Essay on Phototropism

Generally speaking, phototropism is directional plant growth in which the direction of growth is determined by the direction of the light source. (Takagi 2003) Formerly it was sometimes called heliotropism. Phototropism is one of the many plant tropisms or movements in response to external stimuli. Growth toward a light source is a positive phototropism, while growth away from light is called negative phototropism. Even though most plant shoots exhibit positive phototropism, roots usually exhibit negative phototropism. (Takagi 2003) In fact, this type of tropism may play a more important role in root behavior and growth. For instance, some vine shoot tips exhibit negative phototropism, which allows them to grow toward dark, solid objects and climb them.

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Phototropism in ferns is regulated by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. (Takagi 2003) Different organs of the plant can exhibit different phototropic reactions to different wavelengths of light. While stem tips exhibit positive phototropic reactions to blue light, root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red light. (Takagi 2003)

Blue light-induced regulation of cell elongation is a component of the signal response pathway for both phototropic curvature and inhibition of stem elongation in higher plants. (Liscum et al., 1992) To determine if blue light regulates cell elongation in these responses through shared or discrete pathways, phototropism and hypocotyl elongation were investigated in several blue light response mutants. Specifically, the blue mutants that lack blue light-dependent inhibition of hypocotyl elongation were found to exhibit a normal phototropic response. (Khurana, Poff 1989) In contrast, a phototropic null mutant (JK218) and a mutant that has a 20- to 30-fold shift in the fluence dependence for first positive phototropism (JK224) showed normal inhibition of hypocotyl elongation in blue light. (Laskowsky et al 1989) F1 progeny of crosses between the blu mutants and JK218 showed normal phototropism and inhibition of hypocotyl elongation, and approximately 1 in 16 F2 progeny were double mutants lacking both responses. Therefore, blue light-dependent inhibition of hypocotyl elongation and phototropism operate through at least some genetically distinct components. (Cosgrove 1981) Red light does not significantly stimulate phototropism at lower, limiting irradiances of blue light. Phytochrome affects the amount or activity of a photoreceptor for phototropism. (Smith 1990)

Moreover, the amount of light and the phototropic reaction are linked. This connection is known in literature under the terms Bunsen-Roscoe law, product law or reciprocity theorem. (Britz, Carroll 1992) According to the Bunsen-Roscoe law, the product of time and intensity, and consequently the energy amount of the used light, is the measure of the stimulus strength. Therefore, it is of no importance whether a light stimulus of low intensity is applied for a longer period of time or whether a stimulus of high intensity is applied for a short time. (Britz, Carroll 1992) It is known today that the Bunsen-Roscoe law applies only in a very limited sense, since a certain minimal amount of light has to be present in order to trigger the reaction (threshold value), and an increase in light intensity does not always cause an increase in the phototropic reaction, but may in contrast suppress the positive reaction. A continuous increase causes a second positive reaction with a new maximum, that decreases again and rises once more (= 1., 2., and 3. positive phototropic bending). (Britz, Carroll 1992)

The order Polypodiales encompasses the major lineages of polypod ferns, which comprise more than 80% of today’s fern species. (Takagi 2003) This plant is often found growing on walls and tree trunks where it can catch more light. They are found in many parts of the world including tropical, semitropical and temperate areas. Polypodiales is considered to be one of the most evolutionarily advanced orders of ferns, based on recent genetic analysis. The polypodioid ferns apparently derived from the ancestors of the dryopteroid ferns, through the ancestors of the davallioid ferns. (Takagi 2003) They arose and diversified a mere 100 million years ago, probably subsequent to the diversification of the angiosperms.

Even though most plant species from algae to flowering plants use blue light for inducing phototropism and chloroplast movement, many ferns, some mosses, and green algae use red as well as blue light for the regulation of these responses, resulting in better sensitivity at low light levels. (Suetsugu 2005) During their evolution, ferns have created a chimeric photoreceptor (phy3 in Adiantum) between phytochrome (phy) and phototropin (phot) enabling them to use red light effectively. Two genes resembling Adiantum PHY3, NEOCHROME1 and NEOCHROME2 (MsNEO1 and MsNEO2) have been identified in the green alga Mougeotia scalaris, a plant famous for its light-regulated chloroplast movement. (Suetsugu 2005) Like Adiantum PHY3, both MsNEO gene products show phytochrome-typical bilin binding and red/far-red reversibility, the difference spectra matching the known action spectra of light-induced chloroplast movement in Mougeotia. (Suetsugu 2005)  In addition, both genes rescue red-light-induced chloroplast movement in Adiantum phy3 mutants, indicating functional equivalence. (Suetsugu 2005) At the same time, the fern and algal genes seem to have arisen independently in evolution, thus providing an intriguing example of convergent evolution. (Suetsugu 2005)

Plants have developed sophisticated photomovement responses such as chloroplast movement, phototropism, and stomatal opening to optimize photosynthesis. In most plant species these responses are mediated by the blue-light photoreceptor phototropin (phot) (Briggs et al., 2002) Yet cryptogams such as ferns, mosses, and green algae, but not angiosperms, also use red light for monitoring the direction of incident light (Suetsugu 2005). Most of these red-light responses show red/far-red reversibility, indicating phytochrome (phy) involvement. In the case of the fern Adiantum capillus-veneris, a chimeric photoreceptor phytochrome 3 (phy3) has arisen, in which the N terminus consists of a phytochrome sensory module attached to an almost complete phot sequence (Nozue et al., 1998). This photoreceptor mediates red-light-induced phototropism and chloroplast movement in polypodiaceous ferns, perhaps conferring an adaptive advantage in low light under dense canopies (Kawai et al., 2003, Schneider et al., 2004)

