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Plant Physiology 150:1107-1108 (2009) © 2009 American Society of Plant Biologists On the InsideSerotonin Synthesis in Senescent Leaves The pineal gland hormone serotonin (5-hydroxytryptamine) plays a key role in mammals in regulating such behaviors as mood, eating disorders, and alcoholism. Serotonin has also been suggested to be involved in several plant processes, including growth regulation, flowering, xylem sap exudation, and plant morphogenesis. In this issue, Kang et al. (pp. 1380–1393) report that serotonin accumulates strongly in rice (Oryza sativa) leaves undergoing senescence induced by either nutrient deprivation or detachment. Immunolocalization analyses revealed that serotonin was abundant in the vascular parenchyma cells, including companion cells and xylem parenchyma cells, suggesting that it may help maintain the cellular integrity of these cells for facilitating efficient nutrient recycling from senescing leaves to sink tissues during senescence. The synthesis of serotonin in senescing rice leaves is closely coupled with the transcriptional and enzymatic induction of Trp biosynthetic genes and tryptophan decarboxylase (TDC). Moreover, transgenic rice plants that overexpress TDC accumulate higher levels of serotonin than the wild type and show delayed leaf senescence. Conversely, in transgenic rice plants in which the expression of TDC is suppressed by RNA interference, less serotonin is produced and the leaves senesce faster than the wild type. Thus, serotonin clearly seems to retard leaf senescence in rice, possibly by protecting senescing leaves by means of its antioxidant activity.
During endoreplication, cells undergo successive rounds of DNA replication without an intervening mitosis. Endoreplication is often used to expand the genome of cell types characterized by high biosynthetic activity. The Ser-Thr kinase activity of various cyclin-dependent kinase (CDK) complexes is crucial in determining the cycling state of a cell. An essential step in CDK activation involves the association of the kinase with a regulatory cyclin subunit. Cyclins of higher eukaryotes are generally divided into three groups: G1/S (D-type), S/M (A-type), and mitotic (B-type) cyclins. Plants have multiple members of each cyclin class, but the CDK-binding partner and biological significance have been identified for only a few of them. Previous research has demonstrated that the B1-type and B2-type CDKs in Arabidopsis (Arabidopsis thaliana) display a peak of activity at the G2-to-M transition and during mitosis, respectively. The activity level of CDKB1;1 apparently acts as a switch, determining whether cells will divide mitotically or undergo endoreplication. In cells competent for division, the presence of CDKB1;1 triggers cells to enter into mitosis. By contrast, in the absence of CDKB1;1 activity, cells endoreplicate. In this issue, Boudolf et al. (pp. 1482–1493) demonstrate that CDKB1;1 copurifies and associates with the A2-type cyclin CYCA2;3. The coexpression of CYCA2;3 with CDKB1;1 triggers ectopic cell divisions and inhibits endoreplication. Moreover, CYCA2;3 protein stability was found to be controlled by CCS52A1, an activator of the anaphase-promoting complex. The authors propose that CCS52A1 participates in the onset of endoreplication by down-regulating CDKB1;1 activity through the destruction of CYCA2;3.
Genetic engineering of the plastid genome offers several advantages from a biotechnological perspective. First, genetic engineering of the plastid would reduce the likelihood of transgene escape since chloroplasts are almost exclusively inherited maternally. Second, chloroplasts have an extraordinary capacity to synthesize and accumulate extremely high levels of foreign proteins. Surprisingly, little is known about the physiological consequences of such high expression of transgenes in chloroplasts. For example, does this high production and accumulation affect photosynthesis or other vital processes required for plant growth and survival? In most reports, no obvious phenotypic defect has been observed in plastid transformants. Such observations, however, raise a number of fundamental questions. For example, do the recombinant proteins simply supplement the resident chloroplast proteins normally present in chloroplasts? If so, does this mean that plants have the capacity to make significantly more proteins than they currently do? Alternatively, are the transgene products made at the expense of resident proteins? Moreover, how are resources allocated between resident and recombinant proteins when the cell budget is reduced, in particular when there is a limitation in nitrogen supply? Bally et al. (pp. 1474–1481) have investigated the impact that massive transgene expression in tobacco (Nicotiana tabacum) chloroplasts has on development, photosynthesis, leaf proteome, chloroplast transcriptome, and amino acid composition. Analyses were made of plastid transformants that abundantly expressed various transgenic proteins. In leaves, two recombinant proteins were found to accumulate to even higher levels than Rubisco. The high expression of these transgenes, however, had no effect on photosynthesis, growth, or amino acid content. Since the amino acid content is unchanged, then recombinant products must be made at the expense of resident proteins. Indeed, Rubisco, which constitutes the major leaf amino acid store, is the most strongly down-regulated plant protein. This reduction is even more dramatic under conditions of limited nitrogen supply. These results show that plants are able to produce massive amounts of recombinant proteins in chloroplasts without profound metabolic perturbation, and that Rubisco, acting as a nitrogen buffer, is a key player in maintaining homeostasis and limiting pleiotropic effects.
