TILLER ANGLE CONTROL 1 modulates plant architecture in response to photosynthetic signals

Abstract

Light serves as an important environmental cue in regulating plant architecture. Previous work had demonstrated that both photoreceptor-mediated signaling and photosynthesis play a role in determining the orientation of plant organs. TILLER ANGLE CONTROL 1 (TAC1) was recently shown to function in setting the orientation of lateral branches in diverse plant species, but the degree to which it plays a role in light-mediated phenotypes is unknown. Here, we demonstrated that TAC1 expression was light dependent, as expression was lost under continuous dark or far-red growth conditions, but did not drop to these low levels during a diurnal time course. Loss of TAC1 in the dark was gradual, and experiments with photoreceptor mutants indicated this was not dependent upon red/far-red or blue light signaling, but partially required the signaling integrator CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1). Overexpression of TAC1 partially prevented the narrowing of branch angles in the dark or under far-red light. Treatment with the carotenoid biosynthesis inhibitor norflurazon or the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) led to loss of TAC1 expression similar to dark or far-red conditions, but expression increased in response to the PSI inhibitor paraquat. Treatment of adult plants with norflurazon resulted in upward growth angle of branch tips. Our results indicate that TAC1 plays an important role in modulating plant architecture in response to photosynthetic signals.

Introduction

Plant architecture is intimately connected to light. It both influences the ability of the plant to intercept light and adjusts in response to light conditions. Architectural parameters such as organ angles, organ numbers, and branch lengths influence the quantity of light a plant can capture. For example, increased leaf number increases photosynthetic surface area, larger plant size and longer branches can allow plants to avoid shade from their neighbors, and leaf angle changes with respect to the angle of sunlight influence the amount of light captured (Osada and Hiura, 2017). In turn, changes in light quality and quantity result in the modification of these parameters. Growing plants under shaded conditions, for example, results in phenotypes characteristic of shade avoidance syndrome, including upward leaf movement, accelerated elongation of plant organs, and fewer shoot branches (Casal, 2012). In addition to these, shade also leads to more vertically oriented branches in Arabidopsis (Roychoudhry et al., 2017).

Lateral organ orientation, or angle, is an important aspect of plant architecture that has been connected to multiple light signaling pathways. Recent work addressing neighbor detection demonstrated that petiole angle altered in response to far-red (FR) light detection at the leaf margin (Pantazopoulou et al., 2017). These studies showed a connection between red (R)/FR light signaling and architecture. Early work defining gravitropic set point angle, the angle at which organs grow with respect to gravity, identified a regulatory role for photosynthesis using Tradescantia as a model (Digby and Firn, 2002). However, beyond this study, little work has been done to elucidate the connection between photosynthesis and branch angles.

Studies to determine the endogenous genetic components underlying lateral organ orientation identified loci associated with narrow angles in rice, maize, and Brassica (Yu et al., 2007; Ku et al., 2011; Li et al., 2017). A gene repeatedly identified in these studies, TILLER ANGLE CONTROL 1 (TAC1), has been shown to regulate lateral branch angle in Arabidopsis, peach, and plum (Dardick et al., 2013; Hollender et al., 2018). Loss of TAC1 expression, through mutation or silencing, results in more vertical organ orientation in tillers, branches, leaves, and pedicels. In peach canopies, this led to an increased rate of carbon accumulation, as the changes in canopy shape allowed increased light penetrance (Glenn et al., 2015). TAC1 belongs to the IGT family, named for a shared amino acid motif, which also contain LAZY and DEEPER ROOTING (DRO) genes (Hollender and Dardick, 2015). Members of the LAZY and DRO clades have recently been reported to influence both shoot and root organ orientation via changes in gravity response upstream of auxin transport (Yoshihara et al., 2013; Ge and Chen, 2016; Guseman et al., 2017; Taniguchi et al., 2017; Yoshihara and Spalding, 2017). Currently, little is known about the regulation of IGT genes; however, LAZY1 expression in maize was reported to be lower under light conditions (Dong et al., 2013).

Here we address the hypothesis that TAC1 is involved in light regulation of lateral branch angles. Our results show that TAC1 exhibits light-dependent gene expression, which correlates with narrower branch angles in response to prolonged growth in darkness. Constitutive expression of TAC1 could partially, but not fully rescue changes in lateral branch orientation. TAC1 expression was not dependent upon known photoreceptor signaling pathways, but partially required a fully functional CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) gene. Using various photosynthetic inhibitors, we found that TAC1 expression in seedlings was abolished when treated with norflurazon (NF) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and increased in response to paraquat (PQ) treatment. Treatment of adult plants with NF led to narrowed branch tips angles. Together, these results suggest that TAC1 is a target of photosynthetic signals to alter the angle of organs in response to persistent changes in light exposure.

