Phosphatidylinositol 4-phosphate 5-kinases 1 and 2 are involved in the regulation of vacuole morphology during Arabidopsis thaliana pollen development

Identification of cotton PIP5K genes and role of GhPIP5K9a in primary root development

Phosphatidylinositol 4 phosphate 5-kinase (PIP5K) is crucial for the phosphatidylinositol (PI) signaling pathway. It plays a significant role in plant growth and development, as well as stress response. However, its effects on cotton are unknown. This study identified PIP5K genes from four cotton species and conducted bioinformatic analyses, with a particular emphasis on the functions of GhPIP5K9a in primary roots. The results showed that cotton PIP5Ks were classified into four subgroups. Analysis of gene structure and motif composition showed obvious conservation within each subgroup. Synteny analysis suggested that the PIP5K gene family experienced significant expansion due to both whole-genome duplication (WGD) and segmental duplication. Transcriptomic data analysis revealed that the majority of GhPIP5K genes had the either low or undetectable levels of expression. Moreover, GhPIP5K9a is highly expressed in the root and was located in plasmalemma. Suppression of GhPIP5K9a transcripts resulted in longer primary roots, longer primary root cells and increased auxin polar transport-related genes expression, and decreased abscisic acid (ABA) content, indicating that GhPIP5K9a negatively regulates cotton primary root growth. This study lays the foundation for further exploration of the role of the PIP5K genes in cotton.

Genetic interaction between TTG2 and AtPLC1 reveals a role for phosphoinositide signaling in a co-regulated suite of Arabidopsis epidermal pathways

The TTG2 transcription factor of Arabidopsis regulates a set of epidermal traits, including the differentiation of leaf trichomes, flavonoid pigment production in cells of the inner testa (or seed coat) layer and mucilage production in specialized cells of the outer testa layer. Despite the fact that TTG2 has been known for over twenty years as an important regulator of multiple developmental pathways, little has been discovered about the downstream mechanisms by which TTG2 co-regulates these epidermal features. In this study, we present evidence of phosphoinositide lipid signaling as a mechanism for the regulation of TTG2-dependent epidermal pathways. Overexpression of the AtPLC1 gene rescues the trichome and seed coat phenotypes of the ttg2-1 mutant plant. Moreover, in the case of seed coat color rescue, AtPLC1 overexpression restored expression of the TTG2 flavonoid pathway target genes, TT12 and TT13/AHA10. Consistent with these observations, a dominant AtPLC1 T-DNA insertion allele (plc1-1D) promotes trichome development in both wild-type and ttg2-3 plants. Also, AtPLC1 promoter:GUS analysis shows expression in trichomes and this expression appears dependent on TTG2. Taken together, the discovery of a genetic interaction between TTG2 and AtPLC1 suggests a role for phosphoinositide signaling in the regulation of trichome development, flavonoid pigment biosynthesis and the differentiation of mucilage-producing cells of the seed coat. This finding provides new avenues for future research at the intersection of the TTG2-dependent developmental pathways and the numerous molecular and cellular phenomena influenced by phospholipid signaling.

The Silencing of GhPIP5K2 and GhPIP5K22 Weakens Abiotic Stress Tolerance in Upland Cotton (Gossypium hirsutum)

Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks), essential enzymes in the phosphatidylinositol signaling pathway, are crucial for the abiotic stress responses and the overall growth and development of plants. However, the GhPIP5Ks had not been systematically studied, and their function in upland cotton was unknown. This study identified a total of 28 GhPIP5Ks, and determined their chromosomal locations, gene structures, protein motifs and cis-acting elements via bioinformatics analysis. A quantitative real-time PCR (qRT‒PCR) analysis showed that most GhPIP5Ks were upregulated under different stresses. A virus-induced gene silencing (VIGS) assay indicated that the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were significantly decreased, while malondialdehyde (MDA) content were significantly increased in GhPIP5K2- and GhPIP5K22-silenced upland cotton plants under abiotic stress. Furthermore, the expression of the stress marker genes GhHSFB2A, GhHSFB2B, GhDREB2A, GhDREB2C, GhRD20-1, GhRD29A, GhBIN2, GhCBL3, GhNHX1, GhPP2C, GhCBF1, GhSnRK2.6 and GhCIPK6 was significantly decreased in the silenced plants after exposure to stress. These results revealed that the silencing of GhPIP5K2 and GhPIP5K22 weakened the tolerance to abiotic stresses. These discoveries provide a foundation for further inquiry into the actions of the GhPIP5K gene family in regulating the response and resistance mechanisms of cotton to abiotic stresses.

