No corporation has more power than the government. European and British laws prevent the development of monopolies and cartels. The EU Competition Directive came into force at the end of 2002 and there are stringent controls and severe penalties in order to prevent such control occurring. Similar laws are in force in all major industrial countries.
The control of monopoly is a function of government, not of technology. How we use our scientific understanding to best advantage is a matter of public policy, in which we all have a say.
It is a fact of life that very expensive activities - like developing new crop plants, building cars or aeroplanes, or running supermarkets - can often be undertaken only by large companies. It is society generally that has to ensure such activities are properly conducted while recognising that without profits there are no companies, and without companies no products and no jobs. It is worth noting, incidentally, that the largest UK supermarket chain (with more than a quarter of the market share) has a turnover some seven times larger than the most famous GM seed company.
G. Conway (1997). The Doubly Green Revolution; Food for all in the 21st Century. Penguin (see reviews at http://www.isop.ucla.edu/profmex/volume4/3summer99/DGR.html)
Biotechnology for Developing Countries: Results of an Electronic Forum (2001). FAO, (http://www.fao.org/sd/2002/KN0501_en.htm)