The Plant Journal

Nakagawa, Tomomi; Kaku, Hanae; Shimoda, Yoshikazu; Sugiyama, Akifumi; Shimamura, Masayuki; Takanashi, Kojiro; Yazaki, Kazufumi; Aoki, Toshio; Shibuya, Naoto; Kouchi, Hiroshi

doi: 10.1111/j.1365-313X.2010.04411.xpmid: 21223383

Nitrogen‐fixing symbiosis between legumes and rhizobia is initiated by the recognition of rhizobial Nod factors (NFs) by host plants. NFs are diversely modified derivatives of chitin oligosaccharide, a fungal elicitor that induces defense responses in plants. Recent evidence has shown that both NFs and chitin elicitors are recognized by structurally related LysM receptor kinases. Transcriptome analyses of Lotus japonicus roots indicated that NFs not only activate symbiosis genes but also transiently activate defense‐related genes through NF receptors. Conversely, chitin oligosaccharides were able to activate symbiosis genes independently of NF receptors. Analyses using chimeric genes consisting of the LysM receptor domain of a Lotus japonicus NF receptor, NFR1, and the kinase domain of an Arabidopsis chitin receptor, CERK1, demonstrated that substitution of a portion of the αEF helix in CERK1 with the amino acid sequence YAQ from the corresponding region of NFR1 enables L. japonicus nfr1 mutants to establish symbiosis with Mesorhizobium loti. We also showed that the kinase domains of two Lotus japonicus LysM receptor kinases, Lys6 and Lys7, which also possess the YAQ sequence, suppress the symbiotic defect of nfr1. These results strongly suggest that, in addition to adaptation of extracellular LysM domains to NFs, limited alterations in the kinase domain of chitin receptors have played a crucial role in shifting the intracellular signaling to symbiosis from defense responses, thus constituting one of the key genetic events in the evolution of root nodule symbiosis in legume plants.

Choi, Hyunju; Jin, Jun‐Young; Choi, Setbyoul; Hwang, Jae‐Ung; Kim, Yu‐Young; Suh, Mi Chung; Lee, Youngsook

doi: 10.1111/j.1365-313X.2010.04412.xpmid: 21223384

The exine of the pollen wall shows an intricate pattern, primarily comprising sporopollenin, a polymer of fatty acids and phenolic compounds. A series of enzymes synthesize sporopollenin precursors in tapetal cells, and the precursors are transported from the tapetum to the pollen surface. However, the mechanisms underlying the transport of sporopollenin precursors remain elusive. Here, we provide evidence that strongly suggests that the Arabidopsis ABC transporter ABCG26/WBC27 is involved in the transport of sporopollenin precursors. Two independent mutations at ABCG26 coding region caused drastic decrease in seed production. This defect was complemented by expression of ABCG26 driven by its native promoter. The severely reduced fertility of the abcg26 mutants was caused by a failure to produce mature pollen, observed initially as a defect in pollen‐wall development. The reticulate pattern of the exine of wild‐type microspores was absent in abcg26 microspores at the vacuolate stage, and the vast majority of the mutant pollen degenerated thereafter. ABCG26 was expressed specifically in tapetal cells at the early vacuolate stage of pollen development. It showed high co‐expression with genes encoding enzymes required for sporopollenin precursor synthesis, i.e. CYP704B1, ACOS5, MS2 and CYP703A2. Similar to two other mutants with defects in pollen‐wall deposition, abcg26 tapetal cells accumulated numerous vesicles and granules. Taken together, these results suggest that ABCG26 plays a crucial role in the transfer of sporopollenin lipid precursors from tapetal cells to anther locules, facilitating exine formation on the pollen surface.