Materials and methods

Plant material and growth conditions

The Columbia (Col-0) and Landsberg erecta (Ler) ecotypes were used as wild-type lines in all experiments. The tac1 mutant used here is a T-DNA SAIL line (SAIL_605_E02) obtained from the ABRC. The 35S::TAC1 line was generated as previously described in Dardick et al. (2013). Briefly, the TAC1 (At2g46640) CDS (coding sequence) was ligated into the multiple cloning site downstream of the 35S promoter of our in-house pBIN-AFRS expression vector, then transformed into Col-0 Arabidopsis plants via floral dip. Signaling mutants phyA phyB and phyA phyB phyD phyE (Hu et al., 2013), cry1 cry2 (Mockler et al., 1999), phot1 phot2 (Kinoshita et al., 2001), cop1-6 (Ang and Deng, 1994), pif1 pif3 pif4 pif5 (Lilley et al., 2012), and hy5 hfr1 laf1 (Jang et al., 2013) were previously described. For seedling expression studies, seeds were surface sterilized and sown on square plates containing half-strength Murashige and Skoog (MS) medium and 0.8% bactoagar, and grown vertically, following our standard lab practice. Once sown, seedlings were stratified at 4 °C in the dark for 2 d, then placed in growth chambers at 20 °C with a 16 h light/8 h dark photoperiod (~100 µmol m^–2 s^–1). For phenotyping studies, 14-day-old seedlings were transplanted to soil and allowed to grow in these conditions until bolting.

Branch angle measurements

For shoot branch and tip angles, seedlings were grown for 2 weeks on plates, then transplanted into 4 inch pots containing Metromix 360 soil (Sun-Gro Horticulture, http://www.sungro.com, last accessed 27 July 2018) and grown until bolting (~15–18 cm in height). Plants were then transferred to continuous light or dark conditions for 72 h. Bolts were then photographed and pressed to obtain and preserve 2-D angles for later measurement. Images were taken using a Canon EOS Rebel T3 camera (http://global.canon/en/index.html, last accessed 27 July 2018). Angles were manually calculated by measuring the angle of the tangent of each lateral branch point or branch tip, with respect to the upper main stem. Only branches that exceeded 3 cm in length were measured. Averages were calculated by first averaging all branch or tip angles per individual plant, to account for variable branch numbers on each plant, then averaging among each genotype. At least six plants were used for each genotype in each experiment.

For branch and tip angle phenotypes, Student’s t-tests were used to determine the significant differences between light treatments within a given genotype. In some cases a t-test was used to compare dark-treated Col plants with tac1 mutants. Significance is indicated when P<0.05. Error bars represent the SD for all graphs shown.

RNA extraction and quantitative real-time PCR

Arabidopsis seedlings were grown on vertical plates for 10–14 d. Four biological replicates were used. Each biological replicate consisted of a plate of 10–12 seedlings. For experiments using adult plants, lateral apices were collected, including the final ~1 cm of all branch tips per plant. Each biological replicate consisted of 4–6 adult plants. Arabidopsis RNA was extracted using a Directzol RNA Extraction Kit (Zymo Research, http://www.zymoresearch.com, last accessed 27 July 2018). qPCR was performed as previously described by Dardick et al. (2010). Briefly, each reaction was run in triplicate using 50 ng of RNA in a 12 µl reaction volume, using the Superscript III Platinum SYBR Green qRT-PCR Kit (Invitrogen, now ThermoFisher Scientific, https://www.thermofisher.com, last accessed 27 July 2018). The reactions were performed using a 7900 DNA sequence detector (Applied Biosystems, now ThermoFisher Scientific, https://www.thermofisher.com, last accessed 27 July 2018). Quantification of Arabidopsis samples was performed using a relative curve derived from a serially diluted standard RNA run in parallel. UBC21 was used as an internal control to normalize expression in light experiments as it was identified as a highly constitutive gene (Czechowski et al., 2005), and IPP2 was used for circadian experiments, as described previously by Imaizumi et al. (2005).

