Interaction of brassinosteroid functions and sucrose transporter SlSUT2 regulate the formation of arbuscular mycorrhiza

Identification and Characterization of the BZR Transcription Factor Genes Family in Potato (Solanum tuberosum L.) and Their Expression Profiles in Response to Abiotic Stresses

Brassinazole resistant (BZR) genes act downstream of the brassinosteroid signaling pathway regulating plant growth and development and participating in plant stress responses. However, the BZR gene family has not systematically been characterized in potato. We identified eight BZR genes in Solanum tuberosum, which were distributed among seven chromosomes unequally and were classified into three subgroups. Potato and tomato BZR proteins were shown to be closely related with high levels of similarity. The BZR gene family members in each subgroup contained similar conserved motifs. StBZR genes exhibited tissue-specific expression patterns, suggesting their functional differentiation during evolution. StBZR4, StBZR7, and StBZR8 were highly expressed under white light in microtubers. StBZR1 showed a progressive up-regulation from 0 to 6 h and a progressive down-regulation from 6 to 24 h after drought and salt stress. StBZR1, StBZR2, StBZR4, StBZR5, StBZR6, StBZR7 and StBZR8 were significantly induced from 0 to 3 h under BR treatment. This implied StBZR genes are involved in phytohormone and stress response signaling pathways. Our results provide a theoretical basis for understanding the functional mechanisms of BZR genes in potato.

StBIN2 Positively Regulates Potato Formation through Hormone and Sugar Signaling

Potato is an important food crop worldwide. Brassinosteroids (BRs) are widely involved in plant growth and development, and BIN2 (brassinosteroid insensitive 2) is the negative regulator of their signal transduction. However, the function of BIN2 in the formation of potato tubers remains unclear. In this study, transgenic methods were used to regulate the expression level of StBIN2 in plants, and tuber related phenotypes were analyzed. The overexpression of StBIN2 significantly increased the number of potatoes formed per plant and the weight of potatoes in transgenic plants. In order to further explore the effect of StBIN2 on the formation of potato tubers, this study analyzed BRs, ABA hormone signal transduction, sucrose starch synthase activity, the expression levels of related genes, and interacting proteins. The results show that the overexpression of StBIN2 enhanced the downstream transmission of ABA signals. At the same time, the enzyme activity of the sugar transporter and the expression of synthetic genes were increased in potato plants overexpressing StBIN2, which also demonstrated the upregulation of sucrose and the expression of the starch synthesis gene. Apparently, StBIN2 affected the conversion and utilization of key substances such as glucose, sucrose, and starch in the process of potato formation so as to provide a material basis and energy preparation for forming potatoes. In addition, StBIN2 also promoted the expression of the tuber formation factors StSP6A and StS6K. Altogether, this investigation enriches the study on the mechanism through which StBIN2 regulates potato tuber formation and provides a theoretical basis for achieving a high and stable yield of potato.

Tomato growth promotion by the fungal endophytes Serendipita indica and Serendipita herbamans is associated with sucrose de-novo synthesis in roots and differential local and systemic effects on carbohydrate metabolisms and gene expression

Plant growth-promoting and stress resilience-inducing root endophytic fungi represent an additional carbohydrate sink. This study aims to test if such root endophytes affect the sugar metabolism of the host plant to divert the flow of resources for their purposes. Fresh and dry weights of roots and shoots of tomato (Solanum lycopersicum) colonised by the closely related Serendipita indica and Serendipita herbamans were recorded. Plant carbohydrate metabolism was analysed by measuring sugar levels, by determining activity signatures of key enzymes of carbohydrate metabolism, and by quantifying mRNA levels of genes involved in sugar transport and turnover. During the interaction with the tomato plants, both fungi promoted root growth and shifted shoot biomass from stem to leaf tissues, resulting in increased leaf size. A common effect induced by both fungi was the inhibition of phosphofructokinase (PFK) in roots and leaves. This glycolytic-pacing enzyme shows how the glycolysis rate is reduced in plants and, eventually, how sugars are allocated to different tissues. Sucrose phosphate synthase (SPS) activity was strongly induced in colonised roots. This was accompanied by increased SPS-A1 gene expression in S. herbamans-colonised roots and by increased sucrose amounts in roots colonised by S. indica. Other enzyme activities were barely affected by S. indica, but mainly induced in leaves of S. herbamans-colonised plants and decreased in roots. This study suggests that two closely related root endophytic fungi differentially influence plant carbohydrate metabolism locally and systemically, but both induce a similar increase in plant biomass. Notably, both fungal endophytes induce an increase in SPS activity and, in the case of S. indica, sucrose resynthesis in roots. In leaves of S. indica-colonised plants, SWEET11b expression was enhanced, thus we assume that excess sucrose was exported by this transporter to the roots. ‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬.

