ИСТИНА

THE IMPACT OF SHOOT APICAL MERISTEM SIZE ON THE STRUCTURE OF COMPOUND LEAF IN FASCIATED MUTANTS OF Pisum sativum Ekaterina A. Bykova, Vladimir V. Choob Lomonosov Moscow State University, Moscow, Russia katebykova.90@mail.ru The rate of cell division in the central zone of shoot apical meristem depends on signaling cross talk between the rib-zone and upper layers of tunica. Any disturbance of this interaction leads to a change in the number of cells in the meristem. The phenomenon of significant intensification of cell division and consequent increasing of meristem size is called fasciation (Choob, Sinyushin, 2012). This process often occurs via the enrichment of stem cell pool in the central zone of the meristem. During formation of any organ primordia, the meristem ‘allocates’ a certain number of cells, whose fate is rather strictly programmed. It is logical to assume that due to an increase of the cell number in the meristem, the ‘allocated’ cell pool can correlatively change, leading to modifications in organ morphology and arrangement. In previous papers (Gourlay et al., 2000; Bykova et al., 2015) devoted to manifestations of fasciation in P. sativum, the ‘allocated’ portion of meristematic cells intended for the development of a single compound leaf (including its base and stipules) was designated as a blastozone. In the present study, we used seedlings of two fasciated mutant lines of pea: “Shtambovy” and “Rosacrone”. Wild type plants of “Nemchinovskiy–766” variety served as control (isogenic to “Shtambovy”). Anatomical analysis of cells of the shoot apical meristem (SAM) of “Shtambovy” revealed ultrastructural features: irregular thickness of the cell walls and a large number of vesicles with electron dense loose inclusions, which were not characteristic of SAM of non-fasciated plants. In fasciated mutants of pea, the number of leaflets and tendrils of the compound leaf did not change. These numbers increased gradually, depending on the node number, in the same manner as in wild type plants. The changes were only revealed in rachis and stipule number and arrangement. For example, instead of single rachis, two rachises per node are formed. This phenomenon is usually accompanied by development of the additional stipules from both sides of the new rachis. We also recorded incompletely divided stipules in between two rachises of the same node. However, in some cases, these rachises were not independent: we often found incompletely fused rachises. All these observations could be explained by gradual increasing of the blastozone size. 31 Thus, the complexity of leaf structure in certain node could be an indirect method of blastozone measurement. In order to obtain the direct data on the correlation between the changes in the compound leaf structure and the shoot meristem size, we carried out morphometric measurements of SAM in wild type and fasciated mutants. The meristem size in the fasciated line “Rosacrone” increased gradually with the node number, significantly differing from the similar values in the wild type. Nevertheless, the differences between the wild type and another fasciated line “Shtambovy” cannot be unequivocally recognized. At the same time, the plants of “Shtambovy” exhibit a strong correlation between the maximal linear size of the meristem and the number of rachises per node. For example, when three rachises were located on one node, the average linear size of the meristem was 600 μm, if there was a single rachis on the node, the meristem linear size was approximately 170 μm. The difference is almost three-fold. We have not registered similar correlation in the other fasciated line “Rosacrone” probably because of late manifestation of fasciation in development (Bykova et al., 2015). Consequently, as a final conclusion we note that the main impact of the linear meristem size on the compound leaf structure in pea involves mainly the increase of rachis and stipules number per node, rather than changes in leaflet and tendril number. References Bykova E.A., Labunskaya E.A., Choob V.V. 2015. Morphological changes in the structure of blastozones during fasciation of Pisum sativum L. and Arabidopsis thaliana (L.) Heynh. Biol. Bull. 42: 179–185. Choob V.V., Sinyushin A.A. 2012. Flower and shoot fasciation: from phenomenology to the construction of models of apical meristem transformations. Russ. J. Plant Physiol. 59: 530–545. Gourlay C.W., Hofer J.M.I., Ellis T.H.N. 2000. Pea compound leaf architecture is regulated by interactions among the genes UNIFOLIATA, COCHLEATA, AFILA, and TENDRIL-LESS. Plant Cell 12: 1279–1294.