The breakdown of starch in leaves

Abstract

Contents Summary Introduction Structure of the starch granule Initial attack on the granule and the role of glucan, water dikinase Debranching of branched glucans The metabolism of linear glucans Export of starch catabolites Metabolism of glucose and maltose The emerging pathway of starch breakdown and its regulation Acknowledgements References Summary This review describes recent progress in discovering the pathway of starch breakdown in leaves. The synthesis of starch from photo‐assimilated carbon is one of the major biochemical fluxes in plants. Despite this, the pathway through which this starch is remobilized has not been defined. Numerous enzymes that could participate in starch breakdown are present in leaves, but until recently, the relative importance of each had not been determined. Through studies using model species such as Arabidopsis and potato, significant progress has now been made in determining the roles of known enzymes, and in the discovery of novel proteins necessary for breakdown. These data allow a tentative pathway for starch breakdown to be mapped out, involving hydrolysis primarily to maltose and subsequent maltose export to the cytosol. This provides a framework for complete discovery of the pathway and for the analysis of its regulation.

Molecular structural differences between maize leaf and endosperm starches

“…Both leaf and reserve starch have multiple levels of structure (Gilbert, 2011;Zeeman, Smith & Smith, 2004). Those considered here are the distribution of lengths of individual starch chains (first level), the branching structure of amylose molecules and amylopectin molecules (second level) and finally the granular morphology (fifth level).…”

The glucan phosphorylation mediated by α-glucan, water dikinase (GWD) is also essential in the light phase for a functional transitory starch turn-over

“…1 It has been shown that the action of GWD is an initial event of starch degradation. [2][3][4] Glucan phosphorylation by GWD disrupts the crystalline structure of the starch granule surface and stimulates hydrolytic enzyme activities, e.g., AtBAM3. 5 Mutations in the gene coding for GWD, SEX1, affect transitory starch turnover and have a deep impact on plant development.…”

Variação hormonal correlacionada à expressão de enzimas ligadas ao metabolismo do amido durante o desenvolvimento e amadurecimento da manga (Mangifera indica cv Keitt)

“…O grânulo de amido possui uma estrutura complexa, composta por dois tipos de polímeros; a amilose, cadeia de glicose formada por ligações α-1,4, tendo poucas ramificações, e a amilopectina, cadeia de glicose altamente ramificada, formada por ligações α-1,4 como a amilose e por ligações α-1,6 nas ramificações, constituindo a maior parte da massa de grânulos de amido de várias espécies vegetais (TETLOW et al, 2004;ZEEMAN et al, 2004).…”

“…No amadurecimento de frutos já foram identificadas tanto hidrolases (que incluem as α e β-amilases e enzimas desramificadoras) como fosforilases, resultando em reações complexas que produzem metabólitos que podem ser transportados dos plastídeos para o citosol (ZEEMAN et al, 2004) e servirem tanto como substratos energéticos como para a síntese da sacarose, promovendo, desta forma, o adoçamento do fruto.…”

“…Como a enzima não é capaz de quebrar as ligações α-1,6 da amilopectina alguns autores sugerem a existência de outras enzimas em Arabidopsis que possuam tal atividade, dentre as quais estão as enzimas desramificadoras, como as isoamilases (SMITH et al, 2004;SMITH et al, 2005;LLOYD et al, 2005).…”