Auxin–cytokinin interactions during plant developmental processes

Plant growth and development are regulated by small signaling molecules called plant hormones. Plant hormones, including auxin, cytokinin (CK), ethylene, gibberellin, brassinosteroids, strigolactone, abscisic acid or jasmonic acid, collectively control the number of developmental processes such as embryogenesis, root and shoot growth and branching, or flowering. Over the past decades, studies that focused on mechanisms of phytohormone regulation of plant development clearly demonstrated that the individual hormone action is largely determined by complex interactions with other hormonal signaling pathways. Thus, a tightly interconnected hormonal network is generated which governs and coordinates proper plant development. Although the molecular principles of signal perception and transduction for most plant hormones were recognized, the molecular basis for hormonal crosstalk remains largely unknown. To get insights into the molecular mechanism underlying the hormonal crosstalk we focused on the interaction between auxin and CK. Auxin and CK are key hormonal coregulators in major developmental processes, such as shoot apical meristem activity (Reinhardt et al. 2000; Werner et al. 2003), shoot branching (Ongaro et al. 2008), root growth (Sabatini et al. 1999; Dello Ioio et al. 2008) or lateral root (LR) organogenesis (Benkova et al. 2003; Laplaze et al. 2007). The key principles of both the auxin and CK signal transduction pathways have been established (Hwang and Sheen 2001; Kepinski and Leyser 2005) and although mechanisms of their interaction have been proposed (Dello Ioio et al. 2008; Muller and Sheen 2008; Zhao et al. 2010) the crosstalk between auxin and CK is not fully understood yet. In my PhD thesis I focused on the investigation of the molecular mechanisms of auxin and CK interaction using LR organogenesis as a suitable model system. Lateral root organogenesis is regulated by both hormones and accurate balancing is required between the promotive effect of auxin (Laskowski et al. 1995) and the inhibitory effect of CK (Laplaze et al. 2007). Cytokinin modulation of polar auxin transport was found to represent an important mode of auxin and CK interaction (Dello Ioio, et al., 2008, Ruzicka et al., 2009; Pernisova et al., 2009). We further investigated the underlying molecular mechanisms and revealed that besides transcriptional regulation of auxin efflux carriers of the AtPIN family, CK interferes with cellular trafficking of the AtPIN1 protein and its stability. We found that CK regulates recycling of the auxin efflux carrier PIN1 to the plasma membrane by redirecting it for lytic 6 degradation in vacuoles independently of transcription. We proposed that such a rapid post-transcriptional regulation of PIN1 abundance provides a very efficient and precise mechanism to control auxin fluxes and distribution during CK-mediated developmental regulations, including lateral root primordia organogenesis and root meristem differentiation. The results presented in the second part of my PhD demonstrate that the above described CK stimulatory effect on the lytic degradation of PIN1 not only allows overall regulation of auxin transport efficiency, but might be part of the mechanism determining directionality of the auxin stream. We reveal that CK interferes selectively with the trafficking pathway targeting PIN1 towards the basal membrane. Developmental and genetic modulations increasing the proportion of apically localized PIN1, due to an increased phosphorylation, dramatically reduce CK sensitivity. We demonstrate that during developmental processes such as LR organogenesis requiring redirection of the auxin flow, CK might contribute to PIN polarity re-establishment by targeting a specific subset of PIN1 membrane proteins for lytic degradation. Last part of my PhD is focused on the role of the PIN3 (PIN-formed) auxin efflux carrier in the endodermis in the regulation of the early phases of lateral root initiation (LRI). We demonstrate that a local, developmentally specific auxin moves from endodermal to founder cells, so called auxin re-flux, and is required for the progress from founder cell to LRI phase. This auxin re-flux is mediated through the PIN3 auxin efflux carrier and its deficiency causes dramatic defects in the transition from founder cells to LRI. The data demonstrate that the endodermis plays an active role in the regulation of LRI and is part of the control circuit building up the auxin threshold in founder cells, which is required for the transition to the initiation phase

A proxitome-RNA-capture approach reveals that processing bodies repress coregulated hub genes

Cellular condensates are usually ribonucleoprotein assemblies with liquid- or solid-like properties. Because these subcellular structures lack a delineating membrane, determining their compositions is difficult. Here we describe a proximity-biotinylation approach for capturing the RNAs of the condensates known as processing bodies (PBs) in Arabidopsis (Arabidopsis thaliana). By combining this approach with RNA detection, in silico, and high-resolution imaging approaches, we studied PBs under normal conditions and heat stress. PBs showed a much more dynamic RNA composition than the total transcriptome. RNAs involved in cell wall development and regeneration, plant hormonal signaling, secondary metabolism/defense, and RNA metabolism were enriched in PBs. RNA-binding proteins and the liquidity of PBs modulated RNA recruitment, while RNAs were frequently recruited together with their encoded proteins. In PBs, RNAs follow distinct fates: in small liquid-like PBs, RNAs get degraded while in more solid-like larger ones, they are stored. PB properties can be regulated by the actin-polymerizing SCAR (suppressor of the cyclic AMP)-WAVE (WASP family verprolin homologous) complex. SCAR/WAVE modulates the shuttling of RNAs between PBs and the translational machinery, thereby adjusting ethylene signaling. In summary, we provide an approach to identify RNAs in condensates that allowed us to reveal a mechanism for regulating RNA fate.A proxitome-RNA-capture approach captures the transcriptome of the archetypal condensates known as processing bodies and reveals a translational hub based on liquid-liquid phase separation

Re-activation of stem cell pathways for pattern restoration in plant wound healing

A process of restorative patterning in plant roots correctly replaces eliminated cells to heal local injuries despite the absence of cell migration, which underpins wound healing in animals. Patterning in plants relies on oriented cell divisions and acquisition of specific cell identities. Plants regularly endure wounds caused by abiotic or biotic environmental stimuli and have developed extraordinary abilities to restore their tissues after injuries. Here, we provide insight into a mechanism of restorative patterning that repairs tissues after wounding. Laser-assisted elimination of different cells in Arabidopsis root combined with live-imaging tracking during vertical growth allowed analysis of the regeneration processes in vivo. Specifically, the cells adjacent to the inner side of the injury re-activated their stem cell transcriptional programs. They accelerated their progression through cell cycle, coordinately changed the cell division orientation, and ultimately acquired de novo the correct cell fates to replace missing cells. These observations highlight existence of unknown intercellular positional signaling and demonstrate the capability of specified cells to re-acquire stem cell programs as a crucial part of the plant-specific mechanism of wound healing

Spatiotemporal regulation of lateral root organogenesis in Arabidopsis by cytokinin

The architecture of a plant's root system, established postembryonically, results from both coordinated root growth and lateral root branching. The plant hormones auxin and cytokinin are central endogenous signaling molecules that regulate lateral root organogenesis positively and negatively, respectively. Tight control and mutual balance of their antagonistic activities are particularly important during the early phases of lateral root organogenesis to ensure continuous lateral root initiation (LRI) and proper development of lateral root primordia (LRP). Here, we show that the early phases of lateral root organogenesis, including priming and initiation, take place in root zones with a repressed cytokinin response. Accordingly, ectopic overproduction of cytokinin in the root basal meristem most efficiently inhibits LRI. Enhanced cytokinin responses in pericycle cells between existing LRP might restrict LRI near existing LRP and, when compromised, ectopic LRI occurs. Furthermore, our results demonstrate that young LRP are more sensitive to perturbations in the cytokinin activity than are developmentally more advanced primordia. We hypothesize that the effect of cytokinin on the development of primordia possibly depends on the robustness and stability of the auxin gradient

Cytokinin response factors regulate PIN-FORMED auxin transporters

Auxin and cytokinin are key endogenous regulators of plant development. Although cytokinin-mediated modulation of auxin distribution is a developmentally crucial hormonal interaction, its molecular basis is largely unknown. Here we show a direct regulatory link between cytokinin signalling and the auxin transport machinery uncovering a mechanistic framework for cytokinin-auxin cross-talk. We show that the CYTOKININ RESPONSE FACTORS (CRFs), transcription factors downstream of cytokinin perception, transcriptionally control genes encoding PIN-FORMED (PIN) auxin transporters at a specific PIN CYTOKININ RESPONSE ELEMENT (PCRE) domain. Removal of this cis-regulatory element effectively uncouples PIN transcription from the CRF-mediated cytokinin regulation and attenuates plant cytokinin sensitivity. We propose that CRFs represent a missing cross-talk component that fine-tunes auxin transport capacity downstream of cytokinin signalling to control plant development

