Emerging role of the ubiquitin proteasome system in the control of shoot apical meristem function(f)

miR394 and LCR cooperate with TPL to regulate AM initiation

Plant architecture is a main determinate of crop yield, and lateral branching significantly influences the number of inflorescences and seeds. The mechanism of axillary bud initiation remains unclear. This work aimed to examine how miRNAs regulate axillary bud initiation. By constructing a small RNA library and screening a mutant population, we revealed the initiation of axillary buds is specifically induced by miR394 and repressed by its target, LEAF CURLING RESPONSIVENESS (LCR). Using promoter-driven fluorescent tags and in situ hybridization, we showed that miR394 is localized in the center of the leaf axil where AMs are initiated. Through molecular and genetic research, we revealed that miR394/LCR may regulate REVOLUTA (REV) and SHOOT MERISTEMLESS (STM) to establish the axillary meristem. Immunoprecipitation-mass spectrometry studies revealed that LCR, as an F-box protein, may interact with TOPLESS (TPL) proteins and participate in ubiquitinated protein degradation. Our results reveal an important mechanism by which the miR394-regulated LCR accelerates the degradation of TPL to precisely modulate axillary bud initiation.

Overexpression of tomato RING E3 ubiquitin ligase gene SlRING1 confers cadmium tolerance by attenuating cadmium accumulation and oxidative stress

Heavy metal pollution not only decreases crop yield and quality, but also affects human health via the food chain. Ubiquitination-dependent protein degradation is involved in plant growth, development, and environmental interaction, but the functions of ubiquitin-ligase (E3) genes are largely unknown in tomato (Solanum lycopersicum L.). Here, we functionally characterized a RING E3 ligase gene, SlRING1, which positively regulates cadmium (Cd) tolerance in tomato plants. An in vitro ubiquitination experiment shows that SlRING1 has E3 ubiquitin ligase activity. The determination of the subcellular localization reveals that SlRING1 is localized at both the plasma membrane and the nucleus. Overexpression of SlRING1 in tomato increased the chlorophyll content, the net photosynthetic rate, and the maximal photochemical efficiency of photosystem II (Fv/Fm), but reduced the levels of reactive oxygen species and relative electrolyte leakage under Cd stress. Moreover, SlRING1 overexpression increased the transcript levels of CATALASE (CAT), DEHYDROASCORBATE REDUCTASE (DHAR), MONODEHYDROASCORBATE REDUCTASE (MDHAR), GLUTATHIONE (GSH1), and PHYTOCHELATIN SYNTHASE (PCS), which contribute to the antioxidant and detoxification system. Crucially, SlRING1 overexpression also reduced the concentrations of Cd in both shoots and roots. Thus, SlRING1-overexpression-induced enhanced tolerance to Cd is ascribed to reduced Cd accumulation and alleviated oxidative stress. Our findings suggest that SlRING1 is a positive regulator of Cd tolerance, which can be a potential breeding target for improving heavy metal tolerance in horticultural crops.

The E3 ubiquitin ligase gene SlRING1 is essential for plant tolerance to cadmium stress in Solanum lycopersicum

The E3 ubiquitin ligases participate in the degradation of plant proteins and play a regulatory role in stress response. However, the role of tomato E3 ubiquitin ligase genes in plant response to heavy metal stress remains elusive. Here, we identified 17 tomato E3 ubiquitin ligase genes using blast analysis of highly expressed E3 ubiquitin ligase genes of Arabidopsis thaliana. Through organ expression analysis, three E3 ubiquitin ligase genes with higher expression levels in roots were further screened out, and they were named Sl1, SlRHE1, and SlRING1. Among these three genes, SlRING1 expression was the highest in response to cadmium (Cd) stress. Silencing SlRING1 significantly decreased chlorophyll content, Fv/Fm, photosynthetic rate, and biomass accumulation under Cd stress. The levels of H₂O₂, electrolyte leakage, and malondialdehyde significantly increased in SlRING1-silenced plants under Cd stress compared with that in non-silenced tomato plants. Cd stress-induced increases in the transcript levels of antioxidant and detoxification genes such as CAT, DHAR, MDHAR, GSH, and PCS were compromised by SlRING1 silencing. Moreover, Cd accumulation in shoots and roots significantly increased in SlRING1-silenced plants compared with non-silenced tomato plants. These findings suggest that SlRING1 plays a positive role in plant tolerance to Cd stress in tomato.

