Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Priti Krishna

Abstract

Brassinosteroids (BRs) are a group of steroidal plant hormones that are essential for proper plant development and also promote stress tolerance. Without BRs, plants are dwarfs and infertile. To understand the molecular mechanisms underlying BR-mediated stress tolerance, global gene expression analysis of untreated and 24-epibrassinolide (EBR)-treated Arabidopsis thaliana seedlings under non-stress and heat stress (HS) conditions was carried out. Microarray data analysis indicated that stress-related genes were predominant within the EBR up-regulated gene data set. Furthermore, several of these genes were abscisic acid (ABA) and jasmonic acid (JA) related. Measurements of endogenous hormones showed significant increases in the levels ABA and JA in EBR treated vs. untreated A. thaliana seedlings. To understand the relationship between BR and ABA, untreated and EBR-treated ABA-deficient and ABA-insensitive mutants of A. thaliana were subjected to HS. The positive effect of EBR on HS tolerance was significantly greater in the ABA-deficient aba1-1 mutant as compared to WT, indicating that ABA masks BR effects on plant stress responses, which is opposite to the effect of BR on ABA responses.

Functional analysis of T-DNA insertion mutants of a subset of genes identified in the microarray screen showed that three genes encoding jacalin related-lectins, and one gene encoding NFYA5, have stress-related functions. Two genes related to calcium (Ca2+)-signaling, calmodulin-like Ca2+-binding protein 10 (CML10) and calmodulin-binding protein 50 (CaMBP50), were studied in detail using reverse genetics approaches. The T-DNA insertion cml10 mutant flowered earlier, had an average of 25% increase in seed yield, and was more resistant to osmotic stress as compared to WT. The transgenic lines overexpressing CML10 displayed the opposite phenotypes, indicating a negative role of CML10 in growth and stress responses in A. thaliana. On the other hand, the cambp50 mutant was more sensitive to salt stress and produced less seeds as compared to WT, while transgenic lines overexpressing CaMBP50 were more resistant to salt stress and had up to 41% increase in seed yield. Thus, CaMBP50 positively regulates these traits.

In summary, the results of the present study have 1) revealed that BR cross-talks with ABA and JA in conferring stress tolerance, 2) identified new stress-related genes, and 3) highlighted the importance of Ca2+-signaling in BR-mediated stress tolerance and growth in A. thaliana.