Joanna Lado

Root ABA Accumulation in Long-Term Water-Stressed Plants is Sustained by Hormone Transport from Aerial Organs

The reduced pool of the ABA precursors, β,β-carotenoids, in roots does not account for the substantial increase in ABA content in response to water stress (WS) conditions, suggesting that ABA could be transported from other organs. Basipetal transport was interrupted by stem-girdling, and ABA levels were determined in roots after two cycles of WS induced by transplanting plants to dry perlite. Leaf applications of isotope-labeled ABA and reciprocal grafting of ABA-deficient tomato mutants were used to confirm the involvement of aerial organs on root ABA accumulation. Disruption of basipetal transport reduced ABA accumulation in roots, and this decrease was more severe after two consecutive WS periods. This effect was linked to a sharp decrease in the β,β-carotenoid pool in roots in response to water deficit. Significant levels of isotope-labeled ABA were transported from leaves to roots, mainly in plants subjected to water dehydration. Furthermore, the use of different ABA-deficient tomato mutants in reciprocal grafting combinations with wild-type genotypes confirmed the involvement of aerial organs in the ABA accumulation in roots. In conclusion, accumulation of ABA in roots after long-term WS periods largely relies on the aerial organs, suggesting a reduced ability of the roots to synthesize ABA from carotenoids. Furthermore, plants are able to transport ABA basipetally to sustain high hormone levels in roots.

Exploring the diversity in Citrus fruit colouration to decipher the relationship between plastid ultrastructure and carotenoid composition

AbstractMain conclusionDifferentiation of new and characteristic plastid ultrastructures during ripening of citrus fruits in both peel and pulp appears to be strongly correlated with the content and complement of carotenoids. Most of the species of the Citrus genus display a wide range in fruit colouration due to differences in carotenoids; however, how this diversity is related and may contribute to plastid differentiation and ultrastructure is currently unknown. To that end, carotenoid profile and plastid ultrastructure were compared in peel and pulp of three sweet oranges: the ordinary orange-coloured Navel, rich in β,β-xanthophylls, the yellow Pinalate mutant with an elevated content of colourless carotenes and reduced β,β-xanthophylls, and the red-fleshed Cara Cara with high concentration of colourless carotenes and lycopene in the pulp; and two grapefruits: the white Marsh, with low carotenoid content, and the red Star Ruby, accumulating upstream carotenes and lycopene. The most remarkable differences in plastid ultrastructure among varieties were detected in the pulp at full colour, coinciding with major differences in carotenoid composition. Accumulation of lycopene in Cara Cara and Star Ruby pulp was associated with the presence of needle-like crystals in the plastids, while high content of upstream carotenes in Pinalate pulp was related to the development of a novel plastid type with numerous even and round vesicles. The presence of plastoglobuli was linked to phytoene and xanthophyll accumulation, suggesting these structures as the main sites for the accumulation of these pigments. Peel chromoplasts were richer in membranes compared to pulp chromoplasts, reflecting their different biogenesis. In summary, differences in carotenoid composition and accumulation of unusual carotenoids are mirrored by the development of diverse and novel chromoplast types, revealing the plasticity of these organelles to rearrange carotenoids inside different structures to allow massive accumulation and thus contributing to the chemical stability of the carotenoids.

A comparative physiological and transcriptional study of carotenoid biosynthesis in white and red grapefruit (Citrus paradisi Macf.)

Accumulation of lycopene in citrus fruits is an unusual feature restricted to selected mutants. Grapefruit (Citrus paradisi Macf.) is the Citrus specie with greater number of red-fleshed mutants, but the molecular bases of this alteration are not fully understood. To gain knowledge into the mechanisms implicated in this alteration, we conducted a comparative analysis of carotenoid profile and of the expression of genes related to carotenoid biosynthesis and catabolism in flavedo and pulp of two grapefruit cultivars with marked differences in colouration: the white Marsh and the red Star Ruby. Mature green fruit of Marsh accumulated chloroplastic carotenoids, while mature tissues lacked carotenoids. However, accumulation of downstream products such as abscisic acid (ABA) and expression of its biosynthetic genes, 9-cis-epoxycarotenoid dioxygenase (NCED1 and NCED2), increased after the onset of colouration. In contrast, red grapefruit accumulated lycopene, phytoene and phytofluene, while ABA content and NCED gene expression were lower than in Marsh, suggesting a blockage in the carotenoid biosynthetic pathway. Expression analysis of three genes of the isoprenoid pathway and nine of the carotenoid biosynthetic pathway revealed virtually no differences in flavedo and pulp between both genotypes, except for the chromoplast-specific lycopene cyclase 2 (β-LCY2) which was lower in the pulp of the red grapefruit. The proportion in the expression of the allele with high (β-LCY2a) and low (β-LCY2b) activity was also similar in the pulp of both genotypes. Therefore, results suggest that reduced expression of β-LCY2 appears to be responsible of lycopene accumulation in the red Star Ruby grapefruit.

