Transplantámos os rebentos antes de murcharem e voltaram a brotar.

Because it is the 1st person plural of the simple past (pretérito perfeito) of transplantar: we transplanted.

In European Portuguese, many -ar verbs use -ámos in this past form to distinguish it in writing from the present -amos.

• transplantamos = we transplant / we are transplanting
• transplantámos = we transplanted

This is a very common European Portuguese spelling pattern:

• falamos / falámos
• plantamos / plantámos

Because Portuguese often omits subject pronouns when the verb ending already makes the subject clear.

Here, transplantámos clearly tells you the subject is we. So nós is unnecessary.

Both are possible:

• Transplantámos os rebentos...
• Nós transplantámos os rebentos...

The version without nós is more natural unless you want emphasis or contrast.

Here, rebentos means shoots, sprouts, or new plant growth.

It is a plant word, referring to young growth coming out of a plant. Depending on context, English might translate it as:

• shoots
• sprouts
• new shoots
• offshoots

So in this sentence it is talking about the parts of the plant that were transplanted.

Portuguese uses the definite article much more often than English.

So os rebentos does not necessarily sound especially emphatic; it often just means the shoots in a normal, natural way.

Here it suggests the speaker is talking about specific shoots, probably already known from the context.

English often drops the article where Portuguese keeps it:

• Plantámos flores.
• Plantámos as flores.

Both are possible depending on context, but Portuguese generally likes articles more than English does.

Because murcharem is a personal infinitive, and it agrees with the subject os rebentos.

After antes de, Portuguese often uses an infinitive. If the subject of that infinitive is clear and especially if it is plural or different from the main subject, Portuguese can mark it with a personal ending:

• antes de murchar = before wilting / before it wilts / before one wilts
• antes de murcharem = before they wilt

Since the thing that might wilt is os rebentos, plural, murcharem is very natural here.

In this sentence, murcharem is the personal infinitive, 3rd person plural, of murchar.

That is why it matches os rebentos.

A useful point: the form murcharem looks identical to the future subjunctive form, but here it is not future subjunctive, because it comes after the preposition de in antes de. After a preposition, this is understood as the infinitive form.

So here:

• antes de murcharem = before they wilted / before wilting

Yes. That is a perfectly good alternative.

• antes de murcharem
• antes que murchassem

Both can express before they wilted.

The difference is mainly grammatical style:

• antes de + infinitive is very common and compact
• antes que + subjunctive is also common, but a little more explicitly clause-like

In this sentence, antes de murcharem is very natural and elegant.

Because the subject changes in the second part of the sentence.

First clause:

• Transplantámos os rebentos
  Subject = we

Second clause:

• e voltaram a brotar
  Subject = the shoots (os rebentos)

So:

• transplantámos = we transplanted
• voltaram = they came back / they again...

If it were voltámos, it would mean we came back, which would not fit the meaning here.

Again, Portuguese often drops subject pronouns when they are clear from the verb form and the context.

So instead of saying:

• e eles voltaram a brotar

Portuguese very naturally says:

• e voltaram a brotar

The verb voltaram already tells you the subject is they, and context tells you that they = os rebentos.

It uses a very common Portuguese structure:

voltar a + infinitive = to do something again

So:

• voltar a brotar = to sprout again
• voltaram a brotar = they sprouted again / they started growing again

It is often more natural in Portuguese than simply adding a word like again.

Other examples:

• Voltei a tentar. = I tried again.
• Ela voltou a ligar. = She called again.

Yes, in many contexts rebrotaram would also work.

• voltaram a brotar = sprouted again
• rebrotaram = resprouted / sprouted again

The version with voltar a + infinitive is very common and very transparent for learners, because it follows a broad pattern used with many verbs. Rebrotar is also correct, but slightly more lexical/specific.

So the sentence could be phrased differently, but voltaram a brotar is completely natural.

Yes: the clearest European Portuguese feature here is transplantámos with -ámos.

That spelling is a strong clue for European Portuguese. A Brazilian text would very often write transplantamos instead.

The rest of the sentence is broadly standard Portuguese, but the accented past form is the most noticeable European Portuguese feature.