Plant Ethylene Responses Through Cross-Talk With Other Signaling Pathways

Abstract

Phytohormones are signaling molecules produced by plants. They control growth and development of plant by regulating expression of different genes to coordinate important aspects of plants life cycle. Phytohormones not only regulate at cellular level like regulation of cell division, cell elongation, cell differentiation, energy production, reproduction but also participate in responses to environmental changes. Understanding phytohormones mediated gene expression will benefit in the long run to improve crop yield and to identify better adapted varieties. The plant hormone ethylene plays important roles throughout plant growth and development. Ethylene regulates germination, growth, abscission, ripening, senescence, and pathogen responses. Cytokinins like ethylene, affect plants throughout their growth and development by regulating a diverse set of responses, including cell growth and division, photosynthesis, and stress responses. In some cases, such as in the control of senescence, ethylene and cytokinin appear to have antagonistic roles. Therefore, ethylene and Cytokinin are two vital plant hormones that regulate key functions in plants. Ethylene: cytokinin signaling converges at conserved histidine kinases that together regulate plant development. Depending on tissue type cytokinin and ethylene can either act antagonistically or in alliance with each other. Cytokinin positively regulates ethylene biosynthesis at transcriptional level, as well as at post transcriptional level. To unravel the hormonal crosstalk at the transcriptional level through the employment of two component signaling in between ethylene/cytokinin, we conducted a series of experiments. Our initial transcript data revealed significant reduction of the stimulatory effect of ethylene on expression of ERF1 and to a lesser extent on ERS1/ETR2 upon cytokinin concurrent treatment whereas it enhanced the stimulatory effect of ethylene for induction of ARGOS. Similarly, ethylene concurrent treatment reduced the stimulatory effect of cytokinin primary response genes like ARR7 and ARR15. Immunoblot assay revealed that transcription factors are not being degraded by concurrent treatment of hormones and provided evidence of their involvement in transcription of a unique set of genes collectively. Transcriptome analysis (RNA-Seq) was used to determine the suite of genes whose expression is co-regulated by the type-B ARR and EIN3 transcription factor families, Differentially Expressed Genes data set revealed that both EIN3 and Type-B Abstract

Plant Ethylene Responses Through Cross Talk with other Signaling Pathways viii ARRs together upregulate 340 new genes and downregulate 531 genes that were not being regulated by either hormone alone. Thus our transcriptomic data revealed that the same ARRs commonly function in the both cytokinin and ethylene signaling pathways, and the shared use of a transcriptional regulator to regulate genes of these two plant hormones. Interestingly current study unraveled that there exists an ethylene mediated cross-talk with another vital hormone auxin. The GO analysis revealed that ethylene hormone by itself downregulates a large set of SAUR genes that are primary response genes in the auxin signaling. SAURs regulate diverse cellular, physiological, and growth-related processes. We also presented the compelling evidence that SAUR genes are involved in cell elongation of dark grown seedlings by acid growth. In current study we identified that ethylene negatively regulates elongation of dark grown seedlings by down regulation of a set of SAUR genes that otherwise play a fundamental role in hypocotyl elongation. On contrary to SAURs normal regulation, we observed upregulation of IAA genes by ethylene, also primary response genes of auxin hormone, but they are negative regulators of ethylene signaling. Among them IAA20 expression was further enhanced by concurrent treatment, which might be one of the contributing factors in down regulation of auxin mediated responses by ethylene. To get reliable evidence of ethylene: auxin crosstalk we performed series of physiological and morphological experiments, our data showed that ethylene mediated down regulation of SAURs is because of reduction in auxin levels. As DII VENUS a negative reporter of auxin concentration was markedly increased by ethylene treatment indicating corresponding decrease of auxin hormone levels in dark grown seedlings. Stringently ethylene is known for its agricultural significance which regulates such important traits as fruit ripening, senescence, abscission, biomass, and pathogen interactions, therefore elucidation of the transcriptional mechanism unraveled by the current study provides a novel insight into ethylene signal transduction as well as its cross talk with other vital plant hormones like cytokinin and auxin. This study provides fundamental avenues by which researchers can benefit in future by modifying ethylene responses in agriculturally important plants along with better understanding of regulation of other hormone responses mediated by ethylene cross-talk.