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ON THE INSIDE

On the Inside

Germins and germin-like proteins (GLPs) are a large plant gene family first identified in a search for germination-specific proteins. They typically occur as stable, oligomeric glycoproteins in the extracellular matrix. Certain germins and GLPs have oxalate oxidase activity or superoxide dismutase activity, both of which may lead to hydrogen peroxide (H₂O₂) accumulation in plants. H₂O₂ is an important plant defense molecule that may reach levels in plant tissues that are toxic to microbes and herbivores. In addition to contributing to the structural reinforcement of plant cell walls, H₂O₂ also plays a central role in the signal transduction cascades that coordinate various defense responses, such as the elicitation of hypersensitive response and the synthesis of PR proteins, phytoalexins, proteinase inhibitors, and polyphenol oxidases. Germins and GLPs are known to function in pathogen resistance, but their involvement in defense against insect herbivores is poorly understood. In the tobacco Nicotiana attenuata, attack from the specialist herbivore Manduca sexta, or elicitation by the addition of larval oral secretions to plant wounds up-regulates transcripts of a GLP. To understand the function of the single-copy gene that codes for this GLP, Lou and Baldwin (pp. 1126–1136) cloned the full-length NaGLP and silenced its expression in N. attenuata. Plants with silenced NaGLP had reduced constitutive NaGLP in their roots and lower elicited NaGLP transcript levels in their leaves. Silencing NaGLP improved M. sexta larval performance and attenuated the oral secretion-induced H₂O₂, diterpene glycosides, and trypsin proteinase inhibitor responses, but did not influence the oral secretion-elicited jasmonate and salicylate bursts, or the release of volatile organic compounds that function as an indirect defense. These results suggest that NaGLP influences the defense responses of N. attenuata via H₂O₂ and ethylene signaling pathways.

Altered Mitochondria in the Quiescent Center of Roots

Embedded within the root apices of angiosperms is a population of slowly dividing cells that form a region known as the quiescent center (QC). Depending on the species, the QC varies in size from four cells in Arabidopsis (Arabidopsis thaliana) to upwards of 1,000 cells in the root apex of maize (Zea mays). Cells comprising the QC spend a prolonged period in G1, dividing, on average, about once every 200 h. Because divisions are infrequent and result in both a self-renewed QC cell and a sister cell that leaves the QC and repopulates the initial pool, many researchers have suggested that QC cells should be viewed as stem cells. Recent work with animal stem cells points to the redox state as having a central role in modulating the equilibrium between self-renewing divisions and differentiation. It is interesting, therefore, that at the biochemical level, one of the few known properties of the QC is its relatively oxidized redox status. Oxidative stress can compromise many cellular activities, including mitochondrial function. Depending on the severity of the oxidized stress, mitochondria can respond in different ways, including a reduction in the flux capacities of the TCA cycle. Jiang et al. (pp. 1118–1125) have investigated whether alterations in mitochondrial activity occur in the QC. They report that mitochondria in the oxidizing environment of the maize root QC are altered in function, but otherwise structurally normal. Compared to mitochondria in the adjacent, rapidly dividing cells of the proximal root tissues, mitochondria in the QC show marked reductions in the activities of TCA cycle enzymes. They propose that these changes in mitochondrial function may underlie the establishment and maintenance of the QC and serve as a link between auxin accumulation and oxidative stress in the region of the QC.

Mechanism and Consequences of High-Affinity Nitrate Transport

Two contributions in this issue concern the physiological function and mechanisms underlying inducible high-affinity transport system (IHATS) for . Two important adaptive responses of the plant root system to nitrogen limitation are the up-regulation of IHATS and the stimulation of lateral root (LR) growth. Up-regulation of the IHATS by nitrogen starvation is suppressed in the atnrt2.1-1 mutant of Arabidopsis, deleted for both NRT2.1 and NRT2.2 nitrate transporter genes. Remans et al. (pp. 909–921) used the atnrt2.1-1 mutant to determine whether lack of IHATS stimulation affected the response of the root system architecture to low availability. In wild-type plants, moderate nitrogen limitation led to an increase in the number of laterals, while severe nitrogen stress promoted LR length. The root system architectural response of the atnrt2.1-1 mutant to low was markedly different. Under moderately low nitrogen conditions, the stimulated appearance of LRs was abolished in atnrt2.1-1 plants, whereas the increase in LR length was much more pronounced than in wild type. These results suggest that the uptake rate of , rather than its external concentration, is the key factor triggering the observed changes in root system architecture. The mutation of NRT2.1, however, was also found to inhibit initiation of LR primordia in plants subjected to nitrogen limitation, independently of the rate of uptake by the whole root system. This indicates a direct stimulatory role for NRT2.1 in this particular step of LR development. Thus, NRT2.1 has a dual function in coordinating root development with external availability, both indirectly through its role as a major uptake system that determines the nitrogen uptake-dependent root system architectural responses, and directly through a specific action on LR initiation under nitrogen-limited conditions.