Identification and Analysis of the Expression of the PIP5K Gene Family in Tomatoes

To explore the function of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) in tomatoes, members of the tomato PIP5K family were identified and characterized using bioinformatic methods, and their expression patterns were also analyzed under salt stress and in different tissues. Twenty-one PIP5K members-namely, SlPIP5K1-SlPIP5K21-were identified from ten chromosomes, and these were divided into three groups according to a phylogenetic analysis. Further bioinformatic analysis showed four pairs of collinear relationships and fragment replication events among the SlPIP5K family members. To understand the possible roles of the SlPIP5Ks, a cis-acting element analysis was conducted, which indicated that tomato PIP5Ks could be associated with plant growth, hormones, and stress responses. We further validated the results of the in silico analysis by integrating RNA-seq and qRT-PCR techniques for salt- and hormone-treated tomato plants. Our results showed that SlPIP5K genes exhibited tissue- and treatment-specific patterns, and some of the SlPIP5Ks exhibited significantly altered expressions after our treatments, suggesting that they might be involved in these stresses. We selected one of the SlPIP5Ks that responded to our treatments, SlPIP5K2, to further understand its subcellular localization. Our results showed that SlPIP5K2 was located on the membrane. This study lays a foundation for the analysis of the biological functions of the tomato SlPIP5K genes and can also provide a theoretical basis for the selection and breeding of new tomato varieties and germplasm innovation, especially under salt stress.

Engineering crop Phytophthora resistance by targeting pathogen-derived PI3P for enhanced catabolism

Phytophthora pathogens lead to numerous economically damaging plant diseases worldwide, including potato late blight caused by P. infestans and soybean root rot caused by P. sojae. Our previous work showed that Phytophthora pathogens may generate abundant phosphatidylinositol 3-phosphate (PI3P) to promote infection via direct association with RxLR effectors. Here, we designed a disease control strategy for metabolizing pathogen-derived PI3P by expressing secreted Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinase 1 (AtPIP5K1), which can phosphorylate PI3P to PI(3,4)P₂. We fused AtPIP5K1 with the soybean PR1a signal peptide (SP-PIP5K1) to enable its secretion into the plant apoplast. Transgenic soybean and potato plants expressing SP-PIP5K1 showed substantially enhanced resistance to various P. sojae and P. infestans isolates, respectively. SP-PIP5K1 significantly reduced PI3P accumulation during P. sojae and soybean interaction. Knockout or inhibition of PI3 kinases (PI3Ks) in P. sojae compromised the resistance mediated by SP-PIP5K1, indicating that SP-PIP5K1 action requires a supply of pathogen-derived PI3P. Furthermore, we revealed that SP-PIP5K1 can interfere with the action of P. sojae mediated by the RxLR effector Avr1k. This novel disease control strategy has the potential to confer durable broad-spectrum Phytophthora resistance in plants through a clear mechanism in which catabolism of PI3P interferes with RxLR effector actions.

Autophagy and its mediated mitochondrial quality control maintain pollen tube growth and male fertility in Arabidopsis

