Transgenerational Inheritance and Resetting of Stress, Induced Loss of Epigenetic Gene Silencing in Arabidopsis

Plants, as sessile organisms, need to sense and adapt to heterogeneous environments and have developed sophisticated responses by changing their cellular physiology, gene regulation, and genome stability. Recent work dem- onstrated heritable stress effects on the control of genome stability in plants--a phenomenon that was suggested to be of epigenetic nature. Here, we show that temperature and UV-B stress cause immediate and heritable changes in the epi- genetic control of a silent reporter gene in Arabidopsis. This stress-mediated release of gene silencing correlated with pronounced alterations in histone occupancy and in histone H3 acetylation but did not involve adjustments in DNA meth- ylation. We observed transmission of stress effects on reporter gene silencing to non-stressed progeny, but this effect was restricted to areas consisting of a small number of cells and limited to a few non-stressed progeny generations. Further- more, stress-induced release of gene silencing was antagonized and reset during seed aging. The transient nature of this phenomenon highlights the ability of plants to restrict stress-induced relaxation of epigenetic control mechanisms, which likely contributes to safeguarding genome integrity.

Pho-view of Auxin:Reversible Protein Phosphorylation in Auxin Biosynthesis,Transport and Signaling

The phytohormone auxin plays a central role in shaping plant growth and development.With decades of genetic and biochemical studies,numerous core molecular components and their networks,underlying auxin biosynthesis,transport,and signaling,have been identified.Notably,protein phosphorylation,catalyzed by kinases and oppositely hydrolyzed by phosphatases,has been emerging to be a crucial type of post-translational modification,regulating physiological and developmental auxin output at all levels.In this review,we comprehensively discuss earlier and recent advances in our understanding of genetics,biochemistry,and cell biology of the kinases and phosphatases participating in auxin action.We provide insights into the mechanisms by which reversible protein phosphorylation defines developmental auxin responses,discuss current challenges,and provide our perspectives on future directions involving the integration of the control of protein phosphorylation into the molecular auxin network.

TOLs Function as Ubiquitin Receptors in the Early Steps of the ESCRT Pathway in Higher Plants

Protein abundance and localization at the plasma membrane(PM)shapes plant development and mediates adaptation to changing environmental conditions.It is regulated by ubiquitination,a post-translational modification crucial for the proper sorting of endocytosed PM proteins to the vacuole for subsequent degradation.To understand the significance and the variety of roles played by this reversible modification,the function of ubiquitin receptors,which translate the ubiquitin signature into a cellular response,needs to be elucidated.In this study,we show that TOL(TOM1-like)proteins function in plants as multivalent ubiquitin receptors,governing ubiquitinated cargo delivery to the vacuole via the conserved Endosomal Sorting Complex Required for Transport(ESCRT)pathway.TOL2 and TOL6 interact with components of the ESCRT machinery and bind to K63-linked ubiquitin via two tandemly arranged conserved ubiquitin-binding domains.Mutation of these domains results not only in a loss of ubiquitin binding but also altered localization,abolishing TOL6 ubiquitin receptor activity.Function and localization of TOL6 is itself regulated by ubiquitination,whereby TOL6 ubiquitination potentially modulates degradation of PM-localized cargoes,assisting in the fine-tuning of the delicate interplay between protein recycling and downregulation.Taken together,our findings demonstrate the function and regulation of a ubiquitin receptor that mediates vacuolar degradation of PM proteins in higher plants.

Plasma Membrane Protein Ubiquitylation and Degradation as Determinants of Positional Growth in Plants

Being sessile organisms, plants evolved an unparalleled plasticity in their post-embryonic development, allowing them to adapt and fine-tune their vital parameters to an ever-changing environment. Cross-talk between plants and their environment requires tight regulation of information exchange at the plasma membrane （PM）. Plasma membrane proteins mediate such communication, by sensing variations in nutrient availability, external cues as well as by controlled solute transport across the membrane border. Localiza-tion and steady-state levels are essential for PM protein function and ongoing research identified cis- and trans-acting determinants, involved in control of plant PM protein localization and turnover. In this overview, we summarize recent progress in our understanding of plant PM protein sorting and degradation via ubiquitylation, a post-translational and reversible modification of proteins. We highlight characterized components of the machinery involved in sorting of ubiquitylated PM proteins and discuss consequences of protein ubiquitylation on fate of selected PM proteins. Specifically, we focus on the role of ubiquitylation and PM protein degradation in the regulation of polar auxin transport （PAT）. We combine this regulatory circuit with further aspects of PM protein sorting control, to address the interplay of events that might control PAT and polarized growth in higher plants.

PIN FORMED 2 Modulates the Transport of Arsenite in Arabidopsis thaliana

Arsenic contamination is a major environmental issue,as it may lead to serious health hazard.The reduced trivalent formof inorganic arsenic,arsenite,is in generalmore toxic to plants comparedwith the fully oxidized pentavalent arsenate.Theuptakeof arsenite inplants hasbeenshown tobemediatedthrough a large subfamily of plant aquaglyceroporins,nodulin 26-like intrinsic proteins(NIPs).However,the efflux mechanisms,as well as themechanismof arsenite-induced root growth inhibition,remain poorly understood.Usingmolecular physiology,synchrotron imaging,and root transport assay approaches,we show that the cellular transport of trivalent arsenicals inArabidopsis thalianais stronglymodulatedbyPINFORMED2(PIN2)auxin efflux transporter.Root transport assay using radioactive arsenite,X-ray fluorescence imaging(XFI)coupled with X-ray absorption spectroscopy(XAS),and inductively coupled plasma mass spectrometry analysis revealed that pin2 plants accumulate higher concentrations of arsenite in roots comparedwith the wild-type.At the cellular level,arsenite specifically targets intracellular sorting of PIN2 and thereby alters the cellular auxin homeostasis.Consistently,loss of PIN2 function results in arsenite hypersensitivity in roots.XFI coupled with XAS further revealed that loss of PIN2 function results in specific accumulation of arsenical species,but not the other metals such as iron,zinc,or calcium in the root tip.Collectively,these results suggest that PIN2 likely functions as an arsenite efflux transporter for the distribution of arsenical species in planta.