Summer annuals overwinter as seeds in the soil seed bank. This is facilitated by a cold-induced increase in dormancy during seed maturation followed by a switch to a state during seed imbibition in which cold instead promotes germination. Here, we show that the seed maturation transcriptome in Arabidopsis thaliana is highly temperature sensitive and reveal that low temperature during seed maturation induces several genes associated with dormancy, including DELAY OF GERMINATION1 (DOG1), and influences gibberellin and abscisic acid levels in mature seeds. Mutants lacking DOG1, or with altered gibberellin or abscisic acid synthesis or signaling, in turn show reduced ability to enter the deeply dormant states in response to low seed maturation temperatures. In addition, we find that DOG1 promotes gibberellin catabolism during maturation. We show that C-REPEAT BINDING FACTORS (CBFs) are necessary for regulation of dormancy and of GA2OX6 and DOG1 expression caused by low temperatures. However, the temperature sensitivity of CBF transcription is markedly reduced in seeds and is absent in imbibed seeds. Our data demonstrate that inhibition of CBF expression is likely a critical feature allowing cold to promote rather than inhibit germination and support a model in which CBFs act in parallel to a low-temperature signaling pathway in the regulation of dormancy.

Oxylipins including jasmonates are signaling compounds in plant growth, development, and responses to biotic and abiotic stresses. In Arabidopsis (Arabidopsis thaliana) most mutants affected in jasmonic acid (JA) biosynthesis and signaling are male sterile, whereas the JA-insensitive tomato (Solanum lycopersicum) mutant jai1 is female sterile. The diminished seed formation in jai1 together with the ovule-specific accumulation of the JA biosynthesis enzyme allene oxide cyclase (AOC), which correlates with elevated levels of JAs, suggest a role of oxylipins in tomato flower/seed development. Here, we show that 35S::SlAOC-RNAi lines with strongly reduced AOC in ovules exhibited reduced seed set similarly to the jai1 plants. Investigation of embryo development of wild-type tomato plants showed preferential occurrence of AOC promoter activity and AOC protein accumulation in the developing seed coat and the embryo, whereas 12-oxo-phytodienoic acid (OPDA) was the dominant oxylipin occurring nearly exclusively in the seed coat tissues. The OPDA-and JA-deficient mutant spr2 was delayed in embryo development and showed an increased programmed cell death in the developing seed coat and endosperm. In contrast, the mutant acx1a, which accumulates preferentially OPDA and residual amount of JA, developed embryos similar to the wild type, suggesting a role of OPDA in embryo development. Activity of the residual amount of JA in the acx1a mutant is highly improbable since the known reproductive phenotype of the JA-insensitive mutant jai1 could be rescued by wound-induced formation of OPDA. These data suggest a role of OPDA or an OPDA-related compound for proper embryo development possibly by regulating carbohydrate supply and detoxification.

(+)-7-iso-jasmonoyl isoleucine, the bioactive JA, is involved in most JA-dependent processes mediated by the F-box protein COI1 in a proteasome-dependent manner. However, there is an increasing number of examples, where the precursor of JA biosynthesis, cis-(+)-12-oxophytodienoic acid (OPDA) is active in a JA/COI1-independent manner. Here, we discuss those OPDA-dependent processes, thereby giving emphasis on tomato embryo development. Recent data on seed coat-generated OPDA and its role in embryo development is discussed based on biochemical and genetic evidences.The plant hormone jasmonic acid (JA) was initially detected as its methyl ester in the essential oil of flowers of Jasminum grandiflorum.1 In the sixties and the seventies numerous additional jasmonate compounds were detected preferentially in flowers.2 In the last three decades hydroxylated, glucosylated, sulphated, decarboxylated and carboxylated derivatives as well as JA amino acid conjugates have been identified as plant constituents. Upon wounding there is a rapid accumulation of JA, JA-Ile within some minutes, followed by accumulation of their hydroxylated, carboxylated and glucosylated derivatives. [3][4][5] Some of these JA metabolites occur in flower organs and seeds up to three orders of magnitude higher levels than JA. 6 All Another JA/COI1-independent role of OPDA detected in tomato embryo development 9-11 This metabolic conversion of JA and JA-Ile, respectively, leads to compounds being biologically inactive in some processes such as wounding, where JA and JA-Ile are typically involved as signaling compounds. 6,9,11 In contrast, an O-glycosylated JA derivative is active in leaf movement, 12 and decarboxylated JA, called cis-jasmone, leads to expression of a specific set of genes different from those involved in JA-induced gene expression. 13All these aspects contribute to the questions whether or not the different JA compounds exhibit individual specificity in signaling and which of the metabolic conversion of JA or JA-Ile represents a "switch off" in its signaling cascade.In the last couple of years a similar question was already addressed for the precursor of JA, 12-oxophytodienoic acid (OPDA). Cis-(+)-OPDA is formed by the allene oxide cyclase (AOC)-catalyzed step of JA biosynthesis (Fig. 1) leading to establishment of that enantiomeric structure which is a feature of the naturally occurring JA.7,14 OPDA accumulates upon wounding of leaves usually to higher levels than JA and JA-Ile. 3,4,15 Moreover, a large portion of OPDA is esterified within lipid membranes to numerous compounds. 16,17 These compounds are called arabidopsides due to their exclusive occurrence in the 25 The AOC-catalyzed step in JA biosynthesis is required for OPDA and JA generation. Therefore, the flower phenotype of 35S::AOC-RNAi lines was assumed to be caused by any compound formed downstream of the AOC step. Due to the low seed set of these RNAi lines, however, OPDA and JA levels could not be analyzed.Interestingly, the tomato mutant spr2 has a delay in proper embr...