**1. Introduction**

As a carbohydrate-rich staple of more than half the world's diet, rice (*Oryza sativa* L.) is one of the most important food crops on the planet. The Food and Agriculture Organization of the United Nations (FAO) estimates that by 2050, overall global agricultural production may need to be increased by up to 70% to meet the dietary requirements of the world's projected population of nine billion [1]. In order to satisfy the demand corresponding to the FAO's projected population in 2050, global rice production would have to increase by nearly 42% over present-day levels [2]. Bacterial blight (BB) caused by *Xanthomonas oryzae* pv. *oryzae* (*Xoo*) is a disease that poses one of the greatest threats to rice

production worldwide. In Asia, BB has proven to be capable of reducing crop yields by as much as 50% [3] to 80% [4]. The disease, being systemic, affects the photosynthetic areas of plants, which results in a drastically lower yield. Although BB can be managed through the use of fungicides, enhancing the genetic resistance in rice is the most effective and ecological method of overcoming the threat posed by the disease.

To date, 42 BB resistance genes have been identified from diverse sources, of which *Xa4*, *xa5*, *Xa7*, *xa13* and *Xa21* are most frequently utilized in BB resistance breeding programs [5–7]. Although the BB resistance genes *xa5* and *xa13* are recessive in nature, abundant molecular marker resources allow for molecular marker-assisted breeding [8–11]. The *xa5* gene encodes a mutated gamma subunit of basal transcription factor IIA 5 (TFIIAγ5), and along with the dominant resistance gene *Xa7*, has shown strong resistance to a virulent BB strain, Z-173, in China [11,12]. Another broad-spectrum recessive gene, *xa13,* was correlated with a plasma membrane protein conferring recessive resistance to PXO99 [13]. *Xa21,* which encodes a leucine-rich repeat (LRR) receptor kinase-type gene, was identified from *O. longistaminata*; it is one of the most effective genes utilized in breeding programs designed to enhance the BB resistance of rice cultivars [8,14].

Conventional backcross breeding embedded with marker-assisted selection (MAS) has been successfully employed in developing crop varieties exhibiting agronomically important traits. The utility of MAS in pyramiding several resistance genes to develop a variety possessing broad-spectrum durable resistance has been successfully demonstrated against numerous pathotypes [6,15,16]; *Jalmagna*, a high-yield, deep-water rice variety, was improved for BB resistance by pyramiding three resistance genes, *xa5* + *xa13* + *Xa21* [6]; a Korean elite *japonica* variety, *Mangeumbyeo*, improved with the introgression of the *Xa4* + *xa5* + *Xa21* genes, which were shown to possess a wide range of resistance to BB [16]. Recently, *xa5*, *xa13* and *Xa21* genes were introgressed into the hybrid rice maintainer lines CO2B, BO23B and CO24B through MAS, which can form the basis to develop new, widely adaptable heterotic hybrids possessing resistance against the destructive diseases to which rice is vulnerable [17]. In addition, there have been several examples of MAS being utilized to successfully incorporate different genes which provide higher resistance to various biotic and abiotic stresses (for example, the pyramiding of QTLs of submergence tolerance (*Sub1A*), leaf/neck blast (*qBL1* and *qBL11*), brown planthopper (*Bph3*) and BB (*xa5* and *Xa21*) in high-yielding and aromatic rice variety 'Pink3-[18]).

According to the annual report of the Council of Agriculture, of the 271,000 hectares of rice paddy fields in Taiwan, approximately 7% are affected by BB per year. Most Taiwanese *japonica* rice cultivars lack BB resistance genes [10], resulting in significant yield loss in fields severely infected by the disease. Pyramiding multiple *R* genes by MAS provides a rapid and precise way to develop a variety with wide-spectrum and durable resistance [19]. A set of 17 near-isogenic lines (NILs) in IR24 background, having single or two to four pyramided *Xa* genes, were included in the panel to serve as controls of known disease reactions [20]. IRBB66, carrying *Xa4*, *xa5*, *Xa7*, *xa13* and *Xa21*, in an *indica* rice IR24 genetic background, conferred strong resistance to races of BB. In the present study, five BB resistance genes were introgressed from IRBB66 into an elite *japonica* variety, 'Tainung82- (TNG82), using marker-assisted backcrossing (MAB) and marker-assisted background analysis of selected backcross progenies using SSR markers. The aims of this study were to (i) develop five gene pyramiding lines using MAB, (ii) evaluate the effects of BB-resistant lines carrying different R genes after inoculation with BB strain, (iii) select individuals possessing agronomic traits and grain quality performance from the resulting BB-resistant lines. The development of BB-resistant lines with more than three genes pyramided has a promising future in molecular breeding of durable BB-resistant rice cultivars.