Identification and characterization of Mini1, a gene regulating rice shoot development

The aerial parts of higher plants are generated from the shoot apical meristem (SAM). In this study, we isolated a small rice (Oryza sativa L.) mutant that showed premature termination of shoot development and was named mini rice 1 (mini1). The mutant was first isolated from a japonica cultivar Zhonghua11 (ZH11) subjected to ethyl methanesulfonate (EMS) treatment. With bulked segregant analysis (BSA) and map-based cloning method, Mini1 gene was finally fine-mapped to an interval of 48.6 kb on chromosome 9. Sequence analyses revealed a single base substitution from G to A was found in the region, which resulted in an amino acid change from Gly to Asp. The candidate gene Os09g0363900 was predicted to encode a putative adhesion of calyx edges protein ACE (putative HOTHEAD precursor) and genetic complementation experiment confirmed the identity of Mini1. Os09g0363900 contains glucose-methanol-choline (GMC) oxidoreductase and NAD(P)-binding Rossmann-like domain, and exhibits high similarity to Arabidopsis HOTHEAD (HTH). Expression analysis indicated Mini1 was highly expressed in young shoots but lowly in roots and the expression level of most genes involved in auxin biosynthesis and signal transduction were reduced in mutant. We conclude that Mini1 plays an important role in maintaining SAM activity and promoting shoot development in rice.

SLIDE, the protein interacting domain of Imitation Switch remodelers, binds DDT-domain proteins of different subfamilies in chromatin remodeling complexes

The Imitation Switch (ISWI) type adenosine triphosphate (ATP)-dependent chromatin remodeling factors are conserved proteins in eukaryotes, and some of them are known to form stable remodeling complexes with members from a family of proteins, termed DDT-domain proteins. Although it is well documented that ISWIs play important roles in different biological processes in many eukaryotic species, the molecular basis for protein interactions in ISWI complexes has not been fully addressed. Here, we report the identification of interaction domains for both ISWI and DDT-domain proteins. By analyzing CHROMATIN REMODELING11 (CHR11) and RINGLET1 (RLT1), an Arabidopsis thaliana ISWI (AtISWI) and AtDDT-domain protein, respectively, we show that the SLIDE domain of CHR11 and the DDT domain together with an adjacent sequence of RLT1 are responsible for their binding. The Arabidopsis genome contains at least 12 genes that encode DDT-domain proteins, which could be grouped into five subfamilies based on the sequence similarity. The SLIDE domain of AtISWI is able to bind members from different AtDDT subfamilies. Moreover, a human ISWI protein SNF2H is capable of binding AtDDT-domain proteins through its SLIDE domain, suggesting that binding to DDT-domain proteins is a conserved biochemical function for the SLIDE domain of ISWIs in eukaryotes.

TALE and Shape: How to Make a Leaf Different

The Three Amino acid Loop Extension (TALE) proteins constitute an ancestral superclass of homeodomain transcription factors conserved in animals, plants and fungi. In plants they comprise two classes, KNOTTED1-LIKE homeobox (KNOX) and BEL1-like homeobox (BLH or BELL, hereafter referred to as BLH), which are involved in shoot apical meristem (SAM) function, as well as in the determination and morphological development of leaves, stems and inflorescences. Selective protein-protein interactions between KNOXs and BLHs affect heterodimer subcellular localization and target affinity. KNOXs exert their roles by maintaining a proper balance between undifferentiated and differentiated cell state through the modulation of multiple hormonal pathways. A pivotal function of KNOX in evolutionary diversification of leaf morphology has been assessed. In the SAM of both simple- and compound-leafed seed species, downregulation of most class 1 KNOX (KNOX1) genes marks the sites of leaf primordia initiation. However, KNOX1 expression is re-established during leaf primordia development of compound-leafed species to maintain transient indeterminacy and morphogenetic activity at the leaf margins. Despite the increasing knowledge available about KNOX1 protein function in plant development, a comprehensive view on their downstream effectors remains elusive. This review highlights the role of TALE proteins in leaf initiation and morphological plasticity with a focus on recent advances in the identification of downstream target genes and pathways.

TALE and Shape: How to Make a Leaf Different

The Three Amino acid Loop Extension (TALE) proteins constitute an ancestral superclass of homeodomain transcription factors conserved in animals, plants and fungi. In plants they comprise two classes, KNOTTED1-LIKE homeobox (KNOX) and BEL1-like homeobox (BLH or BELL, hereafter referred to as BLH), which are involved in shoot apical meristem (SAM) function, as well as in the determination and morphological development of leaves, stems and inflorescences. Selective protein-protein interactions between KNOXs and BLHs affect heterodimer subcellular localization and target affinity. KNOXs exert their roles by maintaining a proper balance between undifferentiated and differentiated cell state through the modulation of multiple hormonal pathways. A pivotal function of KNOX in evolutionary diversification of leaf morphology has been assessed. In the SAM of both simple- and compound-leafed seed species, downregulation of most class 1 KNOX (KNOX1) genes marks the sites of leaf primordia initiation. However, KNOX1 expression is re-established during leaf primordia development of compound-leafed species to maintain transient indeterminacy and morphogenetic activity at the leaf margins. Despite the increasing knowledge available about KNOX1 protein function in plant development, a comprehensive view on their downstream effectors remains elusive. This review highlights the role of TALE proteins in leaf initiation and morphological plasticity with a focus on recent advances in the identification of downstream target genes and pathways.