BACKGROUND

Carotenoids are common isoprenoid pigments synthesized by all photosynthetic organisms and many heterotrophic bacteria and fungi. Carotenoids are essential constituents of the photosynthetic apparatus due to their vital function in photo-protection and contribution to harnessing light energy, as well as sources for biologically important compounds such as the retinoids. Moreover, these pigments play an ecological role by conferring their colors to many fruits and flowers. Besides these “classical” functions, carotenoids are a ubiquitous source for biologically important compounds, such as the opsin chromophore retinal, and for hormones and signaling molecules including the vertebrate morphogen retinoic acid, the phytohormones abscisic acid (ABA) and strigolactone (SL), the plant plastid retrograde signaling molecule cyclocitral, and the fungal morphogen trisporic acid. All of these derivatives arise by virtue of the extended, conjugated double bond system that makes carotenoids prone to oxidative cleavage. This reaction yields carbonyl products that are generally called apocarotenoids and can be catalyzed by enzymes from the carotenoid cleavage dioxygenase (CCD) family, which break defined C—C double bonds by inserting molecular oxygen. Arabidopsis CCDs are divided into nine-cis-epoxycarotenoid cleavage dioxygenases (NCED2, 3, 5, 6, and 9) that form the ABA precursor xanthoxin from 9-cis-epoxycarotenoids and CCDs with different substrate and regio-specificity. The latter group includes CCD1, which forms a plenitude of C13, C10and C8volatiles from different apocarotenoids and C40-carotenoids, CCD4, which cleaves all-trans-β-carotene into β-ionone (C13) and β-apo-10′-carotenal (C27), the SL biosynthesis enzyme CCD7 (MAX3), which breaks 9-cis-β-carotene into β-ionone (C13) and 9-cis-β-apo-10′-carotenal (C27) that is converted by CCD8 (MAX4) into the SL biosynthesis intermediate carlactone. CCD8 can also cleave all-trans-β-apo-10′-carotenal into the ketone carotenone (d′orenone), but with low activity. Non-enzymatic cleavage, which occurs at each double bond in the carotenoid backbone, is another important route for apocarotenoid formation. This process is triggered by reactive oxygen species (ROS) and can also yield signaling molecules, such as the plant stress signal β-cyclocitral., Which is formed by singlet oxygen (1O2) and mediates gene responses to this ROS. In addition to monocarbonyls, carotenoid cleavage can yield dialdehyde products (diapocarotenoids), as shown for several plants and cyanobacterial CCDs that cleave multiple double bonds within carotenoids or target apocarotenoids. The question of whether diapocarotenoids are also regulatory metabolites has not yet been answered.

Plants need complex and dynamic root systems for anchorage as well as for water and nutrient uptake under variable availabilities. Arabidopsis is an ideal model plant to study root development because of its genetic tractability and the fast-growing and relatively simple root system. Arabidopsis has three highly characterized types of roots: i) a primary root initiated in embryogenesis, ii) lateral roots (LRs) that form from the primary root and other LRs, and iii) adventitious roots, which emerge from non-root tissues:, such as stem, leaves and hypocotyl. Anchor roots (ANRs) constitute a fourth type of roots. ANRs emerge from the collet, a region located just below the root-hypocotyl region, and have remained largely uncharacterized. ANRs are largely uncharacterized and are often classified as either adventitious roots or LRs.

The development of LRs occurs in a series of well-documented stages. LRs are positioned through a process that involves gene expression oscillation mediated by an unidentified carotenoid-derived signal. In Arabidopsis, LRs initiate from xylem pole pericycle cells, which form the cell layer between the vascular bundle and the endodermis. These cells divide to produce LR primordium, which continue to grow until the new roots emerge by pushing through the outer layers of the originating root. The plant hormone auxin plays a central role in root development. For instance, the auxin signaling component MP/ARF5 regulates embryonic root development and the auxin-related transcription factors ARF7 and ARF19 are pivotal for LR initiation In addition to auxin, the carotenoid-derived phytohormones, SL and ABA, have been shown to regulate different aspects of root development. There is a need to identify carotenoid-derived compounds involved in regulation of plant growth and root development.

