UV-B radiation and temperature stress causes variable growth response in Metarhizium anisopliae and Beauveria bassiana isolates

摘要

The biocidal effects exerted through the activity of living organisms are practically more feasible and sustainable than the chemical cure. But one of the major threats for the bio-control agents is the on-field exposure to abiotic stresses. In this context, the present in-vitro study was undertaken to investigate the effect of temperature and time-dependent exposure of UV-B radiations on seventeen Beauveria bassiana and fourteen Metarhizium anisopliae isolates. A comparative analysis based on germination, growth and sporulation was undertaken. It was observed that certain isolates showed significant tolerance to these abiotic stresses. In response to UV-B stress, the B. bassiana isolates were more tolerant than the M. anisopliae isolates. But under temperature stress, the M. anisopliae isolates were more tolerant with isolates showing germination, growth and sporulation potential up to 37oC. Asa conclusion, these stress tolerant entomopathogenic fungal isolates hold high affirmations for commercial market. Introduction Commercial biological control involving entomopathogenic fungi in the present global scenario is a hi-tech venture both in terms of safety and sustenance. But the credibility of hi-fidelity management of insect pests by fungus is besotted upon attaining, maintaining and novel sustaining of such fungal strains, in the turmoil multitude of abiotic stresses (1). The continuance of viability and virulence of fungal inoculum (conidia) after field application is the pre-requisite threshold for their efficacy (2). Various isolates of B. bassiana and M. anisopliae have been the most entrusted entomopathogens that have been heavily researched upon and find appreciable on-field commercial usage for insect pest management (3). Upon field application, the entomopathogens are exposed to an array of abiotic stresses like temperature (4), UV radiations (5), humidity-osmolarity (6), edaphic factors and nutrient source (7) that negatively affect the field use of entomopathogens as biocontrol agents. Continued abiotic stress can either have absolute deleterious effect or force natural selection in them (8).The solar radiation, which includes visible light, ultraviolet radiations, infrared rays and radio waves have been the dominant source in which all organisms evolved and adapted. In biological context, the UV radiations acclaim a special mention in terms of their impact on life (9). Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. When considering the effect of UV radiations on organisms and the environment, the range of UV wavelength is often subdivided into UV-A or long wave or black light (400-315 nm), UV-B or medium wave (315-280 nm), and UV-C or short wave or germicidal (? 280 nm). UV-photons, in particular those belonging to the UV-B type, form covalent bonds between adjacent thymine bases resulting in thymine dimers. Thymine dimers do not base pair normally, which causes distortion of the DNA helix, stalled replication, gaps and misincorporation. These can lead to mutations and ultimately disrupt the normal functioning of the organism (10). Soil temperature is a major factor, which affects the success or failure in the establishment and production of fungal inoculum (11). The entomopathogenic fungi not only have to be tolerant to the soil temperature but also have to survive through thermoregulatory defense response of the host insect (12, 13). It has been demonstrated that stress temperature alters the vegetative growth among isolates of entomopathogenic fungi (14). Dry heat exposure causes DNA damage through base loss leading to depurination and this may cause mutation (15). Wet heat i.e. heat in conjunction with high humidity results in protein denaturation and membrane disorganization. It has been reported that M. anisopliae has temperature tolerance upper limit as 37-40oC (11). B. bassiana on the other hand can survive up to a max