No abstract

SummaryThe bHLH transcription factor MYC2, together with its paralogues MYC3 and MYC4, is a master regulator of the response to the jasmonate (JA) hormone in Arabidopsis (Arabidopsis thaliana). In the absence of JA, JASMONATE ZIM (JAZ) proteins interact with the MYC proteins to block their activity. Understanding of the mechanism and specificity of this interaction is key to unravel JA signalling.We generated mutant MYC proteins and assessed their activity and the specificity of their interaction with the 12 Arabidopsis JAZ proteins.We show that the D94N mutation present in the atr2D allele of MYC3 abolishes the interaction between MYC3 and most JAZ proteins. The same effect is observed when the corresponding conserved Asp (D105) was mutated in MYC2. Accordingly, MYC2 D105N activated target genes in the presence of JAZ proteins, in contrast to wild-type MYC2. JAZ1 and JAZ10 were the only JAZ proteins still showing interaction with the mutant MYC proteins, due to a second MYC interaction domain, besides the classical Jas domain. Our results visualize the divergence among JAZ proteins in their interaction with MYC proteins. Ultimately, the transferability of the Asp-to-Asn amino acid change might facilitate the design of hyperactive transcription factors for plant engineering.

Plant organ size and shape are major agronomic traits that depend on cell division and expansion, which are both regulated by complex gene networks. In several eudicot species belonging to the rosid clade, organ growth is controlled by a repressor complex consisting of PEAPOD (PPD) and KINASE-INDUCIBLE DOMAIN INTERACTING (KIX) proteins. The role of these proteins in asterids, which together with the rosids constitute most of the core eudicot species, is unknown. We used CRISPR-Cas9 genome editing to target SlKIX8 and SlKIX9 in the asterid model species tomato (Solanum lycopersicum) and analyzed loss-of-function phenotypes. Loss of function of SlKIX8 and SlKIX9 led to the production of enlarged, dome-shaped leaves and these leaves exhibited increased expression of putative SlPPD target genes. Unexpectedly, kix8 kix9 mutants carried enlarged fruits with increased pericarp thickness due to cell expansion. At the molecular level, protein interaction assays indicated that SlKIX8 and SlKIX9 act as adaptors between the SlPPD and SlTOPLESS co-repressor proteins. Our results show that KIX8 and KIX9 are regulators of organ growth in asterids and can be used in strategies to improve important traits in produce such as thickness of the fruit flesh.