Subcellular dynamics and protein-protein interactions of plant sucrose transporters

Although extensively studied for their role in long distance transport, plant sucrose transporters are active not only in the phloem but throughout the plant body. Sucrose transporters of the SUT family were first described to be plasma membrane-resident proteins, but recent investigations revealed that subcellular dynamics of these transporters were part of complex regulatory mechanisms. The yeast two-hybrid split-ubiquitin system, tandem-affinity purification, and bimolecular-fluorescence complementation aided in identification of a complex network of SUT-interacting proteins that led to answers to many open questions. We found, for example, interacting proteins localized to other subcellular compartments. Although sucrose transporters were assumed to be localized mainly on the plasma membrane, and the tonoplast in the case of SUT4, the interaction partners were not exclusively predicted to be plasma membrane proteins, but belonged to the extracellular space (cell wall), intracellular vesicles, the ER, tonoplast, nuclei, and peroxisomes, among other cellular compartments. A subset of the SUT-interacting proteins localized exclusively to plasmodesmata. We conclude that (transient) protein-protein interactions of integral membrane proteins help to sequester SUTs to subcellular compartments, such as membrane microdomains, with specific functions to enable subcellular transport and cell-to-cell trafficking via plasmodesmata. Identification of SNARE proteins (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptors) and protein disulfide isomerases support the assumption that the protein-protein interaction plays an important role for the subcellular movement of sugar transporters. It becomes apparent that the interaction partners provide a substantial impact on how and where the transporter is localized or processed for either targeting to a specific cellular or extracellular location, or tagging for degradation or recycling. In this review, interacting proteins, as well as the role of oligomeric complex formation, post-translational modification, and stress responses are summarized for SUTs of higher plants.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Brassinosteroids Benefit Plants Performance by Augmenting Arbuscular Mycorrhizal Symbiosis

Arbuscular mycorrhizal (AM) play an important role in improving plant growth and development. The interaction between phytohormones and AM symbiosis is gradually revealed. Here we examined the effect of Brassinosteroids (BR) on AM symbiosis and discussed the synergistic promotion of plant growth by BR and AM symbiosis. The xylophyta Eucalyptus grandis Hill (E. grandis) was inoculated with AM fungi Rhizoglomus irregularis R197198 (R. irregularis) and treated with different concentrations (0, 1, 10, and 100 nM) of 24-epibrassinolide (24-epiBL) for 6 weeks. With the increase of 24-epiBL concentration, E. grandis growth was firstly promoted and then inhibited, but inoculation with AM fungi alleviated this inhibition. 24-epiBL and R. irregularis colonization significantly improved E. grandis growth and antioxidant system response, and the synergistic effect was the best. Compared with the control group, 24-epiBL treatment significantly increased the mycorrhizal colonization and arbuscular abundance of AM fungi R. irregular in E. grandis roots. The expression of AM symbiosis maker genes was significantly increased by 24-epiBL treatment. Both 24-epiBL treatment and AM colonization upregulated gibberellins (GA) synthesis genes, but no inhibition caused by GA levels was found. 24-epiBL is a kind of synthetic highly active BR. Based on the results of 24-epiBL treatment, we hypothesized that BR actively regulates AM symbiosis regulates AM symbiosis without affecting GA-INSENSITIVE DWARF1 (GID1)-DELLA expression. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. IMPORTANCE Brassinosteroids (BR) and Arbuscular mycorrhizas (AM) symbiosis play an important role in improving plant growth and development. Previous studies have shown that there is a complex regulatory network between phytohormones and AM symbiosis. However, the interactions of BR-signaling and AM symbiosis are still poorly understood. Our results suggest that BR actively regulates the colonization and development of AM fungi, and AM fungal colonization can alleviate the inhibition of plant growth caused by excessive BR. In addition, BR actively regulates AM symbiosis, but does not primarily mediate gibberellins-DELLA interaction. The synergistic treatment of BR and AM symbiosis can significantly promote the growth and development of plants. The conclusions of this study provide a reference for phytohormones-AM symbiosis interaction.

