Growth asymmetry precedes differential auxin response during apical hook initiation in Arabidopsis

Exploring fine tuning between phytohormones and ROS signaling cascade in regulation of seed dormancy, germination and seedling development

In higher plants, seed is a propagule which ensures dissemination and survival of species. Developmental phases of a seed comprise embryogenesis, maturation and germination paving a way to its final fate i.e. seedling establishment. The final stage of seed maturation is marked by dehydration, acquisition of dessication tolerance and induction of dormancy. A precise Abscisic acid (ABA) to Gibberellins (GA) ratio, accumulation of miRNA 156, low level of reactive oxygen species (ROS) and enzyme inactivity govern seed dormancy. This also prevent pre harvest sprouting of the seeds. Overtime, stored seed mRNAs and proteins are degraded through oxidation of specific nucleotides in response to ROS accumulation. This degradation alleviates seed dormancy and transforms a dormant seed into a germinating seed. At this stage, ABA catabolism and degradation accompanied by GA synthesis contribute to low ABA to GA ratio. GA as well as ROS acts downstream, to mobilize reserve food materials, rupture testa, enhance imbibition and protrude radicle. All these events mark seed germination. Further, seedling is established under the governance of auxin and light. ABA and GA are master regulators while auxin, cytokinins, ethylene, jasmonic acid, brassinosteroids act through interdependent pathways to tightly regulate seed dormancy, germination and seedling establishment. In this review, the role of phytohormones and ROS in accordance with environmental factors in governing seed dormancy, promoting seed germination and thus, establishing a seedling is discussed in detail.

HOOKLESS1 acetylates AUTOPHAGY-RELATED PROTEIN18a to promote autophagy during nutrient starvation in Arabidopsis

Acetylation is an important posttranslational modification (PTM) that regulates almost all core processes of autophagy in yeast and mammals. However, the role of protein acetylation in plant autophagy and the underlying regulatory mechanisms remain unclear. Here, we show the essential role of the putative acetyltransferase HOOKLESS1 (HLS1) in acetylation of the autophagy-related protein ATG18a, a key autophagy component that regulates autophagosome formation in Arabidopsis (Arabidopsis thaliana). Loss of HLS1 function suppressed starvation-induced autophagy and increased plant susceptibility to nutrient deprivation. We discovered that HLS1 physically interacts with and directly acetylates ATG18a both in vitro and in vivo. In contrast, mutating putative active sites in HLS1 inhibited ATG18a acetylation and suppressed autophagy upon nutrient deprivation. Accordingly, overexpression of ATG18a mutant variants with lower acetylation levels inhibited the binding activity of ATG18a to PtdIns(3)P and autophagosome formation under starvation conditions. Moreover, HLS1-modulated autophagy was uncoupled from its function in hook development. Taken together, these findings shed light on a key regulator of autophagy and further elucidate the importance of PTMs in modulating autophagy in plants.

Effects of Low Temperature on Pedicel Abscission and Auxin Synthesis Key Genes of Tomato

Cold stress usually causes the abscission of floral organs and a decline in fruit setting rate, seriously reducing tomato yield. Auxin is one of the key hormones that affects the abscission of plant floral organs; the YUCCA (YUC) family is a key gene in the auxin biosynthesis pathway, but there are few research reports on the abscission of tomato flower organs. This experiment found that, under low temperature stress, the expression of auxin synthesis genes increased in stamens but decreased in pistils. Low temperature treatment decreased pollen vigor and pollen germination rate. Low night temperature reduced the tomato fruit setting rate and led to parthenocarpy, and the treatment effect was most obvious in the early stage of tomato pollen development. The abscission rate of tomato pTRV-Slfzy3 and pTRV-Slfzy5 silenced plants was higher than that of the control, which is the key auxin synthesis gene affecting the abscission rate. The expression of Solyc07g043580 was down-regulated after low night temperature treatment. Solyc07g043580 encodes the bHLH-type transcription factor SlPIF4. It has been reported that PIF4 regulates the expression of auxin synthesis and synthesis genes, and is a key protein in the interaction between low temperature stress and light in regulating plant development.

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness. The hook structure is transient but essential for seedlings' survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury. As a superb model system for studying plant differential growth, apical hook has fascinated botanists as early as the Darwin age, and significant advances have been achieved at both the morphological and molecular levels to understand how apical hook development is regulated. Here, we will mainly summarize the research progress at these two levels. We will also briefly compare the growth dynamics between apical hook and hypocotyl gravitropic bending at early seed germination phase, with the aim to deduce a certain consensus on their connections. Finally, we will outline the remaining questions and future research perspectives for apical hook development.

Multiple mechanisms behind plant bending

To survive, plants constantly adapt their body shape to their environment. This often involves remarkably rapid bending of their organs such as stems, leaves and roots. Since plant cells are enclosed by stiff cell walls, they use various strategies for bending their organs, which differ from bending mechanisms of soft animal tissues and involve larger physical forces. Here we attempt to summarize and link different viewpoints on bending mechanisms: genes and signalling, mathematical modelling and biomechanics. We argue that quantifying cell growth and physical forces could open a new level in our understanding of bending and resolve some of its paradoxes.

Modulation of receptor-like transmembrane kinase 1 nuclear localization by DA1 peptidases in Arabidopsis

Signals are often perceived by proteins in one cellular location and transduced to other locations such as the nucleus. Signaling proteins can be cleaved by peptidases to facilitate this movement, but the peptidases involved in this are poorly understood despite their widespread role. We describe a role for the ubiquitin-activated peptidase DA1 in cleaving the membrane-localized receptor-like kinase transmembrane kinase 1 (TMK1) in Arabidopsis. TMK1 is phosphorylated in response to auxin and mediates several auxin responses including growth induction by cell expansion. DA1-mediated cleavage of TMK1 facilitates nuclear localization of its intracellular kinase domain to repress auxin-mediated gene expression, facilitating differential cell expansion during growth. These analyses establish a wider role for DA1 family activities in cell growth.

The cleavage of intracellular domains of receptor-like kinases (RLKs) has an important functional role in the transduction of signals from the cell surface to the nucleus in many organisms. However, the peptidases that catalyze protein cleavage during signal transduction remain poorly understood despite their crucial roles in diverse signaling processes. Here, we report in the flowering plant Arabidopsis thaliana that members of the DA1 family of ubiquitin-regulated Zn metallopeptidases cleave the cytoplasmic kinase domain of transmembrane kinase 1 (TMK1), releasing it for nuclear localization where it represses auxin-responsive cell growth during apical hook formation by phosphorylation and stabilization of the transcriptional repressors IAA32 and IAA34. Mutations in DA1 family members exhibited reduced apical hook formation, and DA1 family-mediated cleavage of TMK1 was promoted by auxin treatment. Expression of the DA1 family-generated intracellular kinase domain of TMK1 by an auxin-responsive promoter fully restored apical hook formation in a tmk1 mutant, establishing the function of DA1 family peptidase activities in TMK1-mediated differential cell growth and apical hook formation. DA1 family peptidase activity therefore modulates TMK1 kinase activity between a membrane location where it stimulates acid cell growth and initiates an auxin-dependent kinase cascade controlling cell proliferation in lateral roots and a nuclear localization where it represses auxin-mediated gene expression and growth.

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening.