Temperature has a large impact on plant growth, development, and defense responses. We are using natural variations in Arabidopsis to reveal molecular mechanisms underlying temperature modulation of plant growth and defense responses. A Genome Wide Association Study on a hundred Arabidopsis thaliana natural accessions identified MORN1 as a candidate gene for natural variations in thermosensitivity of disease resistance. Genetic studies revealed that MORN1 is a regulator of disease resistance, temperature stress tolerance, and plant growth. The loss of function mutant of MORN1 exhibited enhanced susceptibility to virulent Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) at lower temperature. It is also more susceptible to heat and freezing stresses than the wild type. In addition, the loss of function mutant has a slower growth than the wild type while overexpressing of MORN1 leads to enhanced plant growth. The MORN1 gene is highly polymorphic, with variations in both coding sequences and promoter regions among Arabidopsis natural accessions. Using transgenic approach, we show that variations in MORN1 differ in their activities in promoting plant growth and heat tolerance. When transformed into the morn1 mutant, the MORN1 haplotypes from disease susceptible accessions at low temperature exhibited less heat tolerance than haplotypes from disease resistant accessions. The MORN1 gene codes for a protein with several MORN domains that are associated with lipids. We found that the MORN1 protein is localized to lipid droplet. Preliminary study shows that lipid droplet accumulation is altered in the morn1 mutants. These data together suggest a role of lipid droplet in regulating plant growth, immunity and temperature stress tolerance.