Revisiting Citrus Rootstocks Polyploidy as a Means to Improve Drought Resilience: Sometimes Less Is More.

Polyploid varieties have been suggested as an alternative approach to promote drought tolerance in citrus crops. In this study, we compared the responses of diploid and tetraploid Sunki 'Tropical' rootstocks to water deficit when grafted onto 'Valencia' sweet orange trees and subjected to water withholding in isolation or competition experiments under potted conditions. Our results revealed that, when grown in isolation, tetraploid rootstocks took longer to show drought symptoms, but this advantage disappeared when grown in competition under the same soil moisture conditions. The differences in drought responses were mainly associated with variations in endogenous leaf levels of abscisic acid (ABA), hydrogen peroxide (H2O2) and carbohydrates among treatments. Overall, tetraploids were more affected by drought in individual experiments, showing higher H2O2 production, and in competition experiments, rapidly increasing ABA production to regulate stomatal closure and reduce water loss through transpiration. Therefore, our results highlight the crucial importance of evaluating diploid and tetraploid rootstocks under the same soil moisture conditions to better simulate field conditions, providing important insights to improve selection strategies for more resilient citrus rootstocks.

Carotenoids and Tocopherol Profiling in Fleshy Fruits.

Carotenoids and tocopherols are among the most powerful lipophilic antioxidants accumulated in fruit and vegetable crops. This chapter describes a method for the separation and quantification of carotenoids/chlorophylls and tocopherols based on microextraction followed by reverse- and normal-phase HPLC, respectively. Using this method, high-throughput, accurate analysis of these compounds can be performed in leaf and fruit samples.

A tomato B-box protein regulates plant development and fruit quality through the interaction with PIF4, HY5, and RIN transcription factors.

During the last decade, knowledge about BBX proteins has greatly increased. Genome-wide studies identified the BBX gene family in several ornamental, industry, and food crops; however, reports regarding the role of these genes as regulators of agronomically important traits are scarce. Here, by phenotyping a knockout mutant, we performed a comprehensive functional characterization of the tomato locus Solyc12g089240, hereafter called SlBBX20. The data revealed the encoded protein as a positive regulator of light signaling affecting several physiological processes during the life span of plants. Through inhibition of PHYTOCHROME INTERACTING FACTOR 4 (SlPIF4)-auxin crosstalk, SlBBX20 regulates photomorphogenesis. Later in development, it controls the balance between cell division and expansion to guarantee correct vegetative and reproductive development. In fruits, SlBBX20 is transcriptionally induced by the master transcription factor RIPENING INHIBITOR (SlRIN) and, together with ELONGATED HYPOCOTYL 5 (SlHY5), up-regulates flavonoid biosynthetic genes. Finally, SlBBX20 promotes the accumulation of steroidal glycoalkaloids and attenuates Botrytis cinerea infection. This work clearly demonstrates that BBX proteins are multilayer regulators of plant physiology because they affect not only multiple processes during plant development but they also regulate other genes at the transcriptional and post-translational levels.

Tomato miR156-targeted SlSBP15 represses shoot branching by modulating hormone dynamics and interacting with GOBLET and BRANCHED1b.

The miRNA156 (miR156)/SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL/SBP) regulatory hub is highly conserved among phylogenetically distinct species, but how it interconnects multiple pathways to converge to common integrators controlling shoot architecture is still unclear. Here, we demonstrated that the miR156/SlSBP15 node modulates tomato shoot branching by connecting multiple phytohormones with classical genetic pathways regulating both axillary bud development and outgrowth. miR156-overexpressing plants (156-OE) displayed high shoot branching, whereas plants overexpressing a miR156-resistant SlSBP15 allele (rSBP15) showed arrested shoot branching. Importantly, the rSBP15 allele was able to partially restore the wild-type shoot branching phenotype in the 156-OE background. rSBP15 plants have tiny axillary buds, and their activation is dependent on shoot apex-derived auxin transport inhibition. Hormonal measurements revealed that indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations were lower in 156-OE and higher in rSBP15 axillary buds, respectively. Genetic and molecular data indicated that SlSBP15 regulates axillary bud development and outgrowth by inhibiting auxin transport and GOBLET (GOB) activity, and by interacting with tomato BRANCHED1b (SlBRC1b) to control ABA levels within axillary buds. Collectively, our data provide a new mechanism by which the miR156/SPL/SBP hub regulates shoot branching, and suggest that modulating SlSBP15 activity might have potential applications in shaping tomato shoot architecture.

Loss of S-nitrosoglutathione reductase disturbs phytohormone homeostasis and regulates shoot side branching and fruit growth in tomato.

S-Nitrosoglutathione plays a central role in nitric oxide (NO) homeostasis, and S-nitrosoglutathione reductase (GSNOR) regulates the cellular levels of S-nitrosoglutathione across kingdoms. Here, we investigated the role of endogenous NO in shaping shoot architecture and controlling fruit set and growth in tomato (Solanum lycopersicum). SlGSNOR silencing promoted shoot side branching and led to reduced fruit size, negatively impacting fruit yield. Greatly intensified in slgsnor knockout plants, these phenotypical changes were virtually unaffected by SlGSNOR overexpression. Silencing or knocking out of SlGSNOR intensified protein tyrosine nitration and S-nitrosation and led to aberrant auxin production and signaling in leaf primordia and fruit-setting ovaries, besides restricting the shoot basipetal polar auxin transport stream. SlGSNOR deficiency triggered extensive transcriptional reprogramming at early fruit development, reducing pericarp cell proliferation due to restrictions on auxin, gibberellin, and cytokinin production and signaling. Abnormal chloroplast development and carbon metabolism were also detected in early-developing NO-overaccumulating fruits, possibly limiting energy supply and building blocks for fruit growth. These findings provide new insights into the mechanisms by which endogenous NO fine-tunes the delicate hormonal network controlling shoot architecture, fruit set, and post-anthesis fruit development, emphasizing the relevance of NO-auxin interaction for plant development and productivity.

