Sucrose metabolism in coffee fruit / Metabolismo da sacarose em frutos de cafe

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

Since coffee culture productivity is straightly connected to three basic production factors: climate, genetics and physiology, herein some aspects of the carbohydrates metabolism involved in the process of coffee grains filling through its development were analyzed. It is known the carbohydrates composition, mainly the polysaccharides composition of the coffee grain (endosperm) as well as the importance of sugars in the beverage quality, nevertheless the sucrose import from the leaves and its sharing in the fruits are still to be completely clarified. The sugars utilized in the seeds metabolism are extremely important, such as in the regulation of the drain-source relation and in the expression control of genes that codify some enzymes involved in sugars metabolism. The main goal of this work was to study the sucrose metabolism in coffee fruits through their development. Histological analyses and marked compounds supply showed that there are no vascular connections among the tissues of the pericarp, perisperm and endosperm, but conduction vases that run through the funiculus get to the perisperm, enabling an unloading of photoassimilates produced in the leaves. From the perisperm, these assimilates are transferred to the endosperm; The pericarp photosynthesis diminishes through the fruit development. Starch grains were observed in juvenile stages of the perisperm and during the endosperm formation these grains start to disappear in regions close to the forming tissue. The enzymes and the endogenous sugar level evaluated in separated tissues through fruit maturation show some differences among distinct lavoura treatments (?full sun?: ordinary culture conditions, shadowing and reduced loading). The sucrose synthase (SUS) showed activity values much higher than the ones presented by the acid invertases (IAV). The sucrose phosphate synthase (SPS) followed the sucrose accumulation in the pericarp initial stage in coffee fruits. It was shown the existence of two isoforms of SUS codificated by genes called CaSUS1 e CaSUS2 in C. Arabica, that possibly play different metabolic roles in different tissues in coffee fruit. These genes show a differential expression, in which CaSUS1 is expressed in the juvenile stage of perisperm and endosperm development; however, CaSUS2 expression overlaps the detected activity peaks of SUS and the sucrose accumulation in the final stages of pericarp and endosperm development. This fact suggests that the CaSUS1 isoform is related to sucrose degradation while it seems clear that the CaSUS2 isoform is related to sucrose synthesis. Both genes were shown to be also expressed in C.racemosa since CaSUS1 and CaSUS2 C. arabica probes recognized transcripts (mRNA) in this species endosperm. Therefore, CrSUS1 and CrSUS2 genes seem to codify SUS isoforms that play different functions from the ones observed in C. arabica; SUS1 seems to be related to sucrose synthesis. The shadowing has influenced the fruits development cycle duration, turning it longer than in pleno sol (respectively, 260 and 231 days after flowering) and altering the sucrose accumulation in fruits. According to these results, it is clear how complex is the sucrose metabolism in coffee fruits, since it is not always that the same pattern of sugar accumulation is followed by the enzymatic activities and enzyme degradation and sucrose (re)synthesis act simultaneously as well as sugar transference among tissues

ASSUNTO(S)

coffee cafe sucrose synthase phosphate sucrose synthase sacarose sintase fosfato invertase invertase sacarose

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