Arrows represent promotion of processes or expression of the regulators. Bars represent inhibitors of the indicated processes. The positions of loci do not imply the order of gene action.
The number of genes shown by genetic and/or molecular studies to regulate ABA responses has increased to nearly 50 in Arabidopsis alone. The list continues to expand, and the molecular and genetic studies are converging as the various ABA response loci are cloned, the putative interacting proteins are identified by two-hybrid screens, and the roles of suspected signaling intermediates are tested by manipulating their activities in transgenic plants. Studies of orthologs and functional tests in heterologous systems have shown that many of these genes have conserved functions. There also is substantial diversity in signaling within any given species, reflected in both redundant and independent ABA signaling mechanisms. As has been described for many other signaling systems, the ABA response depends on coordinated interactions between positive and negative regulators. Many interactions with pathways that mediate responses to other signals also have been described, but in many cases, it is unclear whether these interactions are direct or indirect.
Together, these results are consistent with the existence of both extracellular and intracellular ABA receptors. However, other interpretations are possible, for example, direct ABA action on plasma and tonoplast membranes (or ion channels) from the cytoplasmic side, higher affinity of an ABA receptor for the protonated form, or pH-dependent pathways.
Although the studies described above focus on the growth-inhibiting effects of high ABA concentrations, the stunted growth of ABA-deficient plants even when well watered implies that the low endogenous ABA levels in unstressed plants actually promote growth. Recent studies in maize and tomato suggest that the stunted growth of ABA-deficient plants is caused by the overproduction of ethylene (Sharp et al., 2000; Spollen et al., 2000), which normally would be inhibited by ABA.
Identities, Roles, and Interactions of Known Regulators Controlling Seed Maturation
During seed maturation in many species, there are two peaks of ABA accumulation. In some species (e.g., Brassica napus), these peaks are correlated with low germinability of isolated embryos (Finkelstein et al., 1985), whereas in others (e.g., maize), there is no correlation (Rivin and Grudt, 1991). Genetic studies in Arabidopsis demonstrated that the first ABA peak is maternally derived and immediately precedes the maturation phase (Karssen et al., 1983). This is important, in conjunction with the leafy-cotyledon FUS3 and LEC genes, for preventing premature germination at the end of the cell division phase of embryogenesis (Raz et al., 2001). However, although this early ABA peak is reduced threefold in fus3 mutants, only the double mutants combining fus3 with ABA deficiency are highly viviparous (Nambara et al., 2000); no ABA-insensitive (abi) or even digenic ABA-deficient (aba3 and aao3) (Seo et al., 2000) Arabidopsis lines are viviparous. In contrast, maize mutants with single defects in either ABA response (vp1) or synthesis (other vp mutants) are viviparous (Robertson, 1955; Robichaud et al., 1980). Although some combinations of abi and leafy-cotyledon mutations also lead to vivipary, there is no good correlation between the degrees of vivipary and seed sensitivity to ABA (e.g., compare abi4 lec1 and abi5 lec1) ( Table 4 ). These results further accentuate the distinction between the mechanisms controlling vivipary and the ABA sensitivity of germination.
At the whole plant level, dehydrative stresses cause reduced stomatal aperture to minimize water loss via transpiration. In addition, mild water stress drives root growth but inhibits shoot growth, whereas severe stress inhibits both root and shoot growth. ABA is a major mediator of these stress responses, and its signaling mechanisms are best characterized with respect to its effects on stomata, where it both promotes stomatal closure and inhibits opening (reviewed by Schroeder et al., 2001). Although these effects lead to the same end result (i.e., closed stomata), opening and closing are not simple reversals of the same process, so the relevant signaling mechanisms differ. In addition to the electrophysiological changes leading to osmotic water loss, guard cells undergo a substantial change in volume accompanied by up to twofold changes in membrane surface area. These large volume and area changes involve vesicle secretion and endocytosis, possibly mediated by annexin-, syntaxin-, and Rho-like gene products (Kovacs et al., 1998; Leyman et al., 1999; Lemichez et al., 2001), and reorganization of the actin cytoskeleton (Eun and Lee, 1997). Like the electrophysiological changes, the ABA effects on actin reorganization are mediated by Ca 2+ signaling and phosphorylation cascades, including ABI1-dependent signaling.
