Gibberellin induced shot berry formation in cv. Early Sweet is a direct consequence of high fruit set.

The 'seedless' table grape industry relies mainly on stenospermocarpic cultivars, in which endosperm abortion results in berries with seed rudiments and low levels of bioactive gibberellin (GA). Application of GA to enhance berry sizing in these cultivars is often accompanied by adverse effects, one of which is increased proportions of very small berries (termed shot berries). Manual removal of these berries, which is essential to improve uniformity and market value, increases production cost and exposes the cluster to damage. Unraveling the physiological causes of shot berry formation is thus of both scientific and practical value. This study focuses on understanding the GA-mediated regulation of shot berry formation in Vitis vinifera cv. Early Sweet, known for a high proportion of shot berries, which severely damage cluster appearance. As GA is known to induce the parthenocarpic fruit set, we first tested the assumption that the parthenocarpic nature of a fruitlet is a primary cause for shot berry development. We then examined the consequence of the flower load on the proportion of shot berries in the cluster. Our data suggests that: (1) contrary to prior assumptions, the parthenocarpic nature of a fruitlet is not the primary cause for shot berry development, demonstrated by the fact that parthenocarpic fruitlets develop into a full-size berries; (2) the proportion of shot berries on a cluster is a function of the initial flower load on the inflorescence, with high initial flower load resulting in greater shot berry percentage in the cluster; (3) GA treatment bypasses the natural regulation of flower load, resulting in high fruitlet density and increased competition among fruitlets; (4) variation of flower load within the cluster influences berry size uniformity to a greater extent than does the variation in number of cluster per vine. The identity of the factors that determine the fate of a given flower on a high-load cluster remains an open question.

Identification of potential post-ethylene events in the signaling cascade induced by stimuli of bud dormancy release in grapevine.

Ethylene signaling appears critical for grape bud dormancy release. We therefore focused on identification and characterization of potential downstream targets and events, assuming that they participate in the regulation of dormancy release. Because ethylene responding factors (ERF) are natural candidates for targets of ethylene signaling, we initially characterized the behavior of two VvERF-VIIs, which we identified within a gene set induced by dormancy release stimuli. As expected, these VvERF-VIIs are localized within the nucleus, and are stabilized upon decreases in oxygen availability within the dormant buds. Less expected, the proteins are also stabilized upon hydrogen cyanamide (HC) application under normoxic conditions, and their levels peak at deepest dormancy under vineyard conditions. We proceeded to catalog the response of all bud-expressed ERFs, and identified additional ERFs that respond similarly to ethylene, HC, azide and hypoxia. We also identified a core set of genes that are similarly affected by treatment with ethylene and with various dormancy release stimuli. Interestingly, the functional annotations of this core set center around response to energy crisis and renewal of energy resources via autophagy-mediated catabolism. Because ERF-VIIs are stabilized under energy shortage and reshape cell metabolism to allow energy regeneration, we propose that: (i) the availability of VvERF-VIIs is a consequence of an energy crisis within the bud; (ii) VvERF-VIIs function as part of an energy-regenerating mechanism, which activates anaerobic metabolism and autophagy-mediated macromolecule catabolism; and (iii) activation of catabolism serves as the mandatory switch and the driving force for activation of the growth-inhibited meristem during bud-break.

The complete chloroplast genome of Acer tutcheri Duthie (Acereae, Sapindaceae): an ornamental tree endemic to China.

Acer tutcheri Duthie is a popular ornamental tree with reddish leaves and bright red fruits. In this study, we report the complete chloroplast genome of it. The cp genome was determined to be 156,973 bp in length, containing a large single copy (LSC) region of 85,356 bp, a small single copy (SSC) region of 18,111 bp and two separated inverted region of 26,753 bp, respectively. It encodes a total of 132 unique genes, including 87 protein-coding genes, 37 tRNA genes and eight rRNA genes. The phylogenetic analysis indicates that A. tutcheri is sister to A. wilsonii Rehd.

Complete chloroplast genome of Senna spectabilis (DC.) H.S. Irwin & Barneby (Fabaceae) and phylogenetic analysis.