Park, Jong‐Jin; Yi, Jakyung; Yoon, Jinmi; Cho, Lae‐Hyeon; Ping, Jin; Jeong, Hee Joong; Cho, Seok Keun; Kim, Woo Taek; An, Gynheung

doi: 10.1111/j.1365-313X.2010.04416.xpmid: 21223385

The plant U‐box (PUB) protein functions as an E3 ligase to poly‐ubiquitinate a target protein for its degradation or post‐translational modification. Here, we report functional roles for OsPUB15, which encodes a cytosolic U‐box protein in the class‐II PUB family. Self‐ubiquitination assays showed that bacterially expressed MBP‐OsPUB15 protein has E3 ubiquitin ligase activity. A T‐DNA insertional mutation in OsPUB15 caused severe growth retardation and a seedling‐lethal phenotype. Mutant seeds did not produce primary roots, and their shoot development was significantly delayed. Transgenic plants expressing the OsPUB15 antisense transcript phenocopied these mutant characters. The abnormal phenotypes were partially rescued by two antioxidants, catechin and ascorbic acid. Germinating seeds in the dark also recovered the rootless defect. Levels of H2O2 and oxidized proteins were higher in the knock‐out mutant compared with the wild type. OsPUB15 transcript levels were increased upon H2O2, salt and drought stresses; plants overexpressing the gene grew better than the wild type under high salinity. These results indicate that PUB15 is a regulator that reduces reactive oxygen species (ROS) stress and cell death.

Yang, Wenyu; Cahoon, Rebecca E.; Hunter, Sarah C.; Zhang, Chunyu; Han, Jixiang; Borgschulte, Trissa; Cahoon, Edgar B.

doi: 10.1111/j.1365-313X.2010.04417.xpmid: 21223386

The biosynthesis of the tocotrienol and tocopherol forms of vitamin E is initiated by prenylation of homogentisate. Geranylgeranyl diphosphate (GGDP) is the prenyl donor for tocotrienol synthesis, whereas phytyl diphosphate (PDP) is the prenyl donor for tocopherol synthesis. We have previously shown that tocotrienol synthesis is initiated in monocot seeds by homogentisate geranylgeranyl transferase (HGGT). This enzyme is related to homogentisate phytyltransferase (HPT), which catalyzes the prenylation step in tocopherol synthesis. Here we show that monocot HGGT is localized in the plastid and expressed primarily in seed endosperm. Despite the close structural relationship of monocot HGGT and HPT, these enzymes were found to have distinct substrate specificities. Barley (Hordeum vulgare cv. Morex) HGGT expressed in insect cells was six times more active with GGDP than with PDP, whereas the Arabidopsis HPT was nine times more active with PDP than with GGDP. However, only small differences were detected in the apparent Km values of barley HGGT for GGDP and PDP. Consistent with its in vitro substrate properties, barley HGGT generated a mixture of tocotrienols and tocopherols when expressed in the vitamin E‐null vte2‐1 mutant lacking a functional HPT. Relative levels of tocotrienols and tocopherols produced in vte2‐1 differed between organs and growth stages, reflective of the composition of plastidic pools of GGDP and PDP. In addition, HGGT was able to functionally substitute for HPT to rescue vte2‐1‐associated phenotypes, including reduced seed viability and increased fatty acid oxidation of seed lipids. Overall, we show that monocot HGGT is biochemically distinct from HPT, but can replace HPT in important vitamin E‐related physiological processes.

Marshall, Richard S.; D’Avila, Francesca; Di Cola, Alessandra; Traini, Roberta; Spanò, Laura; Fabbrini, Maria Serena; Ceriotti, Aldo

doi: 10.1111/j.1365-313X.2010.04413.xpmid: 21223387

The fate of the type I ribosome‐inactivating protein (RIP) saporin when initially targeted to the endoplasmic reticulum (ER) in tobacco protoplasts has been examined. We find that saporin expression causes a marked decrease in protein synthesis, indicating that a fraction of the toxin reaches the cytosol and inactivates tobacco ribosomes. We determined that saporin is largely secreted but some is retained intracellularly, most likely in a vacuolar compartment, thus behaving very differently from the prototype RIP ricin A chain. We also find that the signal peptide can interfere with the catalytic activity of saporin when the protein fails to be targeted to the ER membrane, and that saporin toxicity undergoes signal sequence‐specific regulation when the host cell is subjected to ER stress. Replacement of the saporin signal peptide with that of the ER chaperone BiP reduces saporin toxicity and makes it independent of cell stress. We propose that this stress‐induced toxicity may have a role in pathogen defence.