Light and time-course experiments

For seedling light experiments, plants were grown for 10 d on vertical plates in 16:8 h long day light conditions in a growth chamber before transfer to experimental light conditions. For comparisons between light and dark, plates were moved to chambers with either continuous light or continuous dark conditions for 72 h, then whole seedlings were collected and flash frozen at 10.00 h [Zeitgeber time (ZT4)]. For comparisons between light colors, plates were moved to chambers with continuous white (W), R (660 nm), blue (B; 480 nm), or FR light (738 nm) for 72 h and whole seedlings were collected at 10.00 h (ZT4). Matching growth chambers fitted with white, red, blue, and far-red LED lamps from PARsource (http://parsource.com, last accessed 27 July 2018) were used for light color experiments. Detailed spectral graphs were measured using an Exemplar spectrophotometer from B&WTek (http://www.bwtek.com, last accessed 27 July 2018) and are shown in Supplementary Fig. S1 at JXB online. For circadian experiments, seedlings were grown for 10 d in 12L:12D light cycles, then transferred to continuous light and collected every 4 h for 84 h. For adult phenotypes and expression studies, plants were grown on soil for 5–6 weeks, until bolts reached 15–18 cm in height. Then plants were transferred to continuous W or FR light conditions for 72 h, then imaged and collected.

Chemical treatments

For sucrose experiments, plants were germinated and grown on half-strength MS plates for 10 d, then transplanted to plates containing 1% sucrose. Plates were then moved to continuous light or dark conditions for 72 h and collected at 10.00 h (ZT4). For photosynthesis experiments, plants were grown on vertical MS plates for 7 d, then transplanted onto media containing either NF (5 µM), DCMU (10 µM), PQ (1 µM), or water (mock). Plates were then moved to continuous light or dark conditions for 5 d and collected at 10.00 h (ZT4). For treatment of adult plants, Arabidopsis were grown for 5–6 weeks until bolts reached 15–18 cm in height. Adult plants were then watered once with either 67 µM NF or water (mock) and allowed to absorb treated water fully over 4 d. Adult plants were then photographed, pressed to preserve branch angles, and manually measured.

Chlorophyll fluorescence imaging

All chlorophyll fluorescence was measured using the Maxi-Imaging-PAM Chlorophyll Fluorometer (Walz, Effeltrich, Germany). Maximum PSII quantum yield (F_v'/F_m') was determined using an actinic light pulse (1500 µmol m^–2 s^–1). Average F_v'/F_m' values were calculated for the leaf area of the entire rosette for six seedlings grown on each chemical treatment, using the Maxi-Imaging-PAM software.

Results

TAC1 expression is lost under extended continuous dark conditions

To address whether TAC1 plays a role in light regulation of organ angle, we initially screened the promoter region upstream of TAC1 for the occurrence of light-related cis-elements (Fig. 1A). Using a cis-element database (AGRIS AtcisDB, http://arabidopsis.med.ohio-state.edu/AtcisDB/, last accessed 27 July 2018), we identified several light-related elements. These included GATA motifs, conserved binding sequences found in many light-responsive promoters (Teakle et al., 2002); a G-box, a characterized binding site for bZip transcriptions factors, such as HY5 (Menkens and Cashmore, 1994; Lee et al., 2007); and T-boxes, a light-responsive element originally identified in the promoter of a gene encoding a chloroplast glyceraldehyde phosphate dehydrogenase GAPDH subunit (Chan et al., 2001). Next, we tested the dynamic response of TAC1 expression to plant growth in continuous dark over a 72 h period. At the end of the 3 d period, TAC1 expression was lost while that of a control gene, UBC21, was unaffected (Fig. 1B). Expression levels gradually declined over time, reaching their lowest values by 48 h (Fig. 1B). Seedlings grown for 72 h in continuous dark and then returned to continuous light showed similar expression dynamics. Expression began to increase at ~4 h once transferred back into the light, but did not return to normal levels until 48 h (Fig. 1C). To address whether TAC1 exhibits a diurnal rhythm, we performed a circadian time-course, transferring seedlings previously entrained to a 12L:12D light cycle to continuous light conditions. TAC1 expression exhibits a rhythm over the course of the experiment, but does not drop to levels seen after prolonged dark treatment (Fig. 1D). Taken together, the data suggest that TAC1 expression is dependent on light, but with gradual response dynamics.

