Thought experiment: a hidden signal and an etioreceptor
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SHORT COMMUNICATION
Thought experiment: a hidden signal and an etioreceptor Rajnish Khanna1,2
•
Ulrich Kutschera1,2
Received: 6 August 2020 / Accepted: 23 September 2020 Ó Society for Plant Biochemistry and Biotechnology 2020
Abstract A pioneer in the study of the effect of light on plant development, Wilhelm Pfeffer (1845–1920), outlined both modes of plant development in darkness and in light– today known as skoto- and photo- morphogenesis, respectively. By exploring the current understanding of these pathways in Arabidopsis, we speculate on the possible mechanisms of cell and organ elongation in darkness. We present a thought experiment, which highlights the need for a yet unknown (hidden) external signal, we call aleph (@), and its possible sensor ‘‘etioreceptor’’. Here we propose for the first time that this system is required for growth and developmental patterning to continue in localized spacetime under minimal growth environments, particularly in the absence of light. If true, this mechanism may have played a fundamental role in organism’s survival during Darwinian (adaptive) evolution (‘‘survival of the fittest’’). We present our working model whereby the hidden signal acting through its etioreceptor, increases auxin biosynthesis to facilitate organ expansion, and suggest a function for this system in the auxin biosynthesis pathway. Keywords Auxin Darkness Etiolation Light Photomorphogenesis Spacetime Abbreviations ANT Anthranilate BBX B-Box COP1 CONSTITUTIVELY PHOTOMORPHOGENIC 1 HY5 ELONGATED HYPOCOTYL5 FR Far-red light IAM Indole-3-acetamide IAN Indole-3-acetonitrile IAOx Indole-3 acetaldoxime IGP Indole-3-glycerol phosphate IGs Indole glucosinolates IPyA Indole-3-pyruvic acid PAR Photosynthetically active radiation R Red light SPA SUPPRESSOR OF PHYA-105 TAM Tryptamine Trp Trypthophan YUC YUCCA
& Rajnish Khanna [email protected] 1
i-Cultiver, Inc., 404 Clipper Cove Way, San Francisco, CA 94130, USA
2
Department of Plant Biology, Carnegie Institution for Science, 260 Panama St., Stanford, CA 94305, USA
Arabidopsis has served as a model plant, helping us unravel intricate mechanisms that regulate plant growth and development. Genomics approaches in Arabidopsis led to the identification of the very first single plant gene-based technology, BBX32 (B-Box32), transferred to soybean for improving its field performance and increased grain yield (Khanna et al. 2009; Holtan et al. 2011; Preuss et al. 2012). BBX32 was first identified as a putative negative regulator of photomorphogenesis in a single locus loss-of-function screen with Arabidopsis mutants in genes implicated in light signaling (Khanna et al. 2006). It was determined that BBX32 acts antagonistically to ELONGATED HYPOCOTYL5 (HY5) (Holtan et al. 2011). Recent studies have shown that HY5 mediated transcriptional regulation is dependent upon other members of the BBX family, BBX20, BBX21 and BBX22 (Bursch et al. 2020). Accumulation of HY5 protein is directly linked to photomorphogenesis (Osterlund et al. 2000). In darkness, the CONST
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