Role of the KNOTTED1-LIKE HOMEOBOX protein (KD1) in regulating abscission of tomato flower pedicels at early and late stages of the process

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; KOCHANEK, BETTINA - Volcani Center (ARO); SALIM, SHOSHANA - Volcani Center (ARO); REID, MICHAEL - University Of California, Davis; MEIR, SHIMON - Volcani Center (ARO)

Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2022
Publication Date: 9/20/2022
Citation: Sundaresan, S., Philosoph-Hadas, S., Ma, C., Jiang, C., Riov, J., Kochanek, B., Salim, S., Reid, M.S., Meir, S. 2022. Role of the KNOTTED1-LIKE HOMEOBOX protein (KD1) in regulating abscission of tomato flower pedicels at early and late stages of the process. Physiologia Plantarum. 173(4):2103-2118. https://doi.org/10.1111/ppl.13560.
DOI: https://doi.org/10.1111/ppl.13560

Interpretive Summary: It is well documented that removal of leaf blades or flowers, the major sites of auxin biosynthesis, or application of polar auxin transport inhibitors, induce leaf petiole and flower pedicel abscission, respectively, as a result of reduction in auxin transport to the organ abscission zone (AZ). This leads to altered expression of auxin-related genes and upregulation of ethylene-related genes, as well as to differential expression of other regulatory genes in the AZ as demonstrated in the flower AZ (FAZ) of the model system of tomato (Solanum lycopersicum) plants. These events result in the acquisition of the FAZ cell competence to respond to ethylene signaling. By performing a transcriptome microarray analysis of the tomato FAZ following abscission induction by flower removal, several genes were shown to be specifically expressed in the FAZ and not in the pedicel non-AZ (NAZ) region. Therefore, these genes were suggested to be involved in regulation of the acquisition of the FAZ cell competence to respond to ethylene signaling following auxin depletion. Of these FAZ-specific expressed genes, two genes were further studied for their functional roles in abscission. These genes were KD1, which encodes the KNOTTED1-LIKE HOMEOBOX PROTEIN1 that belongs to class I KNOX family, and THyPRP, which encodes the Tomato Hybrid Proline-Rich Protein, and its silencing delayed pedicel abscission by regulating the FAZ cell competence to respond to ethylene signaling. We previously showed that downregulation of KD1 by antisense significantly delayed pedicel and petiole abscission in tomato plants by affecting auxin levels in the AZ and auxin gradient from the distal to the proximal region of the FAZ, without impairing their growth and development. However, the data obtained in the study were quite limited, and demonstrated its effect only on the expression of auxin-related genes at one time point (4 h) after flower removal. The aim of the present study was to acquire additional information on the involvement of KD1 in flower pedicel abscission, in order to further elucidate its role in this process and better understand its mode of action. Accordingly, we studied the effect of KD1 on the expression of a wide range of genes before flower removal and at various time points after flower removal. For this purpose, we performed a detailed transcriptome analysis of the FAZ and the NAZ of tomato WT plants compared to the FAZ of the TAPG4::antisense KD1 plants, using a newly developed customized AZ-specific microarray. The use of this customized AZ-specific microarray in a time course analysis, allowed to expand the database of changes in gene expression occurring at the early stage of pedicel abscission, since it contains more gene probes than those that were previously used by Ma et al. 2015. We found that in the TAPG4::antisense KD1-silenced plants, KD1 gene expression decreased before flower removal, resulting in altered expression of regulatory genes, such as epigenetic modifiers, transcription factors, post translation regulators, and antioxidative defense factors occurring at zero time and before affecting auxin levels in the FAZ detected at 4 h after flower removal. The expression of additional regulatory genes was altered in the FAZ of KD1-silenced plants at 4-20 h after flower removal, thereby leading to an inhibited abscission phenotype, and downregulation of genes involved in abscission execution and defense processes. Our data suggest that KD1 is a master regulator of the abscission process, which promotes abscission of tomato flower pedicels.

Technical Abstract: The KNOTTED1-LIKE HOMEOBOX PROTEIN1 (KD1) gene is highly expressed in flower and leaf abscission zones (AZs), and KD1 was reported to regulate tomato flower pedicel abscission via alteration of the auxin gradient and response in the flower AZ (FAZ). The present work was aimed to further examine how KD1 regulates signaling factors and regulatory genes involved in pedicel abscission, by using silenced KD1 lines and performing a large-scale transcriptome profiling of the FAZ before and after flower removal, using a customized AZ specific microarray. The results highlighted a differential expression of regulatory genes in the FAZ of KD1-silenced plants compared to the wild type. In the TAPG4::antisense KD1-silenced plants, KD1 gene expression decreased before flower removal, resulting in altered expression of regulatory genes, such as epigenetic modifiers, transcription factors, post translation regulators, and antioxidative defense factors occurring at zero time and before affecting auxin levels in the FAZ detected at 4 h after flower removal. The expression of additional regulatory genes was altered in the FAZ of KD1-silenced plants at 4-20 h after flower removal, thereby leading to an inhibited abscission phenotype, and downregulation of genes involved in abscission execution and defense processes. Our data suggest that KD1 is a master regulator of the abscission process, which promotes abscission of tomato flower pedicels. This suggestion is based on the inhibitory effect of KD1 silencing on flower pedicel abscission that operates via alteration of various regulatory pathways, which delay the competence acquisition of the FAZ cells to respond to ethylene signaling.