The Tomato Hybrid Proline-Rich Protein regulates the abcission zone competence to respond to ethylene signals

Authors: SUNDARESAN, SRIVIGNESH - Volcani Center (ARO); PHILOSOPH-HADAS, SONIA - Volcani Center (ARO); MA, CHAO - University Of California, Davis; Jiang, Cai-Zhong; RIOV, JOSEPH - Hebrew University Of Jerusalem; MUGASIMANGALAM, RAJA - Qtlomics Technologies Pvt Ltd; KOCHANEK, BETTINA - Volcani Center (ARO); SALIM, SHOSHANA - Volcani Center (ARO); REID, MICHAEL - University Of California, Davis; MEIR, SHIMON - Volcani Center (ARO)

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2018
Publication Date: 6/1/2018
Citation: Sundaresan, S., Philosoph-Hadas, S., Ma, C., Jiang, C., Riov, J., Mugasimangalam, R., Kochanek, B., Salim, S., Reid, M.S., Meir, S. 2018. The Tomato Hybrid Proline-Rich Protein regulates the abcission zone competence to respond to ethylene signals. Horticulture Research. 5:28. https://doi.org/10.1038/s41438-018-0033-2.
DOI: https://doi.org/10.1038/s41438-018-0033-2

Interpretive Summary: Abscission, senescence and ripening are plant developmental processes that their timing is determined by tissue sensitivity to ethylene. The biological basis for this increased ethylene sensitivity is still not known, but it has been shown to be modulated also by other plant hormones. In abscission, the interplay between indole-3-acetic acid (IAA) and ethylene is well established. The generally accepted model is that a basipetal IAA flux through the abscission zone (AZ) prevents abscission by rendering the AZ insensitive to ethylene. The molecular mechanisms leading to increased tissue sensitivity to ethylene in response to IAA deficiency in the AZs are still unknown. Tomato (Solanum lycopersicum) is a convenient model system to study the abscission process, since tomato plants develop a distinct AZ in the midpoint of the flower pedicel, referred to as flower AZ (FAZ). In the first transcriptome microarray analysis of the tomato FAZ performed following abscission induction by auxin depletion, several genes were specifically expressed in the FAZ and not in the pedicel non-AZ (NAZ) region, including KNOTTED1-LIKE HOMEOBOX PROTEIN1 (KD1), and TOMATO PROLINE RICH PROTEIN (TPRP-F1). The role of KD1 in tomato flower abscission was reported, and the present study describes an attempt to elucidate the functional role of TPRP-F1 in abscission. The TPRP-F1 gene, first isolated from young tomato fruit, represents a single-copy gene in the tomato genome. Here, we refer to the tomato PRP-F1 protein in the present study as the Tomato Hybrid Proline Rich Protein (THyPRP). Protein domains binding proline-rich motifs are frequently involved in signaling events. The unique properties of proline provide a high discriminatory recognition without requiring high affinities, and therefore the structural features of proline-rich motif binding and specific recognition were investigated. However, although HyPRPs are ubiquitous in plants, little is known about their roles other than the function as cell wall structural proteins. Several reports indicated that subgroups of HyPRPs might have variable functions during specific developmental stages, and in response to biotic and abiotic stresses. Ectopic expression of HyPRP in plants accelerated cell death, led to developmental abnormality with downregulation of reactive oxygen species-scavenging genes, and enhanced the susceptibility to pathogens by suppressing expression of defense-related genes. Recent findings showed that another tomato HyPRP1 gene (solyc12g009650) is a negative regulator of salt and oxidative stresses, and is probably involved in sulfite metabolism. The regulation of the FAZ and the tomato fruit to respond to ethylene involves a cross-talk between auxin and ethylene, as auxin depletion is a prerequisite for acquiring the competence for ripening or abscission induction by ethylene. Thus, THyPRP, which was primarily expressed in immature green tomato fruit that are ethylene insensitive, was sharply downregulated upon transition to mature green fruit that ripen in response to ethylene. Similarly, following flower removal, a significant decreased THyPRP expression in the tomato FAZ was obtained, which was not affected by the ethylene antagonist 1-methylcyclopropene (1-MCP). This indicates that the decreased THyPRP expression is a direct effect of auxin depletion. The properties of THyPRP and its specific gene expression pattern in the tomato FAZ and in tomato fruit suggest, that THyPRP has an important role in regulating the tissue competence to respond to ethylene signals. In the present report we investigated the role of THyPRP in regulating tomato pedicel abscission induced by flower removal. For this purpose, we studied the effects of silencing the THyPRP gene under the control of the AZ-specific promoter, Tomato Abscission Polygalacturonase4 (TAPG4), and performed a transcriptomic a

Technical Abstract: The Tomato Hybrid Proline-Rich Protein (THyPRP) gene was specifically expressed in the tomato (Solanum lycopersicum) flower abscission zone (FAZ), and its stable antisense silencing under the control of an abscission zone (AZ)-specific promoter, Tomato Abscission Polygalacturonase4,significantly inhibited tomato pedicel abscission following flower removal. For understanding the THyPRP role in regulating pedicel abscission, a transcriptomic analysis of the FAZ of THyPRP silenced plants was performed, using a newly developed AZ-specific tomato microarray chip. Decreased expression of THyPRP in the silenced plants was already observed before abscission induction, resulting in FAZ-specific altered gene expression of transcription factors, epigenetic modifiers, post-translational regulators, and transporters. Our data demonstrate that the effect of THyPRP silencing on pedicel abscission was not mediated by its effect on auxin balance, but by decreased ethylene biosynthesis and response. Additionally, THyPRP silencing revealed new players, which were demonstrated for the first time to be involved in regulating pedicel abscission processes. These include: gibberellin perception, Ca2+-Calmodulin signaling, Serpins and Small Ubiquitin-related Modifier proteins involved in post-translational modifications, Synthaxin and SNARE-like proteins, which participate in exocytosis, a process necessary for cell separation. These changes, occurring in the silenced plants early after flower removal, inhibited and/or delayed the acquisition of the competence of the FAZ cells to respond to ethylene signaling. Our results suggest that THyPRP acts as a master regulator of flower abscission in tomato, predominantly by playing a role in the regulation of the FAZ cell competence to respond to ethylene signals.