Elucidation of Molecular Mechanisms Involved in Silicon Uptake and Derived Stress Tolerance in Plants

Abiotic and biotic stresses are among the most important constraints in agriculture production aiming to provide for an ever-increasing world population. The search for a sustainable solution to reduce stress-induced losses is now a prime priority in many crop improvement programs. Among different approaches, the use of silicon (Si) is gaining increased attention since over 1000 studies have reported its benefits against stress on many plant species. However, these benefits are apparent in plant species that can accumulate high Si levels, and negligible in species known as low or non-accumulators. Silicon uptake in plants is known to be regulated by two types of transporters, an influx protein Lsi1, and an efflux protein Lsi2. To better understand the molecular mechanisms involved in Si uptake, and to provide a genetic basis for categorizing plants as accumulators or non-accumulators, we performed an extensive genome-wide analysis in 25 plant species. Our results showed that the prime requirement for a plant to accumulate Si was the presence of a functional Lsi1, i.e. a NIP2 aquaporin with a G-S-G-R selectivity filter and a precise 108 amino acid (AA) spacing between the two NPA domains. This finding provided the first molecular explanation for the inability of Solanaceae in general, and tomato in particular to accumulate Si on the basis of a 109 AA spacing between the NPA domains. As a proof of concept, tomato plants transformed with a functional Lsi1 from wheat were tested for their ability to accumulate Si. Results confirmed an increased accumulation of Si in plants but also confirmed the importance to restrict the expression of the Lsi1 gene in roots. The problem was solved by using a grafting approach with the transformed material to confine the expression in roots. The aerial portion of the grafted material accumulated more Si and showed an elevated tolerance against water stress and powdery mildew. These results confirm the benefits of Si against stress in plants and provide a unique approach to exploit Si even in non-accumulator plants.