AUTHOR=Zhang Bin , Li Pan , Su Tongbing , Li Peirong , Xin Xiaoyun , Wang Weihong , Zhao Xiuyun , Yu Yangjun , Zhang Deshuang , Yu Shuancang , Zhang Fenglan 

TITLE=BrRLP48, Encoding a Receptor-Like Protein, Involved in Downy Mildew Resistance in Brassica rapa

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01708

DOI=10.3389/fpls.2018.01708

ISSN=1664-462X

ABSTRACT=<p>Downy mildew, caused by <italic>Hyaloperonospora parasitica</italic>, is a major disease of <italic>Brassica rapa</italic> that causes large economic losses in many <italic>B. rapa</italic>-growing regions of the world. The genotype used in this study was based on a double haploid population derived from a cross between the Chinese cabbage line BY and a European turnip line MM, susceptible and resistant to downy mildew, respectively. We initially located a locus <italic>Br-DM04</italic> for downy mildew resistance in a region about 2.7 Mb on chromosome A04, which accounts for 22.3% of the phenotypic variation. Using a large F<sub>2</sub> mapping population (1156 individuals) we further mapped <italic>Br-DM04</italic> within a 160 kb region, containing 17 genes encoding proteins. Based on sequence annotations for these genes, four candidate genes related to disease resistance, <italic>BrLRR1, BrLRR2, BrRLP47</italic>, and <italic>BrRLP48</italic> were identified. Overexpression of both <italic>BrRLP47</italic> and <italic>BrRLP48</italic> using a transient expression system significantly enhanced the downy mildew resistance of the susceptible line BY. But only the leaves infiltrated with RNAi construct of <italic>BrRLP48</italic> could significantly reduce the disease resistance in resistant line MM. Furthermore, promoter sequence analysis showed that one salicylic acid (SA) and two jasmonic acid-responsive transcript elements were found in <italic>BrRLP48</italic> from the resistant line, but not in the susceptible one. Real-time PCR analysis showed that the expression level of <italic>BrRLP48</italic> was significantly induced by inoculation with downy mildew or SA treatment in the resistant line MM. Based on these findings, we concluded that <italic>BrRLP48</italic> was involved in disease resistant response and the disease-inducible expression of <italic>BrRLP48</italic> contributed to the downy mildew resistance. These findings led to a new understanding of the mechanisms of resistance and lay the foundation for marker-assisted selection to improve downy mildew resistance in <italic>Brassica rapa</italic>.</p>