Macroautophagy/autophagy, a major catabolic pathway in eukaryotes, participates in plant sexual reproduction including the processes of male gametogenesis and the self-incompatibility response. Rapid pollen tube growth is another essential reproductive process that is metabolically highly demanding to drive the vigorous cell growth for delivery of male gametes for fertilization in angiosperms. Whether and how autophagy operates to maintain the homeostasis of pollen tubes remains unknown. Here, we provide evidence that autophagy is elevated in growing pollen tubes and critically required during pollen tube growth and male fertility in Arabidopsis. We demonstrate that SH3P2, a critical non-ATG regulator of plant autophagy, colocalizes with representative ATG proteins during autophagosome biogenesis in growing pollen tubes. Downregulation of SH3P2 expression significantly disrupts Arabidopsis pollen germination and pollen tube growth. Further analysis of organelle dynamics reveals crosstalk between autophagosomes and prevacuolar compartments following the inhibition of phosphatidylinositol 3-kinase. In addition, time-lapse imaging and tracking of ATG8e-labeled autophagosomes and depolarized mitochondria demonstrate that they interact specifically via the ATG8-family interacting motif (AIM)-docking site to mediate mitophagy. Ultrastructural identification of mitophagosomes and two additional forms of autophagosomes imply that multiple types of autophagy are likely to function simultaneously within pollen tubes. Altogether, our results suggest that autophagy is functionally crucial for mediating mitochondrial quality control and canonical cytoplasm recycling during pollen tube growth.Abbreviations: AIM: ATG8-family interacting motif; ATG8: autophagy related 8; ATG5: autophagy related 5; ATG7: autophagy related 7; BTH: acibenzolar-S-methyl; DEX: dexamethasone; DNP: 2,4-dinitrophenol; GFP: green fluorescent protein; YFP: yellow fluorescent protein; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; PVC: prevacuolar compartment; SH3P2: SH3 domain-containing protein 2.

A phosphoinositide hub connects CLE peptide signaling and polar auxin efflux regulation

Auxin efflux through plasma-membrane-integral PIN-FORMED (PIN) carriers is essential for plant tissue organization and tightly regulated. For instance, a molecular rheostat critically controls PIN-mediated auxin transport in developing protophloem sieve elements of Arabidopsis roots. Plasma-membrane-association of the rheostat proteins, BREVIS RADIX (BRX) and PROTEIN KINASE ASSOCIATED WITH BRX (PAX), is reinforced by interaction with PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K). Genetic evidence suggests that BRX dampens autocrine signaling of CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 45 (CLE45) peptide via its receptor BARELY ANY MERISTEM 3 (BAM3). How excess CLE45-BAM3 signaling interferes with protophloem development and whether it does so directly or indirectly remains unclear. Here we show that rheostat polarity is independent of PIN polarity, but interdependent with PIP5K. Catalytically inactive PIP5K confers rheostat polarity without reinforcing its localization, revealing a possible PIP5K scaffolding function. Moreover, PIP5K and PAX cooperatively control local PIN abundance. We further find that CLE45-BAM3 signaling branches via RLCK-VII/PBS1-LIKE (PBL) cytoplasmic kinases to destabilize rheostat localization. Our data thus reveal antagonism between CLE45-BAM3-PBL signaling and PIP5K that converges on auxin efflux regulation through dynamic control of PAX polarity. Because second-site bam3 mutation suppresses root as well as shoot phenotypes of pip5k mutants, CLE peptide signaling likely modulates phosphoinositide-dependent processes in various developmental contexts.

Functional Differentiation among the Arabidopsis Phosphatidylinositol 4-Phosphate 5-Kinase Genes PIP5K1, PIP5K2 and PIP5K3

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) is involved in regulating various cellular processes through the signaling function of its product, phosphatidylinositol (4,5)-bisphosphate. Higher plants encode a large number of PIP5Ks forming distinct clades in their molecular phylogenetic tree. Although biological functions of PIP5K genes have been analyzed intensively in Arabidopsis thaliana, it remains unclear how those functions differ across clades of paralogs. We performed comparative functional analysis of the Arabidopsis genes encoding PIP5K1, PIP5K2 and PIP5K3, of which the first two and the last belong to closely related but distinct clades, to clarify their conserved and/or differentiated functions. Genetic analysis with their single and multiple mutants revealed that PIP5K1 and PIP5K3 have non-overlapping functions, with the former in total plant growth and the latter in root hair elongation, whereas PIP5K2 redundantly functions in both phenomena. This pattern of functional redundancy is explainable in terms of the overlapping pattern of their promoter activities. In transformation rescue experiments, PIP5K3 promoter-directed PIP5K1-YFP completely rescued the short-root-hair phenotype of pip5k3. However, PIP5K3-YFP could substitute for PIP5K1-YFP only partially in rescuing the severe dwarfism of pip5k1pip5k2 when directed by the PIP5K1 promoter. Phylogenetic analysis of angiosperm PIP5Ks revealed that PIP5K3 orthologs have a faster rate of diversification in their amino-acid sequences compared with PIP5K1/2 orthologs after they arose through a eudicot-specific duplication event. These findings suggest that PIP5K3 specialized to promote root hair elongation and lost some of the protein-encoded functions retained by PIP5K1 and PIP5K2, whereas PIP5K1 differentiated from PIP5K2 only in its promoter-directed expression pattern.

