YODA-HSP90 Module Regulates Phosphorylation-Dependent Inactivation of SPEECHLESS to Control Stomatal Development under Acute Heat Stress in Arabidopsis

Stomatal ontogenesis,patterning,and function are hallmarks of environmental plant adaptation,especially to conditions limiting plant growth,such as elevated temperatures and reduced water availability.The specification and distribution of a stomatal cell lineage and its terminal differentiation into guard cells require a master regulatory protein phosphorylation cascade involving the YODA mitogen-activated protein kinase kinase kinase.YODA signaling results in the activation of MITOGEN-ACTIVATED PROTEIN KINASEs(MPK3 and MPK6),which regulate transcription factors,including SPEECHLESS(SPCH).Here,we report that acute heat stress affects the phosphorylation and deactivation of SPCH and modulates stomatal density.By using complem entary molecular,genetic,biochemical,and cell biology approaches,we provide solid evidence that HEAT SHOCK PROTEINS 90(HSP90s)play a crucial role in transducing heat-stress response through the YODA cascade.Genetic studies revealed that YODA and HSP90.1 are epistatic,and they likely function linearly in the same developmental pathway regulating stomata formation.HSP90s interact with YODA,affectits cellular polarization,and modulate the phosphorylation of downstream targets,such as MPK6 and SPCH,under both normal and heat-stress conditions.Thus,HSP90-mediated specification and differentiation of the stomatal cell lineage couples stomatal development to environmental cues,providing an adaptive heat stress response mechanism in plants.

Alternative Transcription Initiation and the AUG Context Configuration Control Dual-Organellar Targeting and Functional Competence of Arabidopsis Lon 1 Protease

Cellular homeostasis relies on components of protein quality control including chaperones and proteases. In bacteria and eukaryotic organelles, Lon proteases play a critical role in removing irreparably damaged proteins and thereby preventing the accumulation of deleterious degradation-resistant aggregates. Gene expression, live-cell imaging, immunobiochemical, and functional complementation approaches provide conclusive evidence for Lonl dual-targeting to chloroplasts and mitochondria. Dual-organellar deposition of Lonl isoforms depends on both transcriptional regula- tion and alternative translation initiation via leaky ribosome scanning from the first AUG sequence context that deviates extensively from the optimum Kozak consensus. Organelle-specific Lonl targeting results in partial complementation of Arabidopsis Ion1-1 mutants, whereas full complementation is solely accomplished by dual-organellar targeting. Both the optimal and non-optimal AUG sequence contexts are functional in yeast and facilitate leaky ribosome scanning com- plementing the piml phenotype when the mitochondrial presequence is used. Bioinformatic search identified a limited number of Arabidopsis genes with Lonl-type dual-targeting sequence organization. Lon4, the paralog of Lonl, has an ambiguous presequence likely evolved from the twin presequences of an ancestral Lonl-like gene, generating a single dual-targeted protein isoform. We postulate that Lonl and its subfunctional paralog Lon4 evolved complementary sub- sets of transcriptional and posttranscriptional regulatory components responsive to environmental cues for dual-organel- lar targeting.