Rice microtubule-associated protein OsMAP65-3.1, but not OsMAP65-3.2, plays a critical role in phragmoplast microtubule organization in cytokinesis

Transcriptomic analysis offers deep insights into the Increased Grain Length 1 (IGL1) regulation of grain length

BACKGROUND

Although great progress has been made in recent years in identifying novel genes or natural alleles for rice yield improvement, the molecular mechanisms of how these genes/natural alleles regulate yield-associated traits, such as grain length and 1000-grain weight, remain largely unclear. An in-depth understanding of the roles of these genes/natural alleles in controlling yield traits become a necessity to ultimately increase rice yield via novel molecular techniques, such as gene editing.

RESULTS

In this study, the roles of IGL1, which was previously identified through a map-based cloning approach, in the regulation of grain length were investigated by overexpressing and knocking out it in the Nipponbare genetic background. Overexpression and knockout of IGL1 (the resulting transgenic lines were hereafter designated IGL1-OE and IGL1-CR lines, respectively) led to elongation and shortening of grains, respectively. To further elucidate the molecular mechanisms behind the IGL1 action, young panicles from IGL1-OE and IGL1-CR lines were subjected to mRNA sequencing. The results showed that both overexpression and knockout of IGL1 all resulted in a large number of upregulated and downregulated differentially expression genes (DEGs) relative to wild-type NPB control lines. A total of 984 DEGs overlapped between upregulated DEGs from IGL1-OE and downregulated DEGs from IGL1-CR; 1146 DEGs were common to downregulated DEGs from IGL1-OE and upregulated DEGs from IGL1-CR. GO term and KEGG pathway analysis revealed that IGL1-upregulated DEGs were associated with extracellular region, protein ubiquitination, cell-wall modification, BR signaling, cell cycle, etc.; by comparison, the IGL1-downregulated DEGs were connected with extracellular region, response to wounding, flavonoid biosynthesis, jasmonic-acid signaling, glucose/sucrose metabolism, etc. Some phytohormone-associated genes (like OsYUCCA4, OsPIN10b, OsBAK1, and OsDLT), TF genes (like OsMADS1 and OsGASR9), grain length-regulating genes (like An-1, GS9, OsIQD14, and TGW2) showed significant upregulation or downregulation in IGL1-OE or IGL1-CR.

CONCLUSION

Our result clearly demonstrated that IGL1 is an important regulator of grain length, and has profound impacts on genome-wide gene expression, suggesting that it may work together with certain TFs. Overexpression or knockout of IGL1 appears to cause complex expression changes of genes associated with phytohormones, TFs, grain length-regulating factors, which ultimately brings about the grain elongation.

Control of cellularization, nuclear localization, and antipodal cell cluster development in maize embryo sacs

The maize female gametophyte contains four cell types: two synergids, an egg cell, a central cell, and a variable number of antipodal cells. In maize, these cells are produced after three rounds of free-nuclear divisions followed by cellularization, differentiation, and proliferation of the antipodal cells. Cellularization of the eight-nucleate syncytium produces seven cells with two polar nuclei in the central cell. Nuclear localization is tightly controlled in the embryo sac. This leads to precise allocation of the nuclei into the cells upon cellularization. Nuclear positioning within the syncytium is highly correlated with their identity after cellularization. Two mutants are described with extra polar nuclei, abnormal antipodal cell morphology, and reduced antipodal cell number, as well as frequent loss of antipodal cell marker expression. Mutations in one of these genes, indeterminate gametophyte2 encoding a MICROTUBULE ASSOCIATED PROTEIN65-3 homolog, shows a requirement for MAP65-3 in cellularization of the syncytial embryo sac as well as for normal seed development. The timing of the effects of ig2 suggests that the identity of the nuclei in the syncytial female gametophyte can be changed very late before cellularization.

The making of a ring: Assembly and regulation of microtubule-associated proteins during preprophase band formation and division plane set-up

The preprophase band (PPB) is a transient cytokinetic structure that marks the future division plane at the onset of mitosis. The PPB forms a dense cortical ring of mainly microtubules, actin filaments, endoplasmic reticulum, and associated proteins that encircles the nucleus of mitotic cells. After PPB disassembly, the positional information is preserved by the cortical division zone (CDZ). The formation of the PPB and its contribution to timely CDZ set-up involves activities of functionally distinct microtubule-associated proteins (MAPs) that interact physically and genetically to support robust division plane orientation in plants. Recent studies identified two types of plant-specific MAPs as key regulators of PPB formation, the TON1 RECRUITMENT MOTIF (TRM) and IQ67 DOMAIN (IQD) families. Both families share hallmarks of disordered scaffold proteins. Interactions of IQDs and TRMs with multiple binding partners, including the microtubule severing KATANIN1, may provide a molecular framework to coordinate PPB formation, maturation, and disassembly.

Microtubule Regulation in Plants: From Morphological Development to Stress Adaptation

Microtubules (MTs) are essential elements of the eukaryotic cytoskeleton and are critical for various cell functions. During cell division, plant MTs form highly ordered structures, and cortical MTs guide the cell wall cellulose patterns and thus control cell size and shape. Both are important for morphological development and for adjusting plant growth and plasticity under environmental challenges for stress adaptation. Various MT regulators control the dynamics and organization of MTs in diverse cellular processes and response to developmental and environmental cues. This article summarizes the recent progress in plant MT studies from morphological development to stress responses, discusses the latest techniques applied, and encourages more research into plant MT regulation.