Volatile amoebic compounds (VOCs) serve as airy curve of advice amid host plants, pathogens, commensals, association groups, and, with flowers, their pollinators. Studying petunia flowers, Adebesin et al. appearance that VOCs do not irenic broadcast out of the beef but are actively shuttled beyond the claret film by an ABC (ATP-binding cassette) agent (see the Perspective by Eberl and Gershenzon). Disabling the agent after-effects in accident to the cell’s membranes by intracellular accession of VOCs.
Science, this affair p. 1386; see additionally p. 1334
Plants amalgamate a assortment of airy molecules that are important for reproduction and defense, serve as applied articles for humans, and access atmospheric allure and climate. Despite advance in deciphering bulb airy biosynthesis, their absolution from the corpuscle has been ailing understood. The absence acceptance has been that volatiles irenic broadcast out of cells. By assuming of a Petunia hybrida adenosine triphosphate–binding cassette (ABC) transporter, PhABCG1, we authenticate that access of volatiles beyond the claret film relies on alive transport. PhABCG1 down-regulation by RNA arrest after-effects in decreased discharge of volatiles, which accrue to baneful levels in the claret membrane. This abstraction provides absolute affidavit of a biologically advised apparatus of airy emission.
Plants can absolute up to 10% of photosynthetically anchored carbon adjoin biosynthesis of airy amoebic compounds (VOCs) (1), which are low-molecular-weight lipophilic molecules (~100 to 200 Da) with aerial breath pressures at ambient temperature. Bulb VOCs are chemically assorted and mainly represented by terpenoids, blubbery acerbic derivatives, benzenoids, and phenylpropanoids. VOCs comedy capital roles in pollinator and seed-disperser attraction, above- and belowground aegis adjoin herbivores and pathogens, plant-plant signaling, and allelopathy (2, 3). They additionally accommodate aegis adjoin abiotic factors such as aerial light, temperature, and oxidative accent (4). Moreover, emitted bulb VOCs accord to the assembly of accessory amoebic aerosols that act as billow abstract nuclei, appropriately affecting atmospheric allure and altitude (5, 6). Global anniversary discharge of bulb benzenoids abandoned is estimated to be millions of tons, arch to the accession of up to 10 Tg y−1 of aerosols (6). Bulb VOCs are additionally of amount to the floriculture, flavor, cosmetic, and aroma industries.
In flowers and roots, VOCs are primarily produced in epidermal cells, from which they are appear into the atmosphere (7, 8) and rhizosphere (9), respectively. In abundant organs, VOCs are actinic in glandular trichomes (e.g., in tomato, basil, and peppermint) (10) and ability the atmosphere through automated disruption, or they are fabricated in the blade mesophyll (11) and avenue through the stomata (12) or cuticle (13). Irrespective of the biosynthetic armpit and discharge route, at the subcellular level, VOCs charge bisect the cytosol and cantankerous the claret membrane, hydrophilic corpuscle wall, and sometimes the cuticle to avenue the cell. It has been an accessible catechism how VOCs cantankerous these barriers in complete beef to ability the ambiance or intercellular air spaces affiliated to stomata.
The absence acceptance that VOCs artlessly broadcast out of beef (14–17) may be accurate for baby VOCs such as isoprene. However, clay has predicted that discharge of best VOCs apprenticed alone by circulation would advance to baneful VOC accession in membranes as a aftereffect of best administration of these compounds into lipid bilayers (17). Such accession would account about-face of cellular membranes that could advance to arising of organellar or cellular agreeable (18). This raises the antecedent that VOC discharge requires alive carriage (17).
We acclimated Petunia hybrida flowers, which afford benzenoid and phenylpropanoid volatiles in a temporally adapted manner, to investigate whether VOC discharge relies on protein-mediated export. Because assertive nonvolatile berserk compounds, such as waxes and diterpenes, are exported beyond the claret film by adenosine triphosphate–binding cassette (ABC) transporters (19, 20), we searched petunia blade RNA sequencing (RNA-seq) abstracts sets (21) for ABC agent archetype sequences. We begin one awful bidding applicant that was up-regulated 103-fold in petunia flowers amid day –1 (bud stage) and day 2 afterwards aperture (postanthesis), the adorning stages with the everyman and accomplished VOC emission, appropriately (22) (fig. S1). This gene encodes Petunia hybrida ABC subfamily G affiliate 1, PhABCG1, a predicted plasma-membrane agent of alien activity that is bidding about alone in petals of accessible flowers and adapted by the ODORANT1 archetype agency (23), which controls VOC biosynthesis in petunia flowers (24).
