Abstract
Phosphate (Pi) is an essential nutrient for all organisms. Roots are underground organs, but the majority of the root biology studies have been done on root systems growing in the presence of light. Root illumination alters the Pi starvation response (PSR) at different intensities. Thus, we have analyzed morphological, transcriptional and physiological responses to Pi starvation in dark-grown roots. We have identified new genes and pathways regulated by Pi starvation that were not described previously. We also show that Pi-starved plants increase the cis-zeatin (cZ):trans-zeatin (tZ) ratio. Transcriptomic analyses show that tZ preferentially represses cell cycle and PSR genes, whereas cZ induces genes involved in cell and root hair elongation and differentiation. In fact, cZ-treated seedlings show longer root system as well as longer root hairs compared with tZ-treated seedlings, increasing the total absorbing surface. Mutants with low cZ concentrations do not allocate free Pi in roots during Pi starvation. We propose that Pi-starved plants increase the cZ:tZ ratio to maintain basal cytokinin responses and allocate Pi in the root system to sustain its growth. Therefore, cZ acts as a PSR hormone that stimulates root and root hair elongation to enlarge the root absorbing surface and to increase Pi concentrations in roots.
Highlights
Phosphorous is one of the most important nutrient for plant growth and development
Transcriptomic analyses show that tZ preferentially represses cell cycle and Pi starvation response (PSR) genes while cZ induces genes involved in cell and root hair elongation and differentiation
We propose that Pi-starved plants increase the cZ/tZ ratio to maintain basal CK responses and allocate Pi in the root system to sustain its growth
Summary
Phosphorous is one of the most important nutrient for plant growth and development. Plants have evolved complex regulatory mechanisms and crosstalk signaling pathways to improve the uptake inorganic phosphate (PO43-; Pi), the absorbable form of P, from the rhizosphere, and the Pi usage to maintain intracellular its homeostasis (Baek et al, 2017). In poor Pi environments, plants activate a general Pi-starvation response (PSR) aimed to mobilize and uptake Pi from rhizosphere and to reallocate endogenous Pi from storage-tissue This PSR involves metabolic readjustment, morphological changes and the activation of specific transcriptional programs, mainly regulated by the PHR1- miR399-PHO2 module (Rubio et al, 2001; Lopez-Bucio et al, 2003; Bari et al, 2006; Bustos et al, 2010; Peret et al, 2011; Secco & Whelan, 2014). The auxin response factor ARF16 negatively regulates Pi signaling and uptake in response to CK (Shen et al, 2014) These data indicate that CK play an important role in the control of PSR and plant adaptation to low Pi levels. The molecular response to Pi starvation is less inhibited by higher cZ/tZ ratio Likewise, both zeatins increase Pi levels in roots in detriment of Pi content in shoots and higher cZ/tZ ratio favors root growth and lateral root (LR) formation. Genetic analyses show that cZ is needed for root hair elongation and Pi allocation in the root during Pi starvation
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