Membrane trafficking to the plasma membrane (PM) is a highly organized process which enables plant cells to build up their bodies. SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) genes, which encode the proteins involved in membrane trafficking, are much more abundant in the Arabidopsis genome than in that of any other eukaryote. We have previously shown that a large number of SNARE molecules in the Arabidopsis cell are localized predominantly on the PM. In the present study, in order to elucidate the physiological function of each PM-localized SNARE, we analyzed the spatiotemporal expression profiling of nine SYP1s that are resident in the PM of Arabidopsis, and used the information thus acquired to generate transgenic Arabidopsis plants expressing green fluorescent protein-fused Qa-SNAREs under control of their authentic promoters. Among the nine SYP1s, only SYP132 is expressed ubiquitously in all tissues throughout plant development. The expression patterns of the other SYP1s, in contrast, are tissue specific, and all different from one another. A particularly noteworthy example is SYP123, which is predominantly expressed in root hair cells during root development, and shows a focal accumulation pattern at the tip region of root hairs. These results suggest that SYP132 is involved in constitutive membrane trafficking to the PM throughout plant development, while the other SYP1s are involved in membrane trafficking events such as root formation or tip growth of root hair, with some redundancy.
Root hairs are fast-growing tubular protrusions on root epidermal cells that play important roles in water and nutrient uptake in plants. The tip-focused polarized growth of root hairs is accomplished by the secretion of newly synthesized materials to the tip via the polarized membrane trafficking mechanism. Here, we report the function of two different types of plasma membrane (PM) Qa-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), SYP123 and SYP132, in the growth of root hair in Arabidopsis. We found that SYP123, but not SYP132, localizes in the tip region of root hairs by recycling between the brefeldin A (BFA)-sensitive endosomes and the PM of the expanding tip in an F-actin-dependent manner. The vesicle-associated membrane proteins VAMP721/722/724 also exhibited tip-focused localization in root hairs and formed ternary SNARE complexes with both SYP123 and SYP132. These results demonstrate that SYP123 and SYP132 act in a coordinated fashion to mediate tip-focused membrane trafficking for root hair tip growth.
Vacuolar H + -PPase, a membrane bound protontranslocating pyrophosphatase found in various species including plants, some protozoan and prokaryotes, has been demonstrated to be localized to the vacuolar membrane in plants. Using a GUS reporter system and a green fluorescent protein (GFP) fusion protein, we investigated the tissue distribution and the subcellular localization, respectively, of a novel type H + -PPase encoded by AVP2/AVPL1 identified in the Arabidopsis thaliana genome. We showed that AVP2/AVPL1 is highly expressed at the trichome and the filament of stamen. Furthermore, the fluorescence of GFP-tagged AVP2/AVPL1 showed small dotlike structures that were observed throughout the cytoplasm of various Arabidopsis cells under a fluorescent microscope. The distribution of this dot-like fluorescent pattern was apparently affected by a treatment with brefeldin A. Moreover, we demonstrated that most dot-like fluorescent structures colocalized with a Golgi resident protein. These findings suggest that this novel type H + -PPase resides on the Golgi apparatus rather than the vacuolar membrane. ß
Amyloplasts, a subtype of plastid, are found in nonphotosynthetic tissues responsible for starch synthesis and storage. When tobacco (Nicotiana tabacum) Bright Yellow-2 cells are cultured in the presence of cytokinin instead of auxin, their plastids differentiate from proplastids to amyloplasts. In this program, it is well known that the expression of nucleus-encoded starch biosynthesis genes, such as ADP-Glucose Pyrophosphorylase (AgpS) and Granule-Bound Starch Synthase (GBSS), is specifically induced. In this study, we investigated the roles of plastid gene expression in amyloplast differentiation. Microarray analysis of plastid genes revealed that no specific transcripts were induced in amyloplasts. Nevertheless, amyloplast development accompanied with starch biosynthesis was drastically inhibited in the presence of plastid transcription/translation inhibitors. Surprisingly, the expression of nuclear AgpS and GBSS was significantly repressed by the addition of these inhibitors, suggesting that a plastid-derived signal(s) that reflects normal plastid gene expression was essential for nuclear gene expression. A series of experiments was performed to examine the effects of intermediates and inhibitors of tetrapyrrole biosynthesis, since some of the intermediates have been characterized as candidates for plastid-to-nucleus retrograde signals. Addition of levulinic acid, an inhibitor of tetrapyrrole biosynthesis, resulted in the up-regulation of nuclear AgpS and GBSS gene expression as well as starch accumulation, while the addition of heme showed opposite effects. Thus, these results indicate that plastid transcription and/or translation are required for normal amyloplast differentiation, regulating the expression of specific nuclear genes by unknown signaling mechanisms that can be partly mediated by tetrapyrrole intermediates.Plastids are plant-specific, semiautonomous organelles that have important metabolic functions, such as photosynthesis, carbon and nitrogen assimilation, and biosynthesis of lipids and amino acids. In higher plants, plastids are known to differentiate between different subtypes in response to several developmental and environmental conditions. Each differentiated form of plastid plays a specific role in maintaining a variety of metabolic and physiological functions as a component of plant cells or tissues (for review, see Lopez-Juez and Pyke, 2005). For instance, chloroplasts, the most characterized form of plastid, are found in the majority of green tissues and are involved in photosynthesis. Chromoplasts occur in limited organs, such as fruit and floral petals, or root tissues in carrot (Daucus carota), and are engaged in the synthesis and storage of carotenoid pigments. Leucoplasts are colorless plastids mainly existing in root tissues. Several reactions for terpene biosynthesis take place in leucoplasts as well as in chloroplasts. In meristematic tissues, plastids can be identified as an undifferentiated form, termed proplastids, which are specialized for proliferation instead of any m...
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