To actuate whether PhABCG1 is complex in VOC emission, we generated transgenic petunia RNA arrest (RNAi) curve beneath ascendancy of the petal-specific linalool synthase apostle (25) to abatement PhABCG1 announcement (fig. S2). Three absolute curve with PhABCG1 archetype levels bargain by 70 to 80% (Fig. 1A) apparent 52 to 62% decreases in absolute VOC discharge (Fig. 1B), with a accessory 101 to 157% access in absolute centralized VOC pools (Fig. 1B). In PhABCG1-RNAi flowers, discharge of anniversary alone VOC was bargain to a altered admeasurement (fig. S3A), and their agnate centralized pools added (fig. S3B). Amounts of booze glycosides, apery 13% of the absolute VOC centralized pool, were ~2.5 times as aerial in transgenic as in wild-type flowers (fig. S3C).
(A) PhABCG1 mRNA levels bent by quantitative about-face archetype polymerase alternation acknowledgment on day 2 postanthesis in wild-type (WT), abandoned agent ascendancy (EV), and three absolute PhABCG1-RNAi curve (A1, A6, and A7). (B) Absolute VOC discharge amount and centralized pools from WT, EV, and PhABCG1-RNAi flowers on day 2 postanthesis. Airy discharge was calm from 18:00 to 22:00 hours (local time). Centralized pools were abstinent at 22:00 hours. All abstracts are agency ± SEM (n = 3). **P < 0.01; ***P < 0.001 (Student’s t test). FW, beginning weight; h, hours.
To anon assay for carriage activity, we bidding His-StrepII–tagged PhABCG1 (fig. S4) in Nicotiana tabacum Bright Yellow 2 (BY-2) abeyance cells. Of 50 buried transformants, the majority gave a absolute arresting abutting to the accepted atomic accession (75.2 kDa), as detected with antibodies adjoin the His tag (fig. S5). Claret membranes able from microsomal fractions were accomplished in both PhABCG1 and the plasma-membrane brand H -dependent adenosine triphosphatase (fig. S6). As a half-size ABC transporter, PhABCG1 possesses a distinct film area and a distinct nucleotide-binding area and may activity as a homo- or heterodimer (19). Because no added half-size ABCG genes were up-regulated in flowers as abundant as PhABCG1 (fig. S1), we accepted that PhABCG1 functions as a homodimer. Back subjected to size-exclusion chromatography, PhABCG1 (fig. S7) elutes afore the 158-kDa brand (fig. S8), constant with the predicted homodimer admeasurement (2 × 75.2 kDa) in a bactericide micelle (~70 kDa). To assay for PhABCG1 carriage activity, we chose methylbenzoate and benzyl booze as substrates because they are above VOC capacity emitted by petunia flowers (22). On evolution of ascendancy and transgenic BY-2 curve cogent PhABCG1 with 14C-labeled substrates, we abstinent cell-associated radioactivity agnate to the aberration amid acquiescent circulation into and alive carriage out of the cells. The empiric abatement in cell-associated radioactivity in transgenic BY-2 beef about to ascendancy beef demonstrates that PhABCG1 transports methylbenzoate and benzyl booze out of beef (Fig. 2, A and C). Two added transgenic BY-2 curve apparent agnate after-effects (Fig. 2C). The antibody-expressing BY-2 corpuscle band SC6 (26), acclimated as a abrogating control, behaved like wild-type cells. Vanadate, a accepted inhibitor of ABC transporters, added methylbenzoate assimilation by 51% in a PhABCG1-expressing BY-2 line, acknowledging the alive attributes of carriage (Fig. 2B). Carriage assays with labeled monoterpene menthol and an analog of the diterpene sclareol [3H-decahydro-2-hydroxy-2,5,5,8a-tetramethyl-1-naphthalene ethanol, a substrate of the N. tabacum NtPDR1 agent (20)] appear no aberration amid PhABCG1-expressing and wild-type BY-2 beef (Fig. 2C). We achieve that PhABCG1 transports phenylpropanoid and benzenoid compounds.
(A) WT and PhABCG1-expressing (ABCG1-9) BY-2 beef were incubated with [14C]methylbenzoate. Afterwards the adumbrated time, beef were harvested and radioactivity was measured. (B) WT and ABCG1-9 BY-2 beef were incubated for 60 min with [14C]methylbenzoate in the attendance or absence of vanadate. (C) Carriage assays with beef from the WT line, a band cogent an antibiotic (SC6) as a abrogating control, and three PhABCG1-expressing curve (ABCG1-3, ABCG1-5, and ABCG1-9) with radiolabeled methylbenzoate, benzyl alcohol, menthol, and a sclareol analog. Evolution time was 60 min for anniversary compound. Values are agency ± SEM (n = 3). **P < 0.01 (Student’s t test).
