Floral development at the Arabidopsis shoot apical meristem occurs in response to environmental cues that are perceived in different tissues. Photoperiod is detected in the vascular tissue of the leaf (phloem) and promotes production of a systemic signal that induces flowering at the meristem. Vernalization, the response to winter temperatures, overcomes a block on photoperiodic floral induction. In Arabidopsis, this block is caused by inhibitors of flowering that comprise several related MADS-box transcription factors, the most prominent of which is FLC. We show that FLC delays flowering by repressing production in the leaf of at least two systemic signals, one of which is controlled by the RAF kinase inhibitor- In Arabidopsis, reproductive development is initiated by the formation of floral primordia on the flanks of the shoot apical meristem. The time at which flowering occurs is governed by environmental cues such as day length and temperature, and is influenced by endogenous signals related to the age of the plant. Classical physiological experiments demonstrated that environmental signals that influence flowering are perceived in different tissues. For example, day length is detected in expanded leaves, and in response to exposure to day lengths that trigger flowering, a systemic signal termed the floral stimulus travels through the phloem to the shoot apical meristem, where it induces floral development (Knott 1934;Zeevaart 1976;Corbesier and Coupland 2005). Vernalization, the acquisition of competence to flower that results from exposure to extended periods of low temperatures that mimic winter conditions, typically relieves a block to the photoperiod pathway (Lang 1965).Here we describe an analysis of the tissues in which key Arabidopsis genes involved in vernalization act to control flowering and demonstrate the significance of transcriptional regulation in the phloem and the meristem for the vernalization response.Genetic analyses of the control of flowering in Arabidopsis thaliana identified four major floral promotion pathways Boss et al. 2004). Environmental responses studied genetically in Arabidopsis include photoperiodic control of flowering, induction of flowering by response to long days (LD) of spring or early summer, and vernalization, which is the promotion of flowering by extended exposures to low temperatures that mimic winter conditions. The responses to day length (or photoperiod) and vernalization are mediated by pathways that specifically control responses to

Homozygous deletion of the survival motor neuron 1 gene (SMN1) causes spinal muscular atrophy (SMA), the most frequent genetic cause of early childhood lethality. In rare instances, however, individuals are asymptomatic despite carrying the same SMN1 mutations as their affected siblings, thereby suggesting the influence of modifier genes. We discovered that unaffected SMN1-deleted females exhibit significantly higher expression of plastin 3 (PLS3) than their SMA-affected counterparts. We demonstrated that PLS3 is important for axonogenesis through increasing the F-actin level. Overexpression of PLS3 rescued the axon length and outgrowth defects associated with SMN down-regulation in motor neurons of SMA mouse embryos and in zebrafish. Our study suggests that defects in axonogenesis are the major cause of SMA, thereby opening new therapeutic options for SMA and similar neuromuscular diseases.

SummaryVisual screening of a T-DNA mutagenised population of Arabidopsis thaliana for an absence of silique elongation lead to the isolation of the aborted microspores (ams) mutant that shows a sporophytic recessive male sterile phenotype. Homozygous mutant plants are completely devoid of mature pollen. Pollen degeneration occurs shortly after release of the microspores from the tetrad, prior to pollen mitosis I. Premature tapetum and microspore degeneration are the primary defects caused by this lesion, while a secondary effect is visualised in the stamen filaments, which are reduced in length and lie beneath the receptive stigma at flower opening. The disrupted gene was isolated and revealed a T-DNA element to be inserted into the eighth exon of a basic helix-loop-helix (bHLH) gene located on chromosome II. This protein sequence contains a basic DNA binding domain and two alpha helices separated by a loop, typical of a transcription factor belonging to the MYC sub family of bHLH genes. Therefore, AMS plays a crucial role in tapetal cell development and the post-meiotic transcriptional regulation of microspore development within the developing anther.