Prospects of Engineering Male Sterility-Fertility Restoration System for Exploiting Hybrid Vigour in Safflower

Abstract

Safflower, an important oilseed crop, has a tremendous potential for increased production through the development of hybrids. However, attempts to exploit heterosis in this crop using conventional male sterility systems are hampered by problems. An alternative is the development of genetically engineered male sterility. Several factors like the choice of the gene, promoter or availability of an efficient and reproducible transformation protocol determine the probability of success of transgenic male sterility systems. Major strategies successfully adopted to create transgenic male sterility in plants include expression of cytotoxic genes, down regulation of important genes, induction of conditional male sterility, creation of hormonal imbalance, expression of genes causing male sterility in CMS lines of other crops and exploitation of cellular processes like RNA editing. There are several reports on the development of transgenic male sterile plants but very few on the development of appropriate restorer lines which are very essential for the applicability of the system for hybrid seed production programs. Fertility restoration in these systems has been achieved by blocking the male sterility-inducing gene at the level of protein, RNA or DNA. As a complete system, male sterility induction through barnase gene and restoration by using the barstar gene, has been exploited commercially in many crops. In India, this system has been used in Brassica juncea by researchers at both public and private sector and the hybrids are now under field trials. Our efforts are focused on the development of transgenic male sterility-fertility restoration systems in safflower by following by two strategies i.e expressing the orfH522 gene from sunflower and the un-edited versions of the mitochondrial genes (u-nad3 and u-atp9) from safflower. It is envisaged that the male sterility could be induced by ablating the tapetal cell layer in the anther and fertility in such transgenic lines could be achieved by down regulating the genes used for induction of male sterility. Suitable gene constructs have been generated for induction of male sterility as well as fertility restoration by using the selected genes and transgenic plants have been developed by deploying these constructs. Analyses of the transgenics have indicated that both the strategies described, could induce male sterility successfully in safflower and tobacco. In case of orfH522 constructs, the restoration of fertility and segregation of the transgenes has also been demonstrated. As future line of work, further modification in the safflower regeneration and transformation protocols are needed to overcome the problems faced during the present investigation and to achieve the ultimate goal of realizing a good transgenic male sterility-restoration system in safflower. It can be concluded that the transgenic approach for induction of male sterility and fertility restoration has proven to be a technology of immense value and adds a