A Proteasomal Regulator of Viral Systemic Transport and Vascular Formation
In plants, the long-distance transport of viruses occurs via the vascular system, particularly the phloem. However, compared to the great advances that have been made in understanding the cell-to-cell movement of viruses through plants, relatively little is known about the long-distance movement of viruses. By means of a viral transport suppressor screen employing virus-induced gene silencing, Jin et al. (pp. 651–661) have identified a 26S proteasome subunit, RPN9, which is required for broad-spectrum viral systemic transport. The authors trace the inhibition of viral systemic transport in tobacco (Nicotiana benthamiana) after RPN9 silencing to alterations in the vascular tissue. RPN9-silenced plants display extra leaf vein formation with increased xylem and decreased phloem. Thus, RPN9 affects vascular development. The authors provide evidence that RPN9 functions at least in part through the regulation of auxin transport and brassinosteroid signaling, two processes that are crucial for vascular formation. RPN9, a component of the proteasome, may regulate vascular formation by targeting a subset of regulatory proteins for degradation.
Specificity of Pollen Tube Chemoattractants
Synergids, the two cells that flank the egg in embryo sacs, emit chemoattractants that are involved in navigating the growth of the pollen tube toward the embryo sac. In the flowers of Torenia fournieri (Fig. 1), pollen tubes are directly attracted to the protruding embryo sacs, and laser ablation experiments revealed that the source of the diffusible signal is the two synergid cells. Higashiyama et al. (pp. 481–491) have investigated the specificity of the attractant in five closely related species in two genera, including T. fournieri, Torenia baillonii, Torenia concolor, Lindernia crustacea, and Lindernia micrantha. In all five species, ablation experiments confirmed that their synergid cells attract the pollen tube. When ovules of T. fournieri and one of the other species were cultivated together with pollen tubes of each species, pollen tubes were significantly more attracted to synergid cells of the same species. These results suggest that the attractant is a species-specific molecule that is synthesized in the synergid cell. Ca2+ ions, long considered a potential attractant, could not serve as the sole attractant in these species because elevation of the Ca2+ ion concentration did not affect the observed attraction. The species specificity of the attractants may serve as a reproductive barrier in the final step of directional control of the pollen tube.
Figure 1.
The synergids of the embryo sacs in the flowers of T. fournieri produce a pollen-attracting substance that is specific for the species and discriminates against the pollen of closely related species. (Photo by Mo Fayya. Courtesy of the University of Wisconsin-Madison Botanical Garden.)
Plasmodesmatal Coupling via the Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an integral component of plasmodesmata in higher plants and forms the desmotubule linking the ER of neighboring cells. The structure of plasmodesmata offers two potential pathways for transport between cells: the cytoplasmic sleeve for cytosolic solutes and the desmotubule for ER-associated molecules, respectively. Whereas the exchange of solutes and macromolecules through the cytoplasmic sleeve is well established, information regarding the transfer of ER-associated molecules is very limited. By means of a Suc transporter/green fluorescent protein (GFP) construct targeted to the ER of the companion cell (CC-ER), Martens et al. (pp. 471–480) have examined whether the CC-ER and the sieve-element (SE) reticulum (SER) are continuous in tobacco (Nicotiana tabacum). GFP fluorescence from the construct was observed in CCs from source to sink, but the fluorescence stayed put in the ER of CCs and did not migrate into the SER of adjacent SEs. An ER-specific fluorochrome, however, was found to move. The complete lack of GFP transfer between CC and SE indicates that the intraluminal pore-plasmodesma contact has a size exclusion limit below 27 kD. By fluorescence redistribution after photobleaching (FRAP), they found that the rate of fluorescence recovery of the ER-specific fluorochrome was unaffected by cytochalasin. Moreover, FRAP occurred slowly when rate limited by plasmodesmata, whereas intracellular FRAP occurred quickly. The highest degree of ER coupling was measured between CCs and SEs. The plasmodesmata that connect CCs and SEs, which have a particularly large size exclusion limit for phloem-derived proteins, are also shown here offer an intimate ER contact that allows a high exchange rate of ER-associated molecules.
