Plant growth homeostasis and defense responses are regulated by BONZAI1 (BON1), an evolutionarily conserved gene. Here, we show that growth regulation by BON1 is mediated through defense responses. BON1 is a negative regulator of a haplotype-specific Resistance (R) gene SNC1. The bon1-1 loss-of-function mutation activates SNC1, leading to constitutive defense responses and, consequently, reduced cell growth. In addition, a feedback amplification of the SNC1 gene involving salicylic acid is subject to temperature control, accounting for the regulation of growth and defense by temperature in bon1-1 and many other mutants. Thus, plant growth homeostasis involves the regulation of an R gene by BON1 and the intricate interplay between defense responses and temperature responses.