydrates and plant hormones such as auxin and cytokinins are required for both intrinsic root development and modulating root system architecture in response to different growth conditions, thus facilitating the optimisation of root growth in complex, heterogeneous environments.