Megumi Iwano

Direct ligand–receptor complex interaction controls Brassica self-incompatibility

Many higher plants have evolved self-incompatibility mechanisms to prevent self-fertilization. In Brassica self-incompatibility, recognition between pollen and the stigma is controlled by the S locus, which contains three highly polymorphic genes: S-receptor kinase (SRK), S-locus protein 11 (SP11) (also called S-locus cysteine-rich protein; SCR) and S-locus glycoprotein (SLG). SRK encodes a membrane-spanning serine/threonine kinase that determines the S-haplotype specificity of the stigma, and SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen. SP11 is localized in the pollen coat. It is thought that, during self-pollination, SP11 is secreted from the pollen coat and interacts with its cognate SRK in the papilla cell of the stigma to elicit the self-incompatibility response. SLG is a secreted stigma protein that is highly homologous to the SRK extracellular domain. Although it is not required for S-haplotype specificity of the stigma, SLG enhances the self-incompatibility response; however, how this is accomplished remains controversial. Here we show that a single form of SP11 of the S8 haplotype (S8-SP11) stabilized with four intramolecular disulphide bonds specifically binds the stigma membrane of the S8 haplotype to induce autophosphorylation of SRK8, and that SRK8 and SLG8 together form a high-affinity receptor complex for S8-SP11 on the stigma membrane.

Comparative analysis of the self‐incompatibility (S‐) locus region of Prunus mume: identification of a pollen‐expressed F‐box gene with allelic diversity

Background: Self‐incompatibility (SI) in the Solanaceae, Rosaceae and Scrophulariaceae is gametophytically controlled by a single polymorphic locus, termed the S‐locus. To date, the only known S‐locus product is a polymorphic ribonuclease, termed S‐RNase, which is secreted by stylar tissue and thought to act as a cytotoxin that degrades the RNA of incompatible pollen tubes. However, understanding how S‐RNase causes S‐haplotype specific inhibition of pollen tubes has been hampered by the lack of a cloned pollen S‐determinant gene.

Reduced Levels of Chloroplast FtsH Protein in Tobacco Mosaic Virus–Infected Tobacco Leaves Accelerate the Hypersensitive Reaction

In tobacco cultivars resistant to tobacco mosaic virus (TMV), infection results in the death of the infected cells accompanying the formation of necrotic lesions. To identify the genes involved in this hypersensitive reaction, we isolated the cDNA of tobacco DS9, the transcript of which decreases before the appearance of necrotic lesions. The DS9 gene encodes a chloroplastic homolog of bacterial FtsH protein, which serves to maintain quality control of some cytoplasmic and membrane proteins. A large quantity of DS9 protein was found in healthy leaves, whereas the quantity of DS9 protein in infected leaves decreased before the lesions appeared. In transgenic tobacco plants containing less and more DS9 protein than wild-type plants, the necrotic lesions induced by TMV were smaller and larger, respectively, than those on wild-type plants. These results suggest that a decrease in the level of DS9 protein in TMV-infected cells, resulting in a subsequent loss of function of the chloroplasts, accelerates the hypersensitive reaction.

Dual Targeting of Spinach Protoporphyrinogen Oxidase II to Mitochondria and Chloroplasts by Alternative Use of Two In-frame Initiation Codons

Protoporphyrinogen oxidase (Protox) is the final enzyme in the common pathway of chlorophyll and heme biosynthesis. Two Protox isoenzymes have been described in tobacco, a plastidic and a mitochondrial form. We isolated and sequenced spinach Protox cDNA, which encodes a homolog of tobacco mitochondrial Protox (Protox II). Alignment of the deduced amino acid sequence between Protox II and other tobacco mitochondrial Protox homologs revealed a 26-amino acid N-terminal extension unique to the spinach enzyme. Immunoblot analysis of spinach leaf extract detected two proteins with apparent molecular masses of 57 and 55 kDa in chloroplasts and mitochondria, respectively. In vitrotranslation experiments indicated that two translation products (59 and 55 kDa) are produced from Protox II mRNA, using two in-frame initiation codons. Transport experiments using green fluorescent protein-fused Protox II suggested that the larger and smaller translation products (Protox IIL and IIS) target exclusively to chloroplasts and mitochondria, respectively.