CRYPTOCHROME 1a-mediated blue light perception regulates tomato seed germination via changes in hormonal balance and endosperm-degrading hydrolase dynamics.

MAIN CONCLUSION: Blue light exposure delays tomato seed germination by decreasing endosperm-degrading hydrolase activities, a process regulated by CRY1a-dependent signaling and the hormonal balance between ABA and GA. The germination of tomato seeds (Solanum lycopersicum L.) is tightly controlled by an internal hormonal balance, which is also influenced by environmental factors such as light. In this study, we investigated the blue light (BL)-mediated impacts on physiological, biochemical, and molecular processes during the germination of the blue light photoreceptor CRYPTOCHROME 1a loss-of-function mutant (cry1a) and of the hormonal tomato mutants notabilis (not, deficient in ABA) and procera (pro, displaying a GA-constitutive response). Seeds were germinated in a controlled chamber in the dark and under different intensities of continuous BL (ranging from 1 to 25 µmol m-2 s-1). In general, exposure to BL delayed tomato seed germination in a fluency rate-dependent way due to negative impacts on the activities of endosperm-degrading hydrolases, such as endo-ß-mannanase, ß-mannosidase, and α-galactosidase. However, not and pro mutants presented higher germination speed index (GSI) compared to WT despite the BL influence, associated with higher hydrolase activities, especially evident in pro, indicating that the ABA/GA hormonal balance is important to diminish BL inhibition over tomato germination. The cry1a germination percentage was higher than in WT in the dark but its GSI was lower under BL exposure, suggesting that functional CRY1a is required for BL-dependent germination. BL inhibits the expression of GA-biosynthetic genes, and induces GA-deactivating and ABA-biosynthetic genes. The magnitude of the BL influence over the hormone-related transcriptional profile is also dependent upon CRY1a, highlighting the complex interplay between light and hormonal pathways. These results contribute to a better understanding of BL-induced events behind the photoregulation of tomato seed germination.

Phytol recycling: essential, yet not limiting for tomato fruit tocopherol accumulation under normal growing conditions.

Tocopherols are potent membrane-bound antioxidant molecules that are paramount for plant physiology and also important for human health. In the past years, chlorophyll catabolism was identified as the primary source of phytyl diphosphate for tocopherol synthesis by the action of two enzymes, PHYTOL KINASE (VTE5) and PHYTHYL PHOSPHATE KINASE (VTE6) that are able to recycle the chlorophyll-derived phytol. While VTE5 and VTE6 were proven essential for tocopherol metabolism in tomato fruits, it remains unknown whether they are rate-limiting steps in this pathway. To address this question, transgenic tomato plants expressing AtVTE5 and AtVTE6 in a fruit-specific manner were generated. Although ripe transgenic fruits exhibited higher amounts of tocopherol, phytol recycling revealed a more intimate association with chlorophyll than with tocopherol content. Interestingly, protein-protein interactions assays showed that VTE5 and VTE6 are complexed, channeling free phytol and phytyl-P, thus mitigating their cytotoxic nature. Moreover, the analysis of tocopherol accumulation dynamics in roots, a chlorophyll-devoid organ, revealed VTE5-dependent tocopherol accumulation, hinting at the occurrence of shoot-to-root phytol trafficking. Collectively, these results demonstrate that phytol recycling is essential for tocopherol biosynthesis, even in chlorophyll-devoid organs, yet it is not the rate-limiting step for this pathway under normal growth conditions.

Fruit Physiology through Signaling Processes: Latest Advances and Future Challenges.

Fruits are unique to flowering plants and confer a selective advantage to these species by facilitating seed maturation and dispersal [...].

Proteomic Analysis of S-Nitrosation Sites During Somatic Embryogenesis in Brazilian Pine, Araucaria angustifolia (Bertol.) Kuntze.

Cysteine S-nitrosation is a redox-based post-translational modification that mediates nitric oxide (NO) regulation of various aspects of plant growth, development and stress responses. Despite its importance, studies exploring protein signaling pathways that are regulated by S-nitrosation during somatic embryogenesis have not been performed. In the present study, endogenous cysteine S-nitrosation site and S-nitrosated proteins were identified by iodo-TMT labeling during somatic embryogenesis in Brazilian pine, an endangered native conifer of South America. In addition, endogenous -S-nitrosothiol (SNO) levels and S-nitrosoglutathione reductase (GSNOR) activity were determined in cell lines with contrasting embryogenic potential. Overall, we identified an array of proteins associated with a large variety of biological processes and molecular functions with some of them already described as important for somatic embryogenesis (Class IV chitinase, pyruvate dehydrogenase E1 and dehydroascorbate reductase). In total, our S-nitrosoproteome analyses identified 18 endogenously S-nitrosated proteins and 50 in vitro S-nitrosated proteins (after GSNO treatment) during cell culture proliferation and embryo development. Furthermore, SNO levels and GSNOR activity were increased during embryo formation. These findings expand our understanding of the Brazilian pine proteome and shed novel insights into the potential use of pharmacological manipulation of NO levels by using NO inhibitors and donors during somatic embryogenesis.