Priming avocado with sodium hydrosulfide prior to frost conditions induces the expression of genes involved in protection and stress responses.

Priming plants with chemical agents has been extensively investigated as a means for improving their tolerance to many biotic and abiotic stresses. Earlier, we showed that priming young avocado (Persea americana Mill cv. 'Hass') trees with sodium hydrosulfide (NaHS), a donor of hydrogen sulfide, improves the response of photosynthesis to simulated frost (cold followed by high light) conditions. In the current study, we performed a transcriptome analysis to gain insight into the molecular response of avocado 'Hass' leaves to frost, with or without NaHS priming. The analysis revealed 2144 (down-regulated) and 2064 (up-regulated) differentially expressed genes (DEGs) common to both non-primed and primed trees. Non-primed trees had 697 (down) and 559 (up) unique DEGs, while primed trees exhibited 1395 (down) and 1385 (up) unique DEGs. We focus on changes in the expression patterns of genes encoding proteins involved in photosynthesis, carbon cycle, protective functions, biosynthesis of isoprenoids and abscisic acid (ABA), as well as ABA-regulated genes. Notably, the differential expression results depict the enhanced response of primed trees to the frost and highlight gene expression changes unique to primed trees. Amongst these are up-regulated genes encoding pathogenesis-related proteins, heat shock proteins, enzymes for ABA metabolism, and ABA-induced transcription factors. Extending the priming experiments to field conditions, which showed a benefit to the physiology of trees following chilling, suggests that it can be a possible means to improve trees' response to cold stress under natural winter conditions.

Daytime or Edge-of-Daytime Intra-Canopy Illumination Improves the Fruit Set of Bell Pepper at Passive Conditions in the Winter.

Optimal light conditions ensure the availability of sufficient photosynthetic assimilates for supporting the survival and growth of fruit organs in crops. One of the growing uses of light-emitting diodes (LEDs) in horticulture is intra-canopy illumination or LED-interlighting, providing supplemental light for intensively cultivated crops directly within their canopies. Originally developed and applied in environmentally controlled greenhouses in northern latitude countries, this technique is nowadays also being tested and studied in other regions of the world such as the Mediterranean region. In the present work, we applied intra-canopy illumination for bell pepper grown in passive high tunnels in the Jordan Valley using a commercial LED product providing cool-white light. The study included testing of daytime ('LED-D') and edge-of-daytime ('LED-N') illumination, as well as a detailed characterization of fruit set and fruit survival throughout the growth season. We found that both light regimes significantly improved the fruit set and survival during winter, with some benefit of LED-N illumination. Notably, we found that western-facing plants of illuminated sections had a higher contribution toward the increased winter fruit set and spring yield than that of illuminated eastern-facing plants. Greater plant height and fresh weight of western-facing plants of the illuminated sections support the yield results. The differences likely reflect higher photosynthetic assimilation of western-facing plants as compared to eastern-facing ones, due to the higher daily light integral and higher canopy temperature of the former. This study provides important implications for the use of intra-canopy lighting for crops grown at passive winter conditions and exemplifies the significance of geographical positioning, opening additional avenues of investigation for optimization of its use for improving fruit yield under variable conditions.

Alternate bearing in fruit trees: fruit presence induces polar auxin transport in citrus and olive stem and represses IAA release from the bud.

In many fruit trees, heavy fruit load in one year reduces flowering in the following year, creating a biennial fluctuation in yield termed alternate bearing (AB). In subtropical trees, where flowering induction is mostly governed by the accumulation of chilling hours, fruit load is thought to generate a signal (AB signal) that blocks the perception of cold induction. Fruit removal during a heavy-fruit-load year is effective at inducing flowering only if performed one to a few months before the onset of the flowering induction period. We previously showed that following fruit removal, the content of the auxin indoleacetic acid (IAA) in citrus buds is reduced, suggesting that the hormone plays a role in the AB signal. Here, we demonstrate that fruit presence generates relatively strong polar auxin transport in citrus and olive stems. Upon fruit removal, polar auxin transport is reduced and allows auxin release from the bud. Furthermore, using immunolocalization, hormone, and gene expression analyses, we show that in citrus, IAA level in the bud and specifically in the apical meristem is reduced upon fruit removal. Overall, our data provide support for the notion that fruit presence generates an auxin signal in the bud, which may affect flowering induction.