Sucrose starvation induces the degradation of proteins in trans-Golgi network and secretory vesicle cluster in tobacco BY-2 cells

Endomembrane transport system begins at the endoplasmic reticulum (ER), continues to the Golgi apparatus and subsequent compartment called trans-Golgi network (TGN). We found that SUT2, a tobacco sucrose-transporter ortholog and was localized in the TGN, decreased significantly under a sucrose-starvation condition. The tobacco SNARE protein SYP41, localized in the TGN and secretory vesicle cluster (SVC), also decreased under the starvation. Similarly, the SCAMP2-RFP fusion protein, which is localized in TGN, SVC, and plasma membrane (PM), was distributed solely in the PM under the starvation. Under the same starvation condition, protein secretion was not arrested but pectin deposition to cell wall was suppressed. These data indicated that the protein composition in TGN and existence of the SVC are regulated by sugar availability. Furthermore, our findings as well as the involvement of SVC in pectin secretion suggested that synthesis and transport of pectin are regulated by the level of extracellular sugars.

ABBREVIATIONS

ER: endoplasmic reticulum; GI-TGN: Golgi-released independent TGN; GFP: green fluorescent protein; mRFP: monomeric red fluorescent protein; P4H1.1: prolyl 4-hydroxylase 1.1; PM: plasma membrane; SCAMP2: secretory carrier membrane protein 2; SUT2: sucrose transporter 2; SVC: secretory vesicle cluster; SYP41: syntaxin of plant 41; TGN: trans-Golgi network; YFP: yellow fluorescent protein.

Distinct gene expression and secondary metabolite profiles in suppressor of prosystemin-mediated responses2 (spr2) tomato mutants having impaired mycorrhizal colonization

Arbuscular mycorrhizal fungi (AMF) colonization, sampled at 32-50 days post-inoculation (dpi), was significantly reduced in suppressor of prosystemin-mediated responses2 (spr2) mutant tomato plants impaired in the ω-3 FATTY ACID DESATURASE7 (FAD7) gene that limits the generation of linolenic acid and, consequently, the wound-responsive jasmonic acid (JA) burst. Contrary to wild-type (WT) plants, JA levels in root and leaves of spr2 mutants remained unchanged in response to AMF colonization, further supporting its regulatory role in the AM symbiosis. Decreased AMF colonization in spr2 plants was also linked to alterations associated with a disrupted FAD7 function, such as enhanced salicylic acid (SA) levels and SA-related defense gene expression and a reduction in fatty acid content in both mycorrhizal spr2 roots and leaves. Transcriptomic data revealed that lower mycorrhizal colonization efficiency in spr2 mutants coincided with the modified expression of key genes controlling gibberellin and ethylene signaling, brassinosteroid, ethylene, apocarotenoid and phenylpropanoid synthesis, and the wound response. Targeted metabolomic analysis, performed at 45 dpi, revealed augmented contents of L-threonic acid and DL-malic acid in colonized spr2 roots which suggested unfavorable conditions for AMF colonization. Additionally, time- and genotype-dependent changes in root steroid glycoalkaloid levels, including tomatine, suggested that these metabolites might positively regulate the AM symbiosis in tomato. Untargeted metabolomic analysis demonstrated that the tomato root metabolomes were distinctly affected by genotype, mycorrhizal colonization and colonization time. In conclusion, reduced AMF colonization efficiency in spr2 mutants is probably caused by multiple and interconnected JA-dependent and independent gene expression and metabolomic alterations.