ABA accumulation in water-stressed Citrus roots does not rely on carotenoid content in this organ

Sustained abscisic acid (ABA) accumulation in dehydrated citrus roots depends on the transport from aerial organs. Under this condition, the role of the β,β-carotenoids (ABA precursors) to the de novo synthesis of ABA in roots needs to be clarified since their low availability in this organ restricts its accumulation. To accomplish that, detached citrus roots were exposed to light (to increase their carotenoid content) and subsequently dehydrated (to trigger ABA accumulation). Stress imposition sharply decreased the pool of β,β-carotenoids but, unexpectedly, no concomitant rise in ABA content was observed. Contrastingly, roots of intact plants (with low levels of carotenoids) showed a similar decrease of ABA precursor together with a significant ABA accumulation. Furthermore, upon dehydration both types of roots showed similar upregulation of the key genes involved in biosynthesis of carotenoids and ABA (CsPSY3a; CsβCHX1; CsβCHX2; CsNCED1; CsNCED2), demonstrating a conserved transcriptional response triggered by water stress. Thus, the sharp decrease in root carotenoid levels in response to dehydration should be related to other stress-related signals instead of contributing to ABA biosynthesis. In summary, ABA accumulation in dehydrated-citrus roots largely relies on the presence of the aerial organs and it is independent of the amount of available root β,β-carotenoids.

Light avoidance reduces ascorbic acid accumulation in the peel of Citrus fruit

Citrus fruits are highly consumed worldwide and represent one of the most important sources of ascorbic acid (AsA). However, information about the molecular mechanisms regulating AsA accumulation in Citrus fruit and the effects of environmental factors is scarce. In this study we have investigated the effect of fruit shading on AsA content and the expression of AsA biosynthetic, degrading and recycling genes in fruits of different Citrus species. Immature-green fruits were covered at the end of the cell enlargement phase and AsA concentration in the flavedo declined and remained at low levels as compared with light-exposed fruits. Fruit shading marginally altered the expression of genes from the l-galactose pathway and this effect was variable in the four Citrus species. However, specific isoforms (GalUR8 or GalUR12) from the l-galacturonic acid pathway were significantly repressed paralleling the reduction in AsA concentration. No significant effect of shading was detected in transcription of genes of the myo-inositol and l-gulose pathways as well as recycling and degradation. Collectively, results indicate that light avoidance inhibited accumulation of AsA in the flavedo of Citrus fruits and suggest that the l-galacturonic acid pathway has a relevant contribution to AsA content in this tissue.

Analysis of ethylene biosynthesis and perception during postharvest cold storage of Marsh and Star Ruby grapefruits.

Grapefruits are among the citrus species more sensitive to cold and develop chilling injury symptoms during prolonged postharvest storage at temperatures lower than 8 ℃–10 ℃. The plant hormone ethylene has been described either to protect or potentiate chilling injury development in citrus whereas little is known about transcriptional regulation of ethylene biosynthesis, perception and response during cold storage and how the hormone is regulating its own perception and signaling cascade. Then, the objective of the present study was to explore the transcriptional changes in the expression of ethylene biosynthesis, receptors and response genes during cold storage of the white Marsh and the red Star Ruby grapefruits. The effect of the ethylene action inhibitor, 1-MCP, was evaluated to investigate the involvement of ethylene in the regulation of the genes of its own biosynthesis and perception pathway. Ethylene production was very low at the harvest time in fruits of both varieties and experienced only minor changes during storage. By contrast, inhibition of ethylene perception by 1-MCP markedly induced ethylene production, and this increase was highly stimulated during shelf-life at 20 ℃, as well as transcription of ACS and ACO. These results support the auto-inhibitory regulation of ethylene in grapefruits, which acts mainly at the transcriptional level of ACS and ACO genes. Moreover, ethylene receptor1 and ethylene receptor3 were induced by cold while no clear role of ethylene was observed in the induction of ethylene receptors. However, ethylene appears to be implicated in the transcriptional regulation of ERFs both under cold storage and shelf-life.

