  Aneutronic and nonradioactive properties make the proton-boron fusion a
prospective candidate for fusion energy production through reactions following
p+$^{11}$B$\rightarrow$3${\alpha}$ (p-$^{11}$B). However, it is difficult to
achieve a thermal fusion ignition, since the low reaction cross-sections for
center-of-mass energy below $\sim$100 keV. To realize fusion energy gain, it is
essential to consider utilization of the maximum cross-section at the resonant
peak of p-$^{11}$B fusion, and explore the nuclear reactions in plasma
environment. In this work, p-$^{11}$B reactions triggered by interactions
between energetic proton beams and laser-ablated boron plasma have been
investigated. More than 200 times enhancement of ${\alpha}$ particle emission
efficiency (number ratio of escaping ${\alpha}$ particles and boron nuclei) in
plasma has been observed, compared with the cold boron. The proton beam
transport path modulated by strong electro-magnetic fields in plasma could
dominate the enhanced ${\alpha}$ particle generation, due to a longer
collisional length. In addition, an ${\alpha}$ particle yield up to
1$\times$10$^{10}$ /sr has been measured via the pitcher-catcher scheme in
plasma. This work could benefit understanding of the plasma effects on nuclear
reaction dynamics, and also enable opportunities to explore physics in laser
fusion associated with advanced fusion fuels.
