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Plant shoots grow from stem cells within shoot apical meristems (SAMs), which produce lateral organs while maintaining the stem cell pool. In the model flowering plant Arabidopsis , the CLAVATA (CLV) pathway functions antagonistically with cytokinin signaling to control the size of the multicellular SAM via negative regulation of the stem cell organizer WUSCHEL (WUS). Although comprising just a single cell, the SAM of the model moss Physcomitrium patens (formerly Physcomitrella patens ) performs equivalent functions during stem cell maintenance and organogenesis, despite the absence of WUS-mediated stem cell organization. Our previous work showed that the stem cell–delimiting function of the receptors CLAVATA1 (CLV1) and RECEPTOR-LIKE PROTEIN KINASE2 (RPK2) is conserved in the moss P. patens . Here, we use P. patens to assess whether CLV–cytokinin cross-talk is also an evolutionarily conserved feature of stem cell regulation. Application of cytokinin produces ectopic stem cell phenotypes similar to Ppclv1a , Ppclv1b , and Pprpk2 mutants. Surprisingly, cytokinin receptor mutants also form ectopic stem cells in the absence of cytokinin signaling. Through modeling, we identified regulatory network architectures that recapitulated the stem cell phenotypes of Ppclv1a , Ppclv1b , and Pprpk2 mutants, cytokinin application, cytokinin receptor mutations, and higher-order combinations of these perturbations. These models predict that Pp CLV1 and Pp RPK2 act through separate pathways wherein Pp CLV1 represses cytokinin-mediated stem cell initiation, and Pp RPK2 inhibits this process via a separate, cytokinin-independent pathway. Our analysis suggests that cross-talk between CLV1 and cytokinin signaling is an evolutionarily conserved feature of SAM homeostasis that preceded the role of WUS in stem cell organization. 

Abstract Grass leaves develop from a ring of primordial initial cells within the periphery of the shoot apical meristem, a pool of organogenic stem cells that generates all of the organs of the plant shoot. At maturity, the grass leaf is a flattened, strap-like organ comprising a proximal supportive sheath surrounding the stem and a distal photosynthetic blade. The sheath and blade are partitioned by a hinge-like auricle and the ligule, a fringe of epidermally derived tissue that grows from the adaxial (top) leaf surface. Together, the ligule and auricle comprise morphological novelties that are specific to grass leaves. Understanding how the planar outgrowth of grass leaves and their adjoining ligules is genetically controlled can yield insight into their evolutionary origins. Here we use single-cell RNA-sequencing analyses to identify a ‘rim’ cell type present at the margins of maize leaf primordia. Cells in the leaf rim have a distinctive identity and share transcriptional signatures with proliferating ligule cells, suggesting that a shared developmental genetic programme patterns both leaves and ligules. Moreover, we show that rim function is regulated by genetically redundant Wuschel-like homeobox3 (WOX3) transcription factors. Higher-order mutations in maizeWox3genes greatly reduce leaf width and disrupt ligule outgrowth and patterning. Together, these findings illustrate the generalizable use of a rim domain during planar growth of maize leaves and ligules, and suggest a parsimonious model for the homology of the grass ligule as a distal extension of the leaf sheath margin. 

Grass inflorescence branching involves the integration of competing signals that regulate leaf and meristem growth. 

Plants maintain populations of pluripotent stem cells in shoot apical meristems (SAMs), which continuously produce new aboveground organs. We used single-cell RNA sequencing (scRNA-seq) to achieve an unbiased characterization of the transcriptional landscape of the maize shoot stem-cell niche and its differentiating cellular descendants. Stem cells housed in the SAM tip are engaged in genome integrity maintenance and exhibit a low rate of cell division, consistent with their contributions to germline and somatic cell fates. Surprisingly, we find no evidence for a canonical stem-cell organizing center subtending these cells. In addition, trajectory inference was used to trace the gene expression changes that accompany cell differentiation, revealing that ectopic expression of KNOTTED1 ( KN1 ) accelerates cell differentiation and promotes development of the sheathing maize leaf base. These single-cell transcriptomic analyses of the shoot apex yield insight into the processes of stem-cell function and cell-fate acquisition in the maize seedling and provide a valuable scaffold on which to better dissect the genetic control of plant shoot morphogenesis at the cellular level. 

ABSTRACT Introduction Materials and methods Results Discussion Conclusion Acknowledgements Disclosure statement Additional information References ABSTRACT Global health researchers often discount mutual learning and benefit to address shared health challenges across high and low- and middle-income settings. Drawing from a 30-year partnership called AMPATH that started between Indiana University in the US and Moi University in Kenya, we describe an innovative approach and program for mutual learning and benefit coined ‘reciprocal innovation.’ Reciprocal innovation harnesses a bidirectional, co-constituted, and iterative exchange of ideas, resources, and innovations to address shared health challenges across diverse global settings. The success of AMPATH in Kenya, particularly in HIV/AIDS and community health, resulted in several innovations being ‘brought back’ to the US. To promote the bidirectional flow of learning and innovations, the Indiana CTSI reciprocal innovation program hosts annual meetings of multinational researchers and practitioners to identify shared health challenges, supports pilot grants for projects with reciprocal exchange and benefit, and produces educational and training materials for investigators. The transformative power of global health to address systemic health inequities embraces equitable and reciprocal partnerships with mutual benefit across countries and communities of academics, practitioners, and policymakers. Leveraging a long-standing partnership, the Indiana CTSI has built a reciprocal innovation program with promise to redefine global health for shared wellbeing at a global scale. 

Summary The formation of developmental boundaries is a common feature of multicellular plants and animals, and impacts the initiation, structure and function of all organs. Maize leaves comprise a proximal sheath that encloses the stem, and a distal photosynthetic blade that projects away from the plant axis. An epidermally derived ligule and a joint‐like auricle develop at the blade/sheath boundary of maize leaves. Mutations disturbing the ligule/auricle region disrupt leaf patterning and impact plant architecture, yet it is unclear how this developmental boundary is established.Targeted microdissection followed by transcriptomic analyses of young leaf primordia were utilized to construct a co‐expression network associated with development of the blade/sheath boundary.Evidence is presented for proximodistal gradients of gene expression that establish a prepatterned transcriptomic boundary in young leaf primordia, before the morphological initiation of the blade/sheath boundary in older leaves.This work presents a conceptual model for spatiotemporal patterning of proximodistal leaf domains, and provides a rich resource of candidate gene interactions for future investigations of the mechanisms of blade/sheath boundary formation in maize.