The salicylic acid (SA) plays a critical role during the establishment of systemic acquired resistance (SAR) in uninfected plant tissues after localised exposure to a pathogen. Here, we studied SA in Populus tomentosa infected by the plant pathogen Botryosphaeria dothidea. The accumulation of SA and methyl salicylate (MeSA) occurred in chronological order in P. tomentosa. The SA and MeSA contents were greater at infected than uninfected sites. Additionally, a gene expression analysis indicated that SA might be accumulated by phenylalanine ammonialyase (PAL) and converted to MeSA by SA carboxyl methyltransferase (SAMT), while MeSA might convert to SA by SA-binding protein 2 (SABP2). The expressions of SAMT at infected sites and SABP2 at uninfected sites, respectively, were significantly up-regulated. Thus, SA might be converted to MeSA at infected sites and transported as a signalling molecule to uninfected sites, where it is converted to SA for SAR. Moreover, the expressions of pathogenesis-related genes PR-1, PR-2 and PR-5 in P. tomentosa were up-regulated by the B. dothidea infection. Our study determined that variations in SA and MeSA contents occur at infected and uninfected sites in poplar after pathogen infection and contributed to the remote signals for poplar SAR.

A study on the biomass of fine roots and its relationship with water-stable aggregates (WSA) was conducted in two herbaceous models, triploid Populus tomentosa + Lolium multiflorum (TL) and triploid P. tomentosa + natural grass (TN). Both of the model triploid P. tomentosa stands were four years old converted from agriculture. Unconverted steep slope farmland was used as a control site. Results showed that the biomass of fine roots ((1 mm) in different layers varied in the following descending order: upper layer, middle layer and lower layer, at approximate ratios of 50:30:20. The average annual biomass of fine roots in ryegrass was twice that of the mixed natural grass-forest land. The total amount of natural grass roots was 4.4 times that of the ryegrass model. Water-stable aggregates of the upper, middle and lower layers and the unconverted farmland did not show any significant differences, whereas the amounts of waterstable aggregates of big-particles in the upper and middle layers were much larger than those of unconverted lands. The amounts of water-stable aggregates of natural grassforest lands (TN model) were higher than those of managed grass-forest lands (TL model). Two-way analysis of variance indicated that fine roots ((1 mm) could significantly enhance water-stable aggregates and total waterstable aggregates. We conclude that the program of converting agricultural lands to forest-grass lands is an effective way in improving soil anti-erosion capability.

Salicylic acid (SA) is generally considered to be a critical signal transduction factor in plant defenses against pathogens. It could be converted to methyl salicylate (MeSA) for remote signals by salicylic acid methyltransferase (SAMT) and converted back to SA by SA-binding protein 2 (SABP2). In order to verify the function of SAMT in poplar plants, we isolated the full-length cDNA sequence of PagSAMT from 84K poplar and cultivated PagSAMT overexpression lines (OE-2 isolate) to test its role in SA-mediated defenses against the virulent fungal pathogen Botryosphaeria dothidea. Our results showed that after inoculation with B. dothidea, OE-2 significantly increased MeSA content and reduced SA content which is associated with increased expression of SAMT in both infected and uninfected leaves, when compared against the wild type (WT). Additionally, SAMT overexpression plant lines (OE-2) exhibited higher expression of pathogenesis-related genes PR-1 and PR-5, but were still susceptible to B. dothidea suggesting that in poplar SA might be responsible for resistance against this pathogen. This study expands the current understanding of joint regulation of SAMT and SABP2 and the balance between SA and MeSA in poplar responses to pathogen invasion.