Shoot apical meristems.

Quiet Branches is back to regular posts about science. I’ll try to get one post up each week through the holidays, including some guest posts! Excited to bring you more plant science stories. 

Creating a plant

Plants are ubiquitous in our day-to-day lives. We all benefit from the products they produce. Air, food, shelter, clean water, medicines, and cultures all stem from plants. Humans require the bounty plants provide. Which raises a question:

How does all this greenery come about?

Most plants grow from seeds. The seed is an evolutionary innovation allowing plants to store and disperse the next generation. When a seed will germinates and developmental processes soften the seed coat so the embryonic plant can emerge. There are two special clusters of cells in a plant embryo called meristems. One is the root apical meristem that will become the root system, a topic for a future post. The Shoot apical meristem (SAM) gives rise to the part of the plant we most often see: The stems, trunks, leaves, flowers, and fruits.

The seed is an evolutionary innovation allowing plants to store and disperse the next generation

One major difference in plants and animals is that their growth is indeterminate and post-embryonic. This is where we Plants are born with all the ingredients to form a mature plant, but shoots and roots are generated after germination. When plant scientists regenerate whole plants from a tissue culture, they are producing meristems that then form new roots and shoots of a plant.

Plants maintain small populations of embryonic, youthful, stem cells throughout their lives. The maintenance of meristems is under developmental control of the plant. Shoot meristem(s) undergo several transitions, from producing leaves to inflorescences to flowers that become fruit or grain where the meristem terminates. The size of the meristem can determine how big an organ ends up because more stem cells get devoted to each organ as cells get pushed out of the meristem via continuous cell divisions and therefore plant growth. A lot of research is dedicated to factors affecting meristem biology.

Shoot meristem(s) undergo several transitions, from producing leaves to inflorescences to flowers that become fruit or grain where the meristem terminates

Meristems are maintained genetically by the products of 3 genes known as CLAVATA3 (CLV3), CLAVATA1 (CLV1), and WUSCHEL (WUS) (1). The CLV3 protein, like a key fitting a lock, activates the receptor protein CLV1. This turns down the expression of WUSCHEL. WUS, by an unknown mechanisms, turns up the expression of CLV3 to complete the negative feedback loop. The meristem is a 3-D structure. The domain of WUS expression resides below the surface layers of high CLV3 expression. Balance is in part maintained by plant growth regulators like hormones. Experiments have shown loss of WUS results in plants trying to organize a meristem, but failing. Losing CLV genes means an enlarged, mis-formed meristem. The population of stem cells is larger in plants that have lost CLV.

meristem

An international team of researchers recently published an article in Nature Genetics on how a bigger meristem can lead to a bigger tomato (2). They discovered a protein that modifies CLV3 protein with a type of sugar, arabinose. This modification of CLV3 is important for normal function. The partially functioning, unmodified CLV3, does not repress WUS as well, thus leading to more CLV3, an enlarged meristem, and a bigger tomato.

Meristems determine not only important determinants of fruit size, but also grain yield in rice. For instance, a naturally occurring variation due to the loss-of-function in a single gene that degrades the plant hormone cytokinin (3). This results in increases in the number of panicles in the rice inflorescence meristem resulting in more grain. Cytokinin increases in meristems may be good for yield, but high active cytokinin can result in larger awns in the final rice grain (4). Awns are spiky protrusions found in wild rice and other grasses to prevent herbivory. During crop domestication, awns become reduced in size and spikiness, making harvesting grains easier. A recently published report found one gene responsible for awn reduction is an enzyme converting cytokinin from inactive precursor to active cytokinin, promoting cell division and hence a bigger awn. Some varieties of domesticated rice have a mutation causing this gene to be inactive, resulting in reduced awn size, an important factor in domestication.

Meristems may be small, yet important cluster of cells on the frontier in science

Research into fruit size and grain yield is the tip of the iceberg when it comes to meristems. Meristems are where outgrowth happens in dormant buds in the spring, where the transition to flowering takes place, and alter growth habit in response to disease, light conditions, or herbivory. Meristems may be small, yet important cluster of cells on the frontier in science. Ongoing meristem research will be part of helping nourish the projected 9 billion people that will live on Earth by 2050, and beyond.

References:

(1) Schoof H, Lenhard M, Haeckner A, Mayer KF, Jürgens G, Laux T. 2000. The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between the CLAVATA and WUSCHEL genes. Cell 100: 635.

(2) Xu C, Liberatore KL, MacAlister CA, Huang Z, Chu Y, Jiang K, Brooks C, Ogawa-Ohnishi M, Xiong G, Pauly M, Van Eck J, Matsubayashi Y, van der Knaap E, Lippman B. 2015. A cascade of arabinosyltransferases controls shoot meristem size in tomato. Nature Genetics: doi: 10.1038/ng.3309.

(3) Ashikiri M, Sakakibara H, Lin S, Yamamoto T, Takashi T, Nishimura A, Angeles ER, Qian Q, Kitano H, Matsuoka M. 2005. Cytokinin oxidase regulates rice grain production. Science 309: 741.

(4) Hua L, Wang DR, Tan L, Fu Y, Liu F, Xiao L, Zhu Z, Fu Q, Sun X, Gu P, Cai H, McCouch SR, Sun C. 2015. LABA1, a Domestication Gene Associated with Long, Barbed Awns in Wild Rice. Plant Cell: Advanced Online Publication.


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