Riely, Brendan K.; He, Hengbin; Venkateshwaran, Muthusubramanian; Sarma, Birinchi; Schraiber, Joshua; Ané, Jean‐Michel; Cook, Douglas R.

doi: 10.1111/j.1365-313X.2010.04414.xpmid: 21223388

Root hairs play important roles in the interaction of plants with their environment. Root hairs anchor the plant in the soil, facilitate nutrient uptake from the rhizosphere, and participate in symbiotic plant–microbe interactions. These specialized cells grow in a polar fashion which gives rise to their elongated shape, a process mediated in part by a family of small GTPases known as Rops. RopGEFs (GEF, guanine nucleotide exchange factor) activate Rops to effect tip growth in Arabidopsis pollen and root hairs, but the genes mediating tip growth in legumes have not yet been characterized. In this report we describe the Rop and RopGEF gene families from the model legume Medicago truncatula and from the crop legume soybean. We find that one member of the M. truncatula gene family, MtRopGEF2, is required for root hair development because silencing this gene by RNA interference affects the cytosolic Ca2+ gradient and subcellular structure of root hairs, and reduces root hair growth. Consistent with its role in polar growth, we find that a GFP::MtRopGEF2 fusion protein localizes in the apex of emerging and actively growing root hairs. The amino terminus of MtRopGEF2 regulates its ability to interact with MtRops in yeast, and regulates its biological activity in vivo.

Murray, Jeremy D.; Muni, RajaSekhara Reddy Duvvuru; Torres‐Jerez, Ivone; Tang, Yuhong; Allen, Stacy; Andriankaja, Megan; Li, Guangming; Laxmi, Ashverya; Cheng, Xiaofei; Wen, Jiangqi; Vaughan, David; Schultze, Michael; Sun, Jongho; Charpentier, Myriam;

Lin, Pei‐Chi; Pomeranz, Marcelo C.; Jikumaru, Yusuke; Kang, Shin Gene; Hah, Cyrus; Fujioka, Shozo; Kamiya, Yuji; Jang, Jyan‐Chyun

doi: 10.1111/j.1365-313X.2010.04419.xpmid: 21223390

Tandem zinc finger (TZF) proteins are characterized by two zinc‐binding CCCH motifs arranged in tandem. Human TZFs such as tristetraproline (TTP) bind to and trigger the degradation of mRNAs encoding cytokines and various regulators. Although the molecular functions of plant TZFs are unknown, recent genetic studies have revealed roles in hormone‐mediated growth and environmental responses, as well as in the regulation of gene expression. Here we show that expression of AtTZF1 (AtCTH/AtC3H23) mRNA is repressed by a hexokinase‐dependent sugar signaling pathway. However, AtTZF1 acts as a positive regulator of ABA/sugar responses and a negative regulator of GA responses, at least in part by modulating gene expression. RNAi of AtTZF1–3 caused early germination and slightly stress‐sensitive phenotypes, whereas plants over‐expressing AtTZF1 were compact, late flowering and stress‐tolerant. The developmental phenotypes of plants over‐expressing AtTZF1 were only partially rescued by exogenous application of GA, implying a reduction in the GA response or defects in other mechanisms. Likewise, the enhanced cold and drought tolerance of plants over‐expressing AtTZF1 were not associated with increased ABA accumulation, suggesting that it is mainly ABA responses that are affected. Consistent with this notion, microarray analysis showed that over‐expression of AtTZF1 mimics the effects of ABA or GA deficiency on gene expression. Notably, a gene network centered on a GA‐inducible and ABA/sugar‐repressible putative peptide hormone encoded by GASA6 was severely repressed by AtTZF1 over‐expression. Hence AtTZF1 may serve as a regulator connecting sugar, ABA, GA and peptide hormone responses.

