Summary: Bacterial blight (BB) of rice caused byXanthomonas oryzaepvoryzae(Xoo) is one of the major constraints to productivity in South-East Asia. The strategy of using major genes, singly or in combination, continues to be the most effective approach for BB management. Currently, more than two dozen genes have been designated but not all the known genes are effective against all the prevalent pathotypes. The challenge, therefore, is to continue to expand the gene pool of effective and potentially durable resistance genes. Wild species constitute an important reservoir of the resistance genes including BB. An accession ofOryza nivara(IRGC 81825) was found to be resistant to all the sevenXoopathotypes prevalent in northern states of India. Inheritance and mapping of resistance inO. nivarawas studied by using F₂, BC₂F₂, BC₃F₁and BC₃F₂progenies of the cross involvingOryza sativacv PR114 and theO. nivaraacc. 81825 using the most virulentXoopathotype. Genetic analysis of the segregating progenies revealed that the BB resistance inO. nivarawas conditioned by a single dominant gene. Bulked segregant analysis (BSA) of F₂population using 191 polymorphic SSR markers identified a ∼35 centiMorgans (cM) chromosomal region on 4L, bracketed by RM317 and RM562, to be associated with BB resistance. Screening of BC₃F₁and BC₂F₂progenies and their genotyping with more than 30 polymorphic SSR markers in the region, covering Bacterial artificial chromosome (BAC) clone OSJNBb0085C12, led to mapping of the resistance gene between the STS markers based on annotated genes LOC_Os04g53060 and LOC_Os04g53120, which is ∼38·4 kb. Since none of the knownXagenes, which are mapped on chromosome 4L, are effective against theXoopathotypes tested, the BB resistance gene identified and transferred fromO. nivarais novel and is tentatively designated asXa30(t). Homozygous resistant BC₃F₃progenies with smallest introgression region have been identified.

Increased water and labour scarcity in major rice growing areas warrants a shift towards direct seeded rice cultivation under which management of weeds is a major issue. Use of broad spectrum non-selective herbicides is an efficient means to manage weeds. Availability of rice genotypes with complete tolerance against broad-spectrum non-selective herbicides is a pre-requisite for advocating use of such herbicides. In the present study, we developed an EMS induced rice mutant, ‘HTM-N22‘, exhibiting tolerance to a broad spectrum herbicide, ‘Imazethapyr‘, and identified the mutations imparting tolerance to the herbicide. We identified a stable and true breeding rice mutant, HTM-N22 (HTM), tolerant to herbicide, Imazethapyr, from an EMS-mutagenized population of approximately 100,000 M2 plants of an upland rice variety, Nagina 22 (N22). Analysis of inheritance of herbicide tolerance in a cross between Pusa 1656-10-61/HTM showed that this trait is governed by a single dominant gene. To identify the causal gene for Imazethapyr tolerance, bulked segregant analysis (BSA) was followed using microsatellite markers flanking the three putative candidate genes viz., an Acetolactate Synthase (ALS) on chromosome 6 and two Acetohydroxy Acid Synthase (AHAS) genes, one on chromosomes 2 and another on chromosome 4. RM 6844 on chromosome 2 located 0.16 Mbp upstream of AHAS (LOC_Os02g30630) was found to co-segregate with herbicide tolerance. Cloning and sequencing of AHAS (LOC_Os02g30630) from the wild type, N22 and the mutant HTM and their comparison with reference Nipponbare sequence revealed several Single Nucleotide Polymorphisms (SNPs) in the mutant, of which eight resulted in non-synonymous mutations. Three of the eight amino acid substitutions were identical to Nipponbare and hence were not considered as causal changes. Of the five putative candidate SNPs, four were novel (at positions 30, 50, 81 and 152) while the remaining one, S627D was a previously reported mutant, known to result in Imidazolinone tolerance in rice. Of the novel ones, G152E was found to alter the hydrophobicty and abolish an N myristoylation site in the HTM compared to the WT, from reference based modeling and motif prediction studies. A novel mutant tolerant to the herbicide “Imazethapyr” was developed and characterized for genetic, sequence and protein level variations. This is a HTM in rice without any IPR (Intellectual Property Rights) infringements and hence can be used in rice breeding as a novel genetic stock by the public funded organizations in the country and elsewhere.