Auxin homeostasis is maintained by sly-miR167-SlARF8A/B-SlGH3.4 feedback module in the development of locular and placental tissues of tomato fruits

The locular gel, produced by the placenta, is important for fruit flavor and seed development in tomato. However, the mechanism underlying locule and placenta development is not fully understood yet. Here, we show that two SlARF transcription factors, SlARF8B and SlARF8A (SlARF8A/B), promote the development of locular and placenta tissues. The expression of both SlARF8A and SlARF8B is repressed by sly-microRNA167 (sly-miR167), allowing for the activation of auxin downstream genes. In slarf8a, slarf8b, and slarf8a/b mutants, the auxin (IAA) levels are decreased, whereas the levels of inactive IAA conjugates including IAA-Ala, IAA-Asp, and IAA-Glu are increased. We further find that SlARF8B directly inhibits the expression of SlGH3.4, an acyl acid amino synthetase that conjugates the amino acids to IAA. Disruption of such auxin balance by the increased expression of SlGH3.4 or SlGH3.2 results in defective locular and placental tissues. Taken together, our findings reveal an important regulatory module constituted by sly-miR167-SlARF8A/B-SlGH3.4 during the development of locular and placenta tissues of tomato fruits.

The miR167-OsARF12 module regulates rice grain filling and grain size downstream of miR159

Grain weight and quality are always determined by grain filling. Plant microRNAs have drawn attention as key targets for regulation of grain size and yield. However, the mechanisms that underlie grain size regulation remain largely unclear because of the complex networks that control this trait. Our earlier studies demonstrated that suppressed expression of miR167 (STTM/MIM167) substantially increased grain weight. In a field test, the yield increased up to 12.90%-21.94% because of a significantly enhanced grain filling rate. Here, biochemical and genetic analyses revealed the regulatory effects of miR159 on miR167 expression. Further analysis indicated that OsARF12 is the major mediator by which miR167 regulates rice grain filling. Overexpression of OsARF12 produced grain weight and grain filling phenotypes resembling those of STTM/MIM167 plants. Upon in-depth analysis, we found that OsARF12 activates OsCDKF;2 expression by directly binding to the TGTCGG motif in its promoter region. Flow cytometry analysis of young panicles from OsARF12-overexpressing plants and examination of cell number in cdkf;2 mutants verified that OsARF12 positively regulates grain filling and grain size by targeting OsCDKF;2. Moreover, RNA sequencing results suggested that the miR167-OsARF12 module is involved in the cell development process and hormone pathways. OsARF12-overexpressing plants and cdkf;2 mutants exhibited enhanced and reduced sensitivity to exogenous auxin and brassinosteroid (BR) treatment, confirming that targeting of OsCDKF;2 by OsARF12 mediates auxin and BR signaling. Our results reveal that the miR167-OsARF12 module works downstream of miR159 to regulate rice grain filling and grain size via OsCDKF;2 by controlling cell division and mediating auxin and BR signals.

Coordination of floral and fiber development in cotton (Gossypium) by hormone- and flavonoid-signalling associated regulatory miRNAs

Various plant development activities and stress responses are tightly regulated by various microRNAs (miRNA) and their target genes, or transcription factors in a spatiotemporal manner. Here, to exemplify how flowering-associated regulatory miRNAs synchronize their expression dynamics during floral and fiber development in cotton, constitutive expression diminution transgenic lines of auxin-signaling regulatory Gh-miR167 (35S-MIM167) were developed through target mimicry approach. 'Moderate' (58% to 80%)- and 'high' (> 80%)-Gh-miR167 diminution mimic lines showed dosage-dependent developmental deformities in anther development, pollen maturation, and fruit (= boll) formation. Cross pollination of 'moderate' 35S-MIM167 mimic lines with wild type (WT) plant partially restored boll formation and emergence of fiber initials on the ovule surface. Gh-miR167 diminution favored organ-specific transcription biases in miR159, miR166 as well as miR160, miR164, and miR172 along with their target genes during anther and petal development, respectively. Similarly, accumulative effect of percent Gh-miR167 diminution, cross regulation of its target ARF6/8 genes, and temporal mis-expression of hormone signaling- and flavonoid biosynthesis-associated regulatory miRNAs at early fiber initiation stage caused irregular fiber formation. Spatial and temporal transcription proportions of regulatory miRNAs were also found crucial for the execution of hormone- and flavonoid-dependent progression of floral and fiber development. These observations discover how assorted regulatory genetic circuits get organized in response to Gh-miR167 diminution and converge upon ensuing episodes of floral and fiber development in cotton.

