Hi, Mahasem, proline have a diverse roles in plants. Not only during osmotic adjustement. Plesae, look carefully this paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3244962/

Everyone will agree that proline is produced during stress. Many would not know that it is also produced under non stress conditions. Two excellent review papers are attached. I hope this will be helpful.

Yes, Proline is a stress indicator in plants. where subjected to abiotic stress especially salt may lead to increased accumulation of proline in leaves

Dear All,

Thank you very much for your informative answers. Those are very useful.

Mahasen

It is well known that proline is synthesized in plant under non-stress conditions as well as under different stresses. Please read Biologia plantarum (2000) 43: 491-500.

Regards

Proline is naturally present in all plants and may increase ir response to osmotic stresses. That´s not the case of glycine betaine, which is an abiotic stress protectant present only in some species. I send you a recent paper in this subject.  I hope this will be helpfull.

Proline and glycine betaine are in basal levels in non-stressed plants, however when plants are under stress (drought or heat) the common response is to increase the content for proline, glycine betaine, sugars, or another compounds to tolerate the stress.

Regards

In addition to its role in protein synthesis and the plant cells’ response to environmental stresses, circumstantial evidence suggest that proline may also play a role in flowering and development both as a metabolite and as a signal molecule. Although there is a growing consensus that proline is of special importance

throughout the reproductive phase (from flower transition to seed development) a general agreement on the molecular and genetic mechanisms proline is involved in, is yet to be established.

However, ever since the early 80s different research groups found

a significant amount of proline in the reproductive organs of different

plant species, raising the possibility that the accumulation

of this amino acid may also occur in physiological non-stressed

conditions for developmental purposes. Chiang and Dandekar,3 for

example, reported that in Arabidopsis reproductive tissues, such as

florets, pollen, siliques and seeds, proline represents up to 26% of

the total amino acid pool, while in vegetative tissues it only accounts

for 1–3%. Even more striking, Schwacke et al.4 observed that the

content of free proline in tomato flowers was 60-fold higher than

in any other organ analyzed. Similar physiological accumulations

of proline have been reported, at different concentrations, in reproductive

organs of other plant species,3 and in most cases the overall

levels of this amino acid seem too high to be accounted for only by

an increased demand of protein synthesis.

Plants alter metabolism in various ways, including production of compatible solutes (e.g. proline, raffinose, and glycine betaine) that are able to stabilize proteins and cellular structures and/or to maintain cell turgor by osmotic adjustment, and redox metabolism to remove excess levels of ROS and re-establish the cellular redox balance (Bartels and Sunkar, 2005; Valliyodan and Nguyen, 2006;Munns and Tester, 2008; Janska et al.; 2010). At the molecular level, gene expression is modified upon stress (Chinnusamy et al., 2007; Shinozaki and Yamaguchi-Shinozaki, 2007) and epigenetic regulation plays an important role in the regulation of gene expression in response to environmental stress (Hauser et al., 2011; Khraiwesh et al., 2011). Stress-inducible genes comprise genes involved in direct protection from stress, including the synthesis of osmoprotectants, detoxifying enzymes, and transporters, as well as genes that encode regulatory proteins such as transcription factors, protein kinases, and phosphatases.

Proline accumulation occurs in plants in response to environmental stress conditions such as salinity, moisture, heat, water logging, pollution,pesticide etc .Being soluble in water It functions as an osmotic regulator and helps in the osmotic adjustment under stress . Its content increases rapidly during stress and also vanishes after release from  it. Proline is a protein amino (rather imino) acid and therefore expected to be present under non stress conditions also like all other amino acids. But the level may not be remarkable as it is under stress.  

I am sorry for entering into this debate after 26 days. Our research team has been working on various aspects of proline since 1988. I have following comments to make:

1. Yes, proline is very much present in all living systems (without any eincluding plants irrespective of whether thay are under stress or unstressed conditions.

I am sorry for entering into this debate after 26 days. Our research team has been working on various aspects of proline since 1988. I have following comments to make:

1. Yes, proline is very much present in all living systems (without any exception)including plants irrespective of whether thay are under stress or unstressed conditions. Proline (an imino acid) like other amino acids is present at levels desired by cells and their biosynthesis is strictly regulated (we do find feed-back inhibition, whenever its level increases beyond desired level). Due to its presence in relatively low level, you might have had difficulty in determining its levels in plant systems you tested

2. As far as glycinebetaine is concerned, there are large number of plant species, which do not possess any pathway for synthesis of glycinebetaine.

3. Although synthesis of proline seems to be tightly regulated under unstressed conditions, its level in living systems increases under stress as a result of stress induced influence/suppression in feed-back inhibition and/or due to its synthesis through alternate methods. In few of our papers (Arora & Pardha Saradhi 1995, 2002), we have shown the prevalence of mutiple proline biosynthetic pathways. Infact, one such pathway is light regulated.

4. Although we do not debate on the role of proline in osmotic adjustment, our research findings/evidences make us believe that proline has more significant role in regulation of NAD(P)H/NAD(P)+ ratio; scavenging reactive oxygen species; protecting photosystem II against photoinhibition; maintaining carboxylase to oxygenase ratio of Rubisco etc. You may down our following papers from ResearchGate:

Alia & Pardha Saradhi 1991; 1993

Alia et al. 1991, 1993, 1995, 1997

Arora & Pardha Saradhi 1995, 2002

Pardha Saradhi et al. 1993, 1995, 1996

Sivakumar et al. 1998,1999,2000, 2001,2002

Sharmila et al, 2008 etc.

If you have any other specific aspect related to proline, I shall be very happy to address. You may also contact me through my email - [email protected]

how proline content changes in susceptible and resistant germplasm of a plant ?