Advances in Bioscience and Biotechnology, 2013, 4, 15-20 ABB http://dx.doi.org/10.4236/abb.2013.410A3003 Published Online October 2013 (http://www.scirp.org/journal/abb/) Homeobox leucine zipper proteins and cotton improvement Muzna Zahur1,2*, Muhammad Ahsan Asif1, Nadia Zeeshan1, Sajid Mehmood1, Muhammad Faheem Malik1, Abdul R. Asif3 1Department of Biochemistry and Molecular Biology, University of Gujrat, Gujrat, Pakistan 2Department of Neurology, University Medical Center Goettingen, Goettingen, Germany 3Department of Clinical Chemistry, University Medical Center Goettingen, Goettingen, Germany Email: *muzna.zahoor@uog.edu.pk Received 7 July 2013; revised 7 August 2013; accepted 1 September 2013 Copyright © 2013 Muzna Zahur et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Transcription factors play key roles in plant develop- ment and stress responses through their interaction with cis-elements and/or other transcription factors. Homeodomain associated leucine zipper proteins (HD-Zip) constitute a family of transcription factors that are characterized by the presence of a DNA- binding domain closely linked with leucine zipper motif functioning in dimer formation. This type of association is unique to plants and considered as an excellent candidate to activate developmental respons- es to altering environmental conditions. Cotton is the most important fiber plant with a lot of local and commercial uses in the world. HD-Zip proteins not only have key roles in different stag es of vascular and inter-fascicular fiber differentiation of cotton but also are suggested to have an important role against abio- tic stress that is one of the key factors limiting cotton productivity. Plants have developed various strategies to manag e stress cond itions through a combination of metabolic, physiological and morphological adapta- tions. These adaptive changes rely largely on altera- tions in gene expression. Therefore, transcriptional regulators play a crucial role in stress tolerance. Be- ing a transcription factor HD-Zip might be a useful target for genetic engineering to generate multiple stress tolerance in susceptible plants. In the following chapter, we discussed how the HD-Zip proteins would play a useful role for cotton development both in fiber production and stres s adaptati on . Keywords: Cotton; Stress; Transcription Factor; HD-Zip Proteins; Homeobox Leucine Zipper 1. ENVIRONMENTAL STRAINS AND PLANTS Plants are exposed to a variety of stress factors that pre- vent them from attaining their full genetic potential. This can be due to insects, fungal infections, weeds, bacteria or viruses, all of which are known as biotic factors. The abiotic stress factors include drought, salinity, flooding, oxidative stress, heavy metal, cold and high temperature [1,2]. Abiotic stress, in fact, is the major cause of crop failure worldwide. It dips average yields for most major crops by more than 50% [3]. When plants are subjected to the stress, they respond through various cellular signal transduction pathways, which result in accumulation of certain differentially ex- pressed gene products that can be classified as functional and regulatory proteins. Functional proteins include wa- ter channel proteins, key enzymes for osmolyte biosyn- thesis, chaperones, LEA (late embryogenesis abundant) proteins, proteinases and detoxicating enzymes. Regula- tory proteins include transcription factors, protein kinas- es, and phospholipases. Regulatory proteins are involved in the further regulation of signal transduction and gene expression of stress tolerant proteins [4-7]. Improving the crop plant potential to endure different abiotic stress- es will lead to more yields by either enhancing the crop set or expanding crop cultivation in the areas previously refuted due to stress intolerance. 2. GOSSYPIUM (COTTON) Cotton is an important cash crop known as white gold due to its valuable fiber production and oilseeds [8]. A large number of ginning factories and textile mills great- ly depend upon cotton. However, Cotton yield is greatly affected by many factors, such as the variety grown, cul- tivation method, environmental and climatic conditions, *Corresponding author. OPEN ACCESS
M. Zahur et al. / Advances in Bioscience and Biotechnology 4 (2013) 15-20 16 amount and application strategy of fertilizers, time of sowing and availability of irrigation water [9]. There are 50 diverse species of the genus Gossypium. Four species are cultivated, G. hirsutumL. and G. barba- dense L., that are tetraploid (2n = 4x = 52), and G. arbo- reum L. and G. herbaceum L., that are diploid (2n = 2x = 26). The most extensively cultivated species throughout the world is G. hirsutum. Whereas the diploid cotton spe- cies are a pool for important disease resistance and pest control genes, and improved agronomic and fiber quail- ties but also have better opportunities for structural and functional studies of genes through advanced systems of gene knockouts [10]. Asiatic G. arboretum L (Desi cot- ton.) has built-in desirable genes for drought tolerance and resistance to insect pests like bollworms, aphids and diseases like black arm, root rot, reddening of leaves and most importantly, highly destructive leaf curl disease of cotton. Its diploid genome makes it a good choice for the identification of novel genes in genus Gossypium [11]. Some of the cotton genotypes are more tolerant through an intricate set of genetic parameters including sensing, signal transduction and response. Due to the large num- ber of genes participating in response to an external stress, improvement