54Phosphatidylcholine (PC) is a major membrane phospholipid and a precursor for most 55 signalling molecules. Understanding its synthesis is important for improving plant growth, 56 nutritional value and resistance to stress. PC synthesis is complex, involving several 57 interconnected pathways, one of which proceeds from serine-derived phosphoethanolamine 58(PEA) to form phosphocholine (PCho) through three sequential phospho-base methylations 59 catalysed by phosphoethanolamine N-methyltransferases (PEAMTs). The contribution of 60 this pathway to the production of PC and plant growth has been a matter of some debate.

The plant body plan and primary organs are established during embryogenesis. However, in contrast to animals, plants have the ability to generate new organs throughout their whole life. These give them an extraordinary developmental plasticity to modulate their size and architecture according to environmental constraints and opportunities. How this plasticity is regulated at the whole-organism level is elusive. Here we provide evidence for a role for translationally controlled tumour protein (TCTP) in regulating the iterative formation of lateral roots in Arabidopsis. AtTCTP1 modulates root system architecture through a dual function: as a general constitutive growth promoter enhancing root elongation and as a systemic signalling agent via mobility in the vasculature. AtTCTP1 encodes mRNAs with long-distance mobility between the shoot and roots. Mobile shoot-derived TCTP1 gene products act specifically to enhance the frequency of lateral root initiation and emergence sites along the primary root pericycle, while root elongation is controlled by local constitutive TCTP1 expression and scion size. These findings uncover a novel type for an integrative signal in the control of lateral root initiation and the compromise for roots between branching more profusely or elongating further. They also provide the first evidence in plants of an extracellular function of the vital, highly expressed ubiquitous TCTP1.

Detection of potentially pathogenic microbes through recognition by plants and animals of both physical and chemical signals associated with the pathogens is vital for host well-being. Signal perception leads to the induction of a variety of responses that augment pre-existing, constitutive defences. The plant cell wall is a highly effective preformed barrier which becomes locally reinforced at the infection site through delivery of new wall material by the actin cytoskeleton. Although mechanical stimulation can produce a reaction, there is little understanding of the nature of physical factors capable of triggering plant defence. Neither the magnitude of forces nor the contact time required has been quantified. In the study reported here, mechanical stimulation with a tungsten microneedle has been used to quantify the response of Arabidopsis plants expressing an actin-binding protein tagged with green fluorescent protein (GFP) to reveal the organisation of the actin cytoskeleton. Using confocal microscopy, the response time for actin reorganisation in epidermal cells of Arabidopsis hypocotyls was shown to be 116 ± 49 s. Using nanoindentation and a diamond spherical tip indenter, the magnitude of the forces capable of triggering an actin response has been quantified. We show that Arabidopsis hypocotyl cells can detect a force as small as 4 μN applied for as short a time as 21.6 s to trigger reorganisation of the actin cytoskeleton. This force is an order of magnitude less than the potential invasive force determined for a range of fungal and oomycete plant pathogens. To our knowledge, this is the first quantification of the magnitude and duration of mechanical forces capable of stimulating a structural defence response in a plant cell.

13As in animals, the plant body plan and primary organs are established during embryogenesis. 14 However, plants have the ability to generate new organs and functional units throughout their 15 whole life. These are produced through the specification, initiation and differentiation of 16 secondary meristems, governed by the intrinsic genetic program and cues from the 17 environment. They give plants an extraordinary developmental plasticity to modulate their size 18 and architecture according to environmental constraints and opportunities. How this plasticity 19 is regulated at the whole organism level is still largely elusive. In particular the mechanisms 20 regulating the iterative formation of lateral roots along the primary root remain little known. A 21 pivotal role of auxin is well established and recently the role of local mechanical signals and 22 oscillations in transcriptional activity has emerged. Here we provide evidence for a role of 23 Translationally Controlled Tumor Protein (TCTP), a vital ubiquitous protein in eukaryotes. We 24 show that Arabidopsis AtTCTP1 controls root system architecture through a dual function: as 25 a general constitutive growth promoter locally, and as a systemic signalling agent via mobility 26 from the shoot. Our data indicate that this signalling function is specifically targeted to the 27 pericycle and modulates the frequency of lateral root initiation and emergence sites along the 28 primary root, and the compromise between branching and elongating, independent of shoot 29 size. Plant TCTP genes show high similarity among species. TCTP messengers and proteins 30 have been detected in the vasculature of diverse species. This suggests that the mobility and 31 extracellular signalling function of AtTCTP1 to control root organogenesis might be widely 32 conserved within the plant kingdom, and highly relevant to a better understanding of post-33 embryonic formation of lateral organs in plants, and the elusive coordination of shoot and root 34 morphogenesis. 35 36 37 39Plant development is highly plastic. This is essential to survival and adaptation to a wide range 40 of environments from which, being sessile, plants cannot escape. That plasticity manifests itself 41 as an extraordinary capacity of a plant to modify the number, size, shape, patterning and spatial 42 deployment of its organs, above and below ground, to efficiently adapt to environmental 43 constraints. 44 As is typical of dicotyledonous species, the Arabidopsis root system arises from a primary root, 45 initiated in the embryo, and de novo organogenesis of secondary and higher order lateral roots, 46 post-embryonically [1, 2]. Lateral roots constitute the major part of the root system, and are 47 major determinants of its ability to take-up water and nutrients and to further expand into new 48 soil pockets. Despite their high agronomic and ecological relevance, the molecular mechanisms 49 that determine the placement of LRs, in space and time, and their number are still little known. 50LR roots or...