Abiotic and Biotic Stress-Tolerance Mechanisms Operative in Brassicaceae Species

Stress tolerance mechanisms play a vital role in plant productivity and reproduction. Plants are subjected to a myriad of environmental stresses that are categorized into two broad categories: abiotic and biotic. Plant responses to these stresses are complex and involve several morphological, physiological, cellular and molecular adaptations. In this dissertation, stress tolerance mechanisms operative in the Brassica species Arabidopsis and Arugula have been explored. Plant growth promoting rhizobacteria (PGPR) have been shown to mediate growth promotion as well as inducible plant defenses. While enhanced protection against pathogen infection has been extensively studied, the role of PGPR in protecting plants against herbivore attack is just beginning to yield to scientific enquiry. Here a previously unidentified mechanism by which the growth-promoting rhizobacterium Bacillus amyloliquefaciens (GB03) transcriptionally activates genes responsible for sulfur assimilation, increasing sulfur uptake and accumulation in Arabidopsis is reported. More specifically, induced sulfur-rich glucosinolates were observed to confer greater protection against the generalist herbivore, Spodoptera exigua (beet armyworm). With respect to growth promotion by GB03, direct measurements of auxin accumulation and biosynthesis rates were performed utilizing stable isotope labeling. The abiotic environmental stress of soil salinity has also been examined in terms of tolerance via evolved natural selection and inducible responses. Although several key ion channels and transporters involved in Na+ homeostasis have been characterized in Arabidopsis, natural genetic variation of the high-affinity potassium transporter (HKT1) in Brassica members utilized in agriculture have yet to be characterized. Here Eruca sativa, a species closely related to Arabidopsis and now grown as the salad crop arugula and its natural salt tolerant ecotypes were used to examine HKT-mediated salt tolerance. Although high concentrations of NaCl imposes deleterious effects on plant growth, here we examined the implications of mild NaCl concentrations on inducible plant tolerance mechanisms.