Sodium hydrosulfide priming improves the response of photosynthesis to overnight frost and day high light in avocado (Persea americana Mill, cv. 'Hass').

Radiation frost events, which have become more common in the Mediterranean Basin in recent years, inflict extensive damage to tropical/subtropical fruit crops. During radiation frost, sub-zero temperatures are encountered in the dark, followed by high light during the subsequent clear-sky day. One of the key processes affected by these conditions is photosynthesis, which, when significantly inhibited, leads to the enhanced accumulation of reactive oxygen species (ROS) and damage. The use of 'chemical priming' treatments that induce plants' endogenous stress responses is a possible strategy to improve their coping with stress conditions. Herein, we studied the effects of priming with sodium hydrosulfide (NaHS), a donor of hydrogen sulfide (H2 S), on the response of photosynthesis to overnight frost and day high-light conditions in 'Hass' avocado (Persea americana Mill). We found that priming with a single foliar application of NaHS had positive effects on the response of grafted 'Hass' plants. Primed plants exhibited significantly reduced inhibition of CO2 assimilation, a lower accumulation of hydrogen peroxide as well as lower photoinhibition, as compared to untreated plants. The ability to maintain a high CO2 assimilation capacity after the frost was attained on the background of considerable inhibition in stomatal conductance. Thus, it was likely related to the lower accumulation of ROS and photodamage observed in primed 'Hass' plants. This work contributes toward the understanding of the response of photosynthesis in a subtropical crop species to frost conditions and provides a prospect for chemical priming as a potential practice in orchards during cold winters.

Primary Metabolism in Citrus Fruit as Affected by Its Unique Structure.

Citrus is one of the world's most important fruit crops, contributing essential nutrients, such as vitamin C and minerals, to the human diet. It is characterized by two important traits: first, its major edible part is composed of juice sacs, a unique structure among fruit, and second, relatively high levels of citric acid are accumulated in the vacuole of the juice sac cell. Although the major routes of primary metabolism are generally the same in citrus fruit and other plant systems, the fruit's unique structural features challenge our understanding of carbon flow into the fruit and its movement through all of its parts. In fact, acid metabolism and accumulation have only been summarized in a few reviews. Here we present a comprehensive view of sugar, acid and amino acid metabolism and their connections within the fruit, all in relation to the fruit's unique structure.

The zinc-finger transcription factor CcLOL1 controls chloroplast development and immature pepper fruit color in Capsicum chinense and its function is conserved in tomato.

Chloroplast development and chlorophyll content in the immature fruit has a major impact on the morphology and quality in pepper (Capsicum spp.) fruit. Two major quantitative trait loci (QTLs), pc1 and pc10 that affect chlorophyll content in the pepper fruit by modulation of chloroplast compartment size were previously identified in chromosomes 1 and 10, respectively. The pepper homolog of GOLDEN2-LIKE transcription factor (CaGLK2) has been found as underlying pc10, similar to its effect on tomato chloroplast development. In the present study, we identified the pepper homolog of the zinc-finger transcription factor LOL1 (LSD ONE LIKE1; CcLOL1) as the gene underlying pc1. LOL1 has been identified in Arabidopsis as a positive regulator of programmed cell death and we report here on its role in controlling fruit development in the Solanaceae in a fruit-specific manner. The light-green C. chinense parent used for QTL mapping was found to carry a null mutation in CcLOL1. Verification of the function of the gene was done by generating CRISPR/Cas9 knockout mutants of the orthologous tomato gene resulting in light-green tomato fruits, indicating functional conservation of the orthologous genes in controlling chlorophyll content in the Solanaceae. Transcriptome profiling of light and dark-green bulks differing for pc1, showed that the QTL affects multiple photosynthesis and oxidation-reduction associated genes in the immature green fruit. Allelic diversity of three known genes CcLOL1, CaGLK2, and CcAPRR2 that influence pepper immature fruit color, was found to be associated with variation in chlorophyll content primarily in C. chinense.

