The Arabidopsis thaliana basic helix-loop-helix Leu zipper transcription factor (TF) MYC2/JIN1 differentially regulates jasmonate (JA)-responsive pathogen defense (e.g., PDF1.2) and wound response (e.g., VSP) genes. In this study, genomewide transcriptional profiling of wild type and mutant myc2/jin1 plants followed by functional analyses has revealed new roles for MYC2 in the modulation of diverse JA functions. We found that MYC2 negatively regulates Trp and Trp-derived secondary metabolism such as indole glucosinolate biosynthesis during JA signaling. Furthermore, MYC2 positively regulates JA-mediated resistance to insect pests, such as Helicoverpa armigera, and tolerance to oxidative stress, possibly via enhanced ascorbate redox cycling and flavonoid biosynthesis. Analyses of MYC2 cis binding elements and expression of MYC2-regulated genes in T-DNA insertion lines of a subset of MYC2-regulated TFs suggested that MYC2 might modulate JA responses via differential regulation of an intermediate spectrum of TFs with activating or repressing roles in JA signaling. MYC2 also negatively regulates its own expression, and this may be one of the mechanisms used in fine-tuning JA signaling. Overall, these results provide new insights into the function of MYC2 and the transcriptional coordination of the JA signaling pathway.

For almost a century the plant hormone auxin has been central to theories on apical dominance, whereby the growing shoot tip suppresses the growth of the axillary buds below. According to the classic model, the auxin indole-3-acetic acid is produced in the shoot tip and transported down the stem, where it inhibits bud growth. We report here that the initiation of bud growth after shoot tip loss cannot be dependent on apical auxin supply because we observe bud release up to 24 h before changes in auxin content in the adjacent stem. After the loss of the shoot tip, sugars are rapidly redistributed over large distances and accumulate in axillary buds within a timeframe that correlates with bud release. Moreover, artificially increasing sucrose levels in plants represses the expression of BRANCHED1 (BRC1), the key transcriptional regulator responsible for maintaining bud dormancy, and results in rapid bud release. An enhancement in sugar supply is both necessary and sufficient for suppressed buds to be released from apical dominance. Our data support a theory of apical dominance whereby the shoot tip's strong demand for sugars inhibits axillary bud outgrowth by limiting the amount of sugar translocated to those buds.shoot branching | sink demand | decapitation | girdling | long-distance signaling

SummaryWe have investigated the relationship between seed dormancy and abscisic acid (ABA) metabolism in the monocot barley and the dicot Arabidopsis. Whether dormant (D) or non-dormant (ND), dry seed of Arabidopsis and embryos of dry barley grains all had similarly high levels of ABA. ABA levels decreased rapidly upon imbibition, although they fell further in ND than in D. Gene expression profiles were determined in Arabidopsis for key ABA biosynthetic [the 9-cis epoxycarotenoid dioxygenase gene family] and ABA catabolic [the ABA 8¢-hydroxylase gene family (CYP707A)] genes. Of these, only the AtCYP707A2 gene was differentially expressed between D and ND seeds, being expressed to a much higher level in ND seeds. Similarly, a barley CYP707 homologue, (HvABA8¢OH-1) was expressed to a much higher level in embryos from ND grains than from D grains. Consistent with this, in situ hybridization studies showed HvABA8¢OH-1 mRNA expression was stronger in embryos from ND grains. Surprisingly, the signal was confined in the coleorhiza, suggesting that this tissue plays a key role in dormancy release. Constitutive expression of a CYP707A gene in transgenic Arabidopsis resulted in decreased ABA content in mature dry seeds and a much shorter after-ripening period to overcome dormancy. Conversely, mutating the CYP707A2 gene resulted in seeds that required longer afterripening to break dormancy. Our results point to a pivotal role for the ABA 8¢-hydroxylase gene in controlling dormancy and that the action of this enzyme may be confined to a particular organ as in the coleorhiza of cereals.