Red light also induces chloroplast movement, phototropism, and polarotropism in mosses. Although Physcomitrella patens have four conventional PHY and four PHOT genes, scientists have been unsuccessful in detecting a chimera resembling Adiantum PHY3. (Mittman et al., 2004) Since such a gene sequence is also absent from the extensive moss EST and nearly complete Physcomitrella genome databases, it is very unlikely that an Adiantum PHY3 homolog is present in mosses. In any case, targeted knockout of PHY and PHOT genes in Physcomitrella has shown that red-light-induced chloroplast movement in Physcomitrella is mediated by canonical phys with phots acting downstream (Mittman et al., 2004, Kasahara et al 2004)

On this basis it would seem that the AcPHY3 (Ac, Adiantum capillus-veneris) gene arose late in fern evolution. (Suetsugu 2005) However, in the filamentous green alga Mougeotia scalaris, a species famous for its phy-mediated chloroplast rotation, two AcPHY3-like genes are present and, moreover, that they are functionally equivalent to the fern gene in mediating chloroplast movement. Based on this, all three chimeric photoreceptors are placed in a new category, neochrome [hence, MsNEO1 (Ms, Mougeotia scalaris), MsNEO2, and Ac-NEO1 (= AcPHY3)]. Comparison of the algal and fern neos suggests that they have arisen independently, providing a most unusual example of convergent evolution. (Kasahara et al., 2004)

Since neither transgene expression nor mutagenesis techniques are established in ferns, it is difficult to demonstrate directly that neochromes regulate red-light-induced chloroplast movement in that species. At the same time, both MsNEO1 and MsNEO2 cDNAs in the Adi-antum rap2 mutant, in which a lesion in NEO (PHY3) leads to defective red-light-mediated phototropism and chloroplast movement (Kawai et al, 2003, Kadota, Wada 1999). MsNEO1- and/or MsNEO2-mediated rescue of rap2 would indicate functional equivalence. Indeed, when either MsNEO gene was cotransformed with GFP-producing construct, chloroplasts in GFP-expressing cells accumulated in response to red-light irradiation but not those in untransformed cells or in cells transformed with GFP only. (Kawai 2003) Blue light irradiation induced the accumulation response in both GFP-positive and -negative cells (that is, in cells transfected with GFP and NEO and in nontransfected cells), showing that the defect in red-light-induced chloroplast movement in the non-transgenic cells does not result from loss of cell viability or damage to the machinery responsible for chloroplast movement. (Suetsugu 2005) Because transient expression of both MsNEO genes rescue red-light-induced chloroplast movement in rap2 (neo), all three gene products have similar functional properties both in relation to light absorption and signal transduction. (Salomon et al., 2000)

Even though neochromes are now known in several polypodiaceous ferns and in Mougeotia, they are not represented in the genome sequences of any prokaryote, the green alga Chlamydomonas, the red alga Cyanidioschyzon, the diatom Thalassiosira, or the higher plants Arabidopsis, Oryza, or Populus. Furthermore, PCR-based searches or library screening for AcPHY3 homologues in more primitive ferns and the mosses Physcomitrella patens and Ceratodon purpureus were unsuccessful; moreover, such sequences are not present among the >200,000 available moss ESTs or in the currently available 109-bp genome sequence data for Physcomitrella (data not shown). Thus, if neochromes are monophyletic they must either have been retained specifically in very few lineages or have been transferred laterally. Detailed examination implies, rather, that they have arisen separately in ferns and algae. (Suetsugu 2005) Gene rearrangements in different organisms are known to arise via several molecular mechanisms including exon shuffling, duplication, and retrotransposition (Long et al., 2003). The present work together with that of Nozue et al. provides an example of chimeric gene products with the same structure and function having independently arisen twice. (Nozue et al., 1998) The complete absence of introns in fern NEO genes suggested that fern NEO genes may be generated by the disruption of preexisting exon-intron structures of PHOT genes by retrotransposons and subsequent fusion with a PHY fragment (Buzdin 2004) Because MsNEO genes retain several Mougeotia-specific introns in both PHY- and PHOT-like regions, in addition to an intron positioned at the apparent N terminus of the PHOT-like segment, they might be the product of exon shuffling between PHY and PHOT genes in that species.

Finally, phototropic reactions are characteristic for growing tissues, and are less easily detected in fully differentiated ones. (Quail 2002) This is on one hand caused by the cells’ loss of plasticity, and on the other hand by the development of mostly inflexible strengthening elements that set a mechanic resistance against each deformation of the tissue. In the shoot axis they are arranged in the periphery thus bringing about an especially high stability. (Nozaki et al., 2004)

Plants rely on sophisticated mechanisms to interpret the constant bombardment of incoming signals so they can adjust their growth accordingly. (Correll et al., 2002) The environmental cues of gravity and light are particularly important for plant growth and development. As gravitropism has been extensively studied in roots, there has been increased emphasis on understanding the cellular and molecular basis of root phototropism. Apart from the blue-light-based negative phototropism, roots also exhibit a recently discovered positive phototropism in response to red light. (Correll et al., 2002)

References

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