Expression pattern of four storage xyloglucan mobilization-related genes during seedling development of the rain forest tree Hymenaea courbaril L.

Abstract

During seedling establishment, cotyledons of the rain forest tree Hymenaea courbaril mobilize storage cell wall xyloglucan to sustain growth. The polysaccharide is degraded and its products are transported to growing sink tissues. Auxin from the shoot controls the level of xyloglucan hydrolytic enzymes. It is not yet known how important the expression of these genes is for the control of storage xyloglucan degradation. In this work, partial cDNAs of the genes xyloglucan transglycosylase hydrolase (HcXTH1) and β-galactosidase (HcBGAL1), both related to xyloglucan degradation, and two other genes related to sucrose metabolism [alkaline invertase (HcAlkIN1) and sucrose synthase (HcSUS1)], were isolated. The partial sequences were characterized by comparison with sequences available in the literature, and phylogenetic trees were assembled. Gene expression was evaluated at intervals of 6 h during 24 h in cotyledons, hypocotyl, roots, and leaves, using 45-d-old plantlets. HcXTH1 and HcBGAL1 were correlated to xyloglucan degradation and responded to auxin and light, being down-regulated when transport of auxin was prevented by N-1-naphthylphthalamic acid (NPA) and stimulated by constant light. Genes related to sucrose metabolism, HcAlkIN1 and HcSUS1, responded to inhibition of auxin transport in consonance with storage mobilization in the cotyledons. A model is proposed suggesting that auxin and light are involved in the control of the expression of genes related to storage xyloglucan mobilization in seedlings of H. courbaril. It is concluded that gene expression plays a role in the control of the intercommunication system of the source–sink relationship during seeding growth, favouring its establishment in the shaded environment of the rain forest understorey.

Diurnal changes in storage carbohydrate metabolism in cotyledons of the tropical tree Hymenaea courbaril L. (Leguminosae)

“…Such temporal changes in auxin levels suggest its involvement in the control of gibberellin biosynthesis (Ross et al 2000), an important hormone class that regulate gene expression and activity of XTH as well (Smith et al 1996). Altogether, gene expression of XTH in cotyledons at noon (Brandão et al 2009) and our data on its activity variations corroborate this model. On the other hand, these changes could be associated to the growth of the top shoot of the seedling during the night period, which produces the auxin that is transported to the cotyledon .…”

“…A consequence of this is that one would expect that xyloglucan degradation (enzyme activities) would vary according the diurnal cycle of the growing seedling. Indeed, Brandão et al (2009) found that the expression of genes related to XG mobilisation present patterns of variation that corroborate this hypothesis.…”

“…On the other hand, these changes could be associated to the growth of the top shoot of the seedling during the night period, which produces the auxin that is transported to the cotyledon . Growth rates during the night were not measured here (differences are far to small to be measured accurately), but some evidence that genes related to sugar metabolism are active during the night in the top shoot was produced by Brandão et al (2009), suggesting that sugar metabolism is possibly related to respiratory metabolism.…”

“…When sink strength is constant and sugar transport is active between cotyledon and the growing top shoot, there is production of auxin in the leaves, which is transported to the cotyledons where it controls gene expression associated to storage mobilisation in the cotyledons , Brandão et al 2009. Indeed, at the cell level, the production of the hydrolases responsible for xyloglucan degradation and sucrose metabolism has been shown to be controlled by polar transport of auxin in seedlings of H. courbaril (Brandão et al 2009).…”

“…Indeed, at the cell level, the production of the hydrolases responsible for xyloglucan degradation and sucrose metabolism has been shown to be controlled by polar transport of auxin in seedlings of H. courbaril (Brandão et al 2009). However, in the present work, evidence was found that the concentration of auxin in the cotyledons increases from the middle of the night until the next morning, indicating an indirect role of auxin in xyloglucan mobilisation.…”