Lateral branch angles are narrower in response to growth in continuous dark versus light

To test whether the loss of TAC1 expression in dark conditions correlated with changes in Arabidopsis branch angle phenotypes, we grew adult plants in continuous dark for 72 h. Lateral branch angles of wild-type plants were significantly narrower by ~10° compared with continuous light-grown controls (Fig. 2A and B). The tips of the branches showed an even greater difference between light and dark treatments, with branch tips sitting nearly horizontal to the primary stem in light-grown plants, but growing nearly vertical under dark conditions (Fig. 2C). Plants overexpressing TAC1 (35S::TAC1) still showed narrower angles in dark conditions but not to the same degree as Col, suggesting that there are TAC1-dependent and TAC1-independent pathways influencing this process. This is also consistent with previous reports that overexpression of TAC1 does not fully rescue the tac1 mutant branch phenotype (Dardick et al., 2013). tac1 mutant plants exhibited branch and tip angles similar to dark-grown wild-type plants, in both light and dark conditions. To address the dynamics of these angle changes, Col plants were imaged over a time-course of 72 h in dark and light conditions (Fig. 2D). Angles and branches remain similar over time in light conditions, but branch angles and tips narrow over time in the dark, reaching a static, upward position after ~48 h, consistent with the timeline of TAC1 expression loss (Fig. 1B).

TAC1 is lost in FR light, does not require phy, cry, or phot genes, but is reduced in a weak cop1 mutant background

We next sought to determine which aspects of light were required for TAC1 expression. First, we tested the requirement for specific light wavelengths, growing Col seedlings in 72 h of continuous W, R, B, or FR light (Fig. 3B). In comparison with growth in W light, TAC1 expression was not significantly different under R light, and elevated slightly, ~2-fold, under B light. Under FR light, the response was similar to growth in darkness, with very low levels of expression. We tested whether FR treatment reduced expression in adult plants and led to similar changes in tip angles observed in dark-grown plants, as tip angles reflected the dynamic phenotypic changes more clearly in the previous experiment. After 72 h in continuous FR light, adult Col plants showed loss of TAC1 expression, similar to dark conditions, and tip angles were narrower by ~35° (Fig. 3C, D).

The findings prompted us to explore two potential mechanisms by which TAC1 expression could be regulated by light: first, that TAC1 expression requires either R or B light via photoreceptor signaling or, secondly, that TAC1 expression is controlled by another light-related process such as photosynthesis. To test the first, we looked at TAC1 levels in different photoreceptor and light signaling mutant backgrounds, grown under W, R, or B light. We obtained multiple mutants within the phytochrome family, including phyA phyB and phyA phyB phyD phyE, which function as photoreceptors in R/FR light signaling, involved in processes such as photomorphogenesis and shade avoidance syndrome (Fig. 3A) (Li et al., 2011; Casal, 2012). Additionally, we obtained mutants of photoreceptors involved in multiple B light signaling pathways. Cryptochromes are involved in numerous developmental processes, including photomorphogenesis, circadian rhythms, and stomatal opening, while phototropins are crucial for phototropism response (Pedmale et al., 2010; Yu et al., 2010). Finally, we included mutants of several genes crucial for integration of multiple light signaling pathways, including transcription factor genes PHYTOCHROME INTERACTING FACTOR (PIF), ELONGATED HYPOCOTYL 5 (HY5) and related genes, and the E3 ubiquitin ligase gene COP1 (Hoecker, 2017; Paik et al., 2017). In these photoreceptor and light signaling mutant backgrounds, we observed small, but significant changes in TAC1 expression (Fig. 3E, F). However, in no mutant background or light treatment combination did we see a loss of expression similar to dark treatment, suggesting that photoreceptor-mediated light signaling has a minor effect on TAC1 expression. For example, there was a relatively small decrease in TAC1 expression in the phyA phyB mutant in R light; however, this does not mimic dark growth results, and the quadruple phyA phyB phyD phyE mutant did not show a similar effect (Fig. 3E). Similarly, there was a small but significant loss of TAC1 expression in the phot1 phot2 background as compared with the Col wild type in B, however not enough to explain loss of gene expression in the dark (Fig. 3F). In addition, in the triple hy5 hfr1 laf1 mutant and the quadruple pif1 pif3 pif4 pif5 (pifq) mutant (Fig. 3G, H), we saw relatively minor or insignificant changes in TAC1 levels. Together, the data suggest that different aspects of R/FR and B light signaling influence TAC1 expression, but to a relatively minor degree compared with dark treatment.