Phosphatidylinositol 3-phosphate regulates SCAB1-mediated F-actin reorganization during stomatal closure in Arabidopsis

Stomatal movement is critical for plant responses to environmental changes and is regulated by the important signaling molecule phosphatidylinositol 3-phosphate (PI3P). However, the molecular mechanism underlying this process is not well understood. In this study, we show that PI3P binds to stomatal closure-related actin-binding protein1 (SCAB1), a plant-specific F-actin-binding and -bundling protein, and inhibits the oligomerization of SCAB1 to regulate its activity on F-actin in guard cells during stomatal closure in Arabidopsis thaliana. SCAB1 binds specifically to PI3P, but not to other phosphoinositides. Treatment with wortmannin, an inhibitor of phosphoinositide kinase that generates PI3P, leads to an increase of the intermolecular interaction and oligomerization of SCAB1, stabilization of F-actin, and retardation of F-actin reorganization during abscisic acid (ABA)-induced stomatal closure. When the binding activity of SCAB1 to PI3P is abolished, the mutated proteins do not rescue the stability and realignment of F-actin regulated by SCAB1 and the stomatal closure in the scab1 mutant. The expression of PI3P biosynthesis genes is consistently induced when the plants are exposed to drought and ABA treatments. Furthermore, the binding of PI3P to SCAB1 is also required for vacuolar remodeling during stomatal closure. Our results illustrate a PI3P-regulated pathway during ABA-induced stomatal closure, which involves the mediation of SCAB1 activity in F-actin reorganization.

Molecular mechanisms of endomembrane trafficking in plants

Endomembrane trafficking is essential for all eukaryotic cells. The best-characterized membrane trafficking organelles include the endoplasmic reticulum (ER), Golgi apparatus, early and recycling endosomes, multivesicular body, or late endosome, lysosome/vacuole, and plasma membrane. Although historically plants have given rise to cell biology, our understanding of membrane trafficking has mainly been shaped by the much more studied mammalian and yeast models. Whereas organelles and major protein families that regulate endomembrane trafficking are largely conserved across all eukaryotes, exciting variations are emerging from advances in plant cell biology research. In this review, we summarize the current state of knowledge on plant endomembrane trafficking, with a focus on four distinct trafficking pathways: ER-to-Golgi transport, endocytosis, trans-Golgi network-to-vacuole transport, and autophagy. We acknowledge the conservation and commonalities in the trafficking machinery across species, with emphasis on diversity and plant-specific features. Understanding the function of organelles and the trafficking machinery currently nonexistent in well-known model organisms will provide great opportunities to acquire new insights into the fundamental cellular process of membrane trafficking.

Genome-wide identification of PIP5K in wheat and its relationship with anther male sterility induced by high temperature

BACKGROUND

Phosphatidylinositol 4 phosphate 5-kinase (PIP5K) plays a key enzyme role in the inositol signal transduction system and has essential functions in plants in terms of growth, development, and stress responses. However, systematic studies on the wheat PIP5K gene family and its relation to male sterility have not been reported yet.

RESULTS

Sixty-four TaPIP5K genes were identified. The TaPIP5K genes contained similar gene structures and conserved motifs on the same branches of the evolutionary tree, and their cis-regulatory elements were related to MeJA-responsiveness. Furthermore, 49 pairs of collinearity genes were identified and mainly subjected to purification selection during evolution. Synteny analyses showed that some PIP5K genes in wheat and the other four species shared a relatively conserved evolutionary process. The expression levels of many conservative TaPIP5K genes in HT-ms anthers were significantly lower than that in Normal anthers. In addition, HT-ms anthers have no dehiscence, and levels of OPDA and JA-ILE are significantly lower at the trinucleus stage.

