Bacillus pumilus increases boron uptake and inhibits rapeseed growth under boron supply irrespective of phosphorus fertilization

Boron and phosphorus are soil nutrients essential for plant growth, metabolism and development. Boron deficiency is a major factor limiting the growth of at least 132 crops in over 80 countries. Symptoms of boron deficiency are often most visible in rapidly growing tissues and can lead to a reduction in growth rate. However, too much boron can also be toxic to plants and so the application of exogenous boron can easily be lethal. Phosphorus availability is also a major problem across the globe, as this element is adsorbed onto the surface of soil particles, where it readily forms insoluble compounds after reacting with soil cations such as iron, aluminium and calcium. As a result, as much as 90 % of soil phosphorus may no longer be available for uptake.Plant growth-promoting bacteria (PGPB) and other soil microorganisms may improve plant growth by increasing water uptake and nutrient availability, including boron and phosphorus availability.

(a) Rapeseed (Brassica napus) in the field. (b) Schematic diagram of mechanisms responsible for enhanced B uptake by plants subjected to both B. pumilus (bacteria) inoculation and P addition. Image credit: Masood et al.

In a recent study published in AoBP, Masood et al. investigate how PGPB influence the growth and uptake of boron and phosphorus in rapeseed (Brassica napus), a crop particularly susceptible to boron deficiency. Their results indicate that the bacteria Bacillus pumilus enhances boron uptake by rapeseed, which is consistent with previous reports. Interestingly, increased boron accumulation in rapeseed inhibited the growth of rapeseed under boron-sufficient soil conditions. These inhibitory effects of B. pumilus on rapeseed growth were related to soil acidification, which increased the boron availability in the soil–plant system. This novel information on the role of PGPB in boron nutrition and plant growth improves our understanding of how PGPB influence boron uptake. It also has potential implications for the application of PGPB in agriculture. Specifically, the inoculation of rapeseed with B. pumilus to replenish boron nutrition is only recommended when it is limiting plant growth.