In addition to IHATS, influx into plant roots consists of at least three other additive fluxes, including constitutive high-affinity influx (CHATS), constitutive low-affinity influx, and inducible low-affinity influx. Members of the NRT2 family of transporters are involved in the IHATS in fungi, algae, and plants. A T-DNA mutant of Arabidopsis disrupted in the AtNRT2.1 and AtNRT2.2 genes exhibited severe and specific impairment of IHATS function. However, in the green alga Chlamydomonas reinhardtii, NRT2 genes do not act alone; two high-affinity nitrate transporter genes (CrNRT2.1 and CrNRT2.2) require a second gene, CrNAR2, to function in transport. Arabidopsis possesses two genes, AtNRT3.1 and AtNRT3.2, that are similar to the Chlamydomonas NAR2 gene. AtNRT3.1 accounts for greater than 99% of NRT3 mRNA and is induced 6-fold by . Okamoto et al. (pp. 1036–1046) analyzed uptake by roots and the effects of on gene expression in two T-DNA mutants of AtNRT3.1 (Atnrt3.1-1 and Atnrt3.1-2). induction of the nitrate transporter genes AtNRT1.1 and AtNRT2.1 was reduced in Atnrt3.1 mutant plants, and this reduced expression was correlated with reduced concentrations in the tissues. CHATS was reduced by 34% and 89%, respectively, in Atnrt3.1-1 and Atnrt3.1-2 mutant plants, while nitrate-inducible influx (IHATS) was reduced by 92% and 96%, respectively. By contrast, low-affinity influx appeared to be unaffected. These results indicate that the CHATS and IHATS (but not low-affinity influx) of higher plant roots require a functional AtNRT3 (NAR2) gene.

Auxin-Ethylene Crosstalk

Auxin response factors (ARFs) are proteins that bind to the auxin response elements that control gene expression during auxin-induced responses. Elucidating the roles of each ARF gene in auxin responses and plant development has been challenging. Li et al. (pp. 899–908) show that both arf19 and arf7 mutants are auxin resistant and that the arf19arf7 double mutant demonstrates stronger auxin resistance than the single mutants. They present evidence that it is the differences in expression level and pattern and not the differences in protein sequences between the two ARFs that determine the relative contribution of the two ARFs in auxin signaling and plant development. In addition to being auxin resistant, arf19 mutants also have ethylene-insensitive roots. Moreover, ARF19 expression is induced by ethylene treatment. This work provides evidence that ARF19 and ARF7 not only participate in auxin signaling, but also play a critical role in ethylene responses in Arabidopsis roots, indicating that the ARFs serve as a crosstalk point between the two hormones.

Plastoglobule Proteins

Plastoglobules (PGs) are lipid-rich structures present in all plastid types, but their specific functions are unclear. It is not even known whether PGs contain any enzymes or regulatory proteins. Among the molecules found in PGs are quinones, -tocopherol, and lipids, and, in chromoplasts, carotenoids. Ytterberg et al. (pp. 984–997) have employed mass spectrometry to examine the proteome of PGs from chloroplasts of stressed and unstressed leaves of Arabidopsis, as well as from pepper (Capsicum annuum) fruit chromoplasts. They report that the proteome of chloroplast PGs consists of seven fibrillins that provide a protein coat and prevent the coalescence of the PGs, as well as an additional 25 proteins that are likely involved in the metabolism of isoprenoid-derived molecules (quinones and tocochromanols), lipids, and carotenoid cleavage. Four unknown ABC1 kinases were also identified, which may be involved in the regulation of quinone monooxygenases. Most PG proteins have predicted N-terminal chloroplast transit peptides and lack transmembrane domains, consistent with localization in the PG lipid monolayer particles. In the case of the PG proteome of pepper chromoplasts, more than 20 proteins were identified, including four enzymes involved in carotenoid biosynthesis and several homologues of proteins observed in the chloroplast PGs. These findings suggest that PGs in chloroplasts form a functional metabolic link between the inner envelope and thylakoid membranes and play a role in the breakdown of carotenoids. The PG proteins in chromoplasts also play a role in carotenoid conversions.

Peter V. Minorsky

Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522

FOOTNOTES

www.plantphysiol.org/cgi/doi/10.1104/pp.104.900186.

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