In contrast, we saw a greater and significant reduction in expression in cop1-6 mutants (Fig. 3H). We originally chose this mutant as it represents a significant signaling hub within R/FR and B light signaling, serving to degrade key light signaling transcription factors (Hoecker, 2017). However, COP1 is involved in numerous other signaling pathways (Gangappa and Botto, 2016). The loss of TAC1 expression in the cop1-6 mutant background, together with the relatively minor effect of other light signaling mutants, suggests a role for COP1 in TAC1 regulation independent of R/FR and B light signaling pathways. Unfortunately, changes in branch or tip angles in these plants could not be comparably measured due to their severely stunted growth (see Supplementary Fig S2).

Exogenous sucrose does not rescue loss of TAC1 in the dark

Sucrose has been reported to have an effect on lateral organ angle (Willemoes et al., 1988), and dark-grown plants have decreased photosynthetic efficiency, and thus produce less photosynthate. To test whether TAC1 expression is dependent on the products of photosynthesis, we grew seedlings on media supplemented with sucrose and exposed these to continuous light and dark conditions (Fig. 4A). Gene expression was similar when supplemented with sucrose in both conditions, demonstrating that exogenous sucrose was not sufficient to attenuate the loss of TAC1 expression in the dark. This suggests that the previously observed sucrose-mediated alteration of organ angle is TAC1 independent.

Photosynthetic inhibitors have differential effects on TAC1 expression

To test if TAC1 expression is regulated by photosynthetic activity, we treated seedlings with a series of photosynthesis inhibitors. Each of these inhibitory chemicals impairs photosynthesis through different pathways. Treatment with NF inhibits carotenoid biosynthesis, allowing for the formation of triplet chlorophyll and subsequent photooxidating damage within the chloroplast (Gray et al., 2003). DCMU specifically inhibits electron transport by blocking the plastoquinone-binding site of PSII. In contrast, PQ, also known as methyl viologen, acts by shunting electrons from PSI, and producing high levels of reactive oxygen species (ROS). Seven-day-old seedlings transferred to media supplemented with these photosynthetic inhibitors were grown in continuous light or dark and measured for photosynthetic efficiency (F_v/F_m) and TAC1 gene expression (Fig. 4B–D). Treatment with NF led to decreased photosynthetic efficiency, as measured by chlorophyll fluorescence imaging, and abolished TAC1 expression in the light, mimicking the effect observed in dark-grown plants (Fig. 4B–D). DCMU treatment resulted in near total loss of chlorophyll fluorescence, and treated plants showed a similar decrease in TAC1 expression to that seen with NF treatment. PQ treatment displayed an inconsistent reduction in PSII efficiency, but led to variable but significant increases in TAC1 expression (Fig. 4B–D). All seedlings grown under continuous dark conditions exhibited loss of TAC1, regardless of treatment (Fig. 4B).

Norflurazon treatment leads to decreased TAC1 expression and narrower branch tip angles

As our initial expression studies with photosynthetic inhibitors were performed using seedlings, the question remained of whether these chemicals had the same effect on TAC1 expression in adult plants and subsequent branch tip angle phenotypes. To address this, adult Col wild-type and tac1 mutant plants were watered with solutions containing either NF or mock treatment, and allowed to soak up the water fully and continue growing for 4 d (Fig. 5). By this time, chlorophyll loss in leaves of NF-treated leaves and stems became visually evident in the light-grown plants as expected. Col plants watered with NF had significantly narrower branch tip angles, 32° on average, compared with mock-watered controls (Fig. 5B). tac1 mutant plants showed no difference between treatments. TAC1 expression was lower in the apices of NF-treated plants, similar to dark-treated plants (Fig. 5C). Together, this suggests that blocking carotenoid biosynthesis and impairing photosynthesis leads to decreases in TAC1 expression and narrower branch angles.

Discussion

Lateral organ angle is strongly linked to light capture, which has important implications for plant productivity and competition. Previously, a connection between photosynthesis and branch angle was described in Tradescantia by Digby and Firn (2002). We provide evidence that TAC1 is a target of photosynthetic signals, and is partially required for the changes in lateral branch angles that are driven by photosynthesis. Arabidopsis grown in continuous darkness exhibited more vertically oriented lateral branches, phenocopying a tac1 mutant phenotype. TAC1 expression in dark-grown plants was abolished after 24–48 h, suggesting that this mechanism is in place to induce vertical growth when branches are subjected to extended periods of darkness. Consistent with this, Tradescantia plants treated with the photosynthetic inhibitor DCMU grew upward, mimicking their growth in dark conditions (Digby and Firn, 2002). Treatment with NF results in triplet chlorophyll formation, and also decreases nuclear gene expression involved in multiple photosynthetic processes, including the light-harvesting complex, electron transfer chain, PSII oxygen-evolving complex, and the reductive pentose phosphate pathway (Gray et al., 2003), effectively reducing the function of multiple early steps in photosynthesis. The loss of TAC1 expression in response to NF and DCMU treatment may suggest that PSII function is required. PQ effectively reduces PSI function, later in photosynthesis, and also generates ROS production. The increase of expression in response to PQ suggests that TAC1 does not require PSI, and may be sensitive to ROS signaling. Taken together, it is likely that TAC1 functions downstream of photosynthesis as a regulator of branch angle.