The initiation and development of legume nodules induced by compatible Rhizobium species requires a complex signal exchange involving both plant and bacterial compounds. Phytohormones have been implicated in this process, although in many cases direct evidence is lacking. Here, we characterize the root and nodulation phenotypes of various mutant lines of pea (Pisum sativum) that display alterations in their phytohormone levels and/or perception. Mutants possessing root systems deficient in gibberellins (GAs) or brassinosteroids (BRs) exhibited a reduction in nodule organogenesis. The question of whether these reductions represent direct or indirect effects of the hormone deficiency is addressed. For example, the application of GA to the roots of a GA-deficient mutant completely restored its number of nodules to that of the wild type. Grafting studies revealed that a wild-type shoot or root also restored the nodule number of a GA-deficient mutant. These findings suggest that GAs are required for nodulation. In contrast, the shoot controlled the number of nodules that formed in graft combinations of a BR-deficient mutant and its wild type. The root levels of auxin and GA were similar among these latter graft combinations. These results suggest that BRs influence a shoot mechanism that controls nodulation and that the root levels of auxin and GA are not part of this process. Interestingly, a strong correlation between nodule and lateral root numbers was observed in all lines assessed, consistent with a possible overlap in the early developmental pathways of the two organs.Nodulation is a symbiotic process whereby bacteria of the genus Rhizobium invade compatible leguminous host plants (Mylona et al., 1995;Mathesius, 2003). The invasion ultimately leads to the formation of structures called nodules, in which the bacteria fix atmospheric nitrogen to be used by the plant. As with any developmental process, nodulation is multifaceted, requiring specific signaling events regulated temporally and spatially (Ferguson and Mathesius, 2003).Beginning in the 1980s, mutagenesis experiments using pea (Pisum sativum) produced abnormal nodulation phenotypes including nonnodulating (nod2), poorly nodulating (nod6), and hypernodulating (nod11) mutants, as well as those that fix nitrogen poorly or not at all (fix-; see refs. in Borisov et al., 2000). At present, over 200 nodulation mutants exist in pea (Borisov et al., 2000). Nodulation mutants have also been selected for in the model legume species Medicago truncatula and Lotus japonicus, which have smaller genomes than pea, making them more desirable tools for molecular studies. Mutants in these species have since been used to identify genes and gene products involved in nodule formation and functioning. This approach has been successful, and the orthologs of many nodulation genes discovered in M. truncatula or L. japonicus have subsequently been identified in important crop species such as pea (see refs. in Oldroyd and Downie, 2004).Here, we take the reverse approach to investig...

(J.J.R., J.B.R.)Long-distance auxin transport was examined in Medicago truncatula and in its supernodulating mutant sunn (super numeric nodules) to investigate the regulation of auxin transport during autoregulation of nodulation (AON). A method was developed to monitor the transport of auxin from the shoot to the root in whole seedlings. Subsequently, the transport was monitored after inoculation of roots with the nodulating symbiont Sinorhizobium meliloti. The sunn mutant showed an increased amount of auxin transported from the shoot to the root compared to the wild type. The auxin transport capacity of excised root segments was similar in wild type and sunn, suggesting that the difference in long-distance auxin transfer between them is due to loading in the shoot. After inoculation, wild-type seedlings showed decreased auxin loading from the shoot to the root; however, the sunn mutant failed to reduce the amount of auxin loaded. The time of reduced auxin loading correlated with the onset of AON. Quantification of endogenous auxin levels at the site of nodule initiation showed that sunn contained three times more auxin than wild type. Inoculation of sunn failed to reduce the level of auxin within 24 h, as was observed in the wild type. We propose a model for the role of auxin during AON of indeterminate legumes: 1) high levels of endogenous auxin are correlated with increased numbers of nodules, 2) inoculation of roots reduces auxin loading from the shoot to the root, and 3) subsequent reduction of auxin levels in the root inhibits further nodule initiation.

Biologically active gibberellins suppress arbuscule formation in pea roots, and DELLA proteins are essential for this response, indicating that this role occurs within the root cells.

One of the first and most enduring roles identified for the plant hormone auxin is the mediation of apical dominance. Many reports have claimed that reduced stem indole-3-acetic acid (IAA) levels and/or reduced basipetal IAA transport directly or indirectly initiate bud growth in decapitated plants. We have tested whether auxin inhibits the initial stage of bud release, or subsequent stages, in garden pea (Pisum sativum) by providing a rigorous examination of the dynamics of auxin level, auxin transport, and axillary bud growth. We demonstrate that after decapitation, initial bud growth occurs prior to changes in IAA level or transport in surrounding stem tissue and is not prevented by an acropetal supply of exogenous auxin. We also show that auxin transport inhibitors cause a similar auxin depletion as decapitation, but do not stimulate bud growth within our experimental time-frame. These results indicate that decapitation may trigger initial bud growth via an auxin-independent mechanism. We propose that auxin operates after this initial stage, mediating apical dominance via autoregulation of buds that are already in transition toward sustained growth.Decapitated garden pea (Pisum sativum) seedlings, bearing axillary buds in leaf axils separated by long internodes, were one of the first systems used to study apical dominance in plants (Snow, 1931). In pea, several axillary buds respond to decapitation by enlarging, but only a few of these reach sustained growth; dormancy remains imposed or is reimposed in the remainder (Stafstrom and Sussex, 1988). This autoregulation of shoot branching is achieved by longdistance signaling (for review, see Napoli et al., 1999). The transition of axillary buds from dormancy to sustained growth in vegetative shoots involves several developmental stages typified by expression of particular molecular markers (Stafstrom and Sussex, 1988;Napoli et al., 1999;Shimizu-Sato and Mori, 2001). The action of long-distance signals at any one or more of these stages could mediate apical dominance.It is well known that the application of auxin to the stump of decapitated plants inhibits axillary bud outgrowth, although less is known about the stage at which auxin acts. A frequently overlooked feature of this inhibition is that it is rarely complete with axillary buds usually growing a small but measurable amount prior to or during inhibition. The results of experiments with auxin transport inhibitors also appear to be consistent with a key role for auxin in apical dominance. These compounds are reported to promote lateral outgrowth (naphthylphtalamic acid [NPA], Tamas, 1987; 2,3,5-triiodobenzoic acid [TIBA], Panigrahi and Audus, 1966; for review, see Shimizu-Sato and Mori, 2001). In the 1930s, studies of bud outgrowth in plants with two decapitated shoots led Snow (1937) to suggest that auxin inhibits branching via a second messenger moving acropetally. Using radiolabeled indole-3-acetic acid (IAA), Hall and Hillman (1975) also proposed that auxin acts indirectly. Auxin was shown to move predo...

In contrast to climacteric fruits, where ethylene is known to be pivotal, the regulation of ripening in non-climacteric fruits is not well understood. In the non-climacteric strawberry (Fragaria anannassa), auxin and abscisic acid (ABA) are thought to be important, but the roles of other hormones suggested to be involved in fruit development and ripening are not clear. Here changes in the levels of indole-3-acetic acid (IAA), ABA, GA1, and castasterone from anthesis to fully ripened fruit are reported. The levels of IAA and GA1 rise early in fruit development before dropping to low levels prior to colour accumulation. Castasterone levels are highest at anthesis and drop to very low levels well before ripening commences, suggesting that brassinosteroids do not play an important role in ripening in strawberry. ABA levels are low at anthesis and gradually rise through development and ripening. The synthetic auxin, 1-naphthaleneacetic acid (NAA), can delay ripening, but the application of GA3, the gibberellin biosythesis inhibitor paclobutrazol, and ABA had no significant effect. IAA and ABA levels are higher in the developing achenes than in the receptacle tissue and may be important for receptacle enlargement and ripening, and seed maturation, respectively. Contrary to a recent report, the biologically active GA4 was not detected. The pattern of changes in the levels of the hormones are different from those reported in another well studied non-climateric fruit, grape, suggesting that a single consistent pattern of hormone changes does not occur in this group of fruit during ripening.