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Petal, Sepal, Or Tepal? B-Genes and Monocot Flowers

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Petal, Sepal, Or Tepal? B-Genes and Monocot Flowers 6. Javed, S. et al. (2016) A transgenic approach to control tabaci. By contrast, Javed et al. [6] the control of a phloem-specific promoter hemipteran insects by expressing insecticidal genes under expressed a plant defence protein (a lec- should be more effective [6] and be more phloem-specific promoters. Sci. Rep. 6, 34706 7. Raza, A. et al. (2016) RNA interference based approach to tin) and a toxin (Hvt) from the venom of a acceptable from a biosafety standpoint. At down regulate osmoregulators of whitefly (Bemisia tabaci): spider to provide broad-spectrum resis- least one toxin, Tma12, has provided dual potential technology for the control of whitefly. PLoS ONE 11, e0153883 tance against multiple sap-sucking pests, begomovirus-B. tabaci resistance; other 8. Shukla, A.K. et al. (2016) Expression of an insecticidal fern including B. tabaci, mealybug, and toxins and RNAi plants remain to be protein in cotton protects against whitefly. Nat. Biotechnol. aphids. Raza et al. [7] and Thakur et al. tested for effectiveness against begomo- 34, 1046–1051 9. Thakur, N. et al. (2014) Enhanced whitefly resistance in [9] used the RNAi strategy to kill insects by viruses. Shukla et al. [8] tested resistance transgenic tobacco plants expressing double stranded expressing dsRNAs in plants to silence to the important B. tabaci-transmitted RNA of v-ATPase A gene. PLoS ONE 9, e87235 vital B. tabaci osmotic regulators (aqua- viruses causing cotton leaf curl virus dis- 10. Whitfield, A.E. et al. (2015) Insect vector-mediated trans- mission of plant viruses. Virology 479-480, 278–289 porin and alpha glucosidase) and an actin ease [8]. The possible mechanism of ortholog (v-ATPase). To maximize the pro- action of Tma12 is binding to the chitin tection, Raza et al. [7] simultaneously tar- polymers that are a major component of geted two genes through RNAi by fusing the exoskeleton of insects [8]. However, Spotlight RNA sequences derived from two B. the precise mechanism of action of Tma12 tabaci genes. Collectively, these studies needs to be determined. Other economi- Petal, Sepal, or show a novel trend: targeting B. tabaci cally important begomovirus diseases, Tepal? B-Genes and to obtain dual begomovirus and B. tabaci such as tomato yellow leaf curl disease resistance (Figure 1). and cassava mosaic disease, remain to be Monocot Flowers evaluated. Engineering Durable B. tabaci 1,@, Steven Dodsworth * Resistance Taken together, these findings not only Genetically modified (GM) crops express- provide exciting ideas for controlling B. ing Bacillus thuringiensis (Bt) toxins, which tabaci and begomoviruses, but also raise In petaloid monocots expansion of are insecticidal endotoxins of bacterial ori- interesting research questions (Figure 1). If B-gene expression into whorl 1 of gin, is the most widely applied technology followed through, this trend of engineering the flower results in two whorls in the history of GM crops; several Bt dual B. tabaci–begomovirus resistance of petaloid organs (tepals), as transgenic crop species are grown world- using novel toxins and dsRNA could result wide over a vast area. This indicates that opposed to sepals in whorl 1 of in a breakthrough as important as once fl the use of an effective toxin is by far the typical eudicot owers. Recently, provided by crops expressing Bt toxins. most practical approach in the develop- new gene-silencing technologies ment of insect-resistant plants. Lessons fi Resources have provided the rst functional learned from the commercialization of Bt i www.issg.org/pdf/publications/worst_100/english_100_ data to support this, in the genus crops in the past must be considered worst.pdf Tricyrtis (Liliaceae). before taking any other GM crops 1 National Institute for Biotechnology and Genetic fi expressing a toxin to the eld. Pyramiding The ABC Model of Flower Engineering (NIBGE), Faisalabad, Pakistan multiple toxins and/or dsRNAs to interfere Development with different pathways of the target insect *Correspondence: [email protected] Since its inception in the 1990s, modifica- (S. Mansoor). has been the most rational approach for tions to the iconic ABC model of floral http://dx.doi.org/10.1016/j.tplants.2016.11.005 developing long-term resistance in the organ development have been pouring past, and should be considered to engi- in through studying species far removed References neer durable B. tabaci resistance. More- 1. Gilbertson, R.L. et al. (2015) Role of the insect supervectors from the original eudicot models thale Bemisia tabaci and Frankliniella occidentalis in the emer- over, the combined effect of Bt toxins cress (Arabidopsis thaliana) and garden gence and global spread of plant viruses. Annu. Rev. Virol. – (against chewing pests) and the recently 2, 67 93 snapdragon (Antirrhinum majus). Expres- 2. Mansoor, S. et al. (2006) Geminivirus disease complexes: investigated toxins (against sucking