Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a devastating disease of wheat (Triticum aestivum) worldwide. Wheat high-temperature seedling plant (HTSP) resistance to Pst is non-race-specific and durable. WRKY transcription factors have been proven to play important roles in plant defence responses to attacks by several pathogens. However, there is no direct evidence as to whether WRKY transcription factors play a role in HTSP resistance to Pst. We isolated a WRKY gene, named TaWRKY70, from wheat cultivar Xiaoyan 6. The expression level of TaWRKY70 was increased significantly when exposed to high temperatures (HTs) during the initial symptom expression stage of Pst infection. The expression of this gene increased in plants treated with ethylene (ET), salicylic acid (SA) and cold (4°C) stresses, but decreased in plants treated with methyl jasmonate (MeJA) and heat (40°C) stresses. Silencing of TaWRKY70 led to greater susceptibility to Pst (in terms of the increase in length of uredinial pustules and the decrease in the number of necrotic cells) compared with non-silenced plants when exposed to HT during the initial symptom expression stage of Pst infection, coinciding with expression changes of the ET- and SA-responsive genes TaPIE1 and TaPR1.1. In contrast, the expression level of the jasmonic acid (JA)-responsive gene TaAOS was not affected by TaWRKY70. These results indicate that TaWRKY70 is positively involved in HTSP resistance, during which SA and ET signalling are probably activated.
Stripe rust, caused by Puccinia striiformis f. tritici, is an important wheat disease in China. P. striiformis f. sp. tritici overwintering and nonoverwintering regions based on the temperature were described elsewhere ( Shi et al. 2005 ). The temperature limit for P. striiformis f. sp. tritici overwintering is derived from field observations. However, P. striiformis f. sp. tritici has recently been observed to overwinter at sites where overwintering is predicted to be unlikely. We studied P. striiformis f. sp. tritici overwintering across several sites in regions close to or further away from the current P. striiformis f. sp. tritici “overwintering boundary” in China. Plants with P. striiformis f. sp. tritici symptoms and uredinia were tagged in late autumn and moved to the laboratory in early spring the following year for quantification of P. striiformis f. sp. tritici biomass via a quantitative reverse-transcription polymerase chain reaction method and for assessment of P. striiformis f. sp. tritici symptoms and sporulation after incubation in a greenhouse. The molecular method detected P. striiformis f. sp. tritici in leaves and sheath in most samples, much greater than the observed incidence of P. striiformis f. sp. tritici symptoms and sporulation after incubation. Thus, further refinement may been necessary to calibrate this molecular method in order to avoid overestimating P. striiformis f. sp. tritici overwintering potential. Active sporulation (hence, successful overwintering) was observed for all sites except one. Increasing altitude led to decreasing incidence of visible P. striiformis f. sp. tritici symptoms and sporulation; in addition to lower temperatures in high altitudes, wind chill may also explain this negative relationship between P. striiformis f. sp. tritici overwinter potential and altitude. P. striiformis f. sp. tritici sporulation on plants subjected to different treatments (control, two oldest leaves, or all leaves removed) indicated that P. striiformis f. sp. tritici overwinters in young green leaves as latent infection established in late autumn. The present study suggests that using only temperature to predict overwintering potential of P. striiformis f. sp. tritici at a given site is insufficient for mountainous regions.
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