Origin and ontogeny of the xylopodium in Cerrado legumes: Role of the hypocotyl and ecological significance

Bud-bearing underground organs play a role on plant persistence in different fire-prone ecosystems worldwide, but their origin, development and anatomical features are still a matter of inquiry. The xylopodium, for instance, seem to originate from the hypocotyl or primary root, or both, usually associated with tuberous roots. Because this structure is commonly found in legumes across the Brazilian savannas (Cerrado), understanding its formation is important for unveiling its relevance for post-fire resprouting. Here we aimed to investigate the role of the hypocotyl on the development of six Cerrado legume shrubs, as well as histochemical features and ecological implications from seedlings to adult plants in this frequently-burned ecosystem. Seeds of the six study species were collected in central Brazil, set to germinate and cultivated until the stages of four, six, nine, and twelve months. At each stage, the region correspondent to the hypocotyl was delimited and anatomical and histochemical analyses performed. Over time, the hypocotyl region underwent a tuberization process, during which the cortex was progressively replaced by the formation of secondary xylem through intense cambial activity. Before the twelve-month-old stage, all species presented buds, mainly cotyledonary, and reacted to starch, phenols and pectins. Early hypocotyl tuberization drives the formation of the xylopodium from seedlings to adulthood, and the presence of buds and the accumulation of reserves during its formation is a fundamental process driving species’ persistence after fire.