pests) sion studies across a diversity of angio- the threat is spreading. Trends Plant Sci. 11, 209–212 should ideally provide broad-spectrum sperm species have shown that minor 3. Fondong, V.N. et al. (2016) Novel functional genomics resistance. approaches: a promising future in the combat against plant alterations in patterns of MADS-box viruses. Phytopathology 106, 1231–1239 (ABC) gene expression result in substan- 4. Zaidi, S.S. et al. (2016) Engineering plants for geminivirus Bemisia tabaci is a phloem feeder (Fig- fl resistance with CRISPR/Cas9 system. Trends Plant Sci. tial homeotic changes to owers (organ 21, 279–281 ure 1) and most begomoviruses are transformation). These in turn can account 5. Hanley-Bowdoin, L. et al. (2013) Geminiviruses: masters at phloem restricted. For this reason, for a great proportion of floral diversity redirecting and reprogramming plant processes. Nat. Rev. – expressing toxins against B. tabaci under Microbiol. 11, 777 788 seen in nature. In the typical ABC model, 8 Trends in Plant Science, January 2017, Vol. 22, No. 1 A-function defines sepals – leaf-like floral categorised within a broad class of ‘C’- data to back them up, these expression organs of the perianth in whorl 1 of the function (female reproductive organ iden- studies could be seen as merely correla- flower. In whorl 2, the combined expres- tity genes) (the revised ABC model has tive. Otani et al. [6] provide the first func- sion of A- and B-function genes defines been recently reviewed [7]). tional data for a modified ABC model in the petals; in whorl 3 both B- and C-genes genus Tricyrtis – toad lilies (Liliaceae; define stamens (male organs), and in Petaloid Monocots: Expansion of Figure 1A,B). Using chimeric repressor whorl 4 C-function alone dictates carpels B-Gene Expression gene-silencing technology (CRES-T) and (female organs) [1]. Several groups of monocots possess two Agrobacterium-mediated genetic transfor- whorls of petaloid organs (tepals) that vary mation, they developed several transgenic Two further classes of genes were since from identical in tulips to highly differentiated lines of Tricyrtis with partial to severe sup- added to this simple scheme (classes D in orchids. Expansion of B-gene expression pression of B-gene expression (reduced to fi and E). D-class genes are involved in ovule correlates with petaloid tepals in the rst 7–8% of the expression level in wild-type fl development, and act in combination with whorl of these monocot owers. There have plants). This resulted in the conversion of C-class genes. Phylogenetically they are been several studies of B-gene expression whorl 1 and 2 floral organs from petaloid closely related, and D-genes may be best in other petaloid monocots from different tepals to greenish sepal-like organs, and, thought to represent the subfunctionalisa- orders including the Liliales, Asparagales, in addition, the whorl 3 stamens were con- tion of C-function with a role in ovule Alismatales, Commelinales, and Zingiber- verted to carpeloid structures [6], as pre- development only. A similar situation ales [4,5,7,8]. Expansion of the B-gene dicted by the modified ABC model. occurs in B-genes, where TM6 genes expression domain is a common feature fl have a subfunctionalised B-function role of oral development in monocots that More B-Genes, More Petaloid in stamen development alone. TM6 cop- have whorl 1 tepals as opposed to sepals Possibilities? ies have been lost from both Arabidopsis (for example tulips, muscari, agapanthus, In other monocots, paralogous (duplicate) and Antirrhinum, but they are found in lilies, orchids). However, without functional B-genes often display unique expression many other eudicots including all Solana- ceae (tomatoes and relatives). (A) (C) A Revised ABC Model E-class genes (SEPALLATA) are required for conferring floral identity, and expres- sion of ABC genes in combination with E- genes is sufficient to produce ectopic flowers instead of vegetative organs [2]. (D) Determination of the floral state, in other words floral meristem identity, is inextrica- bly linked with the ground-state of floral organs, in other words sepals. A-function itself appears to be independently acquired in Arabidopsis and, in all other species tested, A-function mutations affect sepal identity and meristem identity (B) W1 W2 W3 W4 W3 W2 W1 together [3]. Further evidence for a revised (E) Ot It St Ca St It Ot A-function comes from the fact that the A- A function genes are broadly expressed, B and indeed this is also the case in mono- C cots, where A-function genes are E expressed in most whorls of the flower and other vegetative organs [4–6]. E- genes are also closely related to A-genes Figure 1. (A) Floral morphology of Tricyrtis sp. ‘Jasmin’, and (B) modified ABC model adapted from Otani et al. phylogenetically. Therefore, A and E-func- (2016) showing the different expression domains of ABC genes in outer tepals (Ot), inner tepals (It), stamens (St), tion can be categorised within a broad and carpels (Ca), in whorls 1–4 of the flower.
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