To Breed or Not to Breed? Self-Incompatibility Prevents Inbreeding in Flowering Plants

By Robert C. Augustine

Genetic diversity is important for maintaining the health and survival of species. Humans often unwittingly reduce genetic diversity in domesticated crops and animals by selecting for traits of interest. For example, inbreeding has maintained the stereotypical qualities we have come to know and love about certain dog breeds, such as the extremes of stature in Chihuahuas and Great Danes.

While this inbreeding comes with predictability, it also increases the risk of costly genetic diseases. The principle is simple, organisms have genetic mistakes in their DNA that are usually hidden because they inherit two copies of every gene — one from mom and one from dad. A defective gene from one parent will be supplemented by a good copy from the other. The likelihood that both parents share the same defective gene is rare, unless of course mom and dad are related, as is the case with purebred dogs.

For plants, this problem is potentially more acute. Most flowering plants are hermaphrodites having both male and female sex organs together on the same plant, often within the same flower, and sometimes separated by only millimeters. While some plants seem to tolerate self-fertilization, others have gone to great lengths to avoid it. Nearly half of all flowering plants have evolved a fail-safe molecular strategy termed ‘self-incompatibility’ (or SI, for short), which completely prevents fertilization by self or even close relatives. SI promotes genetic diversity, and research by Chen and colleagues (2018) reveals new insights about the inner workings of the SI system that has potential implications for commercial fruit production.

Let’s talk about sex…

Before we delve into SI, you will first need a brief flowering plant sex education tutorial. Sperm cells, contained within pollen grains produced by the anthers (the male organs of the flower), must somehow get delivered to the ovules (the female organs of the flower) for fertilization to take place. Pollen grains first land on the female stigma, but then they must traverse the length of the style to reach the ovaries where the ovules reside (see figure). To do so the pollen grain forms an outgrowth called a pollen tube.

Pollen tubes are truly extraordinary. They are among the fastest growing cells and can achieve lengths up to hundreds of times greater than typical cells. Their mission is simple, but the journey is not. Pollen tubes must force their way through all the tissues of the pistil (stigma, style, ovary) to deliver two immobile sperm cells to the ovule. Guidance cues released by the female tissues help the pollen tubes navigate their way to the ovule, and once there the pollen tube explodes releasing the sperm cells for fertilization.

Pollen landing on a stigma can be blown in on the wind or delivered by way of a courier (such as bumblebee) from a distant individual. However, in the case of hermaphrodite plants, the pollen could come from the same flower or another flower on the same plant. Self-incompatibility allows the female organs to tell the difference and prevent growth of pollen tubes from “self” pollen.

A race to the finish line

Pollen tubes rely on a rapid form of polarized growth called ‘tip growth’, which magnificently balances cell wall deposition with the powerful forces of turgor pressure to drive cell expansion. Grow too quickly and risk prematurely bursting open; too slowly and other pollen tubes reach the prized ovule first.

Key to the growth of pollen tubes is the actin cytoskeleton (see figure). Actin forms tiny filaments that serve as supports and highways, which enable organelle movement and cargo delivery necessary for tip growth. Whereas human skeletons provide rigid frameworks that maintain structural integrity, the actin cytoskeleton is a highly-dynamic scaffolding capable of growing, shrinking, and remodeling on the order of seconds.

Actin dynamics are required for pollen tube growth, and actin disassembly is one of the major symptoms associated with the SI response, presumably because it quickly stops pollen tubes in their tracks. The factor(s) driving this rapid disassembly have remained elusive until now, with this study by Chen et al. (2018) identifying an obvious, but previously overlooked, enzyme.

The molecular sieve

Self-incompatibility is essentially a molecular sieve that allows unrelated (compatible) pollen tubes to pass, but sifts out closely related (incompatible) donors. The sieve is not a true physical barrier, but rather an enzyme released by the female and internalized by all pollen tubes as they grow through the pistil. The enzyme in this case is an S-RNase that degrades RNA molecules, eventually stimulating cell death within the pollen tube. However, pollen tubes are armed with a potential antidote, a specific F-box protein that recognizes and degrades the S-RNase depending on genetic background (Hua et al., 2008). Only F-box proteins from compatible pollen donors (i.e. unrelated to female) provide immunity by binding to the S-RNase and destroying it. In this way, the female can selectively allow passage of unrelated pollen tubes and increase the genetic diversity of its offspring.

Using pear trees as an SI model system, Chen and colleagues (2018) found that in addition to degrading RNA, S-RNases also sever actin filaments to promote cytoskeletal breakdown and cell death in incompatible pollen tubes. S-RNases were overlooked as actin disassembly factors because they are primarily associated with RNA destruction. This new discovery now provides another route by which S-RNases impinge upon SI. Whether S-RNases from other plants share this ability to breakdown actin remains an open question and a likely direction of future investigation.

The authors also found that levels of a fat molecule called phosphatidic acid (PA) becomes elevated within minutes of triggering the SI response. This rapid PA accumulation was predicted to initiate the cascade of events that eventually leads to pollen tube death. However, the PA surge was instead shown to prolong the survival of incompatible pollen tubes by preventing disassembly of the actin cytoskeletal network (see figure). One potential connection was that PA directly inhibits the ability of S-RNase to sever actin filaments, however this hypothesis was not supported by experimentation, leaving open the question of how PA stabilizes actin. The purpose of this protective mechanism is also unclear. One idea is that it might help to avoid inadvertent cell death in compatible pollen tubes.

Fruit for thought

The effectiveness of the SI mechanism in species such as pears, apples, peaches, plums, and cherries means that they never self-cross and therefore maintain diverse genetic pools resistant to genetic and biological diseases (Orcheski and Brown, 2012). However, this diversity also comes with its downside. For instance, apples must be propagated by grafting because the offspring from out-crosses never taste like their parents. In addition, breeders must carefully plan their orchards to ensure that there are ample unrelated varieties available to serve as pollen donors for fruit production.

The discovery of this protective mechanism might open new strategies to trick the female flowers into accepting pollen from related donors. Not only would this improve pollination efficiency and fruit yields, it could potentially bypass the need for grafting, and might even result in formation of new hybrid varieties.

Robert C. Augustine

Department of Biology

Washington University in St. Louis

St. Louis, USA

ORCID ID: 0000–0002–9614–1558

email: raugustine@wustl.edu

Read the research article upon which this story is based:

Chen, J., Wang, P., de Graaf, B.H.J., Zhang, H., Jiao, H., Tang, C., Zhang, S., and Wu, J. (2018). Phosphatidic acid mitigates S-RNase signaling in pollen by stabilizing the actin cytoskeleton. The Plant Cell. Published April 2018, https://doi.org/10.1105/tpc.18.00021.

Other Cited References:

Hua, Z.-H., Fields, A., Kao, T.-H. (2008) Biochemical models for S-RNase-based self-incompatibility. Mol. Plant 1(4): 575–585.

Orcheski, B. and Brown, S. (2012). A grower’s guide to self and cross-incompatibility in apple. New York Fruit Quarterly. 20(2): 25–28.