Regulation of Carotenoid Biosynthesis During Fruit Development.

Carotenoids are recognized as the main pigments in most fruit crops, providing colours that range from yellow and pink to deep orange and red. Moreover, the edible portion of widely consumed fruits or their derived products represent a major dietary source of carotenoids for animals and humans. Therefore, these pigments are crucial compounds contributing to fruit aesthetic and nutritional quality but may also have protecting and ecophysiological functions in coloured fruits. Among plant organs, fruits display one of the most heterogeneous carotenoids patterns in terms of diversity and abundance. In this chapter a comprehensive list of the carotenoid content and profile in the most commonly cultivated fleshy fruits is reported. The proposed fruit classification systems attending to carotenoid composition are revised and discussed. The regulation of carotenoids in fruits can be rather complex due to the dramatic changes in content and composition during ripening, which are also dependent on the fruit tissue and the developmental stage. In addition, carotenoid accumulation is a dynamic process, associated with the development of chromoplasts during ripening. As a general rule, carotenoid accumulation is highly controlled at the transcriptional level of the structural and accessory proteins of the biosynthetic and degradation pathways, but other mechanisms such as post-transcriptional modifications or the development of sink structures have been recently revealed as crucial factors in determining the levels and stability of these pigments. In this chapter common key metabolic reactions regulating carotenoid composition in fruit tissues are described in addition to others that are restricted to certain species and generate unique carotenoids patterns. The existence of fruit-specific isoforms for key steps such as the phytoene synthase, lycopene β-cyclases or catabolic carotenoid cleavage dioxygenases has allowed an independent regulation of the pathway in fruit tissues and a source of variability to create novel activities or different catalytic properties. Besides key genes of the carotenoid pathway, changes in carotenoid accumulation could be also directly influenced by differences in gene expression or protein activity in the pathway of carotenoid precursors and some relevant examples are discussed. The objective of this chapter is to provide an updated review of the main carotenoid profiles in fleshy fruits, their pattern of changes during ripening and our current understanding of the different regulatory levels responsible for the diversity of carotenoid accumulation in fruit tissues.

Involvement of Plant Hormones in Cold Stress Tolerance

The adaptation and survival of plants in challenging environments involve changes at the cellular and molecular levels. In particular, cold acclimation that is due to gradual exposure to low nonfreezing temperatures includes structural and morphological modifications, changes in cell membrane composition, and an accumulation of compatible solutes among other cryoprotective compounds. These processes are governed by plant hormones that are also involved in the adaptation to other abiotic and biotic stresses. Also, other hormones not usually associated with the response to stress have been shown to be participated in the plant cold response. Here, we review the latest information regarding the involvement of plant hormones and its cross talk during cold tolerance.

Metabolic and Hormonal Responses of Plants to Cold Stress

The adaptation and survival of plants in challenging environments involves changes at cellular and molecular levels. Plants inherently possess sophisticated mechanisms to timely perceive environmental stimuli and respond accordingly. Temperature and water availability are the major environmental factors that significantly influence geographical distribution of plants. In particular, cold acclimation includes structural and morphological modifications, changes in cell membrane composition and the accumulation of compatible solutes among others cryoprotective compounds. In order to survive in such non-optimal conditions, plants have developed complex mechanisms to perceive external signals and trigger crucial responses. These phenomena are mediated by complex phytohormone networks that are also involved in the adaptation to other abiotic and biotic stresses. Through hormonal signaling, cold stress modifies biomass and bioactive compounds accumulation in medicinal plants, both of special interest regarding their biological activity and therefore, their pharmacological potential. In this chapter we review the latest information regarding cold signaling in plants, phytohormones and how integrated crosstalk underpins cold tolerance together with a special mention of cold effect on bioactive compounds accumulation, with emphasis in medicinal and aromatical plants.