The chloroplastic DEVH‐box RNA helicase INCREASED SIZE EXCLUSION LIMIT 2 involved in plasmodesmata regulation is required for group II intron splicing

INCREASED SIZE EXCLUSION LIMIT 2 (ISE2) encodes a putative DEVH‐box RNA helicase originally identified through a genetic screening for Arabidopsis mutants altered in plasmodesmata (PD) aperture. Depletion of ISE2 also affects chloroplasts activity, decreases accumulation of photosynthetic pigments and alters expression of photosynthetic genes. In this work, we show the chloroplast localization of ISE2 and decipher its role in plastidic RNA processing and, consequently, PD function. Group II intron‐containing RNAs from chloroplasts exhibit defective splicing in ise2 mutants and ISE2‐silenced plants, compromising plastid viability. Furthermore, RNA immunoprecipitation suggests that ISE2 binds in vivo to several splicing‐regulated RNAs. Finally, we show that the chloroplast clpr2 mutant (defective in a subunit of a plastidic Clp protease) also exhibits abnormal PD function during embryogenesis, supporting the idea that chloroplast RNA processing is required to regulate cell–cell communication in plants.     

The Skp1‐like protein SSK1 is required for cross‐pollen compatibility in S‐RNase‐based self‐incompatibility

The self‐incompatibility (SI) response occurs widely in flowering plants as a means of preventing self‐fertilization. In these self/non‐self discrimination systems, plant pistils reject self or genetically related pollen. In the Solanaceae, Plantaginaceae and Rosaceae, pistil‐secreted S‐RNases enter the pollen tube and function as cytotoxins to specifically arrest self‐pollen tube growth. Recent studies have revealed that the S‐locus F‐box (SLF) protein controls the pollen expression of SI in these families. However, the precise role of SLF remains largely unknown. Here we report that PhSSK1 (Petunia hybrida SLF‐interacting Skp1‐like1), an equivalent of AhSSK1 of Antirrhinum hispanicum, is expressed specifically in pollen and acts as an adaptor in an SCF(Skp1‐Cullin1‐F‐box)^SLF complex, indicating that this pollen‐specific SSK1‐SLF interaction occurs in both Petunia and Antirrhinum, two species from the Solanaceae and Plantaginaceae, respectively. Substantial reduction of PhSSK1 in pollen reduced cross‐pollen compatibility (CPC) in the S‐RNase‐based SI response, suggesting that the pollen S determinant contributes to inhibiting rather than protecting the S‐RNase activity, at least in solanaceous plants. Furthermore, our results provide an example that a specific Skp1‐like protein other than the known conserved ones can be recruited into a canonical SCF complex as an adaptor.     

A stearoyl‐acyl carrier protein desaturase,NbSACPD‐C,is critical for ovule development in Nicotiana benthamiana

Stearoyl‐acyl carrier protein desaturase (SACPD) activity is essential for production of the major unsaturated fatty acids (UFAs) in plant lipids. We report here the characterization of three SACPD genes from Nicotiana benthamiana, NbSACPD‐A, ‐B, and ‐C. All three genes share high similarity to AtSSI2/FAB2 (Suppressor of Salicylic acid‐Insensitivity2/Fatty Acid Biosynthesis2), the primary SACPD isoform in Arabidopsis. Knocking down the expression of individual or combinations of NbSACPDs by an artificial microRNA approach resulted in significantly reduced accumulation of 18C UFAs and elevated levels of 18:0‐FA (Fatty acids) in leaves, indicating that all three genes participated in fatty acid desaturation. The triple knockdown (KD) plants displayed severe growth phenotypes, including spontaneous cell death and dwarfing. While no vegetative morphologic abnormality was observed in NbSACPD‐A, ‐B, or ‐C KD plants, strikingly, NbSACPD‐C KD plants produced small fruits with aborted ovules. Reciprocal crosses with wild‐type and NbSACPD‐C KD plants revealed that knocking down NbSACPD‐C expression caused female, but not male, sterility. Furthermore, arrested ovule development and significantly altered lipid composition in ovaries were observed in NbSACPD‐C KD plants, consistent with the predominant NbSACPD‐C expression in ovules. The ovule development defect was fully complemented by coexpressing an amiRNA‐resistant NbSACPD‐C variant in the NbSACPD‐C KD background, further supporting a specific requirement for NbSACPD‐C in female fertility. Our results thus indicated that NbSACPD‐C plays a critical role maintaining membrane lipid composition in ovule development for female fertility in N. benthamiana, complementing and extending prior understanding on the well‐demonstrated roles of SACPDs in biotic and abiotic stresses.     