We articular 16 added ABCG agent candidates in our RNA-seq abstracts sets (fig. S1). All had lower archetype levels than PhABCG1 on day 2 postanthesis, and alone two candidates (Ph4681 and Ph5139) apparent up-regulation on day 2 against day –1, (fig. S1, A and B). Whereas announcement of Ph4681 was begin to be 0.5% that of PhABCG1, announcement of Ph5139 was 17% that of PhABCG1. However, brief RNAi down-regulation of Ph5139 had no furnishings on VOC discharge or centralized pools (fig. S9).
Phylogenetic assay of all petunia annual ABCG transporters articular by RNA-seq and half-size Arabidopsis thaliana transporters with accepted activity appear PhABCG1 to accept one carefully accompanying petunia protein, Ph19708 (65% amino acerbic identity), and no Arabidopsis ortholog (fig. S10). Ph19708 announcement was 0.2% that of PhABCG1 (fig. S1), advertence that it has little, if any, addition to VOC emission. Two added PhABCG1 homologs (Ph13519 and Ph9795, administration ~45% amino acerbic character with PhABCG1) amassed with three Arabidopsis ABCG transporters accepted for alteration wax precursors (AtABCG11, AtABCG12, and AtABCG13) (27) and amassed alone from PhABCG1 and Ph19708 (fig. S10). To analysis whether PhABCG1 contributes to wax forerunner transport, we analyzed wax in wild-type and PhABCG1-RNAi flowers (fig. S11). We begin no statistically cogent differences in wax composition, quantity, and thickness, advertence that PhABCG1 is not complex in consign of wax constituents.
To assay our archetypal anticipation that accumulated VOCs accept adverse furnishings on film candor (17), we decrepit petunia petals of 2-day-old wild-type and PhABCG1-RNAi flowers with two compounds accepting altered modes of action. The first, propidium iodide, diffuses into beef and stains nucleic acids alone if the claret film is disrupted (28). The second, fluorescein diacetate, passes through complete corpuscle membranes and charcoal achromatic until the acetate atom is removed by intracellular esterases. The deacetylated fluorescein is beaming and clumsy to canyon through complete cellular membranes (29). We begin that in PhABCG1-RNAi petals, propidium iodide decrepit nuclei, constant with accident to the claret film by accumulated VOCs (Fig. 3A). Mimicking this effect, wild-type flowers apparent agnate staining back fed with aerial concentrations (15 to 30 mM) of benzaldehyde (fig. S12). We additionally begin beneath fluorescein staining in PhABCG1-RNAi petals (Fig. 3B), accouterment absolute affirmation for disruption of the corpuscle membrane. Flowers from PhABCG1-RNAi curve were abate and had bargain beginning weight about to controls (fig. S13). However, scanning electron microscopy of the epidermal conical beef of transgenic and wild-type petals appear no aberration in basal bore of beef or corpuscle appearance (fig. S14). This suggests that adverse and amplification of these beef booty abode afore VOC accession and appropriately are unaffected.
(A) Confocal microscopy images of 2-day-old WT and PhABCG1 transgenic flowers decrepit with propidium iodide (scale bars, 50 μM). (B) Confocal microscopy images of 2-day-old WT and PhABCG1 transgenic flowers decrepit with fluorescein diacetate (scale bars, 20 μM). (C) Quantification of fluorescence acuteness of fluorescein in WT and PhABCG1 A1 curve decrepit with fluorescein diacetate. Values are agency ± SE (n = 19). ***P < 0.001 (Student’s t test).
Here we appearance that VOC discharge from petunia flowers depends not on acquiescent circulation but rather on protein-mediated carriage beyond the claret membrane. Transporters may adapt discharge of VOCs in added plants and in bacilli throughout added kingdoms of life, area VOCs comedy important roles in actinic communication.
Acknowledgments: This assignment was accurate by admission IOS-1655438 from the National Science Foundation to J.A.M. and N.D. and in allotment by grants from the Belgian National Fund for Scientific Research and the Interuniversity Poles of Allure Program (Belgium). M.Y. was accurate by NSF admission MCB-1121893 to D. B. Szymanski. We acknowledge J. Nader for accomplished abstruse advice and A. Withrow for acceptable with manual electron microscopy. The added abstracts accommodate added data.
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