Nexus between spermidine and floral organ identity and fruit/seed set in tomato

Authors: NAMBEESAN, SAVITHRI - Purdue University; Mattoo, Autar; HANDA, AVTAR - Purdue University

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2019
Publication Date: 7/25/2019
Citation: Nambeesan, S., Mattoo, A.K., Handa, A.K. 2019. Nexus between spermidine and floral organ identity and fruit/seed set in tomato. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2019.01033.
DOI: https://doi.org/10.3389/fpls.2019.01033

Interpretive Summary: Biogenic amines called polyamines are now recognized as plant hormones that are pro-growth in nature and also enhance resistance of plants to abiotic stresses. Our goal is to understand the dynamics of polyamine function in tomato vis a vis plant development and fruit ripening. For this purpose, we have generated transgenic tomato lines that either constitutively upregulate the levels of polyamines throughout the plant or, instead, have only fruit-specific expression. We obtained independent transgenic lines with variable flower phenotypes and others that did not set fruit or lacked seed setting. This manuscript describes the findings using these different genetic lines, which indicate that the triamine spermidine influences floral organ identity via controlling the expression of homeotic genes involving the synthesis and perception of the plant hormone gibberellins. This novel information is of interest to biologists, academicians, industry, mammalian biologists and scientists.

Technical Abstract: Polyamines (PAs) constituting putrescine (Put), spermidine (Spd) and spermine (Spm) are ubiquitous in all organisms and play essential roles in the growth and developmental processes in living organisms, including plants. Evidences obtained through genetic, biochemical and transgenic approaches suggest a tight homeostasis for cellular PA levels. Altered cellular PA homeostasis is associated with abnormal phenotypes. However, the mechanisms involved for observed abnormalities are not yet understood, nor the importance of changes in the cellular ratios of different polyamines is clear. We expressed a yeast spermidine synthase gene (ySpdSyn) under a constitutive promoter CaMV35S as well as under the fruit-specific promote SlE8 in tomato and studied the different phenotypes that developed. The constitutive expression of ySpdSyn resulted in variable flower phenotypes in independent transgenic lines and some of these lacked fruit and seed set. In contrast, the independent transgenic lines obtained through the fruit-specific expression of ySpdSyn resulted in uniform phenotypes and set fruit as well as seeds. Quantification of PA levels in the developing flowers showed that the transgenic plants without fruit and seed set had significantly reduced Spd levels as well as low ratios of Spd/Put and Spd/Spm compared to the transgenic lines (fruit-specific transgene expression), as well as WT plants both of which set fruit and seeds. Transcript levels of several floral homeotic genes, namely, TM4, TM5, TM6, TM29 and TAG1, were significantly higher and that of SlDELLA, GA-20oxidase-1 and GA-3oxidase-2 significantly reduced in the flowers of transgenic lines that lacked fruit and seed set. Collectively, these findings indicate that PAs, particularly Spd, influence floral organ identity and fruit set in tomato by modifying the genetic program(s) that control the expression of homeotic genes likely via influencing gibberellin (GA) biosynthesis and perception. Our studies presented here suggest a nexus between PA ratios and regulation of developmental programs in plants.