Potential of Entomopathogenic Bacillus thuringiensis as Plant Growth Promoting Rhizobacteria and Biological Control Agents for Tomato Fusarium Wilt

Tomato (Solanum lycopersicum) is an important vegetable crop. In tomato cultivate, it is often hampered by vascular wilt caused by Fusarium oxysporum f.sp. lycopersici (FOL)W.C. Synder and H.N.Hans. FOL is a serious fungal pathogen of the tomato plant. The most important symptom of this pathogen is vascular wilt. FOL can cause a huge loss of tomato quantity and quality, and it has the ability to colonize the roots of a large number of fireweeds and to produce resistant spore structures, so it can exist in mostsoils (Ailton et al., 2007). This pathogen is difficult to control with traditional methods, such as preplant soil fumigation and other cultural methods, so it is a better way to choose bio control to control this disease (Archana et al., 2010).

 In general, some effective means of controlling FOL include disinfection of the soil and planting material by using fungicidal chemicals, crop rotation with non-hosts of the fungus, or by using resistant cultivars (Ana et al., 1997; Benhamou et al., 1998). Since the current available control methods, such as chemical control, are either low efficient or difficult to apply, methods to control Fusarium wilts caused by FOL have been studied for a long time (Kotan et al., 2009). The chemical control measures destroy balances in the microbial community, which may cause the loss of abundance beneficial organisms, such as insect natural enemies, and may also lead to the evolution of resistant strains of the pathogen. Breeding resistant plant varieties can be difficult in the absence of dominant genes and development of new races of the pathogen that overcome host resistance (Jetiyanon and Kloepper, 2002). Furthermore, increasingly, there are more restrictions on utilization of fungicides due to public concern about harmful effects on the human health and environment, and residues in the food. Hence, there is a need for developing attractive management strategies that are virtually efficient and environmentally safe (Sudhamoy et al., 2009). 

Bacillus thuringiensis has been used as an effective bioinsecticide (Roh et al., 2007; Schnepf et al., 1998). The specificity ofB. Thuringiensis is showed highly beneficial in agricultural biotechnology. Unlike most insecticides, B. thuringiensis insecticides are highly toxic against target insects and friendly towards beneficial insects, non-target organisms such as humans and wildlife (Bravoet al., 2011). It is also not harmful to the environment. B. thuringiensis has been used as an alternative to chemical pesticides for decades by organic farmers to control insects. At present, B. thuringiensis is the only "microbial insecticide" in widespread use (Cherif et al., 2003, 2008; Dong et al., 2002). 

Plant growth promoting rhizobacteria (PGPR) promote plant growth and suppress plant disease by colonizing plant roots, reducing plant pathogen populations in the soil, and maintaining a beneficial effect on plant growth. Similarly, the sporulating gram-positive bacteria such as Bacillus spp. have also been used successfully as potential biological control agents (BCAs) to control plant disease (Kloepper et al., 2004). Also, some studies have reported that Bt was successful endophytic colonization insoybean, cotton, cabbage, rice bean and so on, even with concomitant production of Crytoxins; the efficient Bt colonization of cabbage seedlings roots suggests this might be in fact the main route of its penetration in the plant (Argôlo-Filho, et al., 2014). Similarly, Bt was able to colonize the roots of certain legumes, which resulted in an increase of nodulation and growth of the plants and Bt produces toxins that can reduce pests or diseases attacks (Mishra, et al., 2009). Recently, B. thuringiensis has also attracted great attention as a biological control agent to suppress plant diseases (Zhou, Yet al., 2008). Therefore, the new view is that the insecticide B. thuringiensis can be used as PGPR to control plant disease. Moreover, the activity of B. thuringiensis which can suppress bacterial wilt disease has been examined in tomato plants (Hyakumachi et al., 2013). 

The objective of this study was to confirm whether B. thuringiensis could suppress the wilt disease caused by Fusarium oxysporum f.sp. lycopersici (FOL), and to verify if B. thuringiensis can act as plant growth promoting rhizobacteria (PGPR), which can promote plant shoot elongation and seed germination.