Both sucrose treatment and photoreceptor-mediated light signaling play roles in setting lateral organ angles (Willemoes et al., 1988; Pantazopoulou et al., 2017; Roychoudhry et al., 2017). However, neither had a strong influence on TAC1 expression. Growth in FR light both decreased TAC1 expression and led to narrower branch angles, but TAC1 remained relatively unaffected by R/FR signaling components. Recent work demonstrated that PIF4 is not required for shade-induced reduction in lateral branch angle (Roychoudhry et al., 2017). Our finding that TAC1 expression is unchanged in a pifq mutant background is consistent with this finding. B light led to elevated levels of TAC1 in several experiments. However, large increases in expression, in the case of 35S::TAC1 plants, had little effect on increasing branch angle. Together, these data suggest that the influence of both sucrose, and B and R/FR light signaling on organ orientation is largely TAC1 independent.

TAC1 expression exhibited some rhythm in a circadian time-course; however, the loss of TAC1 expression that is correlated with branch angle changes only occurred after a prolonged period of darkness. This is consistent with the observation that these major phenotypic angle changes do not occur over a daily time span, but rather after prolonged shade or dark. We propose that TAC1 is involved in a mechanism to influence changes to organ angles only when long-term unfavorable photosynthetic conditions have been sensed by the plant.

Of the light signaling mutants tested, cop1-6 mutants had the strongest effect on TAC1 gene expression. However, the effect of COP1 appears to be independent of phytochrome or cyptochrome-mediated signaling, as other mutants within these pathways exhibited little to no change. Recent work has implicated COP1 in chloroplast retrograde signaling, revealing that COP1 degrades ABI4 in the light during de-etiolation (Xu et al., 2016). The requirement for COP1 raises the question of whether TAC1 is regulated by retrograde signaling. Similarly, the differential responses of TAC1 expression to chemical inhibitors of photosynthesis, particularly the decrease in expression with NF treatment, and the increase in response to PQ, suggest that retrograde signals involving ROS production may be involved in regulating TAC1 and influencing branch angles. Previous work from our group has demonstrated TAC1 expression as strongest in branch tips and in leaves (Dardick et al., 2013), placing TAC1 in appropriate locations to be involved in these processes; however, it is unclear how communication between leaves and branch tips could be occurring. The data presented here suggest a possible signaling pathway from photosynthesis through COP1 or ROS signals and TAC1 to regulate branch angles. Here we present a simple model (Fig. 6), proposing how TAC1 may function as part of a feedback mechanism by which plants modify branch orientations to optimize light capture and photosynthetic efficiency. When branches experience a prolonged loss of light exposure, possibly due to severe shading by neighboring branches or other physical objects in the landscape, slow loss of TAC1 expression results in eventual upward branch growth. Once the branch grows above the barrier and light capture is regained, TAC1 expression is re-activated and the shaded branch regains a more horizontal growth habit to maximize light exposure and photosynthetic capacity. This model proposes that TAC1 is a key player in a novel mechanism by which plants modify their architecture in response to the environment to improve light capture conditions for photosynthetic efficiency.

Supplementary data

Supplementary data are available at JXB online

Fig. S1. Light spectra of LED bulbs used in this study.

Fig. S2. The dwarfed cop1-6 phenotype in comparison with Col wild-type plants.

Acknowledgements

We would like to thank the labs of Kerry Franklin, Chentao Lin, Ken-ichiro Shimazaki, Xing Wang Deng, Jennifer Nemhauser, and In-Cheol Jang for providing seeds of the light signaling mutants. We thank Amy Tabb for her assistance with light spectral measurements. The work at AFRS was supported by Agriculture and Food Research Initiative Competitive grant 10891264 from the USDA National Institute of Food and Agriculture and by the National Science Foundation grant number 1339211.

Author contributions

JMW designed experiments and performed analyses. JMW wrote the manuscript with help from CD.

References