During development, cells interpret complex, often conflicting signals to make optimal decisions. Plant stomata, the cellular interface between a plant and the atmosphere, develop according to positional cues including a family of secreted peptides, EPIDERMAL PATTERNING FACTORS (EPFs). How these signaling peptides orchestrate pattern formation at a molecular level remains unclear. Here we report that Stomagen/EPF-LIKE9 peptide, which promotes stomatal development, requires ERECTA (ER)-family receptor kinases and interferes with the inhibition of stomatal development by the EPF2-ER module. Both EPF2 and Stomagen directly bind to ER and its co-receptor TOO MANY MOUTHS. Stomagen peptide competitively replaced EPF2 binding to ER. Furthermore, application of EPF2, but not Stomagen, elicited rapid phosphorylation of downstream signaling components in vivo. Our findings demonstrate how a plant receptor agonist and antagonist define inhibitory and inductive cues to fine-tune tissue patterning on the plant epidermis.

Chloroplast biogenesis is an essential light-dependent process involving the differentiation of photosynthetically competent chloroplasts from precursors that include undifferentiated proplastids in leaf meristems, as well as etioplasts in dark-grown seedlings. The mechanisms that govern these developmental processes are poorly understood, but entail the coordinated expression of nuclear and plastid genes. This coordination is achieved, in part, by signals generated in response to the metabolic and developmental state of the plastid that regulate the transcription of nuclear genes for photosynthetic proteins (retrograde signaling). Variegation mutants are powerful tools to understand pathways of chloroplast biogenesis, and over the years our lab has focused on immutans (im) and variegated2 (var2), two nuclear gene-induced variegations of Arabidopsis. im and var2 are among the best-characterized chloroplast biogenesis mutants, and they define the genes for plastid terminal oxidase (PTOX) and the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, respectively. To gain insight into the function of these proteins, forward and reverse genetic approaches have been used to identify second-site suppressors of im and var2 that replace or bypass the need for PTOX and AtFtsH2 during chloroplast development. In this review, we provide a brief update of im and var2 and the functions of PTOX and AtFtsH2. We then summarize information about second-site suppressors of im and var2 that have been identified to date, and describe how they have provided insight into mechanisms of photosynthesis and pathways of chloroplast development.

The immutans (im) variegation mutant of Arabidopsis has green and white-sectored leaves due to the absence of fully functional plastid terminal oxidase (PTOX), a plastoquinol oxidase in thylakoid membranes. PTOX appears to be at the nexus of a growing number of biochemical pathways in the plastid, including carotenoid biosynthesis, PSI cyclic electron flow, and chlororespiration. During the early steps of chloroplast biogenesis, PTOX serves as an alternate electron sink and is a prime determinant of the redox poise of the developing photosynthetic apparatus. Whereas a lack of PTOX causes the formation of photooxidized plastids in the white sectors of im, compensating mechanisms allow the green sectors to escape the effects of the mutation. This manuscript provides an update on PTOX, the mechanism of im variegation, and findings about im compensatory mechanisms.

Cell-fate in eukaryotes is regulated by MAP Kinases (MAPKs) that translate external cues to cellular responses. In plants, two MAPKs, MPK3/6, regulate diverse processes of development, environmental response, and immunity. Yet, the mechanism bridging these shared signaling components with a specific target remains unresolved. Focusing on the development of stomata, epidermal valves for gas exchange and transpiration, we report here that the bHLH protein SCREAM functions as a scaffold by recruiting MPK3/6 to downregulate SPEECHLESS, a transcription factor initiating stomatal cell lineages.SCREAM directly binds with MPK3/6 through an evolutionarily-conserved yet unconventional bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 at the 2.75Å resolution unraveled bipartite binding of SCREAM with MPK6, that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism directly linking extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell-fate, and highlight a unique substrate-binding mode adopted by plant MAPKs.