Jasmonates are best known for their role in defense against herbivores and certain pathogens. Since stressful conditions often require that plants adjust their development and reallocate resources toward defense, it might be expected that jasmonate defense signaling would be tied to growth regulation. Indeed, this appears to be the case: The exogenous application of jasmonic acid (JA) has been shown to inhibit growth, and both JA biosynthesis and jasmonate signaling mutants display growth abnormalities. JA also exists in conjugated forms in plants. Early physiological studies suggested that some JA conjugates might have biological activity, but most were considered inactive or of minor importance compared with JA or its methyl ester (JA-Me). That perspective changed dramatically with the discovery that the Ile conjugate of JA (JA-Ile) is a key jasmonate signal. While evaluating the possible jasmonate activity of a variety of JA-amino acid conjugates, Staswick (pp. 1310–1321) discovered that the Trp conjugate of JA (JA-Trp) caused agravitropic root growth in seedlings, unlike JA or nine other JA-amino acid conjugates. The Trp conjugates of indole-3-acetic acid (IAA) also caused agravitropic root growth. Although conjugates are generally considered inactive metabolites of the hormone, this research suggests that the Trp conjugates of JA and IAA are naturally occurring auxin antagonists that interfere with a broad range of IAA-mediated processes. Mutant studies suggest that Trp conjugates require the auxin receptor TIR1 for full activity. These results show that JA-Trp and IAA-Trp constitute a previously unrecognized mechanism to regulate auxin action.
Alternative splicing contributes to protein diversity and the quantitative regulation of gene expression. In Arabidopsis, at least 23.5% of genes undergo alternative splicing. The mode of alternative splicing in plants differs from that in animals in that the majority of splicing events involve intron retention rather than exon inclusion. The Ser-Arg-rich (SR) proteins constitute a conserved family of pre-mRNA splicing factors. SR proteins function both in splice site recognition and in spliceosome assembly. In Arabidopsis, SR proteins are encoded by 19 genes, most of which are alternatively spliced themselves. Although several of these SR genes arose by genome duplication events, their products are not functionally redundant. Arabidopsis SR proteins are distributed differently in a spatial and temporal-specific manner. Recently, several studies have described SR45's interaction with other proteins and its dynamic localization within the nucleus. However, the mechanisms by which SR45 participates in different processes are still largely unknown. In the particular case of SR45, two alternatively spliced isoforms are produced: isoform 1 (SR45.1) and isoform 2 (SR45.2). A previously described loss-of-function mutant affecting both isoforms, sr45-1, shows several developmental defects, including defects in petal development and root growth. Zhang and Mount (pp. 1450–1458) tested the ability of each SR45 isoform to complement the sr45-1 mutant by overexpression of isoform-specific GFP fusion proteins. SR45.1-GFP complements the flower petal phenotype but not the root growth phenotype. Conversely, SR45.2-GFP complements root growth but not floral morphology. Thus, the two alternatively spliced isoforms of SR45 have distinct biological functions, with SR45.1 playing a major role in flower petal development and SR45.2 playing a major role in root growth.
Cytokinins (CKs) maintain stomata open, thereby increasing stomatal conductance and transpiration. In general, there is a decrease in CK accumulation during drought stress, and the reduction in CKs can increase the shoot responses to increasing abscisic acid concentrations, leading to an increase in stomatal resistance. These stress-induced changes in CKs and abscisic acid may also lead to leaf abscission, thereby creating a smaller canopy and reducing water loss. Although such strategies work to mitigate drought and enhance plant survival, they also reduce crop yield. Previous work has shown that leaf senescence is delayed in transgenic plants expressing isopentenyltransferase (IPT), an enzyme that catalyzes the rate-limiting step in CK synthesis. Transgenic plants expressing IPT under the control of constitutive or various inducible promoters, however, lost their apical dominance and showed poor root development. In an effort to overcome the adverse physiological effects resulting from the manipulation of CKs, Rivero et al. (pp. 1530–1540) have developed transgenic plants that expressed IPT under the control of Senescence-Associated Receptor Kinase (SARK), a maturation- and stress-inducible promoter. Previously, it has been shown that following severe drought, the production of CKs in transgenic plants expressing PSARK::IPT led to enhanced photosynthetic rates and water use efficiency. Moreover, the transgenic plants displayed minimal yield losses when watered with only a fraction of the optimal water requirement. In this issue, the authors have investigated further the effects of PSARK::IPT expression and CK production on several aspects of photosynthesis in transgenic tobacco plants grown under optimal or restricted watering regimes. Their results show that during water stress, the production of CKs resulted in the protection of biochemical processes associated with photosynthesis and in the induction of photorespiration, which may contribute to the protection of photosynthesis during water stress.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.109.900296 Related articles in Plant Physiol.:
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