Chapter 6: Plant–Microbe Interactions

Plant–Microbe Interactions explores the biochemistry of nitrogenase, the intricate symbiotic relationship between rhizobia and leguminous plants (involving bacteroids and leghemoglobin), and the genetic regulation of nif and nod genes to improve nitrogen assimilation or expand host ranges. A significant portion of the chapter is dedicated to the mechanics of gene transfer, highlighting Agrobacterium tumefaciens as nature's genetic engineer. It explains the biology of the Ti plasmid, the function of the vir region's two-component regulatory system (VirA and VirG), and the Type IV secretion system used to inject T-DNA into plant cells. The chapter differentiates between cointegrate and binary vector systems and discusses alternative direct transfer methods like biolistics (gene gun) and electroporation, which are essential for monocots and chloroplast transformation. Furthermore, it addresses the challenges of gene silencing and RNA interference (RNAi) in transgenic expression. The discussion concludes with a survey of commercial applications, including herbicide resistance (glyphosate and phosphinothricin), insect resistance via codon-optimized Bacillus thuringiensis (Bt) toxins, virus resistance through coat protein expression, and stress tolerance mechanisms involving compatible osmolytes like glycine betaine. Finally, it reviews efforts to improve nutritional quality and yield, such as the development of Golden Rice enriched with beta-carotene, iron fortification, and the modification of photosynthetic pathways.