Biology subjects

Bartlett, M. E.

Publications and source records attributed to Bartlett, M. E..

3 recordsLinked to original sources

bioRxiv2023

Duplicate transcription factors GT1 and VRS1 regulate branching and fertile flower number in maize and Brachypodium distachyon

Crop engineering and de novo domestication using genome editing are new frontiers in agriculture. However, outside of well-studied crops and model systems, prioritizing engineering targets remains challenging. Evolution can serve as our guide, revealing high-priority genes with deeply conserved roles. Indeed, GRASSY TILLERS1 (GT1), SIX-ROWED SPIKE1 (VRS1), and their homologs have repeatedly been targets of selection in domestication and evolution. This repeated selection may be because these genes have an ancient, conserved role in regulating growth repression. To test this, we determined the roles of GT1 and VRS1 homologs in maize (Zea mays) and the distantly related grass brachypodium (Brachypodium distachyon) using CRISPR-Cas9 gene editing and mutant analysis. GT1 and VRS1 have roles in floral development in maize and barley, respectively. Grass flowers are borne in branching structures called spikelets. In maize spikelets, carpels are suppressed in half of all initiated ear flowers. These spikelets can only produce single grains. We show that gt1; vrs1-like1 (vrl1) mutants have derepressed carpels in ear flowers. Importantly, these plants can produce two grains per spikelet. In brachypodium, bdgt1; bdvrl1 mutants have more branches, spikelets, and flowers than wildtype plants, indicating conserved roles for GT1 and VRS1 homologs in growth suppression. Indeed, maize GT1 can suppress growth in Arabidopsis thaliana, separated from the grasses by ca. 160 million years of evolution. Thus, GT1 and VRS1 maintain their potency as growth regulators across vast timescales and in distinct developmental contexts. Modulating the activity of these and other conserved genes may be critical in crop engineering.

plant biology↗

bioRxiv2023

Asymmetric evolution of protein domains in the leucine-rich repeat receptor-like kinase (LRR-RLK) family of plant developmental coordinators

The coding sequences of developmental genes are expected to be conserved over deep time, with cis-regulatory change driving the modulation of gene function. In contrast, proteins with roles in defense are expected to evolve rapidly, in molecular arms-races with pathogens. However, some gene families include both developmental and defense genes. In these families, does the tempo and mode of evolution differ between developmental and defense genes, despite shared ancestry and structure? The leucine-rich repeat receptor-like kinase (LRR-RLKs) protein family includes many members with roles in plant development and defense, thus providing an ideal system for answering this question. LRR-RLKs are receptors that traverse plasma membranes. LRR domains bind extracellular ligands, RLK domains initiate intracellular signaling cascades in response to ligand binding. In LRR-RLKs with roles in defense, LRR domains evolve faster than RLK domains. To determine whether this asymmetry extends to developmental LRR-RLKs, we assessed evolutionary rates and tested for selection acting on eleven clades of LRR-RLK proteins, using deeply sampled protein trees. To assess functional evolution, we performed heterologous complementation assays using Arabidopsis thaliana (arabidopsis) LRR-RLK mutants. We found that the LRR domains of developmental LRR-RLK proteins evolved faster than their cognate RLK domains. LRR-RLKs with roles in development and defense had strikingly similar patterns of molecular evolution. Heterologous transformation experiments revealed that the evolution of developmental LRR-RLKs likely involves multiple mechanisms, including changes to cis-regulation, coding sequence evolution, and escape from adaptive conflict. Our results indicate similar evolutionary pressures acting on developmental and defense signaling proteins, despite divergent organismal functions. In addition, deep understanding of the molecular evolution of developmental receptors can help guide targeted genome engineering in agriculture.

plant biology↗

bioRxiv2021

Recruitment of an ancient branching program to suppress carpel development in maize flowers

Floral morphology is immensely diverse. One developmental process acting to shape this diversity is growth suppression. For example, grass flowers exhibit extreme diversity in floral sexuality, arising through differential suppression of stamens or carpels. In maize, carpels undergo programmed cell death in half of the flowers initiated in ears and in all flowers in tassels. The HD-ZIP I transcription factor gene GRASSY TILLERS1 (GT1) is one of only a few genes known to regulate this process. To identify additional regulators of carpel suppression, we performed a gt1 enhancer screen, and found a genetic interaction between gt1 and ramosa3 (ra3). RA3 is a classic inflorescence meristem determinacy gene that encodes a trehalose-6-phosphate (T6P) phosphatase (TPP). Dissection of floral development revealed that ra3 single mutants have partially derepressed carpels, whereas gt1; ra3 double mutants have completely derepressed carpels. Surprisingly, gt1 suppresses ra3 inflorescence branching, revealing a role for gt1 in meristem determinacy. Supporting these genetic interactions, GT1 and RA3 proteins colocalize to carpel nuclei in developing flowers. Global expression profiling revealed common genes misregulated in single and double mutant flowers, as well as in derepressed gt1 axillary meristems. Indeed, we found that ra3 enhances gt1 vegetative branching, similar to the roles for the trehalose pathway and GT1 homologs in the eudicots. This functional conservation over ~160 million years of evolution reveals ancient roles for GT1-like genes and the trehalose pathway in regulating axillary meristem suppression, later recruited to mediate carpel suppression. Our findings expose hidden pleiotropy of classic maize genes, and show how an ancient developmental program was redeployed to sculpt floral form.

plant biology↗