On the Inside

Peter V. Minorsky

doi:10.1104/pp.114.900489

Published July 2014. DOI: https://doi.org/10.1104/pp.114.900489

Reactive Oxygen Species and Lateral Root Development

Most of a plant’s overall root architecture is generated by the de novo formation of lateral roots (LRs). These lateral organs are specified at regular intervals along the main root from a limited number of pericycle cells, called founder cells. One of the main questions for root biologists is how plants control the number of lateral root primordia (LRP) and their emergence through the main root. Because very few pericycle cells contribute to LR formation, an LR-inducible system has been developed previously that allows synchronized LRP formation along the entire pericycle. More recently, cell sorting of GFP-labeled root cells coupled with transcriptomics have contributed to our understanding of LRP development. To further refine these existing transcriptomic data sets by focusing only on those cells that actually participate in LRP development, Manzano et al. (pp. 1105–1119) have used the S-phase kinase-associated protein2 (SKP2Bp):GFP marker line to isolate only those cells that are intimately associated with LRP development. SKP2B encodes an F-box ubiquitin ligase that regulates cell division and founder cell division. In the current study, the authors isolated SKP2Bp:GFP-expressing cells, without the use of LRP-promoting drug or hormonal treatments, by means of cell sorting. A transcriptomics analysis identified a large number of genes that are likely involved in LR development. Loss-of-function mutants of many of these genes caused changes in root length, LR number, or development. The key finding of the authors is that a large proportion of genes involved in redox activity (reactive oxygen species signaling) seem to be involved in LR formation. Consequently, the authors’ attention shifted to this class of enzymes, and they were able to demonstrate, using genetic and chemical inhibitor studies, that peroxidase activity and reactive oxygen species signaling are specifically required during LR emergence but, intriguingly, not for primordium specification itself.

Strigolactone Stereoisomers

Strigolactones (SLs) are carotenoid-derived phytohormones that mediate various aspects of plant development in addition to symbiotic and parasitic interactions in the rhizosphere. Originally identified as seed germination stimulants of root-parasitic weeds, SLs have now been implicated in several processes, including bud inhibition, regulation of leaf morphology and root architecture, control of secondary growth in the cambium, and association of plant roots with symbiotic fungi and nodulating bacteria. The stereochemistry of SLs can play an important role in regulating their various biological functions. Related to SLs are the abiotic signals known as karrikins (KARs), which are found in plant-derived smoke. Two hydrolases, KARRIKIN INSENSITIVE2 (KAI2) and Arabidopsis thaliana DWARF14 (AtD14), are necessary for responses to SLs and KARs in Arabidopsis (Arabidopsis thaliana). Although KAI2 mediates responses to KARs and some SL analogs, AtD14 mediates SL but not KAR responses. To further determine the specificity of these proteins, Scaffidi et al. (pp. 1221–1232) assessed the ability of naturally occurring deoxystrigolactones to inhibit Arabidopsis hypocotyl elongation, regulate seedling gene expression, suppress outgrowth of secondary inflorescences, and promote seed germination. Neither 5-deoxystrigol nor 4-deoxyorobanchol was active in KAI2-dependent seed germination or hypocotyl elongation, but both were active in AtD14-dependent hypocotyl elongation and secondary shoot growth. The nonnatural enantiomer of 5-deoxystrigol was active through KAI2 in growth and gene expression assays. The four stereoisomers of the synthetic SL analog GR24 had similar activities to their deoxystrigolactone counterparts. These results support the conclusion that KAI2-dependent signaling does not respond to canonical SLs. Furthermore, racemic mixtures of chemically synthesized SLs and their analogs such as GR24, should be used with caution because they can activate responses that are not specific to naturally occurring SLs. The results of this study reveal that the use of specific stereoisomers might provide valuable information concerning the specific perception systems operating in different plant functions.

Evolution of Phosphoenolpyruvate Carboxylase in Flaveria spp.

C₄ plants evolved from C₃ plants by developing a spatial separation for the process of carbon fixation in the leaves and carrying it out in two cell types, mesophyll and bundle sheath cells. This special leaf anatomy is known as Kranz anatomy and includes enlarged, chloroplast-rich bundle sheath cells around the closely spaced veins to ensure an intense contact between the mesophyll and bundle sheath cells. During C₄ photosynthesis, atmospheric CO₂ is initially fixed by phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cells. The PEPC in the C₄ mesophyll has evolved from nonphotosynthetic PEPC found in C₃ ancestors. In all plants, PEPC is phosphorylated by phosphoenolpyruvate carboxylase protein kinase (PPCK). However, differences in the phosphorylation pattern exist among plants with these photosynthetic types, and it is still not clear if they are because of interspecies differences or depend on photosynthetic type. The genus Flaveria contains closely related C₃, C₃-C₄ intermediate, and C₄ species, which are evolutionarily young and thus well suited for comparative analysis. To characterize the evolutionary differences in PPCK between plants with C₃ and C₄ photosynthesis, Aldous et al. (pp. 1076–1091) prepared transcriptome libraries from nine Flaveria spp. This enabled them to identify a two-member PPCK family (PPCKA and PPCKB). Quantitative analysis of transcriptome data revealed that PPCKA and PPCKB exhibit inverse diel expression patterns and that C₃ and C₄ Flaveria spp. differ in the expression levels of these genes. PPCKA has maximal expression levels during the day, whereas PPCKB has maximal expression during night. Phosphorylation patterns of PEPC also varied among C₃ and C₄ Flaveria spp., with PEPC from the C₄ species being predominantly phosphorylated throughout the day, whereas in the C₃ species, phosphorylation level was maintained during the entire 24 h. Because C₄ Flaveria spp. evolved from C₃ ancestors, these findings link the evolutionary changes in PPCK expression and phosphorylation pattern to an evolutionary phase shift of kinase activity from a C₃ to a C₄ mode.

