Light-triggered and phosphorylation-dependent 14-3-3 association with NONPHOTOTROPIC HYPOCOTYL 3 is required for hypocotyl phototropism

ABSTRACTNON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key component of the phototropic response, acting downstream of the primary photoreceptor phototropin and upstream of auxin redistribution. Despite the obvious physiological significance of the blue light-induced differential growth process, the molecular mode of NPH3 action is poorly understood. Light-triggered dephosphorylation of NPH3, however, is thought to constitute a major signaling event. Here, we show that NPH3 directly binds to polyacidic phospholipids via a polybasic motif in its C-terminal domain, allowing for plasma membrane association in darkness. We further demonstrate that blue light induces phosphorylation of a C-terminal 14-3-3 binding motif in NPH3. Subsequent binding of 14-3-3 to the phosphorylated NPH3 in turn is required for light-triggered release of NPH3 from the plasma membrane. In the cytosol, NPH3 undergoes a dynamic transition from a dilute to a condensed state. Intriguingly, the dephosphorylated state of the 14-3-3 binding site as well as NPH3 plasma membrane association are recoverable in darkness. Given that NPH3 variants constitutively localizing either to the plasma membrane or to cytosolic condensates are non-functional, the phototropin-triggered and 14-3-3 mediated dynamic change in the subcellular localization of NPH3 seems to be crucial for its function. Taken together, our data demonstrate a fundamental role for 14-3-3 members in regulating NPH3 localization and auxin-dependent phototropic responses.

LAZY Gene Family in Plant Gravitropism

Adapting to the omnipresent gravitational field was a fundamental basis driving the flourishing of terrestrial plants on the Earth. Plants have evolved a remarkable capability that not only allows them to live and develop within the Earth’s gravity field, but it also enables them to use the gravity vector to guide the growth of roots and shoots, in a process known as gravitropism. Triggered by gravistimulation, plant gravitropism is a highly complex, multistep process that requires many organelles and players to function in an intricate coordinated way. Although this process has been studied for several 100 years, much remains unclear, particularly the early events that trigger the relocation of the auxin efflux carrier PIN-FORMED (PIN) proteins, which presumably leads to the asymmetrical redistribution of auxin. In the past decade, the LAZY gene family has been identified as a crucial player that ensures the proper redistribution of auxin and a normal tropic response for both roots and shoots upon gravistimulation. LAZY proteins appear to be participating in the early steps of gravity signaling, as the mutation of LAZY genes consistently leads to altered auxin redistribution in multiple plant species. The identification and characterization of the LAZY gene family have significantly advanced our understanding of plant gravitropism, and opened new frontiers of investigation into the novel molecular details of the early events of gravitropism. Here we review current knowledge of the LAZY gene family and the mechanism modulated by LAZY proteins for controlling both roots and shoots gravitropism. We also discuss the evolutionary significance and conservation of the LAZY gene family in plants.

IGT/LAZY family genes are differentially influenced by light signals and collectively required for light-induced changes to branch angle

Plants adjust their growth orientations in response to environmental signals such as light and gravity in order to optimize photosynthesis and access to nutrients. However, given the fixed nature of gravity, understanding how light and gravity signals are integrated is challenging. Branch orientation, or gravitropic set point angle, is a key aspect of plant architecture, set with respect to gravity and shown to be altered by changes in light conditions. The IGT gene family, also known as the LAZY family, contains important components for branch angle and gravity responses, including three gene clades: LAZY, DEEPER ROOTING (DRO), and TILLER ANGLE CONTROL (TAC). LAZY and DRO genes promote upward branch orientations downstream of amyloplast sedimentation, and upstream of auxin redistribution in response to gravity. In contrast, TAC1 promotes downward branch angles in response to photosynthetic signals. Here, we investigated the influence of different light signaling pathways on LAZY and DRO gene expression, and their role in light regulation of branch angle responses. We found differential effects of continuous light and dark, circadian clock, photoreceptor-mediated signaling, and photosynthetic signals on LAZY and DRO gene expression. Phenotypic analysis revealed that LAZY and DRO genes are collectively required for branch angle responses to light.HighlightLAZY and DRO gene expression responds differentially to changes in light regime and signaling. Loss of multiple LAZY and DRO genes leads to loss of branch angle response to light.

Expression Profile of PIN-Formed Auxin Efflux Carrier Genes during IBA-Induced In Vitro Adventitious Rooting in Olea europaea L.

Exogenous auxins supplementation plays a central role in the formation of adventitious roots (AR) for several plant species. However, the molecular mechanisms underlying the process of adventitious rooting are still not completely understood and many plants with economic value, including several olive cultivars, exhibit a recalcitrant behavior towards cutting propagation, which limits its availability in plant nurseries. PIN-formed proteins are auxin efflux transporters that have been widely characterized in several plant species due to their involvement in many developmental processes including root formation. The present study profiled the expression of the OePIN1a-c, OePIN2b, OePIN3a-c, OePIN5a-c, OePIN6, and OePIN8 gene members during indole-3-butyric acid (IBA)-induced in vitro adventitious rooting using the olive cultivar ‘Galega vulgar’. Gene expression analysis by quantitative real time PCR (RT-qPCR) showed drastic downregulation of most transcripts, just a few hours after explant inoculation, in both nontreated and IBA-treated microcuttings, albeit gene downregulation was less pronounced in IBA-treated stems. In contrast, OePIN2b showed a distinct expression pattern being upregulated in both conditions, and OePIN5b was highly upregulated in IBA-induced stems. All transcripts, except OePIN8, showed different expression profiles between nontreated and IBA-treated explants throughout the rooting experiment. Additionally, high levels of reactive oxygen species (ROS) were observed soon after explant preparation, decreasing a few hours after inoculation. Altogether, the results suggest that wounding-related ROS production, associated with explant preparation for rooting, may have an impact on auxin transport and distribution via changes in OePIN gene expression. Moreover, the application of exogenous auxin may modulate auxin homeostasis through regulation of those genes, leading to auxin redistribution throughout the stem-base tissue, which may ultimately play an important role in AR formation.

Heterogeneous phosphate supply influences maize lateral root proliferation by regulating auxin redistribution

Background and Aims Roots take up phosphorus (P) as inorganic phosphate (Pi). Enhanced root proliferation in Pi-rich patches enables plants to capture the unevenly distributed Pi, but the underlying control of root proliferation remains largely unknown. Here, the role of auxin in this response was investigated in maize (Zea mays). Methods A split-root, hydroponics system was employed to investigate root responses to Pi supply, with one (heterogeneous) or both (homogeneous) sides receiving 0 or 500 μm Pi. Key results Maize roots proliferated in Pi-rich media, particularly with heterogeneous Pi supply. The second-order lateral root number was 3-fold greater in roots of plants receiving a heterogeneous Pi supply than in roots of plants with a homogeneous Pi supply. Root proliferation in a heterogeneous Pi supply was inhibited by the auxin transporter inhibitor 1-N-naphthylphthalamic acid (NPA). The proliferation of lateral roots was accompanied by an enhanced auxin response in the apical meristem and vascular tissues at the root tip, as demonstrated in a DR5::RFP marker line. Conclusions It is concluded that the response of maize root morphology to a heterogeneous Pi supply is modulated by local signals of Pi availability and systemic signals of plant P nutritional status, and is mediated by auxin redistribution.