CONCLUSION

These results indicate that the PIP5K gene family may be associated with male sterility induced by HT, and the reduction of JA-ILE levels and the abnormal levels of these genes expression may be one reason for the HT-ms anthers having no dehiscence, ultimately leading to the abortion of the anthers.

Lipid kinases PIP5K7 and PIP5K9 are required for polyamine-triggered K+ efflux in Arabidopsis roots

Polyamines, such as putrescine, spermidine and spermine (Spm), are low-molecular-weight polycationic molecules present in all living organisms. Despite their implication in plant cellular processes, little is known about their molecular mode of action. Here, we demonstrate that polyamines trigger a rapid increase in the regulatory membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ), and that this increase is required for polyamine effects on K⁺ efflux in Arabidopsis roots. Using in vivo ³² P_i -labelling of Arabidopsis seedlings, low physiological (μm) concentrations of Spm were found to promote a rapid PIP₂ increase in roots that was time- and dose-dependent. Confocal imaging of a genetically encoded PIP₂ biosensor revealed that this increase was triggered at the plasma membrane. Differential ³² P_i -labelling suggested that the increase in PIP₂ was generated through activation of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) activity rather than inhibition of a phospholipase C or PIP₂ 5-phosphatase activity. Systematic analysis of transfer DNA insertion mutants identified PIP5K7 and PIP5K9 as the main candidates involved in the Spm-induced PIP₂ response. Using non-invasive microelectrode ion flux estimation, we discovered that the Spm-triggered K⁺ efflux response was strongly reduced in pip5k7 pip5k9 seedlings. Together, our results provide biochemical and genetic evidence for a physiological role of PIP₂ in polyamine-mediated signalling controlling K⁺ flux in plants.

Genome-wide systematic characterization and expression analysis of the phosphatidylinositol 4-phosphate 5-kinases in plants

Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) are key enzymes in the process of phosphatidylinositol signaling pathway and have essential functions in growth, development, and biotic and abiotic stresses responses in plants. However, the evolutionary history and patterns of PIP5K genes in plants have not been examined systematically. Here, we use whole-genome sequences from eight plant species of land plants and algae to define the evolutionary relationships between these proteins in plants. 85 PIP5K genes were identified and divided into two subfamilies based on phylogenetic analyses. PIP5K members in subfamily II underwent several duplication events in land plants, resulting in multiple gene copies in angiosperms, while PIP5K members in subfamily I displayed low-copy numbers and lost in eudicots. Furthermore, PIP5K genes within the same subfamily had similar motifs and intron/exon features. Nine duplicated soybean gene pairs, four duplicated Arabidopsis gene pairs and two rice duplicated gene pairs were identified and many of them localized in synteny genomic regions. These duplicate events were formed by Whole-genome duplication (WGD)/segmental duplications. In addition, the ratios of non-synonymous to synonymous substitutions (Ka/Ks) showed that the PIP5K family had undergone purifying selection in higher plants. Expression analysis showed that PIP5K genes had complex and variable expression patterns in different developmental stages. The specificity of these genes is utilized to provide evidence for selective expression in the evolutionary process.

DWT1/DWL2 act together with OsPIP5K1 to regulate plant uniform growth in rice

Uniform growth of the main shoot and tillers significantly influences rice plant architecture and grain yield. The WUSCHEL-related homeobox transcription factor DWT1 is a key regulator of this important agronomic trait, disruption of which causes enhanced main shoot dominance and tiller dwarfism by an unknown mechanism. Here, we have used yeast-two-hybrid screening to identify OsPIP5K1, a member of the rice phosphatidylinositol-4-phosphate 5-kinase family, as a protein that interacts with DWT1. Cytological analyses confirmed that DWT1 induces accumulation of OsPIP5K1 and its product PI(4,5)P₂ , a phosphoinositide secondary messenger, in nuclear bodies. Mutation of OsPIP5K1 compounds the dwarf dwt1 phenotype but abolishes the main shoot dominance. Conversely, overexpression of OsPIP5K1 partially rescues dwt1 developmental defects. Furthermore, we showed that DWL2, the homologue of DWT1, is also able to interact with OsPIP5K1 and shares partial functional redundancy with DWT1 in controlling rice uniformity. Overall, our data suggest that nuclear localised OsPIP5K1 acts with DWT1 and/or DWL2 to coordinate the uniform growth of rice shoots, likely to be through nuclear phosphoinositide signals, and provides insights into the regulation of rice uniformity via a largely unexplored plant nuclear signalling pathway.

