auxin

Ljung, Karin et al. 2001

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Ljung, Karin, Rishikesh P. Bhalerao, and Göran Sandberg. 2001. Sites and homeostatic control of auxin biosynthesis in Arabidopsis during vegetative growth. The Plant Journal 28:465-474.

This article focuses on auxin levels and how they affect plant growth and development. It was observed that auxin levels must be regulated and maintained in order to permit continued growth. The study suggests that IAA (auxin) is synthesized at specific sources via a feedback-inhibition mechanism. Through observation, it was found that leaf size and IAA levels were inversely related. In young Arabidopsis seeds, IAA levels start high and drop off post leaf expansion, which means that younger leaves have a higher capacity to synthesize IAA and higher levels of IAA correlate with higher rates of cell division.

http://onlinelibrary.wiley.com/doi/10.1046/j.1365-313X.2001.01173.x/full

Sakata et al 2010

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Sakata, Tadishi, Takeshi Oshino, Shinya Miura, Mari Tomabechi, Yuta Tsunaga, Nahoko Higashitani, Yutaka Miyazawa, Hideyuki Takahashi, Masao Watanabe, Atsushi Higashitani. 2010. Auxin reverse plant male sterility caused by high temperatures. PNAS Vol 107:8569-8574.

http://www.pnas.org/content/107/19/8569.full.pdf+html

The activation of auxin biosynthesis with increased temperature has been reported in many plant tissues. Under high temperatures, it has been found that endogenous auxin levels have decreased in the anthers of both barley and Arabidopsis. Increased temperature also suppression YUCCA expression and the application of auxin completely reversed the sterility of the plant species. This study suggests that tissue specific auxin reduction is the cause of injury to a plant due to increased temperature. This in turn leads to the plants abortion of pollen development, however, when temperatures were lowered, the plant became fertile once more. The expression of IAA under high temperatures decreased by 50%, showing the damage that  high temperature has on the expression of YUCCA and IAA in the biosynthesis of auxin.

Thingnaes et al 2003

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Thingnaes, Elin, Sissel Torre, Arild Ernstsen, Roar Moe.  2003. Day and Night Temperature Responses in Arabidopsis: Effects on Gibberellin and Auxin Content, Cell Size, Morphology, and Flowering Time. Annals of Botany Vol 92:601-612.

This paper looks at the effects of sixteen different day and night temperature levels on the rosette growth, flower stem elongation, and flowering time of Arabidopsis. As night temperature was increased, the final leaf length decreased because both elongation rate and period were reduced. Day temperature had no effect on leaf length.  Increased day temperature resulted in an increased stem length because of the increased elongation rate. However, it decreased with an increase in night temperature due to the shortened elongation period. Transitioning to a flowering period was increased by the increase of night temperatures. Cell volume was increased in flower stems during D22/N12 as compared to D12/N22 because of higher auxin levels under the D22/N12 temperature settings.  IAA is an essential factor for stem elongation. This study has implied that auxin plays a role in the elongation rate of the flower stem and circadian growth rhythm of the flower stem in Arabidopsis. The findings indicate that thermoperiodic responses on stem elongation may be mediated by altered IAA levels.

Shibasaki et al. 2009

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Shibasaki, Kyohei, et al. “Auxin Response In Arabidopsis Under Cold Stress: Underlying Molecular Mechanisms.” Plant Cell 21.12 (2009): 3823-3838. Academic Search Premier. Web. 11 Feb. 2015.

In this experiment, Shibasaki et al. examined the impact on root growth and gravity response after exposing Arabidopsis thaliana seedlings to cold temperatures for differing periods of time to determine the role of auxin in cold stressed plants. After exposure to cold, the roots were observed for 9 hours as the temperature returned to 23°C and the plant was rotated.  The findings suggest that cold stress affects auxin transport pathways.  A transport assay showed that auxin accumulation occurred in the meristem, resulting in inhibited root growth and gravitropism because basipetal polar transport of auxin was inhibited.  These findings also support previous results that exposure to cold does not affect auxin signaling.  The cold response in the plants was temporary and after returning to normal temperatures the roots returned to normal gravity response and growth rate.

This experiment then focused on the proteins responsible for auxin redistribution in the root meristem through the use of mutants.  Although PIN3 has previously been pinpointed as having a key role in auxin redistribution, Shibaskaki et al. believe that it is only partially responsible and that actin may play a key role in the inhibition of PIN protein trafficking under cold conditions.

http://www.plantcell.org.ezproxy.sju.edu/content/21/12/3823.full.pdf+html

Dane Gallagher

Galweiler et al 1998

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Galweiler, Leo, Changhui Guan, Andreas Muller, Ellen Wisman, Kurt Mendgen, Alexander Yephremov, Klaus Palme. 1998. Regulation of Polar Auxin Transport by AtPIN1 in Arabidopsis Vascular Tissue. Science. Vol. 282:2226-2230.

Auxin is transported downward from the plant tip providing directional information, differentiation of the vascular tissues, apical development, regeneration of organs, tropic growth, and cellular elongation. The chemiosmotic hypothesis suggests that the driving force of auxin transport is provided by the transmembrane proton motive force and that the efflux of auxin anions is controlled by auxin-specific carriers in shoots at the basal end of transport cells. The pin-formed mutants if Arabidopsis inhibit auxin transport to some degree in the inflorescence axes. For plants that do not have this mutation, auxin flow is vital for the forming of spatially organized patterns of vascular tissue. PIN mutants show an increase of vascular tissue just below where young leaves connect to the axial vascular system, reducing the drainage of auxin from the leaves causing an increase in xylem growth there. Similar variations were seen in terms of axial vascular growth as well.

