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

Abstract

Background The KNOTTED1-LIKE HOMEOBOX PROTEIN1 ( KD1 ) gene is highly expressed in flower and leaf abscission zones (AZs). RNA-antisense silencing of KD1 was shown to delay tomato pedicel and petiole abscission, induced by flower or leaf removal, respectively. KD1 was found to regulate flower pedicel abscission via alteration of auxin gradient through the flower AZ (FAZ), and disruption of the auxin response at the early stages of the abscission process. The present work was aimed to further understand how KD1 regulates signaling factors and regulatory genes involved in the delay of pedicel abscission using the silenced KD1 lines. For this purpose we performed a large scale transcriptome profiling of the FAZ at various time points after flower removal, using a customized AZ-specific microarray. Results The results highlighted a differential expression of regulatory genes in the FAZ of KD1 -silenced plants compared to the wild type (WT). These genes were controlled by KD1 before and after abscission induction. In the KD1 -silenced plants, KD1 expression already decreased at zero time before flower removal, resulting in altered expression of regulatory genes, including epigenetic modifiers, transcription factors (TFs), post-translation regulators, and antioxidative defense components. The increased expression of these regulatory genes, and genes related to exocytosis and gibberellin perception, observed in the WT FAZ, was inhibited in the KD1- silenced plants at 4 h after flower removal. This response led to an inhibited abscission phenotype and downregulation of genes involved in abscission execution and defense in the KD1 -silenced plants. Conclusions The data suggest that KD1 is a master regulator in tomato flower abscission. Some of the altered genes might directly affect auxin homeostasis and transport, resulting in slowing down auxin depletion in the FAZ. This probably delayed the subsequent cascade of the molecular events described above. The data obtained suggest that the inhibitory effect of KD1 silencing on flower pedicel abscission is not limited to manipulation of auxin levels and response as previously reported, but it probably also operates via alteration of other regulatory pathways that delay the acquisition of the competence of the FAZ cells to respond to ethylene signaling.