Recent studies have shown that ABA sensitizes these channels, apparently by enhancing the production of reactive oxygen species (ROS; e.g., H2O2) that can serve as secondary messengers leading to channel activation (Pei et al., 2000; Zhang et al., 2001). ROS production is a common Rop-dependent response to several stresses leading to stomatal closure, including drought and pathogen attack (Lee et al., 1999), and the ROS-dependent pathway of response may be shared by multiple stresses (Yang, 2002). Consistent with this view, constitutive activation of an H2O2-activated mitogen-activated protein kinase cascade confers enhanced tolerance of abiotic stresses (Kovtun et al., 2000). Finally, although ABA signaling can result in sustained steady state [Ca 2+ ]cyt increases, various signals that affect stomatal aperture induce [Ca 2+ ]cyt oscillations with distinct periodicity, and responses in ABA mutants can be “rescued” by imposing the correct periodicity with exchanges of external buffer solutions (Allen et al., 2000, 2001).
ABA SIGNALING IN VEGETATIVE STRESS RESPONSES
Aspects of maturation such as reserve accumulation and late-embryogenesis-abundant (LEA) gene expression are controlled largely by the coordinated action of transcription factors. Promoter sequences for storage protein and LEA genes contain elements essential to confer hormone responsiveness and stage and tissue specificity (reviewed by Rock and Quatrano, 1995; Busk and Pages, 1998). Although many DNA binding factors that interact with these promoters have been identified by biochemical or yeast one-hybrid approaches, the functional roles of most of them still must be tested by reverse genetic analyses. To date, only six classes of transcription factors have been demonstrated by genetic analyses to be essential for some ABA- or seed-specific gene expression: ABI3/VP1, ABI4, ABI5, LEC1, LEC2, and FUS3.
Mutants Defective in ABA Synthesis or Response
East China Normal University, Shanghai, 200241, China
Yang, B. & Kallio, H. P. Fatty acid composition of lipids in sea buckthorn (Hippophaë rhamnoides L.) berries of different origins. J. Agric. Food Chem. 49(4), 1939–1947 (2001).
Figure 3 and Table 3 show that the tissues displayed high variation for FA composition in tree peony seeds. For P. ludlowii, a total of 22, 14 and 7 FAs were found respectively in embryo, endosperm and seed coat. Identical results were obtained for P. ostii and P. rockii, except that only 6 FAs were observed in their seed coat and 19 FAs were found in P. rockii embryo. As the minor FAs, heneicosanoic (C21:0, peak 15), eicosadienoic (C20:2 Δ11,14 , peak 16), behenic (C22:0, peak 17), erucic (C22:1 Δ13 , peak 18), tricosanoic (C23:0, peak 19), tetracosanoic (C24:0, peak 20), tetracosaenoic (C24:1 Δ15 , peak 21) and pentacosanoic acids (C25:0, peak 22) were only detected in embryo, but not in endosperm, and only one (C21:0) in the P. ludlowii seed coat. The TFA content varied dramatically among different tissues of P. ostii, P. rockii and P. ludlowii at 264.3, 199.5 and 238.4 mg g −1 in embryo, 342.1, 271.1 and 174.1 mg g −1 in endosperm and 1.61, 3.46 and 4.13 mg g −1 in seed coat, respectively (Table 3).
Simopoulos, A. P. The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomed. Pharmacother. 56, 365–379 (2002).
