Importance of Rubisco activase in maize productivity based on mass selection procedure

Rogers et al., 1996; Fischer et al., 1997). In these plants, enzymes responsible for sucrose uptake and assimilation The original maize (Zea mays L.) var. Zacatecas 58 (Z 0 ) on one side, and photosynthetic rates, sucrose synthesis and five composites of cycles 5, 10, 15, 20, and 23 of and transport on the other, have been described as stratified mass selection (Z 5 , Z 10 , Z 15 , Z 20 , and Z 23 ) for participants in the mechanisms determining plant yield improved productivity applied to the original variety (Gifford and Evans, 1983; Daie, 1985; Hay and Walker, were used as the model system. Ribulose-1,51989; Jones et al., 1996). Indeed, it has been demonstrated bisphosphate carboxylase/oxygenase (EC 4.1.1.39) that in higher plants, carbohydrate depletion up-regulates (Rubisco) activity and the rates of net photosynthesis genes for photosynthesis, remobilization and export, in the leaves above the ear were compared in all the indicating a mechanism for controlling resource distribucomposites during the grain filling period. Leaves were tion among tissues and organs ( Koch, 1996). sampled weekly from anthesis to physiological maturSeveral studies in C3 plants have pointed to ribuloseity. Results showed a significant and gradual increase 1,5-bisphosphate carboxylase/oxygenase (Rubisco) actin Rubisco activity for the improved populations. In ivity and photosynthetic capacity of the source leaves vivo photosynthesis measured by IRGA during the as possible limiting factors for plant growth and fruit same period also showed increased levels associated development (Jang and Sheen, 1994; Rogers et al., with increases of grain yield. The highest Rubisco activity, photosynthetic rate and grain yield were found 1996). On the other hand, scarce information is availin the Z 23 population. Western blot analysis for Rubisco able about these mechanisms in 4 plants, as maize, protein did not show significant differences either suggest there is not a relationship between photosynthetic during the filling period or between populations. The rate and grain yield (Crosbie et al., 1981; Hageman and same analysis, however, for Rubisco activase protein Lambert, 1988). Moreover, to determine which enzymatic showed increasing contents in the improved populaactivities control carbohydrate flux is further complicated tions. These data confirm and complement previous by the additional complexity of cell specialization findings, indicating that the stratified visual mass (Furbank and Taylor, 1995; Dai et al., 1995). selection procedure applied to maize plants is associThe role of Rubisco activase (R-A) in this process ated with leaf content of Rubisco activase protein. The might be relevant since this protein is known to activate possible regulatory role of Rubisco activase during in vivo Rubisco (for reviews see Portis, 1990, 1992) from grain filling is discussed. an inactive complex (Rubisco: Pentose-P) by restoring the active Rubisco structure (Andrews et al., 1995;