Brassinosteroids and sucrose transport in mycorrhizal tomato plants

Silencing of SlSUT2 expression in tomato plants leads to a dwarfed phenotype, reduced pollen vitality and reduces pollen germination rate. Male sterility of flowers, together with a dwarfed growth behavior is reminiscent to brassinosteroid defective mutant plants. Therefore we aimed to rescue the SlSUT2 silencing phenotype by local brassinosteroid application. The phenotypical effects of SlSUT2 down-regulation could partially be rescued by epi-brassinolide treatment suggesting that SlSUT2 interconnects sucrose partitioning with brassinosteroid signaling. We previously showed that SlSUT2 silenced plants show increased mycorrhization and, this effect was explained by a putative sucrose retrieval function of SlSUT2 at the periarbuscular membrane. More recently, we reported that the symbiotic interaction between Solanaceous hosts and AM fungi is directly affected by watering the roots with epi-brassinolide. Here we show that the SlSUT2 effects on mycorrhiza are not only based on the putative sucrose retrieval function of SlSUT2 at the periarbuscular membrane. Our analyses argue that brassinosteroids as well as SlSUT2 per se can impact the arbuscular morphology/architecture and thereby affect the efficiency of nutrient exchange between both symbionts and the mycorrhizal growth benefit for the plant.

Linking lipid transfer with reduced arbuscule formation in tomato roots colonized by arbuscular mycorrhizal fungus under low pH stress

Arbuscules are the core structures of arbuscular mycorrhizae (AM), and arbuscule development is regulated by environmental stress, e.g., low pH. Recent studies indicate that lipid transfer from plants is essential for AM fungal colonization; however, the role of lipid transfer in arbuscule formation and the dynamics of lipid accumulation in arbuscules under low pH stress are far from well understood. In the symbiosis of tomato and Rhizophagus intraradices under contrasting pH conditions (pH 4.5 vs. pH 6.5), we investigated arbuscule formation, nutrient uptake, alkaline phosphatase activity and lipid accumulation; examined the gene expression involved in phosphate transport, lipid biosynthesis and transfer and sugar metabolism; and visualized the lipid dynamics in arbuscules. Low pH greatly inhibited arbuscule formation, in parallel with reduced phospholipid fatty acids accumulation in AM fungus and decreased P uptake. This reduction was supported by the decreased expression of plant genes encoding lipid biosynthesis and transfer. More degenerating arbuscules were observed under low pH conditions, and neutral lipid fatty acids accumulated only in degenerating arbuscules. These data reveal that, under low pH stress, reduced lipid transfer from hosts to AM fungi is responsible for the inhibited arbuscule formation.

Brassinosteroids Affect the Symbiosis Between the AM Fungus Rhizoglomus irregularis and Solanaceous Host Plants

Together with several proteins involved in brassinosteroid (BR) signaling and synthesis, the membrane steroid binding protein 1 (MSBP1) was identified within the interactome of the sucrose transporter of tomato (SlSUT2). We asked whether MSBP1 is also involved in BR signaling as assumed for the AtMSBP1 protein from Arabidopsis and whether it impacts root colonization with arbuscular mycorrhizal (AM) fungi in a similar way as shown previously for SlSUT2. In addition, we asked whether brassinosteroids per se affect efficiency of root colonization by AM fungi. We carried out a set of experiments with transgenic tobacco plants with increased and decreased MSBP1 expression levels. We investigated the plant and the mycorrhizal phenotype of these transgenic plants and tested the involvement of MSBP1 in BR metabolism by application of epi-brassinolide and brassinazole, an inhibitor of BR biosynthesis. We show that the phenotype of the transgenic tobacco plants with increased or reduced MSBP1 expression is consistent with an inhibitory role of MSBP1 in BR signaling. MSBP1 overexpression could be mimicked by brassinazole treatment. Interestingly, manipulation of MSBP1 expression in transgenic tobacco plants not only affected plant growth and development, but also the host plant responses toward colonization with AM fungi, as well as arbuscular architecture. Moreover, we observed that brassinosteroids indeed have a direct impact on the nutrient exchange in AM symbiosis and on the biomass production of colonized host plants. Furthermore, arbuscular morphology is affected by changes in MSBP1 expression and brassinolide or brassinazole treatments. We conclude that host plant growth responses and nutrient exchange within the symbiosis with AM fungi is controlled by brassinosteroids and might be impeded by the MSBP1 protein.

