Screening genome-editing knockouts reveals the receptor-like kinase ASX role in regulations of secondary xylem development in Populus

center dot In trees, secondary xylem development is essential for the growth of perennial stem increments. Many signals regulate the process of development, but our knowledge of the molecular components involved in signal transduction is still limited. center dot In this study, we identified Attenuation of Secondary Xylem (ASX) knockouts by screening genome-editing knockouts of xylem-expressed receptor-like kinases (RLKs) in Populus. The ASX role in secondary xylem development in Populus was discovered using biochemical, cellular, and genomic analyses. center dot The ASX knockout plants had abnormal secondary stem growth but had little effect on shoot apical primary growth. ASX and SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK)2/4 were co-precipitated in developing xylem. Through their interaction, ASX is phosphorylated by SERK. Transcriptome analysis of developing xylem revealed that ASX deficiency inhibited the transcriptional activity of genes involved in xylem differentiation and secondary cell wall formation. By forming a complex, ASX and SERK may function as a signaling module for signal transduction required in the regulation of secondary xylem development in trees. center dot This study shows that ASX, which encodes a RLKs, is required for secondary xylem development and sheds light on regulatory signals found in tree stem secondary growth.