Histone Methyltransferases Regulate Responses to Biotic and Abiotic Factors in Tomato

Carol Nunurai Bvindi, Purdue University

Abstract

Plants are constantly exposed to biotic and abiotic factors throughout their developmental stages which threaten their growth and productivity. Environmental stresseslimit crop productivity and are likely to increase in severity due to the drastic and rapid changes in global climate. In this project, we studied the genetic factors that contribute to plant adaption to pathogens and other environmental factors in tomato. The results of these are presented in chapters 2-4 of this thesis. Chapter 1 covers background information and the review of the current literature in plant responses to biotic and abiotic stress. Chapter 2 deals with functional analysis of tomato histone methyltransferases sdg33 and sdg34 and their role in plant defense and stress tolerance. Chapter 3 focuses on the role of sdg33 and sdg34on plant responses to Nitrogen. Finally, Chapter 4 summarizes the results from a reverse genetic screen using CRISPR cas9 genome editing to identify Receptor Like Cytoplasmic Kinases (RLCKs) required for plant resistance to fungal pathogens. Plant responses to environmental cues are underpinned by rapid and extensive transcriptional reprogramming. Post translational modification of histones orchestrate these reprogramming and cellular responses by altering chromatin structure and establishing permissive or repressive states. Histone lysine methylation (HLM) is a principal modification of chromatin that affects various cellular processes. HLM is mediated by histone methyltransferases (HMTs) that deposit methyl groups to specific lysine residues on n-terminal histones tails. Although it is known that chromatin modifications occur in response to environmental cues, the mechanisms by which this is achieved, and the biological functions of HMTs are poorly understood. The function of tomato histone methyltransferases Set Domain Group (SDG)33 and SDG34 in biotic and abiotic stress responses were studied using tomato mutants generated through CRISPR/cas9 genome editing. SDG33 and SDG34 genes were induced by pathogens, drought stress, the plant hormones methyl jasmonate, salicylate and abscisic acid. The sdg33 and sdg34 mutants display altered global HLMs. SDG34 is required for global H3K36 and H3K4 mono, di- and tri-methylation while SDG33 is primarily responsible for di- and tri- H3K36 and H3K4 methylation. Tomato SDG33 and SDG34 are orthologues of the Arabidopsis SDG8, an H3K4 and H3K36 methyl transferase previously implicated in plant immunity and plant growth through epigenetic control of Carotenoid Isomerase (CCR2) and other target genes. However, the tomato sdg33 or sdg34 single mutants showed no altered responses to fungal and bacterial pathogens likely due to functional redundancy of the tomato SDG33 and SDG34 genes consistent with their overlapping biochemical activities. Interestingly, tomato SDG33 or SDG34 genes rescued the disease susceptibility and early flowering phenotypes of Arabidopsis sdg8 mutant. Expression of CCR2 gene is completely inhibited in Arabidopsis sdg8 mutant attributed to loss of H3K36 di- and tri methylation at CCR2 chromatin. CCR2 gene expression was partially restored by transgenic expression of tomato SDG33 or SDG34 genes in Arabidopsis sdg8. In tomato, the single CCR2 gene is expressed independent of SDG33 or SDG33 genes suggesting that the genomic targets of the tomato HMTs are different. Unexpectedly, sdg33 and sdg34 plants were more tolerant to osmotic stress, maintain a higher water status during drought which translated to better survival after drought. Tolerance of sdg33 and sdg34to drought stress is accompanied by higher expression of drought responsive genes. Collectively, our data demonstrate the critical role of tomato HLM in pathogen and stress tolerance likely through the regulation of gene expression.

Degree

Ph.D.

Advisors

Mengiste, Purdue University.

Subject Area

Physiology|Biochemistry|Bioinformatics|Endocrinology|Genetics|Pathology|Plant sciences|Systematic biology|Virology

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