The Role of Gibberellins and Brassinosteroids in Nodulation and Arbuscular Mycorrhizal Associations

Plant hormones play key roles in nodulation and arbuscular mycorrhizal (AM) associations. These two agriculturally and ecologically important symbioses enable plants to gain access to nutrients, in particular, nitrogen in the case of nodulation and phosphorous in the case of AM. Work over the past few decades has revealed how symbioses with nitrogen-fixing rhizobia, restricted almost exclusively to legumes, evolved in part from ancient AM symbioses formed by more than 80% of land plants. Although overlapping, these symbiotic programs also have important differences, including the de novo development of a new organ found only in nodulation. One emerging area of research is the role of two plant hormone groups, the gibberellins (GAs) and brassinosteroids (BRs), in the development and maintenance of these symbioses. In this review, we compare and contrast the roles of these hormones in the two symbioses, including potential interactions with other hormones. This not only focuses on legumes, most of which can host both symbionts, but also examines the role of these in AM development in non-legumes. GA acts by suppressing DELLA, and this regulatory module acts to negatively influence both rhizobial and mycorrhizal infection but appears to promote nodule organogenesis. While an overall positive role for BRs in nodulation and AM has been suggested by studies using mutants disrupted in BR biosynthesis or response, application studies indicate that BR may play a more complex role in nodulation. Given the nature of these symbioses, with events regulated both spatially and temporally, future studies should examine in more detail how GAs and BRs may influence precise events during these symbioses, including interactions with other hormone groups.

Unraveling the Initial Plant Hormone Signaling, Metabolic Mechanisms and Plant Defense Triggering the Endomycorrhizal Symbiosis Behavior

Arbuscular mycorrhizal (AM) fungi establish probably one of the oldest mutualistic relationships with the roots of most plants on earth. The wide distribution of these fungi in almost all soil ecotypes and the broad range of host plant species demonstrate their strong plasticity to cope with various environmental conditions. AM fungi elaborate fine-tuned molecular interactions with plants that determine their spread within root cortical tissues. Interactions with endomycorrhizal fungi can bring various benefits to plants, such as improved nutritional status, higher photosynthesis, protection against biotic and abiotic stresses based on regulation of many physiological processes which participate in promoting plant performances. In turn, host plants provide a specific habitat as physical support and a favorable metabolic frame, allowing uptake and assimilation of compounds required for the life cycle completion of these obligate biotrophic fungi. The search for formal and direct evidences of fungal energetic needs raised strong motivated projects since decades, but the impossibility to produce AM fungi under axenic conditions remains a deep enigma and still feeds numerous debates. Here, we review and discuss the initial favorable and non-favorable metabolic plant context that may fate the mycorrhizal behavior, with a focus on hormone interplays and their links with mitochondrial respiration, carbon partitioning and plant defense system, structured according to the action of phosphorus as a main limiting factor for mycorrhizal symbiosis. Then, we provide with models and discuss their significances to propose metabolic targets that could allow to develop innovations for the production and application of AM fungal inocula.

The plasma membrane proteome of Medicago truncatula roots as modified by arbuscular mycorrhizal symbiosis

In arbuscular mycorrhizal (AM) roots, the plasma membrane (PM) of the host plant is involved in all developmental stages of the symbiotic interaction, from initial recognition to intracellular accommodation of intra-radical hyphae and arbuscules. Although the role of the PM as the agent for cellular morphogenesis and nutrient exchange is especially accentuated in endosymbiosis, very little is known regarding the PM protein composition of mycorrhizal roots. To obtain a global overview at the proteome level of the host PM proteins as modified by symbiosis, we performed a comparative protein profiling of PM fractions from Medicago truncatula roots either inoculated or not with the AM fungus Rhizophagus irregularis. PM proteins were isolated from root microsomes using an optimized discontinuous sucrose gradient; their subsequent analysis by liquid chromatography followed by mass spectrometry (MS) identified 674 proteins. Cross-species sequence homology searches combined with MS-based quantification clearly confirmed enrichment in PM-associated proteins and depletion of major microsomal contaminants. Changes in protein amounts between the PM proteomes of mycorrhizal and non-mycorrhizal roots were monitored further by spectral counting. This workflow identified a set of 82 mycorrhiza-responsive proteins that provided insights into the plant PM response to mycorrhizal symbiosis. Among them, the association of one third of the mycorrhiza-responsive proteins with detergent-resistant membranes pointed at partitioning to PM microdomains. The PM-associated proteins responsive to mycorrhization also supported host plant control of sugar uptake to limit fungal colonization, and lipid turnover events in the PM fraction of symbiotic roots. Because of the depletion upon symbiosis of proteins mediating the replacement of phospholipids by phosphorus-free lipids in the plasmalemma, we propose a role of phosphate nutrition in the PM composition of mycorrhizal roots.