Comparative transcriptional analysis of Capsicum flower buds between a sterile flower pool and a restorer flower pool provides insight into the regulation of fertility restoration

BACKGROUND

Cytoplasmic male sterility (CMS) and its restoration of fertility (Rf) system is an important mechanism to produce F₁ hybrid seeds. Understanding the interaction that controls restoration at a molecular level will benefit plant breeders. The CMS is caused by the interaction between mitochondrial and nuclear genes, with the CMS phenotype failing to produce functional anthers, pollen, or male gametes. Thus, understanding the complex processes of anther and pollen development is a prerequisite for understanding the CMS system. Currently it is accepted that the Rf gene in the nucleus restores the fertility of CMS, however the Rf gene has not been cloned. In this study, CMS line 8A and the Rf line R1, as well as a sterile pool (SP) of accessions and a restorer pool (RP) of accessions analyzed the differentially expressed genes (DEGs) between CMS and its fertility restorer using the conjunction of RNA sequencing and bulk segregation analysis.

RESULTS

A total of 2274 genes were up-regulated in R1 as compared to 8A, and 1490 genes were up-regulated in RP as compared to SP. There were 891 genes up-regulated in both restorer accessions, R1 and RP, as compared to both sterile accessions, 8A and SP. Through annotation and expression analysis of co-up-regulated expressed genes, eight genes related to fertility restoration were selected. These genes encode putative fructokinase, phosphatidylinositol 4-phosphate 5-kinase, pectate lyase, exopolygalacturonase, pectinesterase, cellulose synthase, fasciclin-like arabinogalactan protein and phosphoinositide phospholipase C. In addition, a phosphatidylinositol signaling system and an inositol phosphate metabolism related to the fertility restorer of CMS were ranked as the most likely pathway for affecting the restoration of fertility in pepper.

CONCLUSIONS

Our study revealed that eight genes were related to the restoration of fertility, which provides new insight into understanding the molecular mechanism of fertility restoration of CMS in Capsicum.

Physiological Functions of Phosphoinositide-Modifying Enzymes and Their Interacting Proteins in Arabidopsis

The integrity of cellular membranes is maintained not only by structural phospholipids such as phosphatidylcholine and phosphatidylethanolamine, but also by regulatory phospholipids, phosphatidylinositol phosphates (phosphoinositides). Although phosphoinositides constitute minor membrane phospholipids, they exert a wide variety of regulatory functions in all eukaryotic cells. They act as key markers of membrane surfaces that determine the biological integrity of cellular compartments to recruit various phosphoinositide-binding proteins. This review focuses on recent progress on the significance of phosphoinositides, their modifying enzymes, and phosphoinositide-binding proteins in Arabidopsis.

Phosphoinositides control the localization of HOPS subunit VPS41, which together with VPS33 mediates vacuole fusion in plants

The vacuole is an essential organelle in plant cells, and its dynamic nature is important for plant growth and development. Homotypic membrane fusion is required for vacuole biogenesis, pollen germination, stomata opening, and gravity perception. Known components of the vacuole fusion machinery in eukaryotes include SNARE proteins, Rab GTPases, phosphoinositides, and the homotypic fusion and vacuolar protein sorting (HOPS) tethering complex. HOPS function is not well characterized in plants, but roles in embryogenesis and pollen tube elongation have been reported. Here, we show that Arabidopsis HOPS subunits VPS33 and VPS41 accumulate in late endosomes and that VPS41, but not VPS33, accumulates in the tonoplast via a wortmannin-sensitive process. VPS41 and VPS33 proteins bind to liposomes, but this binding is inhibited by phosphatidylinosiltol-3-phosphate [PtdIns(3)P] and PtdIns(3,5)P₂, which implicates a nonconserved mechanism for HOPS recruitment in plants. Inducible knockdown of VPS41 resulted in dramatic vacuole fragmentation phenotypes and demonstrated a critical role for HOPS in vacuole fusion. Furthermore, we provide evidence for genetic interactions between VPS41 and VTI11 SNARE that regulate vacuole fusion, and the requirement of a functional SNARE complex for normal VPS41 and VPS33 localization. Finally, we provide evidence to support VPS33 and SYP22 at the initial stage for HOPS-SNARE interactions, which is similar to other eukaryotes. These results highlight both conserved and specific mechanisms for HOPS recruitment and function during vacuole fusion in plants.

