| Branching through lateral bud outgrowth plays a pivotal role in the development of the plant architechture. In addition, branching is closely related to the accumulation of plant biomass and fruit yields, and thus, lateral branching number is considered as an important agronomic trait for crop improvement. Branching number is one of the direct factors that determine the crop yield. However, excessive branches impair the shoot growth ability. More seriously, excessive branches favor disease occurrence and thus aggravating yield loss. In recent years, significant progress has been made in the regulation of branching and development by using physiological, biochemical, functional genomics and comparative genomics approaches. Such studies are mostly conducted in Arabidopsis, rice, corn and peas with less focus on the mechanisms of branching in horticultural crops particularly in tomato. Tomato is an important horticultural crop having high nutritional and commercial values worldwide. In tomato cultivation, pruning is indispensable technique for getting good quality fruits and high yield. As this practiceis time-consuming and labour intensive, strategies for developing new efficient techniques are of great significance. Studies on the genetic and molecular mechanisms of tomato lateral branching will not only deepen our knowledge in plant branching development, but also help tomato breeders to develop varieties with high yield and goodquality. In this study, tomato has been used as an experimental material to elucidate the effect of H2O2on growth of lateral branching. By using VIGS approach, we investigated the branching hormone-related gene expression, H2O2signals, and the hormone levels to gain insights into the mechanism of auxin and strigolactone involved in hydrogen peroxide-controled lateral branching outgrowth in tomato plant.The main results are as follows: 1. The gene silencing technique was used to study the the role of H2O2in tomato lateral branching development. Silencing the tomato NADPH oxidase gene RESPIRATORY BURST OXIDASE HOMOLOGS1(RBOHl) reduced H2O2content within the cytoplasm and promoted the growth of lateral buds. Exogenous application of a certain concentration of H2O2on to the leaf enhanced apical dominance and suppressed the lateral branching outgrowth. These results suggest that H2O2has an important role in the growth and development of lateral branching in tomato.2. Plant hormone auxin (e.g. indole-3-acetic acid; IAA) is well recognized as the key factor for apical dominance. We then investigated the involvement of IAA in H2O2-mediated regulation of lateral branching. Silencing RBOHl and WHITEFLYINDUCED1(WFI1), which are involved in H2O2production in the apoplast and enhanced bud outgrowth, respectively, decreased transcript level of genes involved in IAA biosynthesis, auxin responses, polar auxin transport (PAT) and IAA content; however, increased the transcript of IPT2, which is involved in cytokmin (CTK) biosynthesis. These effects were folly abolished by the application of exogenous H2O2. In addition, both decapitation and the silencing of FZY, a rate-limiting gene in IAA biosynthesis, promoted bud outgrowth, and down regulated and upregulated the gene transcription in PAT and CTK biosynthesis, respectively. However, these effects were not blocked by treatment with exogenous H2O2but by napthalene acetic acid (NAA) treatment. These results suggest that RBOH-dependent apoplastic H2O2production plays a critical role in the regulation of auxin biosynthesis.3. Finally, we studied the role of strigolactone and its crosstalk with H2O2and IAA in controlling the lateral branching outgrowth. Results show that silencing RBOH1and FZY genes lead to reduced expression of strigolactone synthetic gene and its endogenous content. Spraying exogenous strigolactone synthetic analogues GR24and phosphorus deficiency treatment can significantly inhibit the growth of lateral branching in the plant development. Further analysis suggests that strigolactone can inhibit the expression of cytokinins synthetic gene IPT2, at the same time promotes the expression of BRC1in bud. Additionally, GR24inhibition of lateral branching has nothing to do with IAA polar transport and signaling, but with the change of cytokinin synthesis genes IPT2in stems and the expression of BRC1in buds. Silencing strigolactone biosynthetic genes CCD7, CCD8, MAX1and signaling gene MAX2also promoted the growth of lateral branching. However, strigolactone synthetic genes-silenced plants were not affected by exogenous application of H2O2and IAA. Importantly, only GR24could completely inhibit the growth of lateral bud in strigolactone biosynthetic gene-silenced plants, but not in pTRV-M4X2-silenced plants. These results suggest that strigolactone may act downstream of H2O2and IAA, and inhibit cytokinins biosynthesis and regulate the expression of the bud negative regulator of lateral branching growth and development in tomato. |
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