TY - JOUR
AU - Minorsky, Peter V.
AB - Nitrate Transporters and Growth To acquire nitrate efficiently from various soils, plants have evolved two nitrate uptake systems: a low-affinity nitrate transport system and a high-affinity nitrate transport system. Three NRT2 transporters (NRT2.1, NRT2.2, and NRT2.4) and two NRT1 transporters (CHL1 and NRT1.2) are involved in nitrate uptake (CHL1 is so named because when mutated it confers resistance to the herbicide chlorate). Although most of the NRT1 nitrate transporters are low-affinity nitrate transporters, CHL1 (NRT1.1) is distinct in that it is a dual-affinity nitrate transporter involved in both the low-affinity nitrate transport system and the high-affinity nitrate transport system. Moreover, CHL1 also functions as a nitrate sensor responsible for the nitrate-regulated transcriptional response. Hsu and Tsay (pp. 844–856) show that two NRT1 nitrate transporters, NRT1.11 and NRT1.12 in Arabidopsis (Arabidopsis thaliana), mediate the xylem-to-phloem transfer of nitrate and participate in nitrate redistribution from mature and larger expanded leaves to the youngest tissues. Functional analysis in Xenopus laevis oocytes showed that both NRT1.11 and NRT1.12 are low-affinity nitrate transporters. Further measurements revealed that they are expressed preferentially in expanded leaves and serve as plasma membrane transporters in the companion cells of the major vein. In nrt1.11 nrt1.12 double mutants, root-fed 15NO3 was translocated preferentially to expanded leaves but less to the youngest tissues, suggesting that NRT1.11 and NRT1.12 are required for transferring root-derived nitrate into phloem in the major veins of mature leaves for redistribution to the youngest tissues. Distinct from the wild type, nrt1.11 nrt1.12 double mutants show no increase of plant growth at high nitrate supply. These data suggest that NRT1.11 and NRT1.12 are involved in xylem-to-phloem transfer for the purpose of redistributing nitrate into developing leaves and that nitrate redistribution of this sort is a critical step for optimal plant growth enhanced by increasing external nitrate. Tetraspanins in Reproductive Tissues Tetraspanins are evolutionary conserved transmembrane proteins present in all multicellular organisms. Tetraspanins are known to act as membrane organizers, interacting with other tetraspanins and recruiting binding partners, such as key adhesion molecules and signaling receptors, which assemble in macromolecular membrane microdomains termed tetraspanin-enriched microdomain complexes or “tetraspanin webs.” In animals, they facilitate diverse biological processes, such as cell proliferation, movement, adhesion, and fusion. The genome of Arabidopsis encodes 17 members of the tetraspanin family; however, little is known about their functions in plant development. Boavida et al. (pp. 696–712) have analyzed their phylogeny, protein topology, and domain structure and surveyed their expression and localization patterns in reproductive tissues. They show that, despite their low sequence identity with metazoan tetraspanins, plant tetraspanins display the typical structural topology and most signature features of tetraspanins in other multicellular organisms. Using bioinformatics and a survey of reproductive tissue-related microarray data sets, the authors conclude that Arabidopsis tetraspanins show a distinct and often overlapping expression at the cell surface of specific reproductive cell types and tissue domains, and this expression is responsive to pollination. Comparable to tetraspanins in metazoans, Arabidopsis tetraspanins can assemble in homo- and heterodimers when expressed in yeast. These results indicate that plant tetraspanins, despite their evolutionary divergence from other eukaryotic tetraspanins, have maintained molecular features that are functionally relevant in the context of intercellular interactions and, moreover, that tetraspanins may be important in plant reproductive development. A Regulator of Programmed Cell Death Programmed cell death (PCD) plays important roles in plant development and defense. The most studied PCD process in plants is the hypersensitive response (HR) to avirulent biotrophic pathogens. The HR is characterized by the rapid death of cells in the local region surrounding an infection to restrict the growth and spread of pathogens to other parts of the plant. The HR is triggered by the plant when it recognizes a pathogen and is accompanied by the accumulations of specific signaling molecules, including ion fluxes, reactive oxygen species (ROS), salicylic acid, and reactive nitrogen intermediates. The HR not only induces the local response but also systemic acquired resistance. In Arabidopsis, many lesion mimic mutants have been isolated that show various defects in regulating PCD. One of the