Gurbani says everything is made by same clay, but it also divides living beings into 5 forms based on their awareness of life and activity of 5 elements.
Plants and Animals differ in their characteristics, so do Humans/animals/plants/birds. Killing plants incur a karma which have to be paid but it is lesser as compared to killing animals. Even people without much spiritual inclination understand this. Thats why plucking a flower from someones' house does not have that much punishment as is of killing someone's dog or cat , and killing a dog or cat does not have that severe punishment as of killing a person.
On what scale does one measure the worth of one life form vs another? If any such scale exists, then it was surely made by man and not the Divine. It is man who places himself at the top of the totem pole and then dictates the worth of other life forms according to his own created system of virtues. In this respect, Humans are the Brahmans of the Animal Kingdom. Karma doesn't reside in action, it lies in intention. Killing an ant out of spite is much worse than killing a dog to put it out if it's misery. It is the intention behind the action that takes one closer or further to the Divine in one's mind. We exchange selfishness for Divinity. This is the only price to be paid.
GGS, page 988:
ਸਭੈ ਘਟ ਰਾਮੁ ਬੋਲੈ ਰਾਮਾ ਬੋਲੈ ॥
सभै घट रामु बोलै रामा बोलै ॥
Sabẖai gẖat rām bolai rāmā bolai.
Within all hearts, the Lord speaks, the Lord speaks.
ਰਾਮ ਬਿਨਾ ਕੋ ਬੋਲੈ ਰੇ ॥੧॥ ਰਹਾਉ ॥
राम बिना को बोलै रे ॥१॥ रहाउ ॥
Rām binā ko bolai re. ||1|| rahā▫o.
Who else speaks, other than the Lord? ||1||Pause||
ਏਕਲ ਮਾਟੀ ਕੁੰਜਰ ਚੀਟੀ ਭਾਜਨ ਹੈਂ ਬਹੁ ਨਾਨਾ ਰੇ ॥
एकल माटी कुंजर चीटी भाजन हैं बहु नाना रे ॥
Ėkal mātī kunjar cẖītī bẖājan haiʼn baho nānā re.
Out of the same clay, the elephant, the ant, and the many sorts of species are formed.
ਅਸਥਾਵਰ ਜੰਗਮ ਕੀਟ ਪਤੰਗਮ ਘਟਿ ਘਟਿ ਰਾਮੁ ਸਮਾਨਾ ਰੇ ॥੧॥
असथावर जंगम कीट पतंगम घटि घटि रामु समाना रे ॥१॥
Asthāvar jangam kīt paṯangam gẖat gẖat rām samānā re. ||1||
In stationary life forms, moving beings, worms, moths and within each and every heart, the Lord is contained. ||1||
ਏਕਲ ਚਿੰਤਾ ਰਾਖੁ ਅਨੰਤਾ ਅਉਰ ਤਜਹੁ ਸਭ ਆਸਾ ਰੇ ॥
एकल चिंता राखु अनंता अउर तजहु सभ आसा रे ॥
Ėkal cẖinṯā rākẖ ananṯā a▫or ṯajahu sabẖ āsā re.
Remember the One, Infinite Lord; abandon all other hopes.
ਪ੍ਰਣਵੈ ਨਾਮਾ ਭਏ ਨਿਹਕਾਮਾ ਕੋ ਠਾਕੁਰੁ ਕੋ ਦਾਸਾ ਰੇ ॥੨॥੩॥
प्रणवै नामा भए निहकामा को ठाकुरु को दासा रे ॥२॥३॥
Paraṇvai nāmā bẖa▫e nihkāmā ko ṯẖākur ko ḏāsā re. ||2||3||
Naam Dayv prays, I have become dispassionate and detached; who is the Lord and Master, and who is the slave? ||2||3||
Plants do suffer but it is degree of consciousness and amount of active elements pertaining in them which differentiate them from rest of the creation. They only have element of water being most active and other are almost null. that results in us humans having less empathy with plants than animals.
The meaning of empathy is the ability to understand and share the feelings of another. It has nothing to with the proportion of the 5 elements in another being with relation to yourself. The 5 elements are a primitive way of understanding the makeup of the universe. It is pseudoscience. If Gurbani were written today, instead of hundreds of years ago, it would surely have replaced the 5 elements with a mention of carbon, oxygen, hydrogen, nitrogen, etc. These elements are the fundamental building blocks of all life forms. This is the clay that Gurbani alludes to.
Plants provide the oxygen that makes life on earth possible for animals like us. Without them, we wouldn't exist. They give us much more than the sacred cow does, yet people kill the plants and worship the cow.