Crop diseases bring great pressure to food security. Among methods of reducing grain loss caused by pathogens, breeding varieties with broad spectrum resistance (BSR) is commonly known as the most economical and effective approach. Recently, many progresses have been obtained on BSR studies, such as developing BSR genes and revealing their molecular mechanisms. In this review, we summarized the advances, opportunities, and challenges of crop BSR studies. Additionally, we made perspectives of studying crop BSR in the future.
The Food and Agriculture Organization of the United Nations (FAO) estimates that loss of grain caused by crop diseases amounts to 10% every year. However, the continuous growth of world population will result into food crisis in 2050 . To guarantee food security, breeding broad spectrum resistant (BSR) varieties is the most effective way to defend against fungi, bacteria, and viruses. Therefore, it is necessary to enhance BSR of crop. Developing the BSR genes is the basis of breeding BSR varieties.
The plant innate immune system has two layers, pathogen-associated molecular patterns (PAMPs)-triggered immunity (PTI) and effector-triggered immunity (ETI) . Factors in PTI mainly include PAMPs and their receptors, while factors in ETI mainly include effectors and nucleotide-binding domain and leucine-rich repeat receptors (NLRs) . However, lots of factors, such as some transcription factors and non-coding RNAs, are not clearly divided into the two layers [4, 5]. Previous studies often indicated that there is clear border between PTI and ETI, and PTI usually confer durable and broad-spectrum resistance (BSR), whereas ETI confer strong but race specific resistance (RSR). However, recent studies showed that the signaling crosstalk between PTI and ETI is present, and some NLRs in ETI can also trigger BSR . It shows the difficulty and complexity of studying plant immunity, especially BSR, but it also provides us with opportunities to recognize BSR.
The resistance conferred by RSR genes often breaks down within 3-5 years due to the high variability and fast evolving populations of the fungus . It further highlights the importance of developing BSR genes. Currently, some BSR genes are developed from crops, such as Xa21, Pi-d2 and Ptr in rice [6, 8], Yr36, Pm21 and Fhb7 in wheat [9-11], Rp1-D, Rxo1, ZmFBL41Chang7-2 in maize [12-14]. Additionally, some susceptibility genes in plant can confer BSR when losing their functions [6, 15]. However, application of BSR genes in breeding crop is inadequate because of an acute shortage of developing BSR genes and elucidating their resistant mechanisms.
Rice blast and bacterial blight of rice are devastating diseases of rice. Li et al. has reported recent advances in BSR to the rice blast disease, which included the identified BSR genes and their mechanisms . Among the BSR genes conferring resistance to Xanthomonas oryzae pv. oryzae (Xoo), Xa21 is the first cloned and the most important BSR genes. It encodes a transmembrane receptor-like kinase, which can recognize RaXX of Xoo to confer BSR resistance . In wheat, four genes including Lr34/Yr18/Pm38/Sr57, Lr27/Yr30/Sr2, Lr46/Yr29/Pm39/Sr58 and Lr67/Yr46/Pm46/Sr55 confer resistance to wheat rust and powdery mildew . In maize, ZmCCoAOMT2 encoding a caffeoyl-CoA O-methyltransferase confers quantitative resistance to three important foliar maize diseases, such as southern leaf blight, gray leaf spot and northern leaf blight . Among these BSR genes,
just several genes are applied in breeding crop, such as rice pi21
 and Pigm , barley Mlo . Wheat Fhb7 and rice bsr-d1
have also great potential of application in breeding because Fhb7
and bsr-d1 confer broad resistance to Fusarium and Magnaporthe
oryzae species, respectively, without yield penalty [4, 11].
Biological technologies have advanced at a breathless pace,
which provide great opportunities for researchers to study
resistance mechanisms, such as reconstitution and structure of the
Resistosome using the cryoelectron microscopy , elucidating
resistant mechanism of tomato using multi-omic , editing
genes using TALEN or CRISPR/CAS9 technology . Prospects
for the development of plant immunity have improved markedly.
Previous studies have established a framework for the plant
BSR system. However, to ensure grain yield and food security,
some important studies are still needed to keep going forward.
1. Screening BSR materials that provide new BSR genes.
2. Using computational biology to develop the BSR genes.
3. Identifying elite BSR genes without negative effects for
plant growth and development.
4. Developing and promoting new technologies to refine
5. Keeping crop yield and quality while enhancing disease
The project is supported by the Outstanding Young
Scientific and Technological Talents Project in Sichuan Province
(2019JDJQ0045) and Tianfu Ten-thousand Talents Program
(Tianfu science and technology elite project).