ZmSTK1 and ZmSTK2, encoding receptor-like cytoplasmic kinase, are involved in maize pollen development with additive effect

Pollen germination and pollen tube growth are important physiological processes of sexual reproduction of plants and also are involved in signal transduction. Our previous study reveals that ZmSTK1 and ZmSTK2 are two receptor-like cytoplasmic kinases (RLCK) homologs in Zea mays as members of receptor-like protein kinase (RLK) subfamily, sharing 86% identity at the amino acid level. Here, we report that ZmSTK1 and ZmSTK2, expressed at late stages of pollen development, regulate maize pollen development with additive effect. ZmSTK1 or ZmSTK2 mutation exhibited severe pollen transmission deficiency, which thus influenced pollen fertility. Moreover, the kinase domains of ZmSTKs were cross-interacted with C-terminus of enolases detected by co-immunoprecipitation (Co-IP) and yeast two-hybrid system (Y2H), respectively. Further, the detective ZmSTK1 or ZmSTK2 was associated with decreased activity of enolases and also reduced downstream metabolite contents, which enolases are involved in glycolytic pathway, such as phosphoenolpyruvate (PEP), pyruvate, ADP/ATP, starch, glucose, sucrose and fructose. This study reveals that ZmSTK1 and ZmSTK2 regulate maize pollen development and indirectly participate in glycolytic pathway.

WRKY23 is a component of the transcriptional network mediating auxin feedback on PIN polarity

Auxin is unique among plant hormones due to its directional transport that is mediated by the polarly distributed PIN auxin transporters at the plasma membrane. The canalization hypothesis proposes that the auxin feedback on its polar flow is a crucial, plant-specific mechanism mediating multiple self-organizing developmental processes. Here, we used the auxin effect on the PIN polar localization in Arabidopsis thaliana roots as a proxy for the auxin feedback on the PIN polarity during canalization. We performed microarray experiments to find regulators of this process that act downstream of auxin. We identified genes that were transcriptionally regulated by auxin in an AXR3/IAA17- and ARF7/ARF19-dependent manner. Besides the known components of the PIN polarity, such as PID and PIP5K kinases, a number of potential new regulators were detected, among which the WRKY23 transcription factor, which was characterized in more detail. Gain- and loss-of-function mutants confirmed a role for WRKY23 in mediating the auxin effect on the PIN polarity. Accordingly, processes requiring auxin-mediated PIN polarity rearrangements, such as vascular tissue development during leaf venation, showed a higher WRKY23 expression and required the WRKY23 activity. Our results provide initial insights into the auxin transcriptional network acting upstream of PIN polarization and, potentially, canalization-mediated plant development.

The plant hormone auxin belongs to the major plant-specific developmental regulators. It mediates or modifies almost all aspects of plant life. One of the fascinating features of the auxin action is its directional movement between cells, whose direction can be regulated by auxin signaling itself. This plant-specific feedback regulation has been proposed decades ago and allows for the self-organizing formation of distinct auxin channels shown to be crucial for processes, such as the regular pattern formation of leaf venation, organ formation, and regeneration of plant tissues. Despite the prominent importance of this so called auxin canalization process, the insight into the underlying molecular mechanism is very limited. Here, we identified a number of genes that are transcriptionally regulated and act downstream of the auxin signaling to mediate the auxin feedback on the polarized auxin transport. One of them is the WRKY23 transcription factor that has previously been unsuspected to play a role in this process. Our work provides the first insights into the transcriptional regulation of the auxin canalization and opens multiple avenues to further study this crucial process.

