Development rate, the rate at which plants produce new nodes/leaves, is crucial for determining crop readiness for harvest. Enhancing our genetic understanding and ability to manipulate this rate can lead to faster crop cycles, increased yield, and more efficient production, particularly for plants with short lifecycles or those harvested in the vegetative stage. Despite existing knowledge from other crops like Arabidopsis and rice, significant gaps remain in our understanding of development rate control. This dissertation investigates the genetic mechanisms controlling development rate in Petunia × hybrida and Stevia rebaudiana to improve crop timing and yield. Petunia, a popular annual bedding plant, often relies on heated greenhouse production in cooler climates. Identifying genetic factors regulating development rate in petunia is essential for reducing production costs and accelerating development rate at sub-optimal temperatures. The first objective was to evaluate the effect of candidate genes identified in previous studies by using virus-induced gene silencing (VIGS). Despite variable silencing efficiency and phenotypic data variability, the MEI2-like1 RNA binding protein emerged as a promising candidate, warranting further investigation with stable transformation methods. The second study employed an F7 recombinant inbred line (RIL) population developed from P. axillaris and P. exserta (AE population) to identify genes differentially expressed between fast- and slow-developing AE RILs. DEGs included genes related to auxin polar transport, gibberellin signaling, MATE efflux transporters, and the 2OG-Fe(II) dependent oxygenase superfamily, among others. Common DEGs between AE and a previous RIL study involved cell-wall mechanics related genes such as PECTINACETYLESTERASE FAMILY PROTEIN and L-ASCORBATE OXIDASE, providing crucial insights into genetic factors influencing development rate. Stevia, known for its zero-calorie sweetening compounds produced by leaf, benefits from breeding faster-developing varieties to enable multiple harvests per season. Stevia leaf yield depends on morphological traits such as leaf size, leaf production rate, plant canopy width and branch production. An F1 mapping population of 200 individuals was evaluated over two years at two field locations in Michigan for leaf-yield related traits. We generated a novel high-density SNP-based linkage map with eleven linkage groups encompassing eleven chromosomes. QTLs were identified at all four environments for traits such as maximum width, secondary branching, leaf length, and plant vigor, explaining 7-15% of phenotypic variation. Overlapping QTL regions were identified for traits including secondary branching, minimum canopy width, and leaf width explaining 7-15% of phenotypic variation. Differential expression analysis of F1 lines with contrasting development rates highlighted genes related to auxin efflux carrier proteins (PIN-like 2), auxin biosynthesis (YUC2), and cell wall loosening enzymes (EXPANSIN). Notably, CYP78A10, an ortholog of the Arabidopsis KLUH gene known to slow development rate, was upregulated in the slow development rate lines. Results from both species suggest that development rate is a complex process regulated by multiple factors, starting in the shoot apical meristem (SAM) through auxin polar transport, cell wall mechanics, and communication signals from emerging leaf primordia to the SAM. This research lays the foundation for breeding programs aimed at accelerating crop timing and increasing yield, enhancing overall crop production efficiency.
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