A knockdown mutation of YELLOW-GREEN LEAF2 blocks chlorophyll biosynthesis in rice.

KEY MESSAGE: An insert mutation of YELLOW-GREEN LEAF2 , encoding Heme Oxygenase 1 , results in significant reduction of its transcript levels, and therefore impairs chlorophyll biosynthesis in rice. Heme oxygenase (HO) in higher plants catalyzes the degradation of heme to synthesize phytochrome precursor and its roles conferring the photoperiodic control of flowering in rice have been revealed. However, its involvement in regulating rice chlorophyll (Chl) synthesis is not fully explored. In this study, we isolated a rice mutant named yellow-green leaf 2 (ygl2) from a (60)Co-irradiated population. Normal grown ygl2 seedlings showed yellow-green leaves with reduced contents of Chl and tetrapyrrole intermediates whereas an increase of Chl a/b ratio. Ultrastructural analyses demonstrated grana were poorly stacked in ygl2 mutant, resulting in underdevelopment of chloroplasts. The ygl2 locus was mapped to chromosome 6 and isolated via map-based cloning. Sequence analysis indicated that it encodes the rice HO1 and its identity was verified by transgenic complementation test and RNA interference. A 7-Kb insertion was found in the first exon of YGL2/HO1, resulting in significant reduction of YGL2 expressions in the ygl2 mutant. YGL2 was constitutively expressed in a variety of rice tissues with the highest levels in leaves and regulated by temperature. In addition, we found expression levels of some genes associated with Chl biosynthesis and photosynthesis were concurrently altered in ygl2 mutant. These results provide direct evidence that YGL2 has a vital function in rice Chl biosynthesis.

OsARG encodes an arginase that plays critical roles in panicle development and grain production in rice.

Nitrogen is a crucial nutrient for plant growth and development. Arginine is considered to be an important amino acid for nitrogen transport and storage, playing a crucial role during plant seedling development. However, little is known about the role of arginine in nitrogen remobilization at the reproductive stage. We isolated a rice mutant nglf-1 with reduced plant height, small panicle and grain size, and low seed-setting rate (10% in nglf-1 compared to 93% in wild-type). Map-based cloning revealed that the mutant phenotype was caused by loss of function of a gene (OsARG) encoding an arginine hydrolysis enzyme, which is consistent with arginine accumulation in the mutant. The phenotype was partially corrected supplying exogenous nitrogen, and fully corrected by expression of a wild-type OsARG transgene. Over-expression of OsARG in rice (cv. Kitaake) increased grain number per plant under nitrogen-limited conditions. OsARG was ubiquitously expressed in various organs, but most strongly in developing panicles. The OsARG protein was localized in the mitochondria, consistent with other arginases. Our results suggest that the arginase encoded by OsARG, a key enzyme in Arg catabolism, plays a critical role during panicle development, especially under conditions of insufficient exogenous nitrogen. OsARG is a potential target for crop improvement.