PATELLINS are regulators of auxin-mediated PIN1 relocation and plant development in Arabidopsis thaliana

Coordinated cell polarization in developing tissues is a recurrent theme in multicellular organisms. In plants, a directional distribution of the plant hormone auxin is at the core of many developmental programs. A feedback regulation of auxin on the polarized localization of PIN auxin transporters in individual cells has been proposed as a self-organizing mechanism for coordinated tissue polarization, but the molecular mechanisms linking auxin signalling to PIN-dependent auxin transport remain unknown. We used a microarray-based approach to find regulators of the auxin-induced PIN relocation in Arabidopsis thaliana root, and identified a subset of a family of phosphatidylinositol transfer proteins (PITPs), the PATELLINs (PATLs). Here, we show that PATLs are expressed in partially overlapping cell types in different tissues going through mitosis or initiating differentiation programs. PATLs are plasma membrane-associated proteins accumulated in Arabidopsis embryos, primary roots, lateral root primordia and developing stomata. Higher order patl mutants display reduced PIN1 repolarization in response to auxin, shorter root apical meristem, and drastic defects in embryo and seedling development. This suggests that PATLs play a redundant and crucial role in polarity and patterning in Arabidopsis.

Vacuolar trafficking and biogenesis: a maturation in the field

The vacuole is a prominent organelle that is essential for plant viability. The vacuole size, and its role in ion homeostasis, protein degradation and storage, place significant demands for trafficking of vacuolar cargo along the endomembrane system. Recent studies indicate that sorting of vacuolar cargo initiates at the ER and Golgi, but not the trans-Golgi network/early endosome, as previously thought. Furthermore, maturation of the trans-Golgi network into pre-vacuolar compartments seems to contribute to a major route for plant vacuolar traffic that works by bulk flow and ends with membrane fusion between the pre-vacuolar compartment and the tonoplast. Here we summarize recent evidence that indicates conserved and plant-specific mechanisms involved in sorting and trafficking of proteins to this major organelle.

Perturbing phosphoinositide homeostasis oppositely affects vascular differentiation in Arabidopsis thaliana roots

The plant vascular network consists of specialized phloem and xylem elements that undergo two distinct morphogenetic developmental programs to become transport-functional units. Whereas vacuolar rupture is a determinant step in protoxylem differentiation, protophloem elements never form a big central vacuole. Here, we show that a genetic disturbance of phosphatidylinositol 4,5-bis-phosphate [PtdIns(4,5)P₂] homeostasis rewires cell trafficking towards the vacuole in Arabidopsis thaliana roots. Consequently, an enhanced phosphoinositide-mediated vacuolar biogenesis correlates with premature programmed cell death (PCD) and secondary cell wall elaboration in xylem cells. By contrast, vacuolar fusion events in protophloem cells trigger the abnormal formation of big vacuoles, preventing cell clearance and tissue functionality. Removal of the inositol 5' phosphatase COTYLEDON VASCULAR PATTERN 2 from the plasma membrane (PM) by brefeldin A (BFA) treatment increases PtdIns(4,5)P₂ content at the PM and disrupts protophloem continuity. Conversely, BFA application abolishes vacuolar fusion events in xylem tissue without preventing PCD, suggesting the existence of additional PtdIns(4,5)P₂-dependent cell death mechanisms. Overall, our data indicate that tight PM phosphoinositide homeostasis is required to modulate intracellular trafficking contributing to oppositely regulate vascular differentiation.

Guilt by Association: A Phenotype-Based View of the Plant Phosphoinositide Network

Eukaryotic membranes contain small amounts of phospholipids that have regulatory effects on the physiological functions of cells, tissues, and organs. Phosphoinositides (PIs)-the phosphorylated derivatives of phosphatidylinositol-are one example of such regulatory lipids. Although PIs were described in plants decades ago, their contribution to the regulation of physiological processes in plants is not well understood. In the past few years, evidence has emerged that PIs are essential for plant function and development. Recently reported phenotypes associated with the perturbation of different PIs suggest that some subgroups of PIs influence specific processes. Although the molecular targets of PI-dependent regulation in plants are largely unknown, the effects of perturbed PI metabolism can be used to propose regulatory modules that involve particular downstream targets of PI regulation. This review summarizes phenotypes associated with the perturbation of the plant PI network to categorize functions and suggest possible downstream targets of plant PI regulation.