Study On The Physiological Mechanism Of The Rice Ribless Mutant Dl2and Preliminary Mapping Of DL2Regulating Midrib Formation

In higher plants, leaves, which consist of epidermis, mesophyll and veins, are themain organ of photosynthesis. Vein are the branch vascular bundles in the stem, whichdistribute to various parts of the leaves via petiole and have various morphology anddistribution in the different plants. In higher plants, water and photosynthates transportis accomplished by a very complex and sophisticated network, the occurrence anddevelopment of the veins is a regularly distributed in the leaves, thereby forming aconduit in the leaf network and plays an important role in transporting nutrients andmechanical support for leaf. Although the leaf vein development mechanism of thedicotyledonous model plant Arabidopsis has been largely known, the vein developmentof the monocotyledonous modle plant rice remains poorly understood. Rice is one of themost important food crops in the word, and its vein developmental condition directlyaffects the water and nutrients supply, as well as the light-receiving ratio for leaf, whichultimately control the photosynthetic efficiency of rice. Therefore, elucidating themolecular mechanism underlying vein development of the rice leaf would improve ourfuture understanding of developmental mechanism in the plant vascular system andbroaden our theoretical knowledge of the molecular mechanism of vein development inmonocots, which might further benefit us the biological design of the plant leafarchitecture to improve light-receiving ratio of plant leaves and enhance photosynthesis.Taken together, probing the molecular mechanism of the vein development in rice andother monocots possesses holds great theoretical and application promise.Here we characterized a classic rice ribless mutant with drooping leaf2(named dl2)in indica N45-2, and the results about drooping leaf2mutant are as follows:⑴The dl2allele affects both the leaf venation development and the leaf venationpatternA comparison between mature wild-type and dl2leaves revealed that the venationpattern parameters were altered in the mutant. Notably, there were no clear cells on themidrib structure in dl2mutant, and the diameters of metaxylem and mesophyll cell sizeof the central large vascular bundles in middle vein and large vein were reducedsignificantly in dl2mutant. In addition, the total number of small veins per leaf wasincreased, and the average number of small veins between two adjacent large veins wasincreased. ⑵The dl2allele affects the developmental patterns of the central vascular bundlesin the early stagesCompared to the wild-type, the differentiation of the central vascular bundles in thedl2mutant significantly delayed, the specific cell proliferation in the central region didnot occur when the central vascular bundle was being formed and the cell accumulationalong the adaxial-abaxial axis in the center of the dl2leaf was reduced significantly. Afailure to accumulation enough cells in the central region seems to be the primary causelacking midribs.⑶The dl2allele affects the developmental of the rootsCompared to the wild-type, the dl2mutant developed fewer adventitious andlateral roots and the number of xylem poles in the central vascular cylinder was reduced.Furthermore, the diameter of the cortical cell was also reduced.⑷The dl2allele affects the polar auxin transport activityA large number of studies had demonstrated that plant hormone auxin, especiallyits characteristic polar transport, plays a key role during pattern formation of thevascular system in both dicot and monocot plants. The polar auxin transport activity indl2mutant is significantly reduced, the basipetal auxin transport activity in dl2coleotiles was reduced to approximately29.2%of the wild-type plants. In addition, theexpression of OsPIN1is significantly reduced. The similar result was obtained whentreated with PATIs.⑸The dl2allele affects the sensitivity towards auxinCompared to the wild-type, the dl2mutant showed a reduced auxin response inboth callus induction and subculture.⑹The genetic analysis and preliminary mapping of DL2regulating midribformationStudies have shown that the phenotype of drooping leaf is controlled by a singlerecessive gene DL2. Further DL2was located between markers RM247and RM3483onchromosome12by map-based cloning.