Auxin plays a role in the adaptation of rice to anaerobic germination and seedling establishment

Auxin is well known to stimulate coleoptile elongation and rapid seedling growth in the air. However, its role in regulating rice germination and seedling establishment under submergence is largely unknown. Previous studies revealed that excessive levels of indole-3-acetic acid(IAA) frequently cause the inhibition of plant growth and development. In this study, the high-level accumulation of endogenous IAA is observed under dark submergence, stimulating rice coleoptile elongation but limiting the root and primary leaf growth during anaerobic germination (AG). We found that oxygen and light can reduce IAA levels, promote the seedling establishment and enhance rice AG tolerance. miRNA microarray profiling and RNA gel blot analysis results show that the expression of miR167 is negatively regulated by submergence; it subsequently modulates the accumulation of free IAA through the miR167-ARF-GH3 pathway. The OsGH3-8 encodes an IAA-amido synthetase that functions to prevent free IAA accumulation. Reduced miR167 levels or overexpressing OsGH3-8 increase auxin metabolism, reduce endogenous levels of free IAA and enhance rice AG tolerance. Our studies reveal that poor seed germination and seedling growth inhibition resulting from excessive IAA accumulation would cause intolerance to submergence in rice, suggesting that a certain threshold level of auxin is essential for rice AG tolerance.

The auxin signaling pathway contributes to phosphorus-mediated zinc homeostasis in maize

Although the interaction between P and Zn has long been recognized in plants, the physiological and molecular mechanisms underlying P and Zn interactions are poorly understood. We show here that P supply decreases the Zn concentration in maize shoots and roots. Compared to +P + Zn (addition of both P and Zn), +P-Zn reduced and -P-Zn increased the total length of 1° lateral roots (LRs). Under +P + Zn, both P and Zn concentrations were lower in the sl1 mutant roots than in wild-type (WT) maize roots, and P accumulation did not reduce the Zn concentration in ll1 mutant roots. Transcriptome profiling showed that the auxin signaling pathway contributed to P-mediated Zn homeostasis in maize. Auxin production and distribution were altered by changes in P and Zn supply. Cytosolic Zn co-localized with auxin accumulation under +P + Zn. Exogenous application of 1-NAA and L-Kyn altered the P-mediated root system architecture (RSA) under Zn deficiency. -P-Zn repressed the expression of miR167. Overexpression of ZmMIR167b increased the lengths of 1° LRs and the concentrations of P and Zn in maize. These results indicate that auxin-dependent RSA is important for P-mediated Zn homeostasis in maize.HighlightAuxin-dependent RSA is important for P-mediated Zn homeostasis in maize.

Advances in the regulation of plant development and stress response by miR167

MicroRNAs (miRNAs) are a class of endogenous, non-coding small RNA that cleavage mRNA targets in sequence-specific manner or the inhibition of translation, which regulates gene expression at the post-transcriptional level. miRNAs are involved in the regulation of plant growth, metabolism and stress response. miR167 family is one of the highly conserved miRNA families in plants. It functions mainly by regulating the auxin response factors (ARFs) and IAA-Ala resistant3 (IAR3) genes, and participates in regulating the development of roots, stems, leaves and flowers, flowering time, embryonic development, seed development and stress response. Here, we reviewed the biological functions of miR167 family and its target genes in plant growth and development and stress response, and further discussed the application prospect of miR167 in agricultural production. Furthermore, this review provides references for the further study of miR167 family in plants.