through conventional breeding is very difficult. Conventional breeding has developed many new cultivars and varieties; however it has some limita- tions like thousands of genes getting transferred in each cross and the barriers for gene transfer through incom- patibility and species differences [12]. Genetic engineer- ing technology has made possible the insertion of desired foreign gene(s) to overcome problems of sexual incom- patibility and species barriers between organisms. This technology helps the breeders and molecular biologists to introduce only the gene of interest with more selective modification and represents a significant advance [13]. In this background we require those cotton varieties which resist these biotic and abiotic stresses. This resistance in cotton to various stresses can be gained by improving the cotton plant through stress resistant genes with special emphasis on stress responsive transcription factors con- trolling the multiple genes involved in stresses [14]. Se- veral stress responsive transcription factor genes have been identified in G. hirsutum and G. arboreum such as WRKY, EREBP, NAC, HD-Zip and DREB genes [15- 19]. 3. TRANSCRIPTION FACTOR (TF) AGAINST STRESS Transcription factors are the sequence-specific DNA bin- ding proteins that control the transfer of genetic informa- tion from DNA to mRNA [20]. These are the first line of defense against stress stimuli that in turn activate the expression of other stress responsive genes. These tran- scription factors bind to the specific elements in the pro- moter regions called cis-acting elements and the tran- scription factors that bind to these elements are known as trans-acting factors. Several cis-acting promoter ele- ments and their subsequent binding proteins, each con- taining a distinct type of DNA binding domain, such as AP2/ERF, basic leucine zipper, HD-ZIP, MYB, MYC, and several classes of zinc finger domains, have been in- volved in plant stress responses due to their variable ex- pression under different stress conditions [21]. Combina- torial interactions of promoters DNA cis-acting elements with trans -acting protein factors are chief processes gov- erning spatio-temporal gene expression [22]. Most of the transcription factors are common among different plants in their motif structure and mode of ac- tion [23]. These are potent targets for genetic engineering of stress tolerance because a transcription factor is en- coded by a single gene but regulates the expression of se- veral other genes leading to the activation of complex adaptive mechanisms. Therefore, in transgenic plants transcription factors can confer better stress tolerance than a single gene transfer. This opens an excellent op- portunity to develop stress tolerant crops in future that can contribute to sustainable food and fiber production in the world [24]. Several transcription factor proteins have been identi- fied from different Gossypiun species and analyzed for their role in diverse stress and development conditions. A leucine zipper-containing WRKY protein named GaWRKY1 was isolated from G. arboreum using the CAD1-A, (a gene contributing in cotton sesquiterpene biosynthesis) promoter. In transgenic Arabidopsis plants and transiently transformed tobacco leaves expression of GaWRKY1 triggered expression of the CAD1-A pro- moter, and interruption of the W-box abolished the acti- vation [15]. Duan et al. [16] isolated two EREBP (ethyl- ene response element binding protein) genes named GhEREB2 and GhEREB3 suggesting their role as the positive transcription factors in biotic stress (ethylene and jasmonic acid) signal transduction pathways. A DRE-binding protein, GhDBP2, was isolated from G. hirsutum seedlings that participate in the activation of down-stream genes in response to environmental stresses and ABA treatment [17]. Another DRE binding protein, GhDREB, containing a conserved AP2/EREBP domain reported in G. hirsutum that is induced by drought, high salt and cold stresses in seedlings. GhDREB accumulates higher levels of soluble sugar and chlorophyll in leaves following to drought, high salt, and freezing stress treat- ments in transgenic wheat plants conferring enhanced to- lerance [25]. From the NAC (NAM, ATAF1, −2, and CUC2) gene family, six full-length, intact putative tran- scription factors were isolated from G. hirsutum (GhNAC1-GhNAC6) that showed differential gene re- gulation under dehydration, high salt, cold and ABA Copyright © 2013 SciRes. OPEN ACCESS
M. Zahur et al. / Advances in Bioscience and Biotechnology 4 (2013) 15-20 17 treatments [18]. 4. HD-ZIP PROTEINS HDZip proteins are characterized by the presence of a DNA-binding homeodomain (about 60 amino acid long) with a closely linked leucine zipper motif functioning in dimer formation [26]. The leucine zipper motif adjoining to the C-terminal of the homeodomain is assumed to form an amphipathic α-helix with a series of leucine re- sidues responsible for dimerization of a pair of target DNA contacting surfaces [27]. Leucine zippers are re- sponsible for the interaction of HD-Zip proteins among each other and with other leucine zipper proteins [28]. Homo- and heterodimer interactions may have important role in the function of these proteins [29]. This type of homeodomain association is present only in plants and it is considered that HD-Zip genes originated in plant line- age by exon exchange between a homeodomain gene and a leucine zipper containing sequence [30]. None of the nearly 350 homeobox genes examined in animal system contains a leucine zipper [31]. So far, the homeodomain-leucine zipper