Functional Characterization of PHOSPHATE1 HOMOLOG 2 (PHO1;H2) in Light Mediated Seedling Development in Arabidopsis thaliana

Das, Devyan (2026) Functional Characterization of PHOSPHATE1 HOMOLOG 2 (PHO1;H2) in Light Mediated Seedling Development in Arabidopsis thaliana. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Devyan Das (18IP008)) 18IP008.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

Light is a fundamental environmental cue that shapes plant development from seed germination to flowering. It drives the transition from skotomorphogenesis, a growth mode in darkness characterized by elongated hypocotyls to photomorphogenesis, a light-triggered developmental program that promotes chlorophyll accumulation and greening. This transition is orchestrated by a network of photoreceptors that decode light signals and activate pathways suppressing etiolation while promoting light-responsive growth. Here, we identify a previously uncharacterized role for PHO1 homolog 2 (PHO1;H2) in regulating this developmental switch. We demonstrate that PHO1;H2 functions as a negative regulator of photomorphogenesis by enhancing the transcriptional activity of the growth-promoting factor PIF4. Loss-of-function mutants of PHO1;H2 exhibit pronounced photomorphogenic phenotypes, including shorter hypocotyls and increased chlorophyll and anthocyanin accumulation across diverse light conditions. Consistently, PHO1;H2 promotes the expression of growth-related genes while repressing light-responsive and de-etiolation markers. Mechanistic insights reveal that PHO1;H2 interacts with TOC1, a known suppressor of PIF4, and this interaction is required for PIF4-mediated transcription. Overexpression of PHO1;H2 results in hypocotyl elongation, and we show that its N-terminal SPX domain is both necessary and sufficient to inhibit photomorphogenesis. At the molecular level, the PHO1;H2-SPX domain binds the C-terminal region of TOC1, competitively preventing TOC1–PIF4 interactions. This sequestration liberates PIF4 to bind to promoters of its downstream targets, such as IAA19 and YUC8, thereby promoting growth while attenuating photomorphogenic development. Overall, this study uncovers a novel component of the light signalling pathway and demonstrates the mechanistic basis of its action in light-mediated regulation of seedling photomorphogenesis.

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