The role of ABC transporters in kin recognition in Arabidopsis thaliana

The ability to sense and respond to the surrounding rhizosphere including communications with neighboring plants and microbes is essential for plant survival. Recently, it has been established that several plant species including Arabidopsis thaliana have the ability to recognize rhizospheric neighbors based or their genetic identity. This study investigated the role of ABC transporters in kin recognition in A. thaliana based on previous evidence that root secretions are involved in the kin recognition response and that ABC transporters are responsible for secretion of a number of compounds. Three genes, AtPGP1, AtATH1 and AtATH10, are all implicated to be partially involved in the complex kin recognition response in A. thaliana based on this report. These findings highlight the importance of ABC transporters in understanding root secretions and plant-plant community interactions.Key words: root biology, rhizosphere, kin recognition, root secretions, Arabidopsis     

Kin recognition, not competitive interactions, predicts root allocation in young Cakile edentula seedling pairs

? Recent studies have demonstrated sibling vs stranger differences in group root allocation in plants, suggesting that plants have the potential for kin discrimination in competition. However, morphology differences could potentially be generated by competition-based mechanisms. Here, we tested these hypotheses for the sibling vs stranger differences in root allocation in Cakile edentula. ? Seeds were planted in pairs of either kin (siblings) or strangers, from all combinations of eight families, to give eight kin (sibling) and 28 stranger pair identities. Because the species has a seed dimorphism, the 10 replicates of each pair identity included both seed types. Root allocation, size inequality between seedlings in a pair, and competitive ability were derived from measures of biomass and height. ? Cakile edentula seedlings demonstrated the same kin recognition response previously observed in juvenile plants, with lower root allocation in kin pairs than stranger pairs. The seed dimorphism was not associated with root allocation. ? The two competitive mechanisms, genetic differences in competitive ability and increased size inequality in stranger groups, did not explain the root allocation differences in these seedlings. Kin recognition offered the most probable explanation for the differences in root allocation between sibling and stranger pairs.     

Morphogenic effects of abiotic stress: reorientation of growth in Arabidopsis thaliana seedlings

Abiotic stress responses include changes in physiological and biochemical processes as well as morphological and developmental patterns. It has remained an enigma which mechanisms are responsible for stress-induced morphogenesis. In this paper we demonstrate that stress induced phenotypes comprise a re-orientation rather than a cessation of growth. Moreover, strong similarities between the phenotypes induced by excess copper, paraquat, salicylic acid and a hydrogen peroxide analogue, indicate that a common molecular-physiological response system mediates these morphogenic stress responses. It is proposed that reactive oxygen species play a key role in controlling the architectural changes in stressed Arabidopsis thaliana seedlings.We found that phenotypes of plants exposed to stress resemble, in terms of the redistribution of growth, plants altered in phytohormone metabolism. We also found that plants in which polar auxin transport is blocked with TIBA, strongly resemble, but are not identical to, plants exposed to abiotic stress. Based on the stress induced formation of lateral roots, we surmise that stress induces local auxin accumulation near the root pericycle.     

植物种间和种内的化学作用

植物间相互作用是生态学基础科学问题之一,植物能感受和识别共存同种或异种植物,进而调整生长、繁殖和防御策略。植物种间和种内的感受和识别大多是由植物产生释放的次生物质所介导,这类化学识别通讯可以启动相应的植物化感作用机制。近年发现,植物亲属间也存在着化学识别、地下根系通讯调控地上开花繁殖等植物种间和种内的化学作用关系。目前植物通过地上挥发物介导的植物化学作用已基本澄清,但根分泌物介导的植物地下化学作用机制及其信号物质还所知甚少。地下化学作用不仅决定根系侵入(接近)和躲避(排斥)行为,也能调控地上开花时间和花期。这样,植物间的化学作用还涉及植物地下和地上的协调互作。本文以植物化感作用和植物化学识别通讯及相应的化感物质和信号物质为基点,从植物亲属识别、根系化学识别和行为模式、地下化学作用调控地上开花繁殖3个方面综述植物种间和种内化学作用的研究进展,为全面理解植物间相互作用提供新视野。     

Temporal root responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> L. to chromate reveal structural and regulatory mechanisms involving the SOLITARY ROOT/IAA14 repressor for maintenance of identity meristem genes

The Arabidopsis root system is modified in response to stress generated by high concentrations of nonessential ions such as chromate [Cr(VI)]. In this work, the distribution of auxin and its transporters PIN1 and PIN7, as well as the expression of genes that maintain the identity of the root meristem, were analyzed in Arabidopsis thaliana wild-type (WT) seedlings and in a mutant affected in the SOLITARY ROOT (SLR1/IAA14) locus, which is required for root response to Cr(VI). We show that primary root inhibition, auxin transporter levels, and expression of meristem identity genes were maintained in the slr-1 mutants but not in WT plants in response to Cr(VI) in a time- and concentration-dependent manner. Notably, the outermost single cell layer of the lateral root cap, which normally dies and tends to peel off, remains viable and increases in size following exposure of WT plants, but not slr-1 mutants, to Cr(VI). Our results suggest that (1) the primary root tip senses Cr(VI), (2) the external lateral root cap may play a protective role during Cr(VI) exposure, and (3) Cr(VI) impacts cell division in root meristems via auxin redistribution and SLR1/IAA14 function, influencing the expression of root meristem genes.     