best-characterized mutants is lesion-simulating disease1 (lsd1). The lsd1 mutant shows abnormal cell death triggered by ROS and salicylic acid and presents a runaway cell death phenotype under long photoperiods or after low-titer avirulent pathogen infection, indicating that LSD1 is a negative regulator of PCD. LSD1 encodes a novel zinc finger protein with three LSD1-like zinc finger motifs, but little is known about the biochemical activity of the LSD1 protein. Li et al. (pp. 1059–1070) report the identification of CATALASE3 (CAT3) as an LSD1-interacting protein by affinity purification and mass spectrometry-based proteomic analysis. The Arabidopsis genome contains three homologous catalase genes (CAT1, CAT2, and CAT3). Yeast two-hybrid and coimmunoprecipitation analyses demonstrated that LSD1 interacted with all three catalases both in vitro and in vivo, and the interaction required the zinc fingers of LSD1. The catalase enzymatic activity was reduced in the lsd1 mutant, indicating that the catalase enzyme activity was partially dependent on LSD1. The lsd1 mutant was more sensitive to a specific catalase inhibitor than the wild type, suggesting that the interaction between LSD1 and catalases is involved in the regulation of the ROS produced in the peroxisome. These results suggest that LSD1-catalase interactions play an important role in regulating PCD in Arabidopsis. MADS-box Inhibitor of Tomato Fruit Ripening MADS-box genes encode transcriptional factors that regulate numerous developmental processes in plants. Dong et al. (pp. 1026–1036) have cloned and analyzed the tissue-specific expression of a tomato (Solanum lycopersicum) MADS-box gene called SlMADS1. This gene was highly expressed in sepals and fruits. Its expression level increased during sepal development, whereas the transcript of SlMADS1 decreased during fruit ripening. To further explore the function of SlMADS1, an RNA interference (RNAi) expression vector targeting SlMADS1 was constructed and transformed into tomato plants. The authors observed a shorter ripening time in SlMADS1-silenced tomatoes. Various ethylene biosynthetic genes and ethylene-responsive genes involved in fruit ripening were up-regulated in silenced plants. SlMADS1 RNAi fruits showed approximately 2- to 4-fold increases in ethylene production compared with the wild type. These results suggest that SlMADS1 plays an important role in fruit ripening as a repressive modulator. Although other MADS-box transcription factors have been reported to be involved in ripening, unlike SlMADS1, they are all positive regulators of ripening. Desiccation Tolerance and Longevity in Medicago truncatula Anhydrobiosis is an important factor in the preservation of seed viability and quality during dry storage, but little is known about the regulatory networks underlying seed survival in the dry state. To survive in the dry state, seeds first acquire desiccation tolerance, which occurs for most species during seed filling. Subsequently, during late maturation, seeds progressively acquire longevity, the ability to remain alive in the dry state for extended periods of time. The relative timing between the progressive increase in seed longevity and the end of seed reserve accumulation, seed desiccation, and fruit abscission varies between species. During seed maturation in the model legume M. truncatula, longevity starts once seed reserve accumulation is terminated and increases progressively over 30-fold during a 2-week maturation phase. Subsequently, a final desiccation step occurs concomitantly with pod abscission. Using a combination of physiology, metabolomic, and transcriptomic approaches, Verdier et al. (pp. 757–774) constructed and partially validated a genetic regulatory network that revealed distinct coexpression modules related to the acquisition of desiccation tolerance, longevity, and pod abscission. The acquisition of desiccation tolerance and dormancy module was associated with abiotic stress response genes, including late-embryogenesis abundant genes. The longevity module was enriched in genes involved in RNA processing and translation. During maturation, gulose and stachyose levels increased and were determined to be correlated with longevity. A survey of seed-specific transcription factors identified a set of potential transcriptional regulators involved in the acquisition of desiccation tolerance and longevity that will facilitate future investigations in the understanding of seed quality acquisition. Author notes www.plantphysiol.org/cgi/doi/10.1104/pp.113.900473 © 2013 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - On the Inside
JO - Plant Physiology
DO - 10.1104/pp.113.900473
DA - 2013-10-08
UR - https://www.deepdyve.com/lp/oxford-university-press/on-the-inside-i4xri0BLK3
SP - 635
EP - 636
VL - 163
IS - 2
DP - DeepDyve
ER -