Cytokinin response factors regulate PIN-FORMED auxin transporters

Auxin and cytokinin are key endogenous regulators of plant development. Although cytokinin-mediated modulation of auxin distribution is a developmentally crucial hormonal interaction, its molecular basis is largely unknown. Here we show a direct regulatory link between cytokinin signalling and the auxin transport machinery uncovering a mechanistic framework for cytokinin-auxin cross-talk. We show that the CYTOKININ RESPONSE FACTORS (CRFs), transcription factors downstream of cytokinin perception, transcriptionally control genes encoding PIN-FORMED (PIN) auxin transporters at a specific PIN CYTOKININ RESPONSE ELEMENT (PCRE) domain. Removal of this cis-regulatory element effectively uncouples PIN transcription from the CRF-mediated cytokinin regulation and attenuates plant cytokinin sensitivity. We propose that CRFs represent a missing cross-talk component that fine-tunes auxin transport capacity downstream of cytokinin signalling to control plant development

Genetic approach towards the identification of auxin-cytokinin crosstalk components involved in root development

Phytohormones are important plant growth regulators that control many developmental processes, such as cell division, cell differentiation, organogenesis and morphogenesis. They regulate a multitude of apparently unrelated physiological processes, often with overlapping roles, and they mutually modulate their effects. These features imply important synergistic and antagonistic interactions between the various plant hormones. Auxin and cytokinin are central hormones involved in the regulation of plant growth and development, including processes determining root architecture, such as root pole establishment during early embryogenesis, root meristem maintenance and lateral root organogenesis. Thus, to control root development both pathways put special demands on the mechanisms that balance their activities and mediate their interactions. Here, we summarize recent knowledge on the role of auxin and cytokinin in the regulation of root architecture with special focus on lateral root organogenesis, discuss the latest findings on the molecular mechanisms of their interactions, and present forward genetic screen as a tool to identify novel molecular components of the auxin and cytokinin crosstalk

Pickle Recruits Retinoblastoma Related 1 to Control Lateral Root Formation in Arabidopsis

Lateral root (LR) formation is an example of a plant post-embryonic organogenesis event. LRs are issued from non-dividing cells entering consecutive steps of formative divisions, proliferation and elongation. The chromatin remodeling protein PICKLE (PKL) negatively regulates auxin-mediated LR formation through a mechanism that is not yet known. Here we show that PKL interacts with RETINOBLASTOMA-RELATED 1 (RBR1) to repress the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16) promoter activity. Since LBD16 function is required for the formative division of LR founder cells, repression mediated by the PKL-RBR1 complex negatively regulates formative division and LR formation. Inhibition of LR formation by PKL-RBR1 is counteracted by auxin, indicating that, in addition to auxin-mediated transcriptional responses, the fine-tuned process of LR formation is also controlled at the chromatin level in an auxin-signaling dependent manner

Auxin reflux between the endodermis and pericycle promotes lateral root initiation

Lateral root (LR) formation is initiated when pericycle cells accumulate auxin, thereby acquiring founder cell (FC) status and triggering asymmetric cell divisions, giving rise to a new primordium. How this auxin maximum in pericycle cells builds up and remains focused is not understood. We report that the endodermis plays an active role in the regulation of auxin accumulation and is instructive for FCs to progress during the LR initiation (LRI) phase. We describe the functional importance of a PIN3 (PIN-formed) auxin efflux carrier-dependent hormone reflux pathway between overlaying endodermal and pericycle FCs. Disrupting this reflux pathway causes dramatic defects in the progress of FCs towards the next initiation phase. Our data identify an unexpected regulatory function for the endodermis in LRI as part of the fine-tuning mechanism that appears to act as a check point in LR organogenesis after FCs are specified

An auxin transport mechanism restricts positive orthogravitropism in lateral roots

As soon as a seed germinates, plant growth relates to gravity to ensure that the root penetrates the soil and the shoot expands aerially. Whereas mechanisms of positive and negative orthogravitropism of primary roots and shoots are relatively well understood [1-3], lateral organs often show more complex growth behavior [4]. Lateral roots (LRs) seemingly suppress positive gravitropic growth and show a defined gravitropic set-point angle (GSA) that allows radial expansion of the root system (plagiotropism) [3, 4]. Despite its eminent importance for root architecture, it so far remains completely unknown how lateral organs partially suppress positive orthogravitropism. Here we show that the phytohormone auxin steers GSA formation and limits positive orthogravitropism in LR. Low and high auxin levels/signaling lead to radial or axial root systems, respectively. At a cellular level, it is the auxin transport-dependent regulation of asymmetric growth in the elongation zone that determines GSA. Our data suggest that strong repression of PIN4/PIN7 and transient PIN3 expression limit auxin redistribution in young LR columella cells. We conclude that PIN activity, by temporally limiting the asymmetric auxin fluxes in the tip of LRs, induces transient, differential growth responses in the elongation zone and, consequently, controls root architecture