Raphanusanin-mediated resistance to pathogens is light dependent in radish and Arabidopsis thaliana

Raphanusanin (Ra) is a light-induced inhibitor of hypocotyl growth that responds to unilateral blue light illumination in radish seedlings. We have previously shown that Ra regulates genes that are involved in common defense mechanisms. Many genes that are induced by Ra are also positively regulated by early blue light. To extend the understanding of the role of Ra in pathogen defense, we evaluated the effects of Ra on radish and Arabidopsis thaliana (A. thaliana) infected with the necrotrophic pathogen Botrytis cinerea (B. cinerea) and biotrophic pathogen Pseudomonas syringae (P. syringae). Radish and A. thaliana were found to be resistant to both pathogens when treated with Ra, depending on the concentration used. Interestingly, Ra-mediated resistance to P. syringae is dependent on light because Ra-treated seedlings exhibited enhanced susceptibility to P. syringae infection when grown in the dark. In addition to regulating the biotic defense response, Ra inhibited seed germination and root elongation and enhanced the growth of root hairs in the presence of light in radish and A. thaliana. Our data suggest that Ra regulates the expression of a set of genes involved in defense signaling pathways and plays a role in pathogen defense and plant development. Our results show that light may be generally required not only for the accumulation of Ra but also for its activation during the pathogen defense response.     

内生真菌KLBMP0506对拟南芥的促生作用

【目的】研究植物内生菌Wickerhamomyces sp.KLBMP0506对拟南芥（Arabidopsis thaliana）的促生作用及潜在的促生机制。【方法】本研究以野生型拟南芥为试验材料,将其与菌株KLBMP0506进行平板共培养及盆栽接种试验,并测定拟南芥鲜重、干重、主根长、侧根数、叶绿素含量和可溶性糖含量等生长、生理指标,同时对筛选的与拟南芥侧根、主根形成及生长素合成和运输相关的11个基因进行实时荧光定量聚合酶链式反应（quantitative real-time polymerase chain reaction,qRT-PCR）分析。【结果】接种目的菌株KLBMP0506后,平板试验的拟南芥鲜重及侧根、盆栽试验的拟南芥鲜重、干重、茎长、叶绿素含量和可溶性糖含量均有一定程度的增加;分隔平板试验及菌株KLBMP0506发酵液中促生活性物质分析显示,该菌株产生的挥发性有机物质及其发酵液中的正丁醇和乙酸乙酯提取物均对拟南芥有明显的促生作用;此外,qRT-PCR分析显示KLBMP0506处理后,拟南芥中与侧根形成相关基因ABI4、FLA1的表达出现不同程度的下调,与生长素合成、运输相关基因AUX1、EIR1、YUC4的表达整体呈上调趋势,表明菌株KLBMP0506可能通过调控拟南芥中与侧根形成以及与生长素合成和运输相关基因的表达,而实现对拟南芥的促生作用。【结论】本研究明确了菌株KLBMP0506对模式植物拟南芥的促生作用,为其开发成为微生物菌肥提供理论依据。     

Stimulation of root hair elongation in Arabidopsis thaliana by low phosphorus availability

Low phosphorus availability stimulates root hair elongation in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effect of low phosphorus on the elongation of Arabidopsis thaliana root hairs. Arabidopsis thaliana plants were grown in plant culture containing high (1000 mmol m^?3) or low (1 mmol m^?3) phosphorus concentrations, and root hair elongation was analysed by image analysis. After 15d of growth, low-phosphorus plants developed root hairs averaging 0.9 mm in length while high-phosphorus plants of the same age developed root hairs averaging 0.3 mm in length. Increased root hair length in low-phosphorus plants was a result of both increased growth duration and increased growth rate. Root hair length decreased logarithmically in response to increasing phosphorus concentration. Local changes in phosphorus availability influenced root hair growth regardless of the phosphorus status of the plant. Low phosphorus stimulated root hair elongation in the hairless axr2 mutant, exogenously applied IAA stimulated root hair elongation in wild-type high-phosphorus plants and the auxin antagonist CM PA inhibited root hair elongation in low-phosphorus plants. These results indicate that auxin may be involved in the low-phosphorus response in root hairs.     

Characterisation of root amino acid exudation in white clover (Trifolium repens L.)

