A new study reveals the role of sugars as initial regulator of apical dominance
Everyone loves to decorate their Christmas tree. Have you noted its particular growth pattern? It is broad at the bottom while narrower at the tip which is at the top. This peculiar growth pattern of certain plants is due to a phenomenon known as Apical dominance. Apical dominance can in simpler words be understood by growth of tips towards sunlight. For almost a century, we believed that the main factor responsible for apical dominance was the plant hormone, Auxin. But, according to new report published in the Journal PNAS, scientists claim that apical dominance is correlated with sugar availability to the plant and not apical supply of Auxin. They showed that apical dominance was controlled by the intensive demand of sugar by growing terminal tips which indirectly also limits their supply towards lateral or axillary buds. Therefore, this helps several trees to survive by out-competing others in dense forests in their race to sunlight, a vital component for photosynthesis.
Apical dominance is maintained by limiting the distribution of sugars to axillary buds, thereby making most of the sugar available to terminal growing tips. Garden pea plants of more than 150 cm in height were selected for the decapitations experiments. The approximate distance of shoot tip with the second node of plant was approximately 20 cm, a relatively far distance. Auxin content and axillary bud growth was investigated in intact and decapitated plants. It was found that when the shoot tip was cut or decapitation was done, there was strong correlation with the rapid bud outgrowth in the plant. Scientists used digital-time photography to observe bud growth in buds 40 cm below the shoot tip within 2.5 hour of decapitation. It was observed that sugars rapidly accumulate in axillary buds after the loss of shoot tip and the buds are released. Stem girdling experiments also proved that any shoot-tip derived inhibitor (Auxin in this case) was not the only responsible factor for bud release.
Examination of sugar content of axillary buds by mass spectrometry revealed that total sucrose (sugar) level was increased by 44% within 4 hours of decapitation. This study leads to an understanding which rules out Auxin as the sole and master regulator of apical dominance in plants. Rather, it gives an edge to sugars for being necessary as well as sufficient factor for axillary bud release from apical dominance.
Reference:
Mason MG, Ross JJ, Babst BA, Wienclaw BN, & Beveridge CA (2014). Sugar demand, not auxin, is the initial regulator of apical dominance. Proceedings of the National Academy of Sciences of the United States of America PMID: 24711430
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