CaVIL1, a plant homeodomain gene that promotes flowering in pepper.

KEY MESSAGE: CaVIL1 is a homolog of VIL1, a regulator of vernalization response in Arabidopsis and acts as a flowering promoter in pepper which does not respond to vernalization and photoperiod. As part of our goal to study the genetic and molecular basis of transition to flowering in pepper, we isolated the late-flowering mutant E-2698. Aside from late flowering, multiple pleiotropic alterations of the shoot structure, such as enlarged and distorted leaves, weak apical dominance, and reduced angle of the lateral branches were observed, indicating a broad role for the mutated gene in pepper development. Genetic mapping and sequence analyses revealed that the disrupted gene in E-2698 is the pepper homolog of VERNALIZATION INSENSITIVE 3-LIKE 1 (VIL1) that acts as a regulator of vernalization in Arabidopsis through chromatin modification. The pepper gene, CaVIL1, contains a plant homeodomain motif associated with chromatin modification and a VERNALIZATION INSENSITIVE 3-interacting domain that is truncated in E-2698 and in two other allelic mutants. Because pepper flowering does not respond to vernalization, we postulate that CaVIL1 regulates flowering time via chromatin modification of unknown targets. Expression analysis indicated that CaVIL1 activates the flowering promoter CaFLOWERING LOCUS T and represses the flowering repressor CaAPETALA2. Furthermore, CaVIL1 represses several genes from the FLOWERING LOCUS C (FLC)-LIKE clade that are clustered together in the pepper genome. This indicates their possible involvement in flowering regulation in this species. Our results show that CaVIL1 is a major regulator of flowering and interacts with other flowering promoters and repressors, as well as with FLC-LIKE genes whose function in flowering regulation is not yet known in pepper.

The ambiguous ripening nature of the fig (Ficus carica L.) fruit: a gene-expression study of potential ripening regulators and ethylene-related genes.

The traditional definition of climacteric and non-climacteric fruits has been put into question. A significant example of this paradox is the climacteric fig fruit. Surprisingly, ripening-related ethylene production increases following pre- or postharvest 1-methylcyclopropene (1-MCP) application in an unexpected auto-inhibitory manner. In this study, ethylene production and the expression of potential ripening-regulator, ethylene-synthesis, and signal-transduction genes are characterized in figs ripening on the tree and following preharvest 1-MCP application. Fig ripening-related gene expression was similar to that in tomato and apple during ripening on the tree, but only in the fig inflorescence-drupelet section. Because the pattern in the receptacle is different for most of the genes, the fig drupelets developed inside the syconium are proposed to function as parthenocarpic true fruit, regulating ripening processes for the whole accessory fruit. Transcription of a potential ripening regulator, FcMADS8, increased during ripening on the tree and was inhibited following 1-MCP treatment. Expression patterns of the ethylene-synthesis genes FcACS2, FcACS4, and FcACO3 could be related to the auto-inhibition reaction of ethylene production in 1-MCP-treated fruit. Along with FcMADS8 suppression, gene expression analysis revealed upregulation of FcEBF1, and downregulation of FcEIL3 and several FcERFs by 1-MCP treatment. This corresponded with the high storability of the treated fruit. One FcERF was overexpressed in the 1-MCP-treated fruit, and did not share the increasing pattern of most FcERFs in the tree-ripened fig. This demonstrates the potential of this downstream ethylene-signal-transduction component as an ethylene-synthesis regulator, responsible for the non-climacteric auto-inhibition of ethylene production in fig.