proteins have been identified in many plants such as sunflower [32,33], carrot [34], soybean [35], tomato [36], rice [37] and Ara- bidopsis [38]. These proteins have been suggested as ex- cellent candidates to activate developmental responses to altering environmental conditions, a characteristic fea- ture of plants. Numerous authors have suggested that ex- pression of HD-Zip transcription factors family is regu- lated by diverse external factors such as illumination or drought. HD-Zip proteins are categorized into four class- es (I - IV) based on gene structure, presence of unique domains and function [39]. A few HDZip family mem- bers are supposed to control the development of particu- lar plant regions, such as the vascular system is controll- ed by (ATHB8, class III, [40]; Oshox1, class II, [41] Va- hox1, class I, [42], and root hairs and trichomes (ATHB10, class IV, [43]. The Arabidopsis genes Athb2 and Athb4 (both class II) are highly induced by far-red light, indicating a role in the shade avoidance response [31]; Athb6, Athb7 and Athb12 are inducible by drought as well as ABA, imply- ing their putative function in dehydration responses [44, 45]. From C. plantagineum, two HDZipgenes (CPHB-1 and CPHB-2, class II) are dehydration-inducible, and one of them is ABA-inducible (CPHB-2) [46]. Therefore, they are thought to be involved in regulation of dehydra- tion responses through different branches of the dehydra- tion-induced signalling network, ABA-independent or ABA-dependent. Similar overexpression was observed in five families of Craterostigma plantagineum homeobox leucine zipper genes (CPHB) that were isolated by Deng et al. [47]. All families of CPHB genes modulate their expression against dehydration in leaves and roots. Aka- shi et al. [48] isolated an HD-Zip gene from Wild water- melon (Citrullus lanatus sp.) differentially expressed un- der drought stress. Expression and functional studies on the sunflower Hd-Zip II subfamily with special emphasis on Hahb-10 from sunflower indicated that the members are expressed primarily in mature photosynthetic tissues, and up-regulated by etiolation and gibberellins in seed- lings [49]. In vitro and in vivo binding assays have demonstrated that HDZip proteins from Arabidopsis, C. plantagineum, sunflower and rice preferentially bind to two 9-bp pseu- dopalindromic sequences, CAAT (A/T) ATTG (HDE1) and CAAT(G/C)ATTG (HDE2) [50]. A few other bind- ing sequences relevant to homeodomain proteins were reported in plants like: A soyabean homeodomain leucine zipper proteins bind to CATTAATTAG sequence present in the phosphate response domain of VspB promoter [51] and ATHB6 of plant-specific HD-Zip class targets the core motif (CAATTATTA) present in its own promoter that mediated ABA-dependent gene expression [52]. With the help of cis-acting elements, efforts to identify target genes in planta will contribute greatly to the understand- ing of HDZip function. This is important to provide fundamental molecular in- formation towards understanding of the biological roles of the HD-Zip proteins in cotton and present a valuable source for improving cotton varieties with resistance to abiotic stresses. 5. COTTON HD-ZIP PROTEINS A number of homeodomain leucine zipper protein of dif- ferent classes have been identified in different species of cotton such as GbHB1 from G. barbadense and GaHOX1 and GaHOX2 from G. arboreum that plays a role in fiber development [53] whereas G. hirsutum GhHB1 is involv- ed in root development and salt stress [19]. G. arboreum GaHDZ protein was identified as ABRE binding protein. It showed enhanced expression under salt, heavy metals and drought treatments (Author unpublished data). Re- cently three HD-Zip proteins designated as GhHB2, GhHB3 and GhHB4 were isolated from cotton cDNA library. All these proteins are suggested to be involved in early seedling development whereas expression of these Hb proteins was up-regulated in response to gibberellin signaling [54]. Another HD-Zipn IV family transcription factor, Meristem Layer 1 (GbML1) was isolated and cha- racterized from G. barbadense that interacted with a key regulator of cotton fiber development. When expressed in Arabidopsis, GbML1 increased the number of tri- chomes on stems and leaves and increased the accumula- tion of anthocyanin in leaves [55]. L1 layer-specific HD- ZIP gene from tetraploid G. hirsutum GhHD-1 is express- ed in trichomes and early fibres thus might play a role in cotton fibre initiation. Further microarray analysis of Copyright © 2013 SciRes. OPEN ACCESS
M. Zahur et al. / Advances in Bioscience and Biotechnology 4 (2013) 15-20 18 GhHD-1 lines indicated that it potentially regulates the levels of ethylene and reactive oxidation species (ROS) through a WRKY transcription factor and calcium-sig- nalling pathway genes to activate downstream genes ne- cessary for cell expansion and elongation [56]. 6. CONCLUSION Plants respond and adapt to environmental stresses through not only physiological and biochemical processes but also molecular and cellular processes. Several genes with various functions are induced by drought and cold stress- es, and those various transcription factors are involved in the regulation of these stress-inducible genes through their specific binding to the cis-acting elements of their promoters. Gaining an understanding of the mechanisms that regulate the expression of these genes is a funda- mental issue in plant biology and will be necessary for the genetic improvement of plants cultivated in extreme environments. 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