PtaRHE1, a Populus tremula × Populus alba RING‐H2 protein of the ATL family,has a regulatory role in secondary phloem fibre development

REALLY INTERESTING NEW GENE (RING) proteins play important roles in the regulation of many processes by recognizing target proteins for ubiquitination. Previously, we have shown that the expression of PtaRHE1, encoding a Populus tremula × Populus alba RING‐H2 protein with E3 ubiquitin ligase activity, is associated with tissues undergoing secondary growth. To further elucidate the role of PtaRHE1 in vascular tissues, we have undertaken a reverse genetic analysis in poplar. Within stem secondary vascular tissues, PtaRHE1 and its corresponding protein are expressed predominantly in the phloem. The downregulation of PtaRHE1 in poplar by artificial miRNA triggers alterations in phloem fibre patterning, characterized by an increased portion of secondary phloem fibres that have a reduced cell wall thickness and a change in lignin composition, with lower levels of syringyl units as compared with wild‐type plants. Following an RNA‐seq analysis, a biological network involving hormone stress signalling, as well as developmental processes, could be delineated. Several candidate genes possibly associated with the altered phloem fibre phenotype observed in amiRPtaRHE1 poplar were identified. Altogether, our data suggest a regulatory role for PtaRHE1 in secondary phloem fibre development.     

Phosphatidylinositol‐specific phospholipase C2 functions in auxin‐modulated root development

Nine phosphatidylinositol‐specific phospholipases C (PLCs) have been identified in the Arabidopsis genome; among the importance of PLC2 in reproductive development is significant. However, the role of PLC2 in vegetative development such as in root growth is elusive. Here, we report that plc2 mutants displayed multiple auxin‐defective phenotypes in root development, including short primary root, impaired root gravitropism, and inhibited root hair growth. The DR5:GUS expression and the endogenous indole‐3‐acetic acid (IAA) content, as well as the responses of a set of auxin‐related genes to exogenous IAA treatment, were all decreased in plc2 seedlings, suggesting the influence of PLC2 on auxin accumulation and signalling. The root elongation of plc2 mutants was less sensitive to the high concentration of exogenous auxins, and the application of 1‐naphthaleneacetic acid or the auxin transport inhibitor N‐1‐naphthylphthalamic acid could rescue the root hair growth of plc2 mutants. In addition, the PIN2 polarity and cycling in plc2 root epidermis cells were altered. These results demonstrate a critical role of PLC2 in auxin‐mediated root development in Arabidopsis, in which PLC2 influences the polar distribution of PIN2.     

A cellulose synthase‐like protein is required for osmotic stress tolerance in Arabidopsis

Osmotic stress imposed by soil salinity and drought stress significantly affects plant growth and development, but osmotic stress sensing and tolerance mechanisms are not well understood. Forward genetic screens using a root‐bending assay have previously identified salt overly sensitive (sos) mutants of Arabidopsis that fall into five loci, SOS1 to SOS5. These loci are required for the regulation of ion homeostasis or cell expansion under salt stress, but do not play a major role in plant tolerance to the osmotic stress component of soil salinity or drought. Here we report an additional sos mutant, sos6‐1, which defines a locus essential for osmotic stress tolerance. sos6‐1 plants are hypersensitive to salt stress and osmotic stress imposed by mannitol or polyethylene glycol in culture media or by water deficit in the soil. SOS6 encodes a cellulose synthase‐like protein, AtCSLD5. Only modest differences in cell wall chemical composition could be detected, but we found that sos6‐1 mutant plants accumulate high levels of reactive oxygen species (ROS) under osmotic stress and are hypersensitive to the oxidative stress reagent methyl viologen. The results suggest that SOS6/AtCSLD5 is not required for normal plant growth and development but has a critical role in osmotic stress tolerance and this function likely involves its regulation of ROS under stress.     

Inhibition of fucosylation of cell wall components by 2‐fluoro 2‐deoxy‐l‐fucose induces defects in root cell elongation

Screening of commercially available fluoro monosaccharides as putative growth inhibitors in Arabidopsis thaliana revealed that 2‐fluoro 2‐l ‐fucose (2F‐Fuc) reduces root growth at micromolar concentrations. The inability of 2F‐Fuc to affect an Atfkgp mutant that is defective in the fucose salvage pathway indicates that 2F‐Fuc must be converted to its cognate GDP nucleotide sugar in order to inhibit root growth. Chemical analysis of cell wall polysaccharides and glycoproteins demonstrated that fucosylation of xyloglucans and of N‐linked glycans is fully inhibited by 10 μm 2F‐Fuc in Arabidopsis seedling roots, but genetic evidence indicates that these alterations are not responsible for the inhibition of root development by 2F‐Fuc. Inhibition of fucosylation of cell wall polysaccharides also affected pectic rhamnogalacturonan‐II (RG‐II). At low concentrations, 2F‐Fuc induced a decrease in RG‐II dimerization. Both RG‐II dimerization and root growth were partially restored in 2F‐Fuc‐treated seedlings by addition of boric acid, suggesting that the growth phenotype caused by 2F‐Fuc was due to a deficiency of RG‐II dimerization. Closer investigation of the 2F‐Fuc‐induced growth phenotype demonstrated that cell division is not affected by 2F‐Fuc treatments. In contrast, the inhibitor suppressed elongation of root cells and promoted the emergence of adventitious roots. This study further emphasizes the importance of RG‐II in cell elongation and the utility of glycosyltransferase inhibitors as new tools for studying the functions of cell wall polysaccharides in plant development. Moreover, supplementation experiments with borate suggest that the function of boron in plants might not be restricted to RG‐II cross‐linking, but that it might also be a signal molecule in the cell wall integrity‐sensing mechanism.