Fine-Tuning of the Cytoplasmic Ca2+ Concentration Is Essential for Pollen Tube Growth

Pollen tube growth is crucial for the delivery of sperm cells to the ovule during flowering plant reproduction. Previous in vitro imaging of Lilium longiflorum and Nicotiana tabacum has shown that growing pollen tubes exhibit a tip-focused Ca2+ concentration ([Ca2+]) gradient and regular oscillations of the cytosolic [Ca2+] ([Ca2+]cyt) in the tip region. Whether this [Ca2+] gradient and/or [Ca2+]cyt oscillations are present as the tube grows through the stigma (in vivo condition), however, is still not clear. We monitored [Ca2+]cyt dynamics in pollen tubes under various conditions using Arabidopsis (Arabidopsis thaliana) and N. tabacum expressing yellow cameleon 3.60, a fluorescent calcium indicator with a large dynamic range. The tip-focused [Ca2+]cyt gradient was always observed in growing pollen tubes. Regular oscillations of the [Ca2+]cyt, however, were rarely identified in Arabidopsis or N. tabacum pollen tubes grown under the in vivo condition or in those placed in germination medium just after they had grown through a style (semi-in vivo condition). On the other hand, regular oscillations were observed in vitro in both growing and nongrowing pollen tubes, although the oscillation amplitude was 5-fold greater in the nongrowing pollen tubes compared with growing pollen tubes. These results suggested that a submicromolar [Ca2+]cyt in the tip region is essential for pollen tube growth, whereas a regular [Ca2+] oscillation is not. Next, we monitored [Ca2+] dynamics in the endoplasmic reticulum ([Ca2+]ER) in relation to Arabidopsis pollen tube growth using yellow cameleon 4.60, which has a lower affinity for Ca2+ compared with yellow cameleon 3.60. The [Ca2+]ER in pollen tubes grown under the semi-in vivo condition was between 100 and 500 μm. In addition, cyclopiazonic acid, an inhibitor of ER-type Ca2+-ATPases, inhibited growth and decreased the [Ca2+]ER. Our observations suggest that the ER serves as one of the Ca2+ stores in the pollen tube and cyclopiazonic acid-sensitive Ca2+-ATPases in the ER are required for pollen tube growth.

Ca2+ Dynamics in a Pollen Grain and Papilla Cell during Pollination of Arabidopsis

Ca2+ dynamics in the growing pollen tube have been well documented in vitro using germination assays and Ca2+ imaging techniques. However, very few in vivo studies of Ca2+ in the pollen grain and papilla cell during pollination have been performed. We expressed yellow cameleon, a Ca2+ indicator based on green fluorescent protein, in the pollen grains and papilla cells of Arabidopsis (Arabidopsis thaliana) and monitored Ca2+ dynamics during pollination. In the pollen grain, [Ca2+]cyt increased at the potential germination site soon after hydration and remained augmented until germination. As in previous in vitro germination studies, [Ca2+]cyt oscillations were observed in the tip region of the growing pollen tube, but the oscillation frequency was faster and [Ca2+]cyt was higher than had been observed in vitro. In the pollinated papilla cell, remarkable increases in [Ca2+]cyt occurred three times in succession, just under the site of pollen-grain attachment. [Ca2+]cyt increased first soon after pollen hydration, with a second increase occurring after pollen protrusion. The third and most remarkable [Ca2+]cyt increase took place when the pollen tube penetrated into the papilla cell wall.

BslA(YuaB) forms a hydrophobic layer on the surface of Bacillus subtilis biofilms

Biofilms are surface‐associated bacterial aggregates, in which bacteria are enveloped by polymeric substances known as the biofilm matrix. Bacillus subtilis biofilms display persistent resistance to liquid wetting and gas penetration, which probably explains the broad‐spectrum resistance of the bacteria in these biofilms to antimicrobial agents. In this study, BslA (formerly YuaB) was identified as a major contributor to the surface repellency of B. subtilis biofilms. Disruption of bslA resulted in the loss of surface repellency and altered the biofilm surface microstructure. BslA localized to the biofilm matrix in an exopolysaccharide‐dependent manner. Purified BslA exhibited amphiphilic properties and formed polymers in response to increases in the area of the air–water interface in vitro. Genetic and biochemical analyses showed that the self‐polymerization activity of BslA was essential for its ability to localize to the biofilm matrix. Confocal laser scanning microscopy showed that BslA formed a layer on the biofilm surface. Taken together, we propose that BslA, standing for biofilm‐surface layer protein, is responsible for the hydrophobic layer on the surface of biofilms.