PEPC from Non-Kranz-Type C4 Plants
Since the discovery of C4 photosynthesis, the spatial separation of carbon-fixing enzymes has been conceptually linked to the occurrence of Kranz-type anatomy. Recently, however, several members of the Chenopodiaceae, including Suaeda aralocaspica and Bienertia sinuspersici, were found to have unique mechanisms of C4 photosynthesis that do not require Kranz-type anatomy. In these plants, CO2 fixation by phosphoenolpyruvate carboxylase (PEPC) and by Rubisco is not separated in different cells but rather in different ends of the same chlorenchyma cells. Lara et al. (pp. 673–684) have studied the molecular and functional features of PEPC in two single-cell functioning C4 species (B. sinuspersici and Suaeda aralocaspica) as compared to Kranz-type (Haloxylon persicum, Salsola richteri, and Suaeda eltonica) and C3 (Suaeda linifolia) chenopods. They report that PEPC from both types of C4 chenopods display higher specific activity than that of the C3 species, and show kinetic and regulatory characteristics similar to those of C4 species in other families, in that they are subject to light/dark regulation by phosphorylation and display differential malate sensitivity. Also, the deduced amino acid sequence from leaf cDNA indicates that the single-cell functioning C4 species possess a Kranz-type C4 isoform with a Ser in the amino terminus. The results of this study indicate that B. sinuspersici and S. aralocaspica have a C4-type PEPC similar to that in Kranz C4 plants, and that this type of PEPC may be a requirement for effective C4 photosynthesis in general.
A Role for an Extracellular RNase in Abscission and Senescence
T2 RNases are secreted endoribonucleases that are found outside cells or in compartments of the endomembrane system and that have no absolute substrate base specificity. T2 RNases are the most broadly distributed family of RNA-degrading enzymes known, having been identified in a wide variety of organisms, ranging from viruses and bacteria to mammals. The only plant enzymes in the T2 family for which an in vivo role has been established are the S-RNases, which are involved in gametophytic self-incompatibility. The other major group of T2 plant RNases, the S-like RNases, has been found in all plants so far examined, but the nature of their function is largely unknown. The tomato (Lycopersicon esculentum) protein, LX, is an S-like RNase, the expression of which is associated with phosphate starvation, ethylene responses, and senescence and programmed cell death. Lers et al. (pp. 710–721) have investigated LX function in antisense tomato plants with reduced levels of LX. LX protein levels normally become elevated when leaves senesce. Antisense inhibition of LX retarded the progression of senescence and caused a marked delay of leaf abscission. Thus, LX appears to play an important role in both abscission and senescence.
Is IMMUTANS a Terminal Oxidase?
IMMUTANS encodes a thylakoid membrane protein that has been hypothesized to act as a terminal oxidase that couples the reduction of O2 to the oxidation of the plastoquinone pool of the photosynthetic electron transport chain. Because IMMUTANS shares sequence similarity to the stress-induced mitochondrial alternative oxidase, it has been suggested that the protein encoded by IMMUTANS acts as a safety valve during the generation of excess photosynthetically generated electrons. Rosso et al. (pp. 574–585) have set about testing this hypothesis by combining in vivo chlorophyll fluorescence quenching analyses with measurements of the redox state of P700 in Arabidopsis (Arabidopsis thaliana). This approach allowed them to assess the capacity of IMMUTANS to compete with PSI for electrons during steady-state photosynthesis. Their in vivo data indicate that modulating IMMUTANS expression and polypeptide accumulation does not alter the flux of intersystem electrons to P700+ during steady-state photosynthesis and does not provide any significant photoprotection. Moreover, meta-analyses of published Arabidopsis microarray data indicated that IMMUTANS expression exhibited minimal modulation in response to a diversity of abiotic stresses that are known to be associated with overexcitation of the light reactions. These results do not support the model that IMMUTANS acts as a safety valve to regulate the redox state of the plastoquinone pool during stress and acclimation. Rather, IMMUTANS appears to be strongly regulated by the developmental stage of Arabidopsis.