SUMMARY

In general, the present disclosure features a plant growth regulator composition comprising, in an agronomically acceptable carrier, at least one diapocarotenoid represented by formula (I):
R-A-R  (I)
or a precursor, salt, solvate, stereoisomer or polymorph thereof, wherein R is a monovalent carbonyl moiety selected from the group consisting of aldehydes, ethers, diethers, carboxylic acids, alcohols, and ester carboxylates and A is a bivalent polyene represented by the bivalent moiety —(CRa═CRb)x— wherein x is the number of double bonds in the polyene moiety A, and Raand Rbare, independently, hydrogen, a hydrocarbon, or an alkoxy group. In one or more embodiments, R is an aldehyde, x is an integer between 2-6 and the double bonds of the bivalent polyene moiety A are all trans. In one or more embodiments, the bivalent polyene moiety A is a C8moiety with two methyl substituents and no heteroatoms. In one or more embodiments, the at least one diapocarotenoid plant growth regulator is 12,12′-diapocarotene-12,12′-dial. In one or more embodiments, the at least one diapocarotenoid plant growth regulator is 8,15-diapocarotene-8,15-dial. In one or more embodiments, the bivalent polyene moiety A is a C13moiety with three methyl substituents and no heteroatoms. In one or more embodiments, the at least one diapocarotenoid plant growth regulator is 8,12′-diapocarotene-8,12′-dial. In one or more of the embodiments above, the at least one diapocarotenoid plant growth regulator is 1,1,8,8-tetramethyoxy-2,7-dimethyl-2,4,6-octatriene. In one or more of the embodiments above, the precursor of the at least one diapocarotenoid plant growth regulator is all-trans-3-OH-β-12′-carotenal. In one or more of the embodiments above, the composition comprises about 1-125 ppm, about 100-750 ppm, about 500-1,500 ppm, about 1,000-3,000, or about 2,000 to 10,000 ppm of the at least one diapocarotenoid plant growth regulator or a precursor thereof. In one or more of the embodiments above, the agronomically acceptable carrier comprises a water-miscible organic solvent selected from the group consisting of alcohols, ethers, esters, ketones and acetals. In one or more of the embodiments above, the composition further comprises a plant growth medium.

The present disclosure further features a method of regulating plant growth comprising applying to a seed, plant propagation material, or plant, in need of growth regulation, an effective amount at least one diapocarotenoid plant growth regulator represented by formula (I):
R-A-R  (I)
or a precursor, salt, solvate, stereoisomer, or polymorph thereof; wherein R is a monovalent carbonyl moiety selected from the group consisting of aldehydes, ethers, diethers, carboxylic acids, alcohols, and ester carboxylates and A is a bivalent polyene represented by the bivalent moiety —(CRa=CRb)x— wherein x is the number of double bonds in polyene moiety A, and Raand Rbare, independently, hydrogen, a hydrocarbon, or an alkoxy group. In one or more embodiments, applying to the seed, plant propagation material or plant comprises treating a plant growth medium in contact with the seed, plant propagation material or plant with the at least one diapocarotenoid plant growth regulator or precursor thereof. In one or more embodiments, the plant growth medium is selected from the group consisting of water, peat moss-based growing mix, a greenhouse growing mix, inert support loaded with nutrient solutions, and agar medium formulated for promoting root development. In one or more of the embodiments above, the seed, plant propagation material or plant is in need of increased root development. In one or more of the embodiments above, the seed, plant propagation material, or plant is a difficult-to-propagate plant or from a difficult-to-propagated plant. In one or more embodiments above, the at least one diapocarotenoid plant growth regulator is applied to the plant within 3-8 days after germination. In one or more of the embodiments above, the seed, plant propagation material or plant is in need of increased resistance to abiotic stress. In one or more of embodiments above, the at least one diapocarotenoid plant growth regulator is selected from the group consisting of 12,12′-diapocarotene-12,12′-dial, 1,1,8,8-tetramethoxy-2,7-dimethyl-2,4,6-octatriene, and the combination thereof, and regulating plant growth comprises one or more of promoting root development, increasing nutrient uptake, enhancing resistance to abiotic stress factors, invigorating plant growth, increasing plant yield, and increasing plant biomass. In one or more of the embodiments above, the at least one diapocarotenoid plant growth regulator is selected from the group consisting of 8,15-diapocarotene-8,15-dial, 8,12′-diapocarotene-8,12′-dial, and the combination thereof, and regulating plant growth comprises inhibiting root growth. In one or more of the embodiments above, the precursor of the at least one diapocarotenoid plant growth regulator is all-trans-3-OH-β-12′-carotenal and regulating plant growth comprises promoting root development. In one embodiment, applying further comprises treating land at risk for being colonized by an unwanted plant with the at least one diapocarotenoid plant growth regulator.

The present disclosure further features a kit for regulating plant growth comprising a composition for promoting plant growth comprising, in one or more containers at least one diapocarotenoid plant growth regulator represented by formula (I)
R-A-R  (I)
or a precursor, salt, solvate, stereoisomer, or polymorph thereof; wherein R is a monovalent carbonyl moiety selected from the group consisting of aldehydes, ethers, diethers, carboxylic acids, alcohols, and ester carboxylates and A is a bivalent polyene represented by the bivalent moiety —(CRa=CRb)x— wherein x is the number of double bonds in polyene moiety A, and Raand Rb are, independently, hydrogen, a hydrocarbon, or an alkoxy group and an agronomically acceptable carrier.

The details of one or more examples are set forth in the description below. Other features, objects, and advantages will be apparent from the description and from the claims.