Lateral roots are crucial for the plasticity of root responses to environmental conditions in soil. The bacterivorous microfauna has been shown to increase root branching and to foster auxin producing soil bacteria. However, information on modifications of plant internal auxin content by soil bacteria and bacterivores is missing. Therefore, the effects of a rhizosphere bacterial community and a common soil amoeba (Acanthamoeba castellanii) on root branching and on auxin (indole-3-acetic acid) metabolism in Lepidium sativum and Arabidopsis thaliana were investigated. In a first experimental series, bacteria increased conjugated auxin concentrations in L. sativum shoots, but did not alter free bioactive auxin content nor root branching. In contrast, in presence of soil bacteria plus amoebae free auxin concentrations in shoots and root branching increased, demonstrating that effects of bacteria on auxin metabolism in plants were strongly modified by the bacterivorous amoebae. In a second experiment, A. thaliana reporter plants for auxin (DR5) and cytokinin (ARR5) responded similarly with increased root branching in the presence of amoebae. Surprisingly, in reporter plants cytokinin but not auxin responses were detectable, accompanied by higher soil nitrate concentrations in the presence of amoebae. Likely, increased nitrate concentrations in the rhizosphere led to an accumulation of cytokinin and interactions with free auxin in plants and finally to increased root growth in the presence of amoebae. Altogether, the results show that mutual control mechanisms exist between plant hormone metabolism and microbial signalling, and that effects on hormonal concentrations of plants by free-living bacteria are strongly influenced by bacterial grazers like amoebae.     

Auxin controls the division of root endodermal cells

The root endodermis forms a selective barrier that prevents the free diffusion of solutes into the vasculature; to make this barrier, endodermal cells deposit hydrophobic compounds in their cell walls, forming the Casparian strip. Here, we showed that, in contrast to vascular and epidermal root cells, endodermal root cells do not divide alongside the root apical meristem in Arabidopsis thaliana. Auxin treatment induced division of endodermal cells in wild-type plants, but not in the auxin signaling mutant auxin resistant3-1. Endodermis-specific activation of auxin responses by expression of truncated AUXIN-RESPONSIVE FACTOR5 (ΔARF5) in root endodermal cells under the control of the ENDODERMIS7 promoter (EN7::ΔARF5) also induced endodermal cell division. We used an auxin transport inhibitor to cause accumulation of auxin in endodermal cells, which induced endodermal cell division. In addition, knockout of P-GLYCOPROTEIN1 (PGP1) and PGP19, which mediate centripetal auxin flow, promoted the division of endodermal cells. Together, these findings reveal a tight link between the endodermal auxin response and endodermal cell division, suggesting that auxin is a key regulator controlling the division of root endodermal cells, and that PGP1 and PGP19 are involved in regulating endodermal cell division.

The endodermal auxin response, which is regulated by centripetal auxin flow, determines division of the endodermal cells.     

Arabidopsis ROOT UVB SENSITIVE2/WEAK AUXIN RESPONSE1 Is Required for Polar Auxin Transport

Auxin is an essential phytohormone that regulates many aspects of plant development. To identify new genes that function in auxin signaling, we performed a genetic screen for Arabidopsis thaliana mutants with an alteration in the expression of the auxin-responsive reporter DR5rev:GFP (for green fluorescent protein). One of the mutants recovered in this screen, called weak auxin response1 (wxr1), has a defect in auxin response and exhibits a variety of auxin-related growth defects in the root. Polar auxin transport is reduced in wxr1 seedlings, resulting in auxin accumulation in the hypocotyl and cotyledons and a reduction in auxin levels in the root apex. In addition, the levels of the PIN auxin transport proteins are reduced in the wxr1 root. We also show that WXR1 is ROOT UV-B SENSITIVE2 (RUS2), a member of the broadly conserved DUF647 domain protein family found in diverse eukaryotic organisms. Our data indicate that RUS2/WXR1 is required for auxin transport and to maintain the normal levels of PIN proteins in the root.     

Root Secreted Metabolites and Proteins Are Involved in the Early Events of Plant-Plant Recognition Prior to Competition

The mechanism whereby organisms interact and differentiate between others has been at the forefront of scientific inquiry, particularly in humans and certain animals. It is widely accepted that plants also interact, but the degree of this interaction has been constricted to competition for space, nutrients, water and light. Here, we analyzed the root secreted metabolites and proteins involved in early plant neighbor recognition by using Arabidopsis thaliana Col-0 ecotype (Col) as our focal plant co-cultured in vitro with different neighbors [A. thaliana Ler ecotype (Ler) or Capsella rubella (Cap)]. Principal component and cluster analyses revealed that both root secreted secondary metabolites and proteins clustered separately between the plants grown individually (Col-0, Ler and Cap grown alone) and the plants co-cultured with two homozygous individuals (Col-Col, Ler-Ler and Cap-Cap) or with different individuals (Col-Ler and Col-Cap). In particularly, we observed that a greater number of defense- and stress- related proteins were secreted when our control plant, Col, was grown alone as compared to when it was co-cultured with another homozygous individual (Col-Col) or with a different individual (Col-Ler and Col-Cap). However, the total amount of defense proteins in the exudates of the co-cultures was higher than in the plant alone. The opposite pattern of expression was identified for stress-related proteins. These data suggest that plants can sense and respond to the presence of different plant neighbors and that the level of relatedness is perceived upon initial interaction. Furthermore, the role of secondary metabolites and defense- and stress-related proteins widely involved in plant-microbe associations and abiotic responses warrants reassessment for plant-plant interactions.