[Plant Signaling & Behavior 4:6, 528-529; June 2009]; ©2009 Landes Bioscience
Article Addendum
A molecular basis for the physiological variation in shade avoidance responses A tale of two ecotypes Rashmi Sasidharan,1,* C.C. Chinnappa,2 Laurentius A.C.J. Voesenek1 and Ronald Pierik1 1Plant Ecophysiology; Institute of Environmental Biology; Utrecht University; CA Utrecht, The Netherlands; 2Department of Biological Sciences; University of Calgary; Calgary, AB CA
Key words: shade avoidance, cell wall modification, expansins, XTHs, Stellaria longipes, phenotypic plasticity, light quality
Using two ecotypes of Stellaria longipes with contrasting responses to shade, we found that plants can differ in their responses to similar light cues, reflecting adaptations to their natural habitat. It was also observed that the plants could distinguish between distinct shade signals. Furthermore, the activity of wall modifying proteins, expansins and xyloglucan endotransglucosylase/hydrolase(s) (XTHs) was regulated during these responses. However, only expansin activity and gene expression profiles correlated with observed growth trends. The differential expression of expansins was light signal specific and ecotype specific and could account for both the trends in growth and their magnitude. We have thus established a potential molecular basis for the observed plasticity in responses to shade.
Shade Avoidance and the Struggle for Light Shade avoidance is an escape strategy adopted by plants upon encountering shade.1 Typical changes in light quality occur due to shading resulting from crowding, neighbouring plants. This includes a lowering of the red to far-red ratio (R/FR) due to absorption of red wavelengths and consequent enrichment in the far-red waveband in light scattered by leaves of neighbouring plants.2 Complete overtopping of a plant by neighbours would also result in depletion of blue light wavelengths in addition to a lowered R/FR and total light intensity. Sensing of these light quality changes by shade avoiding plants results in characteristic morphological changes. This includes mainly, an elongation *Correspondence to: Rashmi Sasidharan; Plant Ecophysiology; Institute of Environmental Biology; Utrecht University; Sorbonnelaan 16; Utrecht 3584 CA The Netherlands; Email:
[email protected] Submitted: 03/26/09; Accepted: 03/30/09 Previously published online as a Plant Signaling & Behavior E-publication: http://www.landesbioscience.com/journals/psb/article/8586 Addendum to: Sasidharan R, Chinnappa CC, Voesenek LA, Pierik R. The regulation of cell wall extensibility during shade avoidance: A study using two contrasting ecotypes of Stellaria longipes. Plant Physiol 2008; 148:1557–69; PMID: 18768908; DOI: 10.1104/pp.108.125518.
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growth of stems and/or petioles with a suppression of branching, acceleration of flowering, truncated fruit and seed development and low seed germination.3 The most dramatic of these is however the elongation response, both in magnitude and rapidity1 and is crucial to survive conditions in a crowded canopy. Shoot elongation in turn, requires rapid cellular expansion. This involves an increase in cell wall extensibility in response to turgor, a process termed wall loosening. Wall loosening is aided by the action of certain wall modifying proteins on the plant cell wall.4 Two well known protein families are the expansins5 and xyloglucan endotransglucosylases/hydrolases (XTHs).6 The main premise of our study was that cell wall extensibility is an important regulatory point during shade-induced growth and that differential regulation of wall modifying proteins can provide a molecular basis for the variation in responses to shade. Furthermore, although the importance of expansins and XTHs in growth has been studied extensively,4 our study was the first, to our knowledge, to establish their relevance in the functional context of shade avoidance.7
Stellaria longipes: A Comparative System to Study Shade Avoidance Adding to the novelty of our study was the choice of two ecotypes of the polyploid, plastic species Stellaria longipes: the alpine and the prairie. This selection was based on previous studies that demonstrated the non-responsiveness of alpine plants to low R/FR. Prairie plants on the other hand showed increased stem elongation under these conditions.8 This reflects the adaptation of each of these ecotypes to their respective habitats. Alpine plants grow in a windy, high altitude location with sparse vegetational growth, while prairie plants grow at a lower elevation, surrounded by competing vegetation and are consequently more susceptible to frequent shading.9
Shade Signals Regulate Expansin and XTH Activity and thus Cell Wall Extensibility We used two shade treatments, low R/FR mimicking the onset of shading and green shade (reduced R/FR, blue light and total light intensity) replicating dense canopy conditions. Using
Plant Signaling & Behavior