Helical microtubule arrays in a collection of twisting tubulin mutants of Arabidopsis thaliana

Anisotropic expansion of plant cells requires organized arrays of cortical microtubules. Mutations in microtubule-associated proteins and a particular mutation in α-tubulins were reported to cause abnormal microtubule arrays and result in helical growth in Arabidopsis thaliana. However, the way in which these mutations affect the organization of microtubules remains unknown. We here identified 32 Arabidopsis twisting mutants that have either missense or amino acid deletion mutations in α- or β-tubulins. Mutations were mapped to the GTPase-activating region in α-tubulin, intra- and interdimer interfaces of tubulin heterodimers, and lateral contact regions among adjacent protofilaments. These dominant-negative tubulin mutants were incorporated into the microtubule polymer and formed shallow helical arrays of distinct handedness along the long axis of the root epidermal cells. A striking correlation exists between the direction in which cortical helical arrays are skewed and the growth direction of elongating roots. The GTPase-activating-region mutant had left-handed helical arrays composed of highly stabilized microtubules, which could be decorated along the entire microtubule lattices with the otherwise tip-localized End Binding 1 protein. A mutation at the intradimer interface, on the other hand, generated highly dynamic microtubules and right-handed helical arrays. Cortical microtubules in wild type and these two tubulin mutants were composed mainly of 13 protofilaments. This comprehensive analysis of tubulin mutations provides insights into the mechanism by which tubulin structures influence microtubule dynamics and organization.

The Dominance of Alleles Controlling Self-Incompatibility in Brassica Pollen Is Regulated at the RNA Level

Self-incompatibility (SI) in Brassica is controlled sporophytically by the multiallelic S-locus. The SI phenotype of pollen in an S-heterozygote is determined by the relationship between the two S-haplotypes it carries, and dominant/recessive relationships often are observed between the two S-haplotypes. The S-locus protein 11 (SP11, also known as the S-locus cysteine-rich protein) gene has been cloned from many pollen-dominant S-haplotypes (class I) and shown to encode the pollen S-determinant. However, SP11 from pollen-recessive S-haplotypes (class II) has never been identified by homology-based cloning strategies, and how the dominant/recessive interactions between the two classes occur was not known. We report here the identification and molecular characterization of SP11s from six class II S-haplotypes of B. rapa and B. oleracea. Phylogenetic analysis revealed that the class II SP11s form a distinct group separated from class I SP11s. The promoter sequences and expression patterns of SP11s also were different between the two classes. The mRNA of class II SP11, which was detected predominantly in the anther tapetum in homozygotes, was not detected in the heterozygotes of class I and class II S-haplotypes, suggesting that the dominant/recessive relationships of pollen are regulated at the mRNA level of SP11s.

Two Distinct Forms of M-Locus Protein Kinase Localize to the Plasma Membrane and Interact Directly with S-Locus Receptor Kinase to Transduce Self-Incompatibility Signaling in Brassica rapa

Many flowering plants possess systems of self-incompatibility (SI) to prevent inbreeding. In Brassica, SI recognition is controlled by the multiallelic gene complex (S-haplotypes) at the S-locus, which encodes both the male determinant S-locus protein 11 (SP11/SCR) and the female determinant S-receptor kinase (SRK). Upon self-pollination, the S-haplotype–specific interaction between the pollen-borne SP11 and the cognate stigmatic SRK receptor induces SI signaling in the stigmatic papilla cell and results in rejection of the self-pollen. Our genetic analysis of a self-compatible mutant revealed the involvement of a cytoplasmic protein kinase, M-locus protein kinase (MLPK), in the SI signaling, but its exact physiological function remains unknown. In this study, we identified two different MLPK transcripts, MLPKf1 and MLPKf2, which are produced using alternative transcriptional initiation sites and encode two isoforms that differ only at the N termini. While MLPKf1 and MLPKf2 exhibited distinct expression profiles, both were expressed in papilla cells. MLPKf1 localizes to the plasma membrane through its N-terminal myristoylation motif, while MLPKf2 localizes to the plasma membrane through its N-terminal hydrophobic region. Although both MLPKf1 and MLPKf2 could independently complement the mlpk/mlpk mutation, their mutant forms that lack the plasma membrane localization motifs failed to complement the mutation. Furthermore, a bimolecular fluorescence complementation assay revealed direct interactions between SRK and the MLPK isoforms in planta. These results suggest that MLPK isoforms localize to the papilla cell membrane and interact directly with SRK to transduce SI signaling.