ÀÖÌìÌÃÓÎÏ·ÍøÕ¾

Lovely lemons, tasty oranges, refreshing limes - these juicy fruits grow on citrus trees that, like all plants, must sense their environment and respond. The high school student and artist, John Wu, thought of the human brain, when he saw the 3D shape of this protein and was surprised to find it is from orange trees, not humans (Fig1). His first impression that the protein having ‘a brain shape� is serendipitous because it echoes this protein’s role in plants -- binding one of the 9 plant hormones. Plant hormones are key in enabling plants to sense their environment and respond.

Plant’s alert system for overwatering or neglect:

If you have overwatered or forgotten a potted plant, you have likely triggered this protein. This protein is the binder of a plant's chemical signal to itself of a catastrophe like a flood or drought. In addition, this protein and the plant hormone the protein binds also have roles in a plant’s normal life cycle � e.g. triggering the release of fully-ripened fruit. The two colourful potted plants in the artwork highlight that this protein can contain two components / subunits.

 

 

Discovery and naming of plant hormone, abscisic acid (ABA):

Plants� reaction to drought needs a chemical to help carry signals from its roots to its leaves. The discovery of the plant hormone (/phytohormone) that signals drought did not come from studying plants� response to drought, but rather investigations into what triggers plants to lose their leaves, which is also called ‘leaf abscission�. In the 1960s, a team of researchers in California, USA, were studying cotton and identified a chemical that triggered leaf loss and the release of cotton fruit (as though the cotton had ripened and it was harvest time) (2). Simultaneously, in England, a research team was studying sycamore trees and identified a chemical that triggered leaf loss / seasonal dormancy (3). Each team gave the chemical a different name (Fig 2), but they soon realised that this was the same chemical and they needed to agree on one name, thus the plant hormone became known as ‘abscisic acid� (4).

 

 

Understanding the plant hormone, ABA, and the search for its binder

Scientific discovery is often not straightforward and in the case of ABA, there were multiple failed attempts to identify which plant protein bound ABA and converts the chemical signal into a protein-based signal. In the meantime, the chemical ABA was identified as a signal molecule used by many if not all plants, not just sycamore trees and cotton plants.

Wilted plants having higher concentrations of the chemical ABA was among the first discoveries after ABA was chemically identified (5; Reviews). Knowing ABA’s chemical identity meant that scientists could ‘fingerprint� its presence and absence in plant tissues. ABA concentrations were found to differ in different plant tissues and during the life cycle of the plant (5; Reviews).

ABA was found to trigger a variety of responses in plants, such as seeds not germinating, root growth stopping, and the closure of pores in leaves (5; Reviews). This last example is particularly helpful to your neglected house plants because the closure of these pores means the plants can retain more of their water. Water and oxygen (produced by photosynthesis) diffuse out of these pores, called stomata, and carbon dioxide (used in photosynthesis) diffuses in and thus these pores provide a way for the chemicals used and produced by photosynthesis to easily leave plant leaves.

Under-watering, over-watering, temperature variation, changes in soil salinity, over-exposure to light, and certain herbicides have all been found to result in increased concentrations of ABA (5; Reviews). Consequently, ABA became known as an ‘anti-stress� / ‘stress� plant hormone � aka a chemical generated by plants in reaction to environmental stresses. However, there was still mystery about which protein bound ABA and converted the chemical signal into the protein-based one (5; Reviews).

 

Mystery solved: small plant protein found that binds ABA

More than 40 years after the discovery of ABA, the protein sensor that bound ABA was finally found (6) and it was partially due to an annoying aspect of ABA -- that is ABA’s lack of chemical stability. The lack of stability meant ABA has limited usefulness for agriculture purposes, in contrast to other plant hormones, such as ethylene. Consequently, scientists were looking for other more stable chemicals that might induce an ABA-like response. Enter pyrabactin (6b), a chemical that although different from ABA (Fig3), was discovered to induce a similar response in plants as ABA does. An unexpected consequence of the discovery of pyrabactin was that it enabled scientists to finally figure-out what protein bound to ABA and thus which protein converted the chemical signal into the protein-based signal that linked to the plant responses such as growth stopping and pores closing.

The proteins that were found to bind ABA were initially referred to as Pyrabactin resistance (PYR) or Pyrabactin resistance-like (PYL) proteins. PYL and PYR are still used as one of the names for the proteins that bind ABA. This name is because when these proteins are absent from the plant and the plant is no longer susceptible to pyrabactin due to pyrabactin not being able to trigger an ABA-like response (6b). In most plants studied, more than 10 potential ABA-binding proteins have been identified. Each ABA-binding protein can be thought of as different ‘switches�, for example, one switch corresponding to opening and closing pores, another switch corresponding to the release of ripened fruit. Therefore, having more than one ABA-binding proteins makes sense given the variety of plant responses triggered by ABA.