Plant carbon nourishment of arbuscular mycorrhizal fungi

Reciprocal nutrient exchange between the majority of land plants and arbucular mycorrhizal (AM) fungi is the cornerstone of a stable symbiosis. To date, a dogma in the comprehension of AM fungal nourishment has been delivery of host organic carbon in the form of sugars. More recently a role for lipids as alternative carbon source or as a signalling molecule during AM symbiosis was proposed. Here we review the symbiotic requirement for carbohydrates and lipids across developmental stages of the AM symbiosis. We present a role for carbohydrate metabolism and signalling to maintain intraradical fungal growth, as opposed to lipid uptake at the arbuscule as an indispensible requirement for completion of the AM fungal life cycle.

Review: Post-translational cross-talk between brassinosteroid and sucrose signaling

A direct link has been elucidated between brassinosteroid function and perception, and sucrose partitioning and transport. Sucrose regulation and brassinosteroid signaling cross-talk at various levels, including the well-described regulation of transcriptional gene expression: BZR-like transcription factors link the signaling pathways. Since brassinosteroid responses depend on light quality and quantity, a light-dependent alternative pathway was postulated. Here, the focus is on post-translational events. Recent identification of sucrose transporter-interacting partners raises the question whether brassinosteroid and sugars jointly affect plant innate immunity and plant symbiotic interactions. Membrane permeability and sensitivity depends on the number of cell surface receptors and transporters. More than one endocytic route has been assigned to specific components, including brassinosteroid-receptors. The number of such proteins at the plasma membrane relies on endocytic recycling, internalization and/or degradation. Therefore, vesicular membrane trafficking is gaining considerable attention with regard to plant immunity. The organization of pattern recognition receptors (PRRs), other receptors or transporters in membrane microdomains participate in endocytosis and the formation of specific intracellular compartments, potentially impacting biotic interactions. This minireview focuses on post-translational events affecting the subcellular compartmentation of membrane proteins involved in signaling, transport, and defense, and on the cross-talk between brassinosteroid signals and sugar availability.

Interactions between ethylene, gibberellins, and brassinosteroids in the development of rhizobial and mycorrhizal symbioses of pea

The regulation of arbuscular mycorrhizal development and nodulation involves complex interactions between the plant and its microbial symbionts. In this study, we use the recently identified ethylene-insensitive ein2 mutant in pea (Pisum sativum L.) to explore the role of ethylene in the development of these symbioses. We show that ethylene acts as a strong negative regulator of nodulation, confirming reports in other legumes. Minor changes in gibberellin1 and indole-3-acetic acid levels in ein2 roots appear insufficient to explain the differences in nodulation. Double mutants produced by crosses between ein2 and the severely gibberellin-deficient na and brassinosteroid-deficient lk mutants showed increased nodule numbers and reduced nodule spacing compared with the na and lk single mutants, but nodule numbers and spacing were typical of ein2 plants, suggesting that the reduced number of nodules innaandlkplants is largely due to the elevated ethylene levels previously reported in these mutants. We show that ethylene can also negatively regulate mycorrhizae development when ethylene levels are elevated above basal levels, consistent with a role for ethylene in reducing symbiotic development under stressful conditions. In contrast to the hormone interactions in nodulation, ein2 does not override the effect of lk or na on the development of arbuscular mycorrhizae, suggesting that brassinosteroids and gibberellins influence this process largely independently of ethylene.

Endocytosis and its regulation in plants

Endocytosis provides a major route of entry for membrane proteins, lipids, and extracellular molecules into the cell. Recent evidence indicates that multiple cellular processes require endocytosis, including nutrient uptake, signaling transduction, and plant-microbe interactions. Also, advanced microscopy, combined with biochemical and genetic approaches, has provided more insights into the molecular machinery and functions of endocytosis in plants. Here we review mechanisms of the clathrin-dependent and membrane microdomain-associated endocytic routes in plant cells. In addition, degradation of endocytosed proteins and endosomal sorting complex required for transport (ESCRT)-mediated vesicle formation at the endosome are discussed. Finally, we summarize the essential roles of various regulators during plant endocytosis.