Shaped through light

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Shaped through light

Plants adapt their appearance to their environment. Plants grow, for example, more in hight when standing in the shadow. Light also influences the number of branches. But how this is regulated was up to now not clear. Now researchers from China and the UK showed how light regulates bud outgrow.

How and when branches are needed is hard to predict in advance. Therefore, plants develop side buds at regular intervals. When the circumstances are right, then those buds will grow into branches. One of the influencing factors is if the plant is standing in the shade or sun. Standing in the shade results in fewer branches.

This is what the researchers observed while growing tomato plants, the deeper the shade, the fewer the branches. They saw this reflected in the amount of HY5, which passes on the amount of sunlight vs shade. The deeper the shade, the lower the amount of HY5 the researchers detected in the buds. This was contrasted with what they saw for bud manager BRANCHED1. The deeper the shade, the more BRANCHED1 was present.

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In sunlight HY5 helps to wake up the buds

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To study how HY5 and BRANCHED1 influence each other, the researchers analyse the number of branches in plants that did not have any HY5 or BRANCHED1. While plants without any HY5 had less branches, plants without any BRANCHED1 had more. Moreover, in the absence of both HY5 and BRANCHED1 plants had more branches. Indicating that HY5 influences BRANCHED1, and that BRANCHED1 does not influence HY5 .

By analysing which genes had a different expression profile in plants without any HY5 or BRANCHED1 the researchers found out how these proteins do their job. Here they observed that BRANCHED1 turns off the genes needed to produce the hormones cytokinin and gibberellic acid. The absence of these hormones prevents the buds from waking up. For the buds to wake up HY5 turns on the genes that are needed for making the hormone brassinosteroid. Brassinosteroid in turn turns om manager BZR1 who turns off BRANCHED1.

Indirectly HY5 switches off BRANCHED1 by lots of sunlight, allowing the buds to wake up and grow into branches. It appears a bit of a round about way, with so many intermediate steps. But it prevents the plant form awaking its buds by every perceived sunray. Only when the plant is really shining in the sun, then its branches will emerge.

Literature

Han Dong, Jiachun Wang, Xuewei Song, Chaoyi Hu, Changan Zhu, Ting Sun, Zhiwen Zhou, Zhangjian Hu, Xiaojian Xia, Jie Zhou, Kai Shi, Yanhong Zhou, Christine H. Foyer, and Jingquan Yu (2023) HY5 functions as a systemic signal by integrating BRC1-dependent hormone signaling in tomato bud outgrowth. PNAS 120 (16) e2301879120 https://doi.org/10.1073/pnas.2301879120