The 3D shape of ABA-binding proteins was determined very soon after the proteins that bound ABA were identified, with five different groups of scientists pursuing and publishing structures of PYR1, PYL1 and PYL2 less than a year after protein was identified (7). ABA-binding proteins are surprisingly small (150-200 residues). ABA-binding proteins were found to be shaped similarly and contained subunits with a β-sheet core with surface α-helices (Fig3; 7-8). The first structures of the ABA-binding protein were from the highly-studied thale cress plant (Arabidopsis thaliana) (Fig3; 7-8), rather than the orange tree.

 

The first ABA-binding structures were discovered by research groups from France, China, Japan and USA and like different puzzle pieces each group contributed different aspects to our understanding (7). For example by comparing PYL1 structures from the different groups (Fig3; 7-8), one can find that the structures with ABA or pyrabactin (PDB IDs: 3jrs and 3neg), have captured the protein in two different states � i.e. a dimeric state (two subunits; like the two flower pots in the artwork) or a monomeric state (one subunit; as if there was only one flower pot). PDBe-KB pages contain an overview of all the current publicly available structures for PYL1 from A. thaliana and PYL1 from orange trees.

Being able to change states to a monomeric state is crucial to how ABA-binding proteins� function. You can explore structures showing why this protein needs to be able to change into a monomeric state in links under More Structures.

 

Science and its serendipitous moments

Scientific research is often not a straight-line and has serendipitous moments. The knowledge we have gained on how plants respond to stress is still growing. The potential for farmers to use chemicals to trigger plants� survival strategies in stressful environments is of interest to those concerned about food security and sustainable agriculture.

Pyrabactin did not find much usage in agriculture, because it did not trigger the ABA-response in a way that could be useful. However, more compounds are being explored as potential triggers of the ABA-response (links listed under More Structures).

Imagine if we could helpfully trigger ABA-pathways in crops. What useful tools these chemicals could be for either vulnerable crops that are less adaptive to environmental stress, or crops that are being exposed to unexpected or atypical environmental change. These chemicals could be useful tools to help lessen the impact on food production due to atypical weather that we are encountering with global warming. Additionally, these chemicals could be useful in managing plant growth and/or optimising water usage in greenhouses or other agricultural settings.

Genevieve Evans

About the artwork

John Wu (Year 12) from the Leys School (Cambridge, U.K.) described the protein that inspired his artwork as having an interesting shape, almost symmetrical, and looking like a brain. His initial thoughts were that this must be a protein from a complicated animal or human organ. This protein being found in citrus surprised and sparked the artist’s curiosity.

The sculpture is mixed media with two almost identical but slightly different sculptural pieces corresponding to the almost symmetrical structure of the ABA-binding protein. The main bodies represent the protein and are made with clay with painted straight lines to represent the central β-sheet of the protein and curls for the α-helices. Folded paper flowers are connected via wire to the main bodies. The plant pots and flowers highlight the link between the protein and plant growth.

View the artwork in the .

 

Structures mentioned in this article

Link to the PDB ID for the entries in the images in this publication
PDB ID 5mmq, PDB ID 3jrs, PDB ID 3kay, PDB ID 3neg

 

More Structures to explore

Link to the PDB IDs for the entries of ABA-binding proteins with their partner protein

PYL1 from orange tree with ABA and protein partner - PDB IDs 5mn0, 8ay9
PYL1 from thale cress with ABA and protein partner - PDB IDs 3jrq, 3kdj

Link to the PDB IDs for the entries of ABA-binding proteins with ABA-mimics bound

PDB ID 3wg8
PDB IDs 4lga, 4lgb, 4lg5
PDB ID 4wvo
PDB IDs 5or6, 5or2
PDB IDs 5vr7, 5vro, 5vs5, 5vsq, 5vsr, 5vt7
PDB IDs 5ur5, 5ur6
PDB IDs 6nwb, 6nwc
PDB IDs 7mlc, 7mld
PDB IDs 8ay6, 8ay3, 8ay7, 8ay8, 8ay9, 8aya

 

Sources

Inspiration for artwork

1.

Historical context:
  Identification of ABA / 1960s discoveries

2.

3.

4.

Further reading:
  Insightful reviews on ABA (with open-access to full text)

5. Reviews

(a) (2021)
(b) (2013)
(c) (2010)
(d) (2001)

2009-2010 Breakthroughs:

6.  Identification of ABA-binding protein

(a)
(b)

7.  First structures of ABA-binding proteins

(a)
(b)
(c)
(d)
(e)
(f) Commentary/summary on the five articles:

8.  Structure of ABA-binding protein with pyrabactin

(a)
(b)
(c)
(d)