How plants grow fat­ter – the secrets of ra­dial growth

We all know how humans grow fatter, but and what about plants!?

Radial growth (outward growth) is what makes plants "get fatter"!
Radial growth provides physical support, produces specialized vascular tissues that transport water and nutrients around plants, which yields items like wood and cork, and plays a major role in converting atmospheric carbon into plant biomass.

Bellow ground radial growth is responsible for producing our root and tuber vegetables, such as turnips, carrots, sugar beet and potatoes.

_{Photo: Ondrej Smetana (source)}

Recently the regulatory mechanisms behind how plants thicken their stems and roots by growing radially were partially revealed in two separate papers in Nature (here, and here) which provides the most comprehensive understanding yet of how plants grow radially.

Apical growth in plants occurs during primary development and then radially growth during secondary development, activated by the vascular cambium. When vascular cambium cells proliferate xylem and phloem are formed, xylem is produced inside, and phloem outside of the cambium.

Dr Ari Pekka Mähönen’s team used lineage-tracing and molecular genetic studies in the roots of Arabidopsis thaliana to show "that cells with a xylem identity direct adjacent vascular cambial cells to divide and function as stem cells." Furthermore, they showed, these xylem-identity cells (early-stage xylem cells, which had not yet differentiated) constitute an organizer for the stem cell activity.

Professor Ykä Helariutta’s` team showed that in "Arabidopsis the initiation of radial growth occurs around early protophloem-sieve-element cell files of the root procambial tissue". This means that during the early stage young phloem cells (protophloem) are initiating and organising the primary (procambial) stage of radial growth.

Together their findings could assist with future breeding of agricultural crops and trees to obtain higher economic yields and to maximise atmospheric carbon sequestration by increasing forest biomass.

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Publications mentioned:

Miyashima, S., Roszak, P., Sevilem, I., Toyokura, K., Blob, B., Heo, J., … Helariutta, Y. 2019. Mobile PEAR transcription factors integrate positional cues to prime cambial growth. Nature 565: 490–494. doi:10.1038/s41586-018-0839-y

Smetana, O., Mäkilä, R., Lyu, M., Amiryousefi, A., Sánchez Rodríguez, F., Wu, M.-F., Solé-Gil, A., Leal Gavarrón, M., Siligato, R., Miyashima, S., Roszak, P., Blomster, T., Reed, J.W., Broholm, S., Mähönen, A.P., 2019. High levels of auxin signalling define the stem-cell organizer of the vascular cambium. Nature 565: 485–489. doi:10.1038/s41586-018-0837-0