Rietveld et al. 2000

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Rietveld, Patrick L., Clare Wilkinson, Hanneke Franssen, Peter Balk, Linus van der Plas, Peter Weisbeek, and A. Douwe de Boer. 2000. Low temperature sensing in tulip (Tulipa gesneriana L.) is mediated through an increased response to auxin. Journal of Experimental Botany 51: 587-594

Tulips cannot properly grow without enduring a period of low temperature; however, this article says that the longer the response to a cold environment, the less auxin was needed to achieve the same results. Although warmer temperatures help initiate flowering, the extended cooler temperatures allows the plant to undergo stalk elongation. Auxin is a plant hormone known to aid in stem elongation. In the case of tulips, auxins must interact with gibberellins to facilitate this process.

http://jxb.oxfordjournals.org/content/51/344/587.full.pdf+html

Thingnaes, Elin, et al. 2003

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Thingnaes, Elin, Sissel Torre, Arild Ernstsen, and Roar Moe. 2003. Day and Night Temperature Responses in Arabidopsis: Effects on Gibberellin and Auxin Content, Cell Size, Morphology and Flowering Time. Annals of Botany 92 : 601-612.

This article describes how temperature affects the growth of plants and “thermoperiodism,” which is how varying day and night temperatures cause different responses in plants. Plant growth and development is controlled by multiple hormones, including auxin and gibberellins. When day temperatures are warmer than night temperatures, plants grow more than the reverse conditions. In the experiment, 16 different day and night temperature combinations were tested to observe the effects on plant growth. Looking at A. thaliana, temperature influenced the time and rate of elongation so that warmer daytime temperatures resulted in longer stems but warmer nighttime temperatures decreased growth.

http://aob.oxfordjournals.org/content/92/4/601.full.pdf+html

Leopold, Guernsey, 1953

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Leopold, A. C., and F. S. Guernsey. “Interaction Of Auxin And Temperatures In Floral Initiation.” Science 118.(1953): 215-217. Readers’ Guide Retrospective: 1890-1982 (H.W. Wilson). Web. 9 Feb. 2015.

http://www.jstor.org.ezproxy.sju.edu/stable/1680981?seq=1#page_scan_tab_contents

In this study Leopold and Guernsey looked into the effects of auxin (NAA) treated seeds and leaves on numbers of flowers produced at high and low temperatures, using different plant species.  For the first part of this experiment, Biloxi soybean seeds were soaked in varying concentrations of auxin solution (0-1 ppm) for 24 hours and then exposed to either 3°C or 18°C temperatures.  On an 18 hour daylight cycle, they found that the plants grown under lower temperatures produced a more flowers at greater concentrations of auxin while increasing temperatures produced fewer flowers at greater concentrations of auxin.

The second part of this experiment focused on exposing cut soybean leaves to varying auxin solutions (0-100 ppm) at 10°C and 20°C, and observing the effects on flower number again.  They found that at the lowest auxin concentration (0.1 ppm) there was an increase in flower number in the lower temperature group only, while otherwise increasing auxin concentration resulted in decreased flower numbers for both groups.

The third part of the experiment involved vernalization of winter rye intact seeds and excised embryos followed by growth in 18° or 3° environment, and although Leopold and Guernsey reported no effect of auxin on flower production at the high temperature of the excised group, there was a slight decrease in the auxin exposed group.  The other results were similar to the findings of the first part of the experiment with increased flowering at low temperatures and decreased flowering at high temperatures when exposed to auxin.

Orbovi & Poff 2007

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Orbovi, Vladmir, Kenneth L Poff. Effect of Temperature Growth and Phototropism of Arabidopsis thaliana Seedlings. 2007. Plant Growth Regul.

Arabidopsis were grown at 25 C, and within 150 min demonstrated a growth response to a temperature change. Regardless of whether the change were in a positive or negative direction, the growth rates of the plants was reduced after exposure to the new temperature. The results suggest that changing temperature affects the elongation rate of Arabidopsis seedlings.  Phototropism causes the side of the plant experiencing light to grow at a slower rate than the side that has been shaded in order to cause the plant to bend toward where the light is coming from – this growth response is thought to be regulated by auxin.  The lowest elongation rate was seen at 9 C with the highest being at the standard 25 C.

http://link.springer.com/article/10.1007/s00344-007-9009-4#page-1

van Zanten et al 2009

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Van Zanten, Martijin, Laurentius ACJ Voesenek, Anton JM Peeters, Frank F Millenaar. Hormone and light-mediated regulation of heat-induced differential petiole growth in Arabidopsis.  2009. Plant Physiology

Sudden increases in temperature induce differential petiole growth-drive upward movement of the leaf in Arabidopsis. This article provides implications for the subjugation of heat-induced growth to natural selection.  Geographical observation revealed that Arabidopsis at lower latitudes are more erect than those in more northern areas. Another report confirmed that increases in temperature lead to an increase in the incline of the leaves. Auxin has been shown to  be associated with high temperature response. The experiment tested to determine the involvement of auxin in heat-induced growth by pharmacologically inhibiting auxin efflux carriers. These treatments greatly reduced the growth response to increased temperature. The data collected suggests that auxin signaling is required for heat-induced growth in Arabidopsis.

http://www.plantphysiol.org/content/151/3/1446.full.pdf+html