FLA11 and FLA12 glycoproteins fine-tune stem secondary wall properties in response to mechanical stresses.

Secondary cell walls (SCWs) in stem xylem vessel and fibre cells enable plants to withstand the enormous compressive forces associated with upright growth. It remains unclear if xylem vessel and fibre cells can directly sense mechanical stimuli and modify their SCW during development. We provide data that two Arabidopsis thaliana SCW-specific Fasciclin-Like Arabinogalactan-proteins 11 (FLA11) and 12 (FLA12) are possible cell surface sensors regulating SCW development in response to mechanical stimuli. Plants overexpressing FLA11 (OE-FLA11) showed earlier SCW development compared to wild-type (WT) and altered SCW properties that phenocopy WT plants under compression stress. In contrast, OE-FLA12 stems showed higher cellulose content compared to WT, similar to plants experiencing tensile stress. fla11, OE-FLA11, fla12, OE-FLA12 plants showed altered SCW responses to mechanical stress compared to WT. Q-PCR and RNA-seq analysis revealed the upregulation of genes and pathways involved in stress responses and SCW synthesis and regulation. Analysis of OE-FLA11 nst1 nst3 plants suggests FLA11 regulation of SCWs is reliant on classical transcriptional networks. Our data support the involvement of FLA11 and FLA12 in SCW sensing complexes to fine-tune both the initiation of SCW development and the balance of lignin and cellulose synthesis/deposition in SCWs during development and in response to mechanical stimuli.