Rough and tough. How does silicic acid protect horsetail from fungal infection?

Abstract

Horsetail (Equisetum arvense) plants grew healthily for 10 weeks under both Si-deficient and Si-replete conditions. After 10 weeks, plants grown under Si-deficient conditions succumbed to fungal infection. We have used NanoSIMS and fluorescence microscopy to investigate silica deposition in the tissues of these plants. Horsetail grown under Si-deficient conditions did not deposit identifiable amounts of silica in their tissues. Plants grown under Si-replete conditions accumulated silica throughout their tissues and especially in the epidermis of the outer side of the leaf and the furrow region of the stem where it was continuous and often, as a double layer suggestive of a barrier function. We have previously shown, both in vivo (in horsetail and thale cress) and in vitro (using an undersaturated solution of Si(OH)), that callose is a "catalyst" of plant silica deposition. Here we support this finding by comparing the deposition of silica to that of callose and by showing that they are co-localized. We propose the existence of a synergistic mechanical protection by callose and silica against pathogens in horsetail, whereby the induction of callose synthesis and deposition is the first, biochemical line of defence and callose-induced precipitation of silica is the second, adventitious mechanical barrier.

In defence of the selective transport and role of silicon in plants

“…A special note on Guerriero et al . () is warranted, as (1) the authors give it much weight in their arguments, claiming that it ‘supersedes and augments’ our model, and (2) it was indeed, we readily admit, not cited in our review. The reasons for this are as follows and pertain not to the chemistry of Si but to fundamentals of plant pathology.…”

A reappraisal of biological silicification in plants?

“…Bélanger, in particular, has significantly added to our understanding of silicon's role in protecting plants against biotic stress. The obstructive hypothesis put forward in this review directly addresses this and yet recent research that supersedes and augments this theory is not acknowledged (Guerriero et al ., ). By way of contrast the authors of this review have little experience in silicon and abiotic stress which may explain myriad omissions (Bayliss et al ., ; Hodson & Evans, ).…”

“…A model that is wholly dependent upon the formation of a silica obstruction throughout apoplastic pathways will rely heavily upon understanding of biological silicification (Currie & Perry, ), the mechanism underlying the obstruction in this hypothesis. However, in this review the mechanism of formation and deposition of biogenic silica in plants received only scant attention with much important past and recent research either inexplicably by‐passed or not appropriately cited (Mann et al ., ; Perry & Fraser, ; Law & Exley, ; Guerriero et al ., , ; Brugiére & Exley, ; Kulich et al ., ). This research, if appropriately included and cited, would certainly challenge the authors’ perceived novelty of their proposed model.…”

“…‘Why do some plants deposit biogenic silica in myriad different tissues (Fig. ), whereas others only deposit silica in a limited number of specialised structures while some plants deposit little if any silica anywhere at all?’ While there is not yet an unequivocal answer to this question it has been addressed in some depth in the scientific literature (Hodson et al ., ; Exley, ; Guerriero et al ., ) and these studies, critical to their hypothesis, should have been appraised and cited accordingly. Due consideration of published literature would have informed Bélanger and colleagues that the ‘reinterpretation of silicon's role in plants’ emphasized in their Summary has, in fact, been underway for many years.…”

A new methodology for the determination of silicon in plants by wavelength dispersive X‐ray fluorescence

“…Apart from oxygen, silicon is the most abundant element in the Earth's crust and plays an important role in biological processes. Silicon is found in human and animal tissue in three major forms: as water‐soluble inorganic compounds, easily penetrating through the cell walls; as organic‐soluble compounds, such as silicones and silicon complexes; and as insoluble polymeric compounds, such as polysilicic acids, silicates, amorphous silica, and quartz …”