Szakonyi, Dóra; Byrne, Mary E.

doi: 10.1111/j.1365-313X.2010.04422.xpmid: 21223391

Ribosomal proteins are integral to ribosome biogenesis, and function in protein synthesis. In higher eukaryotes, loss of cytoplasmic ribosomal proteins results in a reduced growth rate as well as developmental defects. To what extent and how ribosomal proteins affect development is currently not known. Here we describe a semi‐dominant mutation in the cytoplasmic ribosomal protein gene RPL27aC that affects multiple aspects of plant shoot development, including leaf patterning, inflorescence and floral meristem function, and seed set. In the embryo, RPL27aC is required to maintain the growth rate and for the transition from radial to bilateral symmetry associated with initiation of cotyledons. rpl27ac‐1d embryos undergo stereotypical patterning to establish a globular embryo. However, a temporal delay in initiation and outgrowth of cotyledon primordia leads to development of an enlarged globular embryo prior to apical domain patterning. Defects in embryo development are coincident with tissue‐specific ectopic expression of the shoot meristem genes SHOOT MERISTEMLESS (STM) and CUP‐SHAPED COTYLEDON2 (CUC2), in addition to delayed expression of the abaxial gene FILAMENTOUS FLOWER (FIL) and mis‐regulation of the auxin efflux effector PIN‐FORMED1 (PIN1). Genetic interactions with other ribosomal protein mutants indicate that RPL27aC is a component of the ribosome. We propose that RPL27aC regulates discrete developmental events by controlling spatial and temporal expression of developmental patterning genes via an as yet undefined process involving the ribosome.

Effendi, Yunus; Rietz, Steffen; Fischer, Urs; Scherer, Günther F. E.

doi: 10.1111/j.1365-313X.2010.04420.xpmid: 21223392

AUXIN‐BINDING PROTEIN 1 (ABP1) is not easily accessible for molecular studies because the homozygous T‐DNA insertion mutant is embryo‐lethal. We found that the heterozygous abp1/ABP1 insertion mutant has defects in auxin physiology‐related responses: higher root slanting angles, longer hypocotyls, agravitropic roots and hypocotyls, aphototropic hypocotyls, and decreased apical dominance. Heterozygous plants flowered earlier than wild‐type plants under short‐day conditions. The length of the main root, the lateral root density and the hypocotyl length were little altered in the mutant in response to auxin. Compared to wild‐type plants, transcription of early auxin‐regulated genes (IAA2, IAA11, IAA13, IAA14, IAA19, IAA20, SAUR9, SAUR15, SAUR23, GH3.5 and ABP1) was less strongly up‐regulated in the mutant by 0.1, 1 and 10 μm IAA. Surprisingly, ABP1 was itself an early auxin‐up‐regulated gene. IAA uptake into the mutant seedlings during auxin treatments was indistinguishable from wild‐type. Basipetal auxin transport in young roots was slower in the mutant, indicating a PIN2/EIR1 defect, while acropetal transport was indistinguishable from wild‐type. In the eir1 background, three of the early auxin‐regulated genes tested (IAA2, IAA13 and ABP1) were more strongly induced by 1 μm IAA in comparison to wild‐type, but eight of them were less up‐regulated in comparison to wild‐type. Similar but not identical disturbances in regulation of early auxin‐regulated genes indicate tight functional linkage of ABP1 and auxin transport regulation. We hypothesize that ABP1 is involved in the regulation of polar auxin transport, and thus affects local auxin concentration and early auxin gene regulation. In turn, ABP1 itself is under the transcriptional control of auxin.