Cell separation is a critical process that takes place throughout the life cycle of a plant. It enables roots to emerge from germinating seeds, cotyledons, and leaves to expand, anthers to dehisce, fruit to ripen, and organs to be shed. The focus of this review is to examine how processes such as abscission and dehiscence are regulated and the ways new research strategies are helping us to understand the mechanisms involved in bringing about a reduction in cell-to-cell adhesion. The opportunities for using this information to manipulate cell separation for the benefit of agriculture and horticulture are evaluated.

No abstract

2Plants can acclimate by using tropisms to link the direction of growth to 41 environmental conditions. Hydrotropism allows roots to forage for water, a process 42 known to depend on abscisic acid (ABA) but whose molecular and cellular basis 43 remains unclear. Here, we show that hydrotropism still occurs in roots after laser 44 ablation removed the meristem and root cap. Additionally, targeted expression 45 studies reveal that hydrotropism depends on the ABA signalling kinase, SnRK2.2, and 46 the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical 47 cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing 48 differential cell-length increases in the cortex, but not in other cell types. We conclude 49 that root tropic responses to gravity and water are driven by distinct tissue-based 50 mechanisms. In addition, unlike its role in root gravitropism, the elongation zone 51 performs a dual function during a hydrotropic response, both sensing a water 52 potential gradient and subsequently undergoing differential growth. 53 3 Tropic responses are differential growth mechanisms that roots use to explore the 54 surrounding soil efficiently. In general, a tropic response can be divided into several steps, 55 comprising perception, signal transduction, and differential growth. All of these steps have 56 been well characterized for gravitropism, where gravity sensing cells in the columella of the 57 root cap generate a lateral auxin gradient, whilst adjacent lateral root cap cells transport 58 auxin to epidermal cells in the elongation zone, thereby triggering the differential growth that 59 drives bending [1][2][3][4] . In gravi-stimulated roots, the lateral auxin gradient is transported 60 principally by AUX1 and PIN carriers [3][4][5] . 61Compared with gravitropism, the tropic response to asymmetric water availability, i.e., 62 hydrotropism, has been far less studied. Previously, it was reported that surgical removal or 63 ablation of the root cap reduces hydrotropic bending in pea [6][7][8] and Arabidopsis thaliana 9 , 64suggesting that the machinery for sensing moisture gradients resides in the root cap. It has 65 also been reported that hydrotropic bending occurs due to differential growth in the 66 elongation zone 7,10 . However unlike gravitropism, hydrotropism in A. thaliana is independent 67 of AUX1 and PIN-mediated auxin transport 11,12 . Indeed, roots bend hydrotropically in the 68 absence of any redistribution of auxin detectable by auxin-responsive reporters 13,14 . 18,19 . 83However it is unclear whether this broad expression pattern is necessary for MIZ1's function 84 in hydrotropism or whether ABA signal transduction components in general have to be 85 expressed in specific root tip tissues for a hydrotropic response. The present study describes 86 a series of experiments in A. thaliana designed to identify the root tissues essential for a 87 hydrotropic response. We report that MIZ1 and a key ABA signal-transduction component 88SnRK2....

Polygalacturonases (PGs) have been proposed to play an important role in the process of cell separation. The Arabidopsis thaliana genome contains 69 annotated genes that by amino acid homology and transcript organization could be classified as putative PGs and these can be grouped into multiple clades. An analysis of five members located in two separate clades, using reporter fusion constructs and reverse transcription-PCR, revealed that whilst these PGs exhibit high sequence similarity they have distinct patterns of spatial and temporal expression. Sites of expression include the aleurone and endosperm cells surrounding the emerging radicle in a germinating seed, the cortical cells adjacent to the developing lateral root, the abscission zones of floral organs, the dehiscence zone of anthers and siliques, and pollen grains. Silencing of an abscission-related PG (At2g41850), using a T-DNA insertion strategy, delayed the time-course of floral organ loss but did not prevent shedding from taking place. These observations are discussed with regard to the contribution that PGs may play during the life cycle of a plant.

Staple crops face major challenges in the near future and a diversification away from over-reliance on staples will be important as part of the progress towards the goal of achieving security of food production. Underutilized or neglected crops species are often indigenous ancient crop species which are still used at some level within the local, national or even international communities, but have the potential to contribute further to the mix of food sources than they currently do. The most cost-effective and easily disseminated changes that can be made to a crop are changes to the genetics, as these are contained within the seed itself and, for many species, the seed is a pure breeding, self-replicating, resource. This article focuses on the potential of underutilized crops to contribute to food security and, in particular, whether genetics and breeding can overcome some of the constraints to the enhanced uptake of these species in the future. The focus here is on overview rather than detail and subsequent articles will examine the current evidence base.

SummarySenescence of plant organs is a genetically controlled process that regulates cell death to facilitate nutrient recovery and recycling, and frequently precedes, or is concomitant with, ripening of reproductive structures. In Arabidopsis thaliana, the seeds are contained within a silique, which is itself a photosynthetic organ in the early stages of development and undergoes a programme of senescence prior to dehiscence. A transcriptional analysis of the silique wall was undertaken to identify changes in gene expression during senescence and to correlate these events with ultrastructural changes. The study revealed that the most highly up-regulated genes in senescing silique wall tissues encoded seed storage proteins, and the significance of this finding is discussed. Global transcription profiles of senescing siliques were compared with those from senescing Arabidopsis leaf or petal tissues using microarray datasets and metabolic pathway analysis software (MapMan). In all three tissues, members of NAC and WRKY transcription factor families were up-regulated, but components of the shikimate and cell-wall biosynthetic pathways were down-regulated during senescence. Expression of genes encoding ethylene biosynthesis and action showed more similarity between senescing siliques and petals than between senescing siliques and leaves. Genes involved in autophagy were highly expressed in the late stages of death of all plant tissues studied, but not always during the preceding remobilization phase of senescence. Analyses showed that, during senescence, silique wall tissues exhibited more transcriptional features in common with petals than with leaves. The shared and distinct regulatory events associated with senescence in the three organs are evaluated and discussed.

No abstract

Natural topsoils in the Appalachian surface mining region are often more difficult to use and less desirable than alternative spoil materials. Parent material effects and initial pedogenic changes over 3 yr were observed in 5 mixes of sandstone (SS) and siltstone (SiS) spoils under grass vegetation. Spoil type controlled initial soil texture, but significant decreases in sand contents and increases in silt contents occurred in several spoil mixes within 2 yr. All mine soils studied were high (≥65%) in coarse fragments. Mine soils derived from spoils high in siltstone content were higher in coarse fragments, pH, extractable cations and iron, fine earth (<2 mm) water holding capacity, and electrical conductivity than sandstone mine soils. Dissolution and leaching, oxidation, and organic matter incorporation were dominant pedogenic processes influencing mine soil properties over the period of this experiment. Distinct surface A horizons formed within 3 yr. Water retention in the <2‐mm fraction increased over time in the surface (0 to 5 cm) of all spoil types except pure sandstone. Extractable Fe and total N increased between 1982 and 1984, while pH, and extractable Ca, Mg and P decreased in most spoil types. These changes reflect rapid pedogenesis in fresh unweathered parent materials in a humid environment.