Opposite Roles of Ethylene Receptors in Seed Germination

Ethylene is a gaseous plant hormone that regulates numerous developmental processes in higher plants from seed germination to senescence and is important for plant responses to biotic and abiotic stresses. In Arabidopsis, ethylene responses are mediated by a family of five receptors that are not entirely redundant in their roles; in some cases, a particular isoform has a unique role in controlling a trait. Wilson et al. (pp. 1353–1366) have used loss-of-function mutants for each receptor isoform to determine the role of individual isoforms in seed germination under salt stress. From this analysis, they found subfunctionalization of the receptors in the control of seed germination during salt stress. Specifically, loss of ETHYLENE RESPONSE1 (ETR1) or ETHYLENE INSENSITIVE4 (EIN4) leads to accelerated germination, loss of ETR2 delays germination, and loss of either ETHYLENE RESPONSE SENSOR1 (ERS1) or ERS2 has no measurable effect on germination. Further analysis indicated that ETR1 and EIN4 function additively with ETR2 to control this trait. Interestingly, regulation of germination by ETR1 requires the full-length receptor. Loss-of-function etr1 mutants have reduced sensitivity to abscisic acid (ABA) and germinate earlier than the wild type, whereas etr2 loss-of-function mutants have increased sensitivity to ABA and germinate more slowly than the wild type. Additionally, the differences in seed germination on salt between the two mutants and the wild type are eliminated by the ABA biosynthetic inhibitor norflurazon. These data suggest that ETR1 and ETR2 have roles independent of ethylene signaling that affect ABA signaling and result in altered germination during salt stress.

Cotyledon Auxin Regulates Hypocotyl Shade Growth

When growing in close proximity to neighboring plants, shade-intolerant plants exhibit a suite of phenotypes collectively referred to as the shade avoidance syndrome. Shade avoidance mechanisms include hypocotyl and stem elongation, reduced root and leaf growth, and reduced defenses against herbivores and pathogens. This adaptive response can be viewed as a competitive strategy to allocate resources toward growth that alters plant architecture to enable better light harvesting. Shade from neighboring plants is perceived as a reduction in the ratio of red (R) to far-red (FR) light. Previous studies have shown that hypocotyl growth in low R to FR shade is largely dependent on the photoreceptor phytochrome B and the phytohormone auxin. However, where shade is perceived in the plant and how auxin regulates growth spatially is less well understood. Procko et al. (pp. 1285–1301) show that the perception of low R to FR shade by the cotyledons triggers hypocotyl cell elongation and auxin target gene expression in Brassica rapa. Following shade perception, elevated auxin levels occur in a basipetal gradient away from the cotyledons, and this is coincident with a gradient of auxin target gene induction. These results show that cotyledon-generated auxin regulates hypocotyl elongation. In addition, the authors found that simulated shade does not affect seed oil composition but may affect seed yield in mature B. rapa plants. This suggests that in field settings where mutual shading between plants may occur, a balance between plant density and seed yield per plant needs to be achieved for maximum oil yield, whereas oil composition might remain constant.

Role of a Ribosomal Protein in Female Gametogenesis

In Arabidopsis, mutations in single ribosomal protein genes are sometimes gametophyte or embryo lethal. However, many ribosomal protein mutants are viable. These mutants typically display a subtle change in leaf shape and may also have distinct developmental defects affecting embryo morphogenesis, inflorescence development, the transition to flowering, and plant stature. Female fertility is also reduced in several ribosomal protein mutants. Why ribosomal protein mutants have specific phenotypes is not fully known, but such defects could potentially result from ribosome insufficiency, ribosome heterogeneity, or extraribosomal functions of ribosomal proteins. Zsögön et al. (pp. 1133–1143) now report that ovule development is sensitive to the level of Ribosomal Protein L27a (RPL27a) and is disrupted by mutations in the two paralogs RPL27aC and RPL27aB. Mutations in RPL27aC result in high levels of female sterility, whereas mutations in RPL27aB have a significant but lesser effect on fertility. Progressive reduction in RPL27a function results in increasing sterility, indicating a dose-dependent relationship between RPL27a and female fertility. RPL27a levels in both the sporophyte and gametophyte affect female gametogenesis, with different developmental outcomes determined by the dose of RPL27a. These results demonstrate that RPL27aC and RPL27aB act redundantly and that appropriate levels of RPL27a in the sporophyte and gametophyte are required for female gametophyte development and plant fertility.