Two metallocarboxypeptidase inhibitors are implicated in tomato fruit development and regulated by the Inner No Outer transcription factor

Involvement of the tomato BBX16 and BBX17 microProteins in reproductive development

BBXs are B-Box zinc finger proteins that can act as transcription factors and regulators of protein complexes. Several BBX proteins play important roles in plant development. Two Arabidopsis thaliana microProteins belonging to the BBX family, named miP1a and miP1b, homotypically interact with and modulate the activity of other BBX proteins, including CONSTANS, which transcriptionally activates the florigen, FLOWERING LOCUS T. Arabidopsis plants overexpressing miP1a and miP1b showed delayed flowering. In tomato, the closest homologs of miP1a and miP1b are the microProteins SlBBX16 and SlBBX17. This study was aimed at investigating whether the constitutive expression of SlBBX16/17 in Arabidopsis and tomato impacted reproductive development. The heterologous expression of the two tomato microProteins in Arabidopsis caused a delay in the flowering transition; however, the effect was weaker than that observed when the native miP1a/b were overexpressed. In tomato, overexpression of SlBBX17 prolonged the flowering period; this effect was accompanied by downregulation of the flowering inhibitors Self Pruning (SP) and SP5G. SlBBX16 and SlBBX17 can hetero-oligomerize with TCMP-2, a cystine-knot peptide involved in flowering pattern regulation and early fruit development in tomato. The increased expression of both microProteins also caused alterations in tomato fruit development: we observed in the case of SlBBX17 a decrease in the number and size of ripe fruits as compared to WT plants, while for SlBBX16, a delay in fruit production up to the breaker stage. These effects were associated with changes in the expression of GA-responsive genes.

Beyond Arabidopsis: BBX Regulators in Crop Plants

B-box proteins represent diverse zinc finger transcription factors and regulators forming large families in various plants. A unique domain structure defines them—besides the highly conserved B-box domains, some B-box (BBX) proteins also possess CCT domain and VP motif. Based on the presence of these specific domains, they are mostly classified into five structural groups. The particular members widely differ in structure and fulfill distinct functions in regulating plant growth and development, including seedling photomorphogenesis, the anthocyanins biosynthesis, photoperiodic regulation of flowering, and hormonal pathways. Several BBX proteins are additionally involved in biotic and abiotic stress response. Overexpression of some BBX genes stimulates various stress-related genes and enhanced tolerance to different stresses. Moreover, there is evidence of interplay between B-box and the circadian clock mechanism. This review highlights the role of BBX proteins as a part of a broad regulatory network in crop plants, considering their participation in development, physiology, defense, and environmental constraints. A description is also provided of how various BBX regulators involved in stress tolerance were applied in genetic engineering to obtain stress tolerance in transgenic crops.

TCMP-2 affects tomato flowering and interacts with BBX16, a homolog of the arabidopsis B-box MiP1b

Flowering and fruiting are processes subject to complex control by environmental and endogenous signals. Endogenous signals comprise, besides classical phytohormones, also signaling peptides and miniproteins. Tomato cystine-knot miniproteins (TCMPs), which belong to a Solanaceous-specific group of Cys-rich protein family, have been recently involved in fruit development. TCMP-1 and TCMP-2 display a highly modulated expression pattern during flower and fruit development. A previous study reported that a change in the ratio of the two TCMPs affects the timing of fruit production. In this work, to investigate TCMP-2 mode of action, we searched for its interacting partners. One of the interactors identified by a yeast two hybrid screen, was the B-box domain-containing protein 16 (SlBBX16), whose closest homolog is the Arabidopsis microProtein 1b implicated in flowering time control. We demonstrated the possibility for the two proteins to interact in vivo in tobacco epidermal cells. Arabidopsis plants ectopically overexpressing the TCMP-2 exhibited an increased level of FLOWERING LOCUS T (FT) mRNA and anticipated flowering. Similarly, in previously generated transgenic tomato plants with increased TCMP-2 expression in flower buds, we observed an augmented expression of SINGLE-FLOWER TRUSS gene, the tomato ortholog of FT, whereas the expression of the antiflorigen SELF-PRUNING was unchanged. Consistently, these transgenic plants showed alterations in the flowering pattern, with an accelerated termination of the sympodial units. Overall, our study reveals a novel function for TCMP-2 as regulatory factor that might integrate, thanks to its capacity to interact with SlBBX16, into the signaling pathways that control flowering, and converge toward florigen regulation.