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Ecotypic variation in light mediated cell wall loosening
two distinct shade signals allowed us to observe whether plants are able to distinguish between the two. Although the stem elongation trends in response to low R/FR agreed with previous studies,8 green shade induced enhanced internodal elongation in both ecotypes.7 We reason that although the alpine ecotype in its natural environment probably never encounters shade, it can only respond to very severe shade conditions. Prairie plants on the other hand are relatively sensitive to shading and have a more fine-tuned shade sensing mechanism. This would help them respond earlier and faster to a shading threat. Furthermore, in this ecotype, green shade responses were much stronger in magnitude compared with low R/FR treatments revealing that plants can sense different levels of shading and respond appropriately. Expansin and XTH activity were measured in the internodes of S. longipes plants grown under different shade conditions. Expansin activity measured as acid-induced extension (AIE) showed a strong correlation with growth trends. In both ecotypes, light conditions that elicited strong internodal elongation also increased AIE values in those internodes. Furthermore, in keeping with the higher growth rates of the prairie plants as compared to its alpine counterpart, corresponding AIE values were also higher.7 In contrast, although XTH activity was higher in internodes of plants grown under green shade conditions, low R/FR did not seem to regulate XTH activity, even in prairie plants (Fig. 1). Further investigations revealed that the green shade induction was due to blue light depletion. Both alpine and prairie plants grown under low blue light conditions showed high XTH activity relative to white light controls correlating with growth trends and comparable to their response to green shade.
Expansins are Differentially Regulated by Shade Signals Five expansin genes cloned from S. longipes were differentially regulated by shade signals (Fig. 1).7 The expression profiles of these genes correlated with both the expansin activity as well as the growth trends observed. The low R/FR non-responsiveness of the alpine ecotype could be explained by the fact that four expansin genes (SlEXPA2, 4, 5 and 7) were downregulated in the internodes relative to white light controls. In contrast, in the prairie ecotype internodes, an expansin gene (SlEXPA4) was upregulated by low R/FR. Green shade conditions resulted in upregulation of all the genes that we tested in both the ecotypes (Fig. 1). These results could potentially explain both the magnitude of plastic variations in the response to shade as well as their specificity in terms of ecotype and light signals.
Conclusion Our results underscore the important role of wall modifying proteins such as expansins in regulating growth responses to environmental stimuli, in this case, shading. Differential regulation of expansins demonstrates how differential regulation of multigene family members could equip a plant with the flexibility it needs to respond to subtle changes in its environment.
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Figure 1. The alpine and prairie ecotypes of Stellaria longipes. The schematic depicts growth trends in the two ecotypes in response to low R/FR and green shade conditions relative to plants grown under white light. Inset boxes show which expansin genes are upregulated (↑) or downregulated (↓) during particular shade treatments. Also included are XTH activity trends that showed upregulation (↑) in response to green shade and no difference (↔) in low R/FR relative to white light controls.
References 1. Franklin KA. Shade avoidance. New Phytol 2008; 179:930-44. 2. Ballare CL, Scopel AL, Sanchez RA. Far-red radiation reflected from adjacent leaves: An early signal of competition in plant canopies. Science 1990; 247:329-32. 3. Franklin KA, Whitelam GC. Phytochromes and shade-avoidance responses in plants. Ann Bot 2005; 96:169-75. 4. Cosgrove DJ. Growth of the plant cell wall. Nat Rev Mol Cell Biol 2005; 6:850-61. 5. Cosgrove DJ. Loosening of plant cell walls by expansins. Nature 2000; 407:321-6. 6. Rose JKC, Braam J, Fry SC, Nishitani K. The XTH family of enzymes involved in xyloglucan endotransglucosylation and endohydrolysis: Current perspectives and a new unifying nomenclature. Plant Cell Physiol 2002; 43:1421. 7. Sasidharan R, Chinnappa CC, Voesenek LACJ, Pierik R. The regulation of cell wall extensibility during shade avoidance: a study using two contrasting ecotypes of Stellaria longipes. Plant Physiol 2008; 148:1557-69. 8. Alokam S, Chinnappa CC, Reid DM. Red/far-red light mediated stem elongation and anthocyanin accumulation in Stellaria longipes: differential response of alpine and prairie ecotypes. Can J Bot 2002; 80:72-81. 9. Chinnappa CC, Donald GM, Sasidharan R, Emery RJN. The biology of Stellaria longipes (Caryophyllaceae). Can J Bot 2005; 83:1367-83.
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