About this article
Published : 31 May 2016
The ω3 and ω6 fatty acids (FAs) are commonly in the form of ALA and linoleic acid (C18:2 Δ9,12 , LA), which are essential FAs for humans and must be obtained in food or dietary supplements 3 . The ω6 and ω3 FAs have reciprocal biological activities, and the ω6/ω3 ratio of consumption is associated with human health 4 . Healthy ratios of ω6 to ω3 FAs should be lower than 5 5 ; it has been recommended that human beings evolve on a diet with a ω6/ω3 FA ratio of ∼ 1 6 . A lack of ω3 FA dietary intake causes a high ratio in our daily diet, such as sunflower ( ∼ 670), peanut ( ∼ 581.6), corn germ ( ∼ 100), olive oil (16.7) and soybean oil ( ∼ 3.9) 7,8,9 , and has been linked to blood lipid, cardiovascular, autoimmune and inflammatory diseases 10,11 . Owing to overharvesting and environmental pollution, fish oil can no longer serve as a source of ALA and fish cannot satisfy the vegetarian diet. Currently, plant seed oil, such as perilla (54%), flax (51%), rapeseed (9%), soybean (7%) and walnut (6%) 7,12 , has become the major source of ALA. However, as herbaceous plants, perilla and flax promotion is limited because of their low seed production and their competition with food crops 7 . As ALA-enriched resources, tree peony (45%), sacha inchi (50%), sea buckthorn (39%) and cypress (35%) are worthy of further exploration; the tree peony seed oil has a low ω6/ω3 ratio and a potential annual seed production of 57,855 tons 1,13,14,15 . In 2013, tree peony seeds were recommended as a new resource of ALA for seed oil production in China. At the beginning of 2015, the General Office of the State Council of China promulgated the opinions on accelerating the development of the woody oil industry, and tree peony seed oil was clearly proposed. The rediscovery of the value of this oil and the rapid development in recent years have brought unprecedented opportunities and challenges for studies on tree peony.
Joët, T. et al. Metabolic pathways in tropical dicotyledonous albuminous seeds: Coffea arabica as a case study. New Phytolo. 182, 146–162 (2009).
1: C14:0; 2: C15:0; 3: C16:0; 4: C16:1 Δ7 ; 5: C16:1 Δ9 ; 6: C17:0; 7: C17:1 Δ10 ; 8, C18:0; 9: C18:1 Δ9 ; 10: C18:1 Δ11 ; 11: C18:2 Δ9,12 ; 12: C20:0; 13: 18:3 Δ9,12,15 ; 14: C20:1 Δ11 ; 15: C21:0; 16: C20:2 Δ11,14 ; 17: C22:0; 18: C22:1 Δ13 ; 19: C23:0; 20: C24:0; 21: C24:1 Δ15 ; 22: C25:0; IS: internal standard, nonadecanoic acid.
Most common plant oils have little α-linolenic acid (C18:3 Δ9,12,15 , ALA) and an unhealthy ω6/ω3 ratio. Here, fatty acids (FAs) in the seeds of 11 species of Paeonia L., including 10 tree peony and one herbaceous species, were explored using gas chromatograph–mass spectrometer. Results indicated that all Paeonia had a ω6/ω3 ratio less than 1.0, and high amounts of ALA (26.7–50%), oleic acid (C18:1 Δ9 , OA) (20.8–46%) and linoleic acid (C18:2 Δ9,12 , LA) (10–38%). ALA was a dominant component in oils of seven subsection Vaginatae species, whereas OA was predominant in two subsection Delavayanae species. LA was a subdominant oil component in P. ostii and P. obovata. Moreover, the FA composition and distribution of embryo (22 FAs), endosperm (14 FAs) and seed coat (6 FAs) in P. ostii, P. rockii and P. ludlowii were first reported. Peony species, particularly P. decomposita and P. rockii, can be excellent plant resources for edible oil because they provide abundant ALA to balance the ω6/ω3 ratio. The differences in the ALA, LA and OA content proportion also make the peony species a good system for detailed investigation of FA biosynthesis pathway and ALA accumulation.
Our systems have detected unusual traffic activity from your network. Please complete this reCAPTCHA to demonstrate that it’s you making the requests and not a robot. If you are having trouble seeing or completing this challenge, this page may help. If you continue to experience issues, you can contact JSTOR support.
Block Reference: #58967ccc-729b-11ec-b1e9-71414f747458
Date and time: Tue, 11 Jan 2022 05:00:07 GMT