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Phosphate (Pi) is an essential nutrient for all organisms. Root are underground organs, but the majority of the root biology studies have been done growing the root system in presence of light.Root illumination alters the Pi starvation response (PSR) at different levels. 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 while 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 than tZ?treated, increasing the total absorbing surface. Mutants with low cZ levels do not allocate free Pi in roots during Pi starvation.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. Therefore, cZ acts as a Pi?starvation response hormone that stimulates root and root hair elongation to enlarge root absorbing surface and to increase Pi levels in roots.

Abscisic acid (ABA) plays a key role in plant responses to abiotic stress, particularly drought stress. A wide number of ABA-hypersensitive mutants is known, however, only a few of them resist/avoid drought stress. In this work we have generated ABA-hypersensitive drought-avoidant mutants by simultaneous inactivation of two negative regulators of ABA signaling, i.e. the protein phosphatases type 2C (PP2Cs) ABA-INSENSITIVE1 (ABI1) and HYPERSENSITIVE TO ABA1 (HAB1). Two new recessive loss-of-function alleles of ABI1, abi1-2 and abi1-3, were identified in an Arabidopsis (Arabidopsis thaliana) T-DNA collection. These mutants showed enhanced responses to ABA both in seed and vegetative tissues, but only a limited effect on plant drought avoidance. In contrast, generation of double hab1-1 abi1-2 and hab1-1 abi1-3 mutants strongly increased plant responsiveness to ABA. Thus, both hab1-1 abi1-2 and hab1-1 abi1-3 were particularly sensitive to ABA-mediated inhibition of seed germination. Additionally, vegetative responses to ABA were reinforced in the double mutants, which showed a strong hypersensitivity to ABA in growth assays, stomatal closure, and induction of ABA-responsive genes. Transpirational water loss under drought conditions was noticeably reduced in the double mutants as compared to single parental mutants, which resulted in reduced water consumption of whole plants. Taken together, these results reveal cooperative negative regulation of ABA signaling by ABI1 and HAB1 and suggest that fine tuning of ABA signaling can be attained through combined action of PP2Cs. Finally, these results suggest that combined inactivation of specific PP2Cs involved in ABA signaling could provide an approach for improving crop performance under drought stress conditions.

CSIC - Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), In Arabidopsis, the root clock regulates the spacing of lateral organs along the primary root through oscillating gene expression. The core molecular mechanism that drives the root clock periodicity and how it is modified by exogenous cues such as auxin and gravity remain unknown. We identified the key elements of the oscillator (AUXIN RESPONSE FACTOR 7, its auxin-sensitive inhibitor IAA18/POTENT, and auxin) that form a negative regulatory loop circuit in the oscillation zone. Through multilevel computer modeling fitted to experimental data, we explain how gene expression oscillations coordinate with cell division and growth to create the periodic pattern of organ spacing. Furthermore, gravistimulation experiments based on the model predictions show that external auxin stimuli can lead to entrainment of the root clock. Our work demonstrates the mechanism underlying a robust biological clock and how it can respond to external stimuli., This work was funded by the Ministerio de Economía y Competitividad of Spain (MINECO) and/or the ERDF (BFU2016-80315-P to M.A.M.-R., BIO2017-82209-R to J.C.d.P., and TIN2016-81079-R to A.R.-P.), the Comunidad de Madrid and/or ERDF and ESF (2017-T1/BIO-5654 to K.W. and S2017/BMD-3691 to A.R.-P.), the Howard Hughes Medical Institute and the NIH (R35-GM131725 to P.N.B.), the Fonds Wetenschappelijk Onderzoek (FWO Flanders) (G022516N, G020918N, and G024118N to T.B.), and the “Severo Ochoa Program for Centres of Excellence in R&D” from the Agencia Estatal de Investigacion of Spain [SEV-2016-0672 (2017–2021)] to K.W., P.P.-G., and M.A.M.-R. through CBGP. M.M. was supported by a postdoctoral contract associated to SEV-2016-0672, E.B.-A. by Ayudante de Investigacion contract PEJ-2017-AI/BIO-7360 from the Comunidad de Madrid, A.S.-C. and L.S.-R. by FPI contracts from MINECO (BES-2014-068852 and BES-2017-080155, respectively), J.C. by a Juan de la Cierva contract from MINECO (FJCI-2016-28607), P.P.-G. by a Juan de la Cierva contract from MINECO (FJCI-2015-24905) and Programa Atraccion Talento from Comunidad Madrid (2017-T2/BIO-3453), A.S. by a Torres Quevedo contract from MINECO (PTQ-15-07915), and K.W. by program PGC2018-093387-A-I00 from the Ministerio de Ciencia e Innovacion (MICIU)