Genomic Dissection of a Wild Region in a Superior Solanum pennellii Introgression Sub-Line with High Ascorbic Acid Accumulation in Tomato Fruit

The Solanum pennellii introgression lines (ILs) have been exploited to map quantitative trait loci (QTLs) and identify favorable alleles that could improve fruit quality traits in tomato varieties. Over the past few years, ILs exhibiting increased content of ascorbic acid in the fruit have been selected, among which the sub-line R182. The aims of this work were to identify the genes of the wild donor S. pennellii harbored by the sub-line and to detect genes controlling ascorbic acid accumulation by using genomics tools. A Genotyping-By-Sequencing (GBS) approach confirmed that no wild introgressions were present in the sub-line besides one region on chromosome 7. By using a dense single nucleotide polymorphism (SNP) map obtained by RNA sequencing (RNA-Seq), the wild region of the sub-line was finely identified; thus, defining 39 wild genes that replaced 33 genes of the ILs genetic background (cv. M82). The differentially expressed genes mapping in the region and the variants detected among the cultivated and the wild alleles evidenced the potential role of the novel genes present in the wild region. Interestingly, one upregulated gene, annotated as a major facilitator superfamily protein, showed a novel structure in R182, with respect to the parental lines. These genes will be further investigated using gene editing strategies.

Genome-Wide Identification and Expression Analysis of the Protease Inhibitor Gene Families in Tomato

The protease inhibitors (PIs) in plants are involved primarily in defense against pathogens and pests and in response to abiotic stresses. However, information about the PI gene families in tomato (Solanumlycopersicum), one of the most important model plant for crop species, is limited. In this study, in silico analysis identified 55 PI genes and their conserved domains, phylogenetic relationships, and chromosome locations were characterized. According to genetic structure and evolutionary relationships, the PI gene families were divided into seven families. Genome-wide microarray transcription analysis indicated that the expression of SlPI genes can be induced by abiotic (heat, drought, and salt) and biotic (Botrytiscinerea and tomato spotted wilt virus (TSWV)) stresses. In addition, expression analysis using RNA-seq in various tissues and developmental stages revealed that some SlPI genes were highly or preferentially expressed, showing tissue- and developmental stage-specific expression profiles. The expressions of four representative SlPI genes in response to abscisic acid (ABA), salicylic acid (SA), ethylene (Eth), gibberellic acid (GA). and methyl viologen (MV) were determined. Our findings indicated that PI genes may mediate the response of tomato plants to environmental stresses to balance hormone signals. The data obtained here will improve the understanding of the potential function of PI gene and lay a foundation for tomato breeding and transgenic resistance to stresses.

Pollination Type Recognition from a Distance by the Ovary Is Revealed Through a Global Transcriptomic Analysis

Sexual reproduction in flowering plants involves intimate contact and continuous interactions between the growing pollen tube and the female reproductive structures. These interactions can trigger responses in distal regions of the flower well ahead of fertilization. While pollination-induced petal senescence has been studied extensively, less is known about how pollination is perceived at a distance in the ovary, and how specific this response is to various pollen genotypes. To address this question, we performed a global transcriptomic analysis in the ovary of a wild potato species, Solanum chacoense, at various time points following compatible, incompatible, and heterospecific pollinations. In all cases, pollen tube penetration in the stigma was initially perceived as a wounding aggression. Then, as the pollen tubes grew in the style, a growing number of genes became specific to each pollen genotype. Functional classification analyses revealed sharp differences in the response to compatible and heterospecific pollinations. For instance, the former induced reactive oxygen species (ROS)-related genes while the latter affected genes associated to ethylene signaling. In contrast, incompatible pollination remained more akin to a wound response. Our analysis reveals that every pollination type produces a specific molecular signature generating diversified and specific responses at a distance in the ovary in preparation for fertilization.