Senna spectabilis (DC.) H.S. Irwin & Barneby is a popular ornamental tree as well as a traditional medical plant in Cameroon. In this study, we sequenced and annotated the complete chloroplast genome of S. spectabilis and reconstructed the phylogenetic relationship of the tribe Cassieae. The length of the chloroplast genome was determined to be 162,754 bp, containing a pair of inverted repeats of 25,413 bp which separated by a small single-copy (SSC) region of 20,161 bp and a large single-copy (LSC) region of 91,767 bp. The cp genome encodes 128 genes, including 83 protein-coding genes, 37 tRNA genes, and eight rRNA genes. The percentage of total GC content of this genome was 35.7%. The phylogenetic analysis indicates that S. spectabilis with the sampled Senna species formed a well-supported monophyletic clade.

Transient induction of a subset of ethylene biosynthesis genes is potentially involved in regulation of grapevine bud dormancy release.

KEY MESSAGE: Transient increases in ethylene biosynthesis, achieved by tight regulation of transcription of specific ACC oxidase and ACC synthase genes, play a role in activation of grapevine bud dormancy release. The molecular mechanisms regulating dormancy release in grapevine buds are as yet unclear. It has been hypothesized that its core involves perturbation of respiration which induces an interplay between ethylene and ABA metabolism that removes repression and allows regrowth. Roles for hypoxia and ABA metabolism in this process have been previously supported. The potential involvement of ethylene biosynthesis in regulation of dormancy release, which has received little attention so far, is now explored. Our results indicate that (1) ethylene biosynthesis is induced by hydrogen cyanamide (HC) and azide (AZ), known artificial stimuli of dormancy release, (2) inhibitors of ethylene biosynthesis and signalling antagonize dormancy release by HC/AZ treatments, (3) ethylene application induces dormancy release, (4) there are two sets of bud-expressed ethylene biosynthesis genes which are differentially regulated, (5) only one set is transiently upregulated by HC/AZ and during the natural dormancy cycle, concomitant with changes in ethylene levels, and (6) levels of ACC oxidase transcripts and ethylene sharply decrease during natural dormancy release, whereas ACC accumulates. Given these results, we propose that transient increases in ethylene biosynthesis prior to dormancy release, achieved primarily by regulation of transcription of specific ACC oxidase genes, play a role in activation of dormancy release.

Abscisic acid catabolism enhances dormancy release of grapevine buds.

The molecular mechanism regulating dormancy release in grapevine buds is as yet unclear. It was formerly proposed that dormancy is maintained by abscisic acid (ABA)-mediated repression of bud-meristem activity and that removal of this repression triggers dormancy release. It was also proposed that such removal of repression may be achieved via natural or artificial up-regulation of VvA8H-CYP707A4, which encodes ABA 8'-hydroxylase, and is the most highly expressed paralog in grapevine buds. The current study further examines these assumptions, and its experiments reveal that (a) hypoxia and ethylene, stimuli of bud dormancy release, enhance expression of VvA8H-CYP707A4 within grape buds, (b) the VvA8H-CYP707A4 protein accumulates during the natural transition to the dormancy release stage, and (c) transgenic vines overexpressing VvA8H-CYP707A4 exhibit increased ABA catabolism and significant enhancement of bud break in controlled and natural environments and longer basal summer laterals. The results suggest that VvA8H-CYP707A4 functions as an ABA degrading enzyme, and are consistent with a model in which the VvA8H-CYP707A4 level in the bud is up-regulated by natural and artificial bud break stimuli, which leads to increased ABA degradation capacity, removal of endogenous ABA-mediated repression, and enhanced regrowth. Interestingly, it also hints at sharing of regulatory steps between latent and lateral bud outgrowth.

Distinct gibberellin functions during and after grapevine bud dormancy release.

