The KAI2 ‐ SMAX1 / SMXL2 Module Regulates Organ Size Through Interaction With Other Hormone Pathways in Arabidopsis thaliana

ABSTRACT Organ size is regulated by multiple genes and hormonal pathways through cell number and size. Karrikins (KARs) are smoke‐derived chemicals and presumed to mimic an unknown endogenous hormone, whose signaling functions in the regulation of plant growth and development. Here, we found that the KAR receptor KAR INSENSITIVE 2 (KAI2)‐deficient kai2 mutant plants were larger than wild‐type (WT) plants in terms of rosette leaves, siliques, petals, and seeds. Consistently, the KAR signaling negative regulator‐deficient mutant plants, suppressor of max2‐1 ( smax1 , s1 ) and smax1‐like 2 ( smxl2 , s2 ) double mutant ( s1 s2 ) plants, showed smaller sizes of the above‐mentioned organs than WT. In pairwise comparisons, ‘ kai2 s1 versus WT’ and ‘ kai2 s2 versus kai2 ’, all these genotypes displayed comparable sizes of these organs. Detailed investigations of cell size indicated that kai2 plants have larger cells than WT plants with respect to leaf mesophyll and seed coat, while s1 s2 plants have smaller cells. Comparative transcriptome analyses of ‘ kai2 versus WT’, ‘ max2 versus WT’, and ‘ s1 s2 versus WT’ using rosette leaves indicated that several pathways related to organ size and hormones, including abscisic acid, auxin, cytokinins, and jasmonic acid, are regulated by KAR signaling. These results suggest that KAR signaling inhibits organ size by restricting cell expansion with the involvement of genes involved in various hormone signaling pathways and organ size determination.