TIC236 gain-of-function mutations unveil the link between plastid division and plastid protein import

Abstract

TIC236 is an essential component of the translocon for protein import into chloroplasts, as evidenced by the embryonic lethality of the knockout mutant. Here, we unveil a TIC236-allied component, the chloroplast outer membrane protein CRUMPLED LEAF (CRL), absence of which impairs plastid division and induces autoimmune responses in Arabidopsis thaliana. A forward genetic screen exploring CRL function found multiple dominant TIC236 gain-of-function (tic236-gf) mutations that abolished crl-induced phenotypes. Moreover, CRL associates with TIC236, and a tic236-knock-down mutant exhibited multiple lesions similar to the crl mutant, supporting their shared functionality. Consistent with the defective plastid division phenotype of crl, CRL interacts with the transit peptides of proteins essential in plastid division, with tic236-gf mutations reinforcing their import via increased TIC236 stability. Ensuing reverse genetic analyses further revealed genetic interaction between CRL and SP1, a RING-type ubiquitin E3 ligase, as well as with the plastid protease FTSH11, which function in TOC and TIC protein turnover, respectively. Loss of either SP1 or FTSH11 rescued crl mutant phenotypes to varying degrees due to increased translocon levels. Collectively, our data shed light on the links between plastid protein import, plastid division, and plant stress responses.