The advent of genetic engineering has revolutionized agriculture remarkably with the development of superior insect-resistant crop varieties harboring resistance against insect pests. Bacillus thuringiensis (Bt) has been used as a main source for insect-resistant genes. In addition to Bt endotoxins, various plant lectins and other non-Bt genes from different sources have also been introduced in crop plants of economic importance. The insect-resistant crops have made a huge economic impact worldwide since their commercial release. The cultivation of insect-resistant cultivars has resulted both in increased crop productivity and in decreased environmental pollution. Although insect-resistant crops have been allowed to be commercialized following proper biosafety guidelines and procedures, still these crops face many challenges in order to be fully adopted and accepted. The degradation kinetics of Bt proteins, horizontal and vertical gene flow, effects on nontarget insects or organisms, antibiotic resistance, and some other unintended effects have been noted and discussed. Although no concrete evidence regarding any significant hazard of genetically engineered crops has been presented so far, the debate still remains intense. Impartial and professionally competent regulatory mechanisms for the evaluation of insect-resistant and other transgenic crops must be fully functionalized. The first part of this review focuses the development of different insect-resistant crops and various strategies adapted to delay resistance development in insect pests, while the second part addresses the challenges and future prospects of insect-resistant crops.

Most of the commercialized Bt crops express cry genes under 35S promoter that induces strong gene expression in all plant parts. However, targeted foreign gene expression in plants is esteemed more important as public may be likely to accept 'less intrusive' expression of transgene. We developed plant expression constructs harboring cry1Ac gene under control of wound-inducible promoter (AoPR1) to confine Bt gene expression in insect wounding parts of the plants in comparison with cry1Ac gene under the control of 35S promoter. The constructs were used to transform four Turkish cotton cultivars (GSN-12, STN-468, Ozbek-100 and Ayhan-107) through Agrobacterium tumefaciens strains GV2260 containing binary vectors p35SAcBAR.101 and AoPR1AcBAR.101 harboring cry1Ac gene under control of 35S and AoPR1, respectively. Phosphinothricin (PPT) was used at concentration of 5 mg L-1 for selection of primary transformants. The primary transformants were analyzed for transgene presence and expression standard molecular techniques. The transformants exhibited appreciable mortality rates against larvae of Spodoptera exigua and S. littoralis. It was found that mechanical wounding of T 1 transgenic plants was effective in inducing expression of cry1Ac protein as accumulated levels of cry1Ac protein increased during post-wounding period. We conclude that use of woundinducible promoter to drive insecticidal gene(s) can be regarded as a valuable insect-resistant management strategy since the promoter activity is limited to insect biting sites of plant. There is no Bt toxin accumulation in unwounded plant organs, seed and crop residues, cotton products and by-products, thus minimizing food and environmental concerns.

Beet necrotic yellow vein virus (BnYVV) is the cause of rhizomania, an important disease of sugar beetaround the world. The multipartite genome of the BNYVV contains four or five single-stranded RNA that has been used to characterize the virus. Understanding genome composition of the virus not only determines the degree of pathogenicity but also is required to development of resistant varieties of sugar beet. Resistance to rhizomania has been conferred to sugar beet varieties by conventional breeding methods or modern genome engineering tools. However, over time, viruses undergo genetic alterations and develop new variants to break crop resistance. Here, we report the occurrence of genetic reassortment and emergence of new variants of BnYVV among the isolates of thrace and Asia Minor (modern-day Turkey). Our findings indicate that the isolates harbor European A-type RNA-2 and RNA-3, nevertheless, RNA-5 is closely related to East Asian J-type. Furthermore, RNA-1 and RNA-4 are either derived from A, B, and P-types or a mixture of them. The RNA-5 factor which enhance the pathogenicity, is rarely found in the isolates studied (20%). The creation of new variants of the virus emphasizes the necessity to develop new generation of resistant crops. We anticipate that these findings will be useful for future genetic characterization and evolutionary studies of BNYVV, as well as for developing sustainable strategies for the control of this destructive disease.Rhizomania is one of the most destructive soil-borne diseases of sugar beet (Beta vulgaris L.) worldwide. Since the first report of rhizomania 1 numerous studies have reported the worldwide infection of sugar beet fields with this disease. Tamada and Baba 2 first identified Beet necrotic yellow vein virus (BNYVV) as the cause of rhizomania when they isolated the virus from infected plants of sugar beet fields in Japan. This disease reduces sugar content by 8%, root yield up to 90%, and sugar yield up to 80% 3,4 . The BNYVV genome is multipartite and composed of four single-stranded RNA species designated as RNA-1, RNA-2, RNA-3, and RNA-4, coating with a 21-kDa protein 5 . In addition, a fifth RNA species (RNA-5) has been identified in some of the European and Asian BNYVV isolates 6-12 . RNA-1 and RNA-2, which contain 6746 and 4612 nt-long RNA species, respectively, encode viral "housekeeping" genes involved in virus replication, assembly, cell-to-cell movement and suppression of post transcriptional gene silencing 13,14 . Therefore, when the virus vector Polymyxa betae Keskin 15 is not present, RNA-1 and RNA-2 are required for the maintenance of BNYVV in the environment 8,14,16 . RNA-3 consisting of a 1775 nt-long RNA species, is involved in viral pathogenicity 7,10,11,17,18 . RNA-4 (1431 nt) plays a key role in transmission of the virus by P. betae 7,11,13,19 . RNA-5 (1342-1347 nt in length) is associated with rhizomania severity, but is not required for virus survival 20,21 . Comparative studies revealed that the RNA-1, RNA-4, and RNA-5 contribute to the development ...

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Alfalfa is a fodder crop that accounts for one of the best sources of protein and is widely cultivated around the world. In vitro regeneration of alfalfa has been studied earlier; however, most of the studies were almost intervened with callus formation. In this study, 3 explant sources (cotyledonary node, hypocotyl, and root crown) of two Turkish cultivars (Nimet and Savaş) were excised from young seedlings. Explants were subjected to different concentrations of BAP, BAP-IBA, and TDZ to evaluate the direct in vitro regeneration potential of selected plant parts. Moreover, we transformed the alfalfa plant with pBin19 harboring 35s.GUS-INT_35s.nptII construct to investigate the transformation efficiency and regeneration frequency after bacterial inoculation. The transformation was carried out by Agrobacterium tumefaciens strain GV2260. The highest mean number of regenerated shoots per explant was recorded as 8.5, 6.66, and 6.33 shoots per explant after cotyledonary node explants were treated with BAP (0.40 mg/L), BAP-IBA (1.25-0.06 mg/L), and TDZ (0.55 mg/L), respectively. The highest gene transformation frequency was 9.52% and 6.19% based on PCR and GUS assays. The regeneration frequency was decreased by up to 48.1% under kanamycin selection pressure. The effect of cultivar on shoot regeneration frequency, mean number of regenerated shoots, and gene transformation efficiency was significant. This study contributes to in vitro regeneration of alfalfa crop and its genetic transformation which could be utilized in future gene transformation studies.