Knock-down of δ-aminolevulinic acid dehydratase via virus-induced gene silencing alters the microRNA biogenesis and causes stress-related reactions in citrus plants

The δ-aminolevulinic acid (δ-ALA) is an intermediate in the biosynthetic pathway of tetrapyrroles. Tetrapyrroles play vital roles in many biological processes such as photosynthesis, respiration, and light-sensing. ALA-dehydratase (ALAD) combines two molecules of δ-ALA to form porphobilinogen. In citrus, the silencing of ALAD caused discrete yellow spots and necrosis in leaves and stems. Additionally, it caused rapid death in developing new shoots. Herein, we hypothesize that the accumulation of δ-ALA results in severe stress and reduced meristem development. For that reason, we investigated the dynamic changes in the expression profiles of 23 microRNA (miRNA) identified through small RNA sequencing, from CTV-tALAD plants in comparison with healthy C. macrophylla and C. macrophylla infiltrated with CTV-wt. Furthermore, we reported the effect of ALAD silencing on the total phenolics, H₂O_2, and reactive oxygen species (ROS) levels, to examine the possibilities of miRNAs involving the regulation of these pathways. Our results showed that the total phenolics content, H₂O₂, and O₂^- levels were increased in CTV-tALAD plants. Moreover, 63 conserved miRNA members belonging to 23 different miRNA families were differentially expressed in CTV-tALAD plants compared to controls. The identified miRNAs are implicated in auxin biosynthesis and signaling, axillary shoot meristem formation and leaf morphology, starch metabolism, and oxidative stress. Collectively, our findings suggested that ALAD silencing initiates stress on citrus plants. As a result, CTV-tALAD plants exhibit reduced metabolic rate, growth, and development in order to cope with the stress that resulted from the accumulation of δ-ALA. This cascade of events led to leaf, stem, and meristem necrosis and failure of new shoot development.

miR167 limits anther growth to potentiate anther dehiscence

In flowering plants, anther dehiscence and pollen release are essential for sexual reproduction. Anthers dehisce after cell wall degradation weakens stomium cell junctions in each anther locule, and desiccation creates mechanical forces that open the locules. Either effect or both together may break stomium cell junctions. The microRNA miR167 negatively regulates ARF6 and ARF8, which encode auxin response transcription factors. Arabidopsis mARF6 or mARF8 plants with mutated miR167 target sites have defective anther dehiscence and ovule development. Null mir167a mutations recapitulated mARF6 and mARF8 anther and ovule phenotypes, indicating that MIR167a is the main miR167 precursor gene that delimits ARF6 and ARF8 expression in these organs. Anthers of mir167a or mARF6/8 plants overexpressed genes encoding cell wall loosening functions associated with cell expansion, and grew larger than wild-type anthers did starting at flower stage 11. Experimental desiccation enabled dehiscence of miR167-deficient anthers, indicating competence to dehisce. Conversely, high humidity conditions delayed anther dehiscence in wild-type flowers. These results support a model in which miR167-mediated anther growth arrest permits anther dehiscence. Without miR167 regulation, excess anther growth delays dehiscence by prolonging desiccation.

Distinct transcriptional and processing regulations control miR167a level in tomato during stress

Besides their definite role in plant developmental processes miR167 also serve as mediator of stress response. Although differential expression of miR167 occurs during stresses, the regulatory-mechanism of biogenesis remained elusive. Therefore, using tomato as the model plant we have explored the mechanism of regulation of miR167a expression during stresses. Fungus or virus infections and exposure to cold stress raised the level of miR167a expression. Whereas, salt, drought and heat treatments resulted in the downregulation, indicating different stresses activated alternative mechanisms for miR167a regulation. Interestingly, the relative expression level of precursors in control versus temperature stressed plants differed from the pattern observed in the mature miR167a expression, suggesting that both transcriptional and processing regulation were important for biogenesis. The promoter-regulatory sequence of the major isoform MIR167a harbours several development and stress-related regulatory sites. Accordingly, promoter assays using transient transformation and transgenic tobacco plants proved stress-dependent regulation of the promoter. Further analyses corroborated the role of tomato DREB2A protein in the transcriptional regulation during temperature stress. Finally, in vitro assays established the importance of processing factors in cold-stress dependent efficient processing of MIR167a precursors. These data confirm distinct role of transcriptional and processing machinery in stress-influenced regulation of tomato miR167a biogenesis.