Alternate bearing (AB) is the process in fruit trees by which cycles of heavy yield (ON crop) one year are followed by a light yield (OFF crop) the next. Heavy yield usually reduces flowering intensity the following year. Despite its agricultural importance, how the developing crop influences the following year's return bloom and yield is not fully understood. It might be assumed that an ‘AB signal’ is generated in the fruit, or in another organ that senses fruit presence, and moves into the bud to determine its fate—flowering or vegetative growth. The bud then responds to fruit presence by altering regulatory and metabolic pathways. Determining these pathways, and when they are altered, might indicate the nature of this putative AB signal. We studied bud morphology, the expression of flowering control genes, and global gene expression in ON- and OFF-crop buds. In May, shortly after flowering and fruit set, OFF-crop buds were already significantly longer than ON-crop buds. The number of differentially expressed genes was higher in May than at the other tested time points. Processes differentially expressed between ON- and OFF-crop trees included key metabolic and regulatory pathways, such as photosynthesis and secondary metabolism. The expression of genes of trehalose metabolism and flavonoid metabolism was validated by nCounter technology, and the latter was confirmed by metabolomic analysis. Among genes induced in OFF-crop trees was one homologous to SQUAMOSA PROMOTER BINDING-LIKE (SPL), which controls juvenile-to-adult and annual phase transitions, regulated by miR156. The expression pattern of SPL-like, miR156 and other flowering control genes suggested that fruit load affects bud fate, and therefore development and metabolism, a relatively long time before the flowering induction period. Results shed light on some of the metabolic and regulatory processes that are altered in ON and OFF buds.

Cold storage is considered the most effective method for prolonging fresh produce storage. However, subtropical fruit is sensitive to cold. Symptoms of chilling injury (CI) in mango include red and black spots that start from discolored lenticels and develop into pitting. The response of ‘Keitt’ mango fruit to chilling stress was monitored by transcriptomic, physiological, and microscopic analyses. Transcriptomic changes in the mango fruit peel were evaluated during optimal (12°C) and suboptimal (5°C) cold storage. Two days of chilling stress upregulated genes involved in the plant stress response, including those encoding transmembrane receptors, calcium-mediated signal transduction, NADPH oxidase, MAP kinases, and WRKYs, which can lead to cell death. Indeed, cell death was observed around the discolored lenticels after 19 days of cold storage at 5°C. Localized cell death and cuticular opening in the lumen of discolored lenticels were correlated with increased general decay during shelf-life storage, possibly due to fungal penetration. We also observed increased phenolics accumulation around the discolored lenticels, which was correlated with the biosynthesis of phenylpropanoids that were probably transported from the resin ducts. Increased lipid peroxidation was observed during CI by both the biochemical malondialdehyde method and a new non-destructive luminescent technology, correlated to upregulation of the α-linolenic acid oxidation pathway. Genes involved in sugar metabolism were also induced, possibly to maintain osmotic balance. This analysis provides an in-depth characterization of mango fruit response to chilling stress and could lead to the development of new tools, treatments and strategies to prolong cold storage of subtropical fruit.

Being composed of several whorls of distinct floral organs, the flower is one of the most complex organs in the plant. As such, the formation and maintenance of boundaries that separate the meristem from the floral organ primordium and adjacent organs are critical for its normal development. In Arabidopsis, the miR164-regulated NAM genes play key roles in floral-boundary specification. By contrast, much less is known about floral-boundary establishment in the model crop tomato. It was found that the miR164-regulated NAM gene GOBLET is expressed in the floral meristem–organ boundaries and its loss-of-function mutant produces flowers with fused organs, indicating its requirement for tomato floral-boundary formation. It was found here that sly-miR164 targets the transcripts of three additional uncharacterized NAM genes in developing flowers. It is shown that, after floral-boundary initiation, the NAM gene Solyc03g115850 (SlNAM2) is expressed as stripes that mark the boundaries between sepals and between different floral whorls. Furthermore, ectopic accumulation of SlNAM2-encoding transcripts caused various growth-suppression and extraorgan phenotypes typically observed in plants over-expressing known boundary genes. Flower-specific silencing of sly-miR164-targeted NAM genes (AP1>>MIR164) caused defects in the separation of sepals and floral whorls indicating abnormal boundary specification. However, supplementing these NAM-deficient flowers with miR164-resistant SlNAM2 suppressed their fusion phenotypes and completely restored floral boundaries. Together, our results strongly suggest that SlNAM2 participates in the establishment of tomato flower whorl and sepal boundaries.

Global warming is predicted to have a negative effect on plant growth due to the damaging effect of high temperatures. In order to address the effect of high temperature environments on olive oil yield and quality, we compared its effect on the fruit development of five olive cultivars placed in a region noted for its high summer temperatures, with trees of the same cultivars placed in a region of relatively mild summers. We found that the effects of a high temperature environment are genotype dependent and in general, high temperatures during fruit development affected three important traits: fruit weight, oil concentration and oil quality. None of the tested cultivars exhibited complete heat stress tolerance. Final dry fruit weight at harvest of the 'Barnea' cultivar was not affected by the high temperature environment, whereas the 'Koroneiki', 'Coratina', 'Souri' and 'Picholine' cultivars exhibited decreased dry fruit weight at harvest in response to higher temperatures by 0.2, 1, 0.4 and 0.2 g respectively. The pattern of final oil concentration was also cultivar dependent, 'Barnea', 'Coratina' and 'Picholine' not being affected by the high temperature environment, whereas the 'Koroneiki' and 'Souri' cultivars showed a decreased dry fruit oil concentration at harvest under the same conditions by 15 and 8% respectively. Regarding the quality of oil produced, the 'Souri' cultivar proved more tolerant to a high temperature environment than any other of the cultivars analyzed in this study. These results suggest that different olive cultivars have developed a variety of mechanisms in dealing with high temperatures. Elucidation of the mechanism of each of these responses may open the way to development of a variety of olives broadly adapted to conditions of high temperatures.

Potato (Solanum tuberosum) tuber, a swollen underground stem, is used as a model system for the study of dormancy release and sprouting. Natural dormancy release, at room temperature, is initiated by tuber apical bud meristem (TAB-meristem) sprouting characterized by apical dominance (AD). Dormancy is shortened by treatments such as bromoethane (BE), which mimics the phenotype of dormancy release in cold storage by inducing early sprouting of several buds simultaneously. We studied the mechanisms governing TAB-meristem dominance release. TAB-meristem decapitation resulted in the development of increasing numbers of axillary buds with time in storage, suggesting the need for autonomous dormancy release of each bud prior to control by the apical bud. Hallmarks of programmed cell death (PCD) were identified in the TAB-meristems during normal growth, and these were more extensive when AD was lost following either extended cold storage or BE treatment. Hallmarks included DNA fragmentation, induced gene expression of vacuolar processing enzyme1 (VPE1), and elevated VPE activity. VPE1 protein was semipurified from BE-treated apical buds, and its endogenous activity was fully inhibited by a cysteinyl aspartate-specific protease-1-specific inhibitor N-Acetyl-Tyr-Val-Ala-Asp-CHO (Ac-YVAD-CHO). Transmission electron microscopy further revealed PCD-related structural alterations in the TAB-meristem of BE-treated tubers: a knob-like body in the vacuole, development of cytoplasmic vesicles, and budding-like nuclear segmentations. Treatment of tubers with BE and then VPE inhibitor induced faster growth and recovered AD in detached and nondetached apical buds, respectively. We hypothesize that PCD occurrence is associated with the weakening of tuber AD, allowing early sprouting of mature lateral buds.