The molecular mechanism regulating dormancy release in grapevine buds is as yet unclear. It has been hypothesized that (i) abscisic acid (ABA) represses bud-meristem activity; (ii) perturbation of respiration induces an interplay between ethylene and ABA metabolism, which leads to removal of repression; and (iii) gibberellin (GA)-mediated growth is resumed. The first two hypothesis have been formally supported. The current study examines the third hypothesis regarding the potential involvement of GA in dormancy release. We found that during natural dormancy induction, levels of VvGA3ox, VvGA20ox, and VvGASA2 transcripts and of GA1 were decreased. However, during dormancy release, expression of these genes was enhanced, accompanied by decreased expression of the bud-expressed GA-deactivating VvGA2ox. Despite indications for its positive role during natural dormancy release, GA application had inhibitory effects on bud break. Hydrogen cyanamide up-regulated VvGA2ox and down-regulated VvGA3ox and VvGA20ox expression, reduced GA1 levels, and partially rescued the negative effect of GA. GA had an inhibitory effect only when applied simultaneously with bud-forcing initiation. Given these results, we hypothesize that during initial activation of the dormant bud meristem, the level of GA must be restricted, but after meristem activation an increase in its level serves to enhance primordia regrowth.

An Intracellular Laccase Is Responsible for Epicatechin-Mediated Anthocyanin Degradation in Litchi Fruit Pericarp.

In contrast to the detailed molecular knowledge available on anthocyanin synthesis, little is known about its catabolism in plants. Litchi (Litchi chinensis) fruit lose their attractive red color soon after harvest. The mechanism leading to quick degradation of anthocyanins in the pericarp is not well understood. An anthocyanin degradation enzyme (ADE) was purified to homogeneity by sequential column chromatography, using partially purified anthocyanins from litchi pericarp as a substrate. The purified ADE, of 116 kD by urea SDS-PAGE, was identified as a laccase (ADE/LAC). The full-length complementary DNA encoding ADE/LAC was obtained, and a polyclonal antibody raised against a deduced peptide of the gene recognized the ADE protein. The anthocyanin degradation function of the gene was confirmed by its transient expression in tobacco (Nicotiana benthamiana) leaves. The highest ADE/LAC transcript abundance was in the pericarp in comparison with other tissues, and was about 1,000-fold higher than the polyphenol oxidase gene in the pericarp. Epicatechin was found to be the favorable substrate for the ADE/LAC. The dependence of anthocyanin degradation by the enzyme on the presence of epicatechin suggests an ADE/LAC epicatechin-coupled oxidation model. This model was supported by a dramatic decrease in epicatechin content in the pericarp parallel to anthocyanin degradation. Immunogold labeling transmission electron microscopy suggested that ADE/LAC is located mainly in the vacuole, with essential phenolic substances. ADE/LAC vacuolar localization, high expression levels in the pericarp, and high epicatechin-dependent anthocyanin degradation support its central role in pigment breakdown during pericarp browning.

Involvement of reactive oxygen species in endosperm cap weakening and embryo elongation growth during lettuce seed germination.

Endosperm cap (CAP) weakening and embryo elongation growth are prerequisites for the completion of lettuce seed germination. Although it has been proposed that the cell wall loosening underlying these processes results from an enzymatic mechanism, it is still unclear which enzymes are involved. Here it is shown that reactive oxygen species (ROS), which are non-enzymatic factors, may be involved in the two processes. In Guasihong lettuce seeds imbibed in water, O2·(-) and H2O2 accumulated and peroxidase activity increased in the CAP, whereas its puncture force decreased. In addition, in the radicle, the increase in embryo growth potential was accompanied by accumulation of O2·(-) and an increase in peroxidase activity. Imbibing seeds in 0.3% sodium dichloroisocyanurate (SDIC) reduced endosperm viability and the levels of O2·(-), H2O2, and peroxidase activity in the CAP, whereas the decrease in its puncture force was inhibited. However, in the embryo, SDIC did not affect the accumulation of O2·(-), peroxidase activity, and the embryo growth potential. As a result, SDIC caused atypical germination, in which the endosperm ruptured at the boundary between the CAP and lateral endosperm. ROS scavengers and ROS generation inhibitors inhibited the CAP weakening and also decreased the embryo growth potential, thus decreasing the percentage of seed germination. Exogenous ROS and ROS generation inducers increased the percentage of CAP rupture to some extent, and